1.
Meiosis
–
This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi. Errors in meiosis resulting in aneuploidy are the known cause of miscarriage. In meiosis, DNA replication is followed by two rounds of division to produce four potential daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as Meiosis I and Meiosis II and this S-phase can be referred to as premeiotic S-phase or meiotic S-phase. Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase, a subset of recombination events results in crossovers, which create physical links known as chiasmata between the homologous chromosomes. During Meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis, in some cases all four of the meiotic products form gametes such as sperm, spores, or pollen. In female animals, three of the four meiotic products are eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse to form a zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, for example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes, half of maternal origin and half of paternal origin. Meiosis produces haploid gametes that contain one set of 23 chromosomes, when two gametes fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the number of chromosomes. First, the cell undergoes DNA replication, so each homolog now consists of two sister chromatids. Then each set of homologs pair with other and exchange DNA by homologous recombination leading to physical connections between the homologs. In the first meiotic division, the homologs are segregated to daughter cells by the spindle apparatus. The cells then proceed to a division without an intervening round of DNA replication. The sister chromatids are segregated to daughter cells to produce a total of four haploid cells. Female animals employ a variation on this pattern and produce one large ovum. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal DNA, furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids
Meiosis
–
A diagram of the meiotic phases
Meiosis
–
In meiosis, the
chromosomes duplicate (during
interphase) and
homologous chromosomes exchange genetic information (
chromosomal crossover) before a first division, called meiosis I. The daughter cells divide again in meiosis II, splitting up sister
chromatids to form haploid
gametes. Male and female gametes fuse during
fertilization, creating a diploid cell with a complete set of paired chromosomes.
2.
Myosotis
–
Myosotis is a genus of flowering plants in the family Boraginaceae. In the northern hemisphere they are commonly called forget-me-nots or scorpion grasses, the common name forget-me-not was calqued from the German Vergissmeinnicht, and first used in English in 1398 AD via King Henry IV. Similar names and variations are found in many languages, Myosotis alpestris is the state flower of Alaska. Plants of this genus are confused with Chatham Islands forget-me-nots which belong to a related genus. Over five hundred species names have been recorded, but there are only 74 accepted species, the rest are either synonyms of currently accepted names or submissions awaiting resolution. The genus is restricted to two discrete geographic centres, western Eurasia with about 60 confirmed species and New Zealand with around 40 confirmed species. Very small numbers of species occur elsewhere including North America, South America, despite this, Mysosotis species are now common throughout temperate latitudes via the planting of cultivars and introductions of alien species. Many are popular in gardens, preferring moist habitats, in areas where they are not native, they have frequently escaped to wetlands and riverbanks. Only those native to the Northern hemisphere are commonly called Forget-me-not and it is equally possible that the genus originated in either the Northern Hemisphere or in New Zealand. One or two European species, especially the wood forget-me-not, Myosotis sylvatica have been introduced into most of the regions of Europe, Asia. Myosotis have 5-merous actinomorphic flowers with 5 sepals and petals, flowers are typically 1 cm diameter, flat, and blue, pink, white or yellow with yellow centres, growing on scorpioid cymes. They may be annual or perennial with alternate leaves and they typically flower in spring or soon after snow-melt in alpine eco-systems. Their root systems are generally diffuse and their seeds are found in small, tulip-shaped pods along the stem to the flower. The pods attach to clothing when brushed against and eventually fall off, seeds can be collected by putting a piece of paper under the stems and shaking the seed pods and some seeds will fall out. Myosotis scorpioides is also known as scorpion grass due to the curve of its inflorescence. Mysosotis are used as food plants by the larvae of some Lepidoptera species including the setaceous Hebrew character, currently 74 species are classified as accepted species at The Plant List, of which 40 are endemic to New Zealand. The full list of species includes the following, In a German legend, God named all the plants when an unnamed one cried out, Forget-me-not. God replied, That shall be your name, in another Greek legend, when the Creator thought he had finished giving the flowers their colours, he heard one whisper Forget me not
Myosotis
–
Myosotis
Myosotis
–
Myosotis sylvatica
Myosotis
3.
Animal cell
–
A eukaryote is any organism whose cells contain a nucleus and other organelles enclosed within membranes. Eukaryotes belong to the taxon Eukarya or Eukaryota, the presence of a nucleus gives eukaryotes their name, which comes from the Greek εὖ and κάρυον. Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, plants and algae contain chloroplasts. Eukaryotic organisms may be unicellular or multicellular, only eukaryotes form multicellular organisms consisting of many kinds of tissue made up of different cell types. Eukaryotes can reproduce asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two identical cells. In meiosis, DNA replication is followed by two rounds of division to produce four daughter cells each with half the number of chromosomes as the original parent cell. These act as sex cells resulting from genetic recombination during meiosis, the domain Eukaryota appears to be monophyletic, and so makes up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features, eukaryotes represent a tiny minority of all living things. However, due to their larger size, eukaryotes collective worldwide biomass is estimated at about equal to that of prokaryotes. Eukaryotes first developed approximately 1. 6–2.1 billion years ago, in 1905 and 1910, the Russian biologist Konstantin Mereschkowsky argued three things about the origin of nucleated cells. Firstly, plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, secondly, the host had earlier in evolution formed by symbiosis between an amoeba-like host and a bacteria-like cell that formed the nucleus. Thirdly, plants inherited photosynthesis from cyanobacteria, the split between the prokaryotes and eukaryotes was introduced in the 1960s. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton, the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1938 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes. However he mentioned this in one paragraph, and the idea was effectively ignored until Chattons statement was rediscovered by Stanier. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in cells in her paper. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA and this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles, mitochondria and chloroplasts
Animal cell
–
Had'n
Animal cell
–
Longitudinal section through the flagellum of
Chlamydomonas reinhardtii
4.
Chromosome
–
A chromosome is a DNA molecule with part or all of the genetic material of an organism. Prokaryotes usually have one single circular chromosome, whereas most eukaryotes are diploid, chromosomes in eukaryotes are composed of chromatin fiber. Chromatin fiber is made of nucleosomes, a nucleosome is a histone octamer with part of a longer DNA strand attached to and wrapped around it. Chromatin fiber, together with associated proteins is known as chromatin, chromatin is present in most cells, with a few exceptions, for example, red blood cells. Occurring only in the nucleus of cells, chromatin contains the vast majority of DNA, except for a small amount inherited maternally. Chromosomes are normally visible under a microscope only when the cell is undergoing the metaphase of cell division. Before this happens every chromosome is copied once, and the copy is joined to the original by a centromere resulting in an X-shaped structure, the original chromosome and the copy are now called sister chromatids. During metaphase, when a chromosome is in its most condensed state, in this highly condensed form chromosomes are easiest to distinguish and study. In prokaryotic cells, chromatin occurs free-floating in cytoplasm, as these cells lack organelles, the main information-carrying macromolecule is a single piece of coiled double-helix DNA, containing many genes, regulatory elements and other noncoding DNA. The DNA-bound macromolecules are proteins that serve to package the DNA, chromosomes vary widely between different organisms. Some species such as certain bacteria also contain plasmids or other extrachromosomal DNA and these are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. Chromosomal recombination during meiosis and subsequent sexual reproduction plays a significant role in genetic diversity. In prokaryotes and viruses, the DNA is often densely packed and organized, in the case of archaea, by homologs to eukaryotic histones, small circular genomes called plasmids are often found in bacteria and also in mitochondria and chloroplasts, reflecting their bacterial origins. Some use the term chromosome in a sense, to refer to the individualized portions of chromatin in cells. However, others use the concept in a sense, to refer to the individualized portions of chromatin during cell division. The word chromosome comes from the Greek χρῶμα and σῶμα, describing their strong staining by particular dyes, schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now so familiar to everyone as chromosomes. The term was coined by von Waldeyer-Hartz, referring to the term chromatin, in a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles were the continuity of chromosomes and the individuality of chromosomes and it is the second of these principles that was so original
Chromosome
–
Walter Sutton (left) and
Theodor Boveri (right) independently developed the chromosome theory of inheritance in 1902.
Chromosome
–
Diagram of a replicated and condensed
metaphase eukaryotic chromosome. (1)
Chromatid – one of the two identical parts of the chromosome after
S phase. (2)
Centromere – the point where the two chromatids touch. (3) Short arm. (4) Long arm.
Chromosome
Chromosome
–
Human chromosomes during
metaphase
5.
Cell nucleus
–
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. Eukaryotes usually have a nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei. Cell nuclei contain most of the genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones. The genes within these chromosomes are the nuclear genome and are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, because the nuclear membrane is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be transported by carrier proteins while allowing free movement of small molecules. Movement of large molecules such as proteins and RNA through the pores is required for gene expression and the maintenance of chromosomes. The best-known of these is the nucleolus, which is involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA, the nucleus was the first organelle to be discovered. What is most likely the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek and he observed a lumen, the nucleus, in the red blood cells of salmon. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei, the nucleus was also described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London. Brown was studying orchids under microscope when he observed an opaque area and he did not suggest a potential function. In 1838, Matthias Schleiden proposed that the plays a role in generating cells. He believed that he had observed new cells assembling around cytoblasts, Franz Meyen was a strong opponent of this view, having already described cells multiplying by division and believing that many cells would have no nuclei. The function of the nucleus remained unclear, between 1877 and 1878, Oscar Hertwig published several studies on the fertilization of sea urchin eggs, showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus. This was the first time it was suggested that an individual develops from a nucleated cell, therefore, the necessity of the sperm nucleus for fertilization was discussed for quite some time. However, Hertwig confirmed his observation in other groups, including amphibians. Eduard Strasburger produced the results for plants in 1884
Cell nucleus
–
HeLa cells stained for nuclear
DNA with the
Blue Hoechst dye. The central and rightmost cell are in
interphase, thus their entire nuclei are labeled. On the left, a cell is going through
mitosis and its DNA has condensed.
Cell nucleus
–
Oldest known depiction of cells and their nuclei by
Antonie van Leeuwenhoek, 1719
Cell nucleus
–
Drawing of a
Chironomus salivary gland cell published by
Walther Flemming in 1882. The nucleus contains
Polytene chromosomes.
Cell nucleus
–
A mouse
fibroblast nucleus in which
DNA is stained blue. The distinct chromosome territories of chromosome 2 (red) and chromosome 9 (green) are stained with
fluorescent in situ hybridization.
6.
Allium
–
Allium is a genus of monocotyledonous flowering plants that includes the cultivated onion, garlic, scallion, shallot and leek as well as chives and hundreds of other wild species. The generic name Allium is the Latin word for garlic, some sources refer to Greek αλεω by reason of the smell of garlic. The cooking and consumption of parts of the plants is due to the variety of flavours. The inclusion of a species to the genus Allium is taxonomically difficult, estimates of the number of species have been as low as 260, and as high as 979, but are most likely about 800–850. The type species for the genus is Allium sativum, Allium species occur in temperate climates of the Northern Hemisphere, except for a few species occurring in Chile, Brazil, and tropical Africa. They vary in height between 5 cm and 150 cm, the flowers form an umbel at the top of a leafless stalk. The bulbs vary in size between species, from small to rather large, some species develop thickened leaf-bases rather than forming bulbs as such. Plants of the Allium genus produce chemical compounds that give them a characteristic onion or garlic taste, many are used as food plants, though not all members of the genus are equally flavorous. In most cases, both bulb and leaves are edible and the taste may be strong or weak, depending on the species, in the rare occurrence of sulfur-free growth conditions, all Allium species lose their usual pungency altogether. In the APG III classification system, Allium is placed in the family Amaryllidaceae, in some of the older classification systems, Allium was placed in Liliaceae. Molecular phylogenetic studies have shown this circumscription of Liliaceae is not monophyletic, Allium is one of about fifty-seven genera of flowering plants with more than 500 species. It is by far the largest genus in the Amaryllidaceae, the genus Allium is characterised by herbaceous geophyte perennials with true bulbs, some of which are borne on rhizomes and an onion or garlic odor and flavor. A small number of species have tuberous roots, the bulbs outer coats are commonly brown or grey, with a smooth texture, and are fibrous, or with cellular reticulation. The inner coats of the bulbs are membranous, many alliums have basal leaves that commonly wither away from the tips downward before or while the plants flower, but some species have persistent foliage. Plants produce from one to 12 leaves, most species having linear, the leaf blades are straight or variously coiled, but some species have broad leaves, including A. victorialis and A. tricoccum. The leaves are sessile, and very rarely narrowed into a petiole, the flowers, which are produced on scapes are erect or in some species pendent, having six petal-like tepals produced in two whorls. The flowers have one style and six stamens, the anthers. The ovaries are superior, and three-lobed with three locules, the fruits are capsules that open longitudinally along the capsule wall between the partitions of the locule
Allium
–
Allium
Allium
–
Allium flavum (yellow) and
Allium carinatum (purple)
Allium
–
Capsule of
Allium oreophilum.
Allium
–
Allium ursinum (wild garlic) in a
Bristol,
England, woodland
7.
Cell biology
–
Cell biology is a branch of biology that studies the different structures and functions of the cell and focuses mainly on the idea of the cell as the basic unit of life. This is done both on a microscopic and molecular level as it encompasses prokaryotic cells and eukaryotic cells, knowing the components of cells and how cells work is fundamental to all biological sciences, it is also essential for research in bio-medical fields such as cancer, and other diseases. Research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology, and developmental biology. The study of the cell is done on a level, however, most of the processes within the cell are made up of a mixture of small organic molecules, inorganic ions, hormones. Approximately 75-85% of the volume is due to water making it an indispensable solvent as a result of its polarity. These molecules within the cell, which operate as substrates, provide an environment for the cell to carry out metabolic reactions. The cell shape varies among the different types of organisms, and are then classified into two categories, eukaryotes and prokaryotes. Cell biology focuses more on the study of cells, and their signalling pathways. This environment of the cell is made up of hydrophilic and hydrophobic regions which allows for the exchange of the above-mentioned molecules and ions. The hydrophilic regions of the cell are mainly on the inside and outside of the cell, the cell membrane consists of lipids and proteins which accounts for its hydrophobicity as a result of being non-polar substances. Therefore, in order for these molecules to participate in reactions, within the cell and they accomplish this process of gaining access to the cell via, osmotic pressure, diffusion, concentration gradients, and membrane channels. Inside of the cell are extensive internal sub-cellular membrane-bounded compartments called organelles, cytoplasm - contents of the main fluid-filled space inside cells, chemical reactions also happen in this jelly-like substance. Nuclear Architecture is important for dictating nuclear function, vacuole - contain cell sap or other storage material. Vesicle - small membrane-bounded spheres inside cells which transport substances, the growth process of the cell does not refer to the size of the cell, but instead the density of the number of cells present in the organism at a given time. Cell growth pertains to the increase in the number of cells present in an organism as it grows and develops, Cells are the foundation of all organisms, they are the fundamental unit of life. The growth and development of the cell are essential for the maintenance of the host, for this process the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication, cell division, regeneration, specialization, and cell death. The cell cycle is divided into four phases, G1, S, G2. The G phases – which is the growth phase - makes up approximately 95% of the cycle
Cell biology
–
Understanding cells in terms of their
molecular components.
Cell biology
–
Endothelial cells under the microscope. Nuclei are stained blue with DAPI, microtubles are marked green by an antibody and actin filaments are labelled red with phalloidin.
Cell biology
–
Electron micrograph.
Cell biology
–
Czech anatomist Jan Evangelista Purkyně is best known for his 1837 discovery of
Purkinje cells.
8.
Cell cycle
–
The cell cycle or cell-division cycle is the series of events that take place in a cell leading to its division and duplication of its DNA to produce two daughter cells. In bacteria, which lack a nucleus, the cell cycle is divided into the B, C. The B period extends from the end of cell division to the beginning of DNA replication, DNA replication occurs during the C period. The D period refers to the stage between the end of DNA replication and the splitting of the cell into two daughter cells. In cells with a nucleus, as in eukaryotes, the cycle is also divided into three periods, interphase, the mitotic phase, and cytokinesis. During interphase, the cell grows, accumulating nutrients needed for mitosis, preparing it for cell division, during the mitotic phase, the chromosomes separate. During the final stage, cytokinesis, the chromosomes and cytoplasm separate into two new daughter cells, to ensure the proper division of the cell, there are control mechanisms known as cell cycle checkpoints. After cell division, each of the cells begin the interphase of a new cycle. The cell cycle consists of four phases, G1 phase, S phase, G2 phase. Activation of each phase is dependent on the progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase, after cell division, each of the daughter cells begin the interphase of a new cycle. G0 is a phase where the cell has left the cycle and has stopped dividing. The cell cycle starts with this phase, the word post-mitotic is sometimes used to refer to both quiescent and senescent cells. Non proliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time and this is very common for cells that are fully differentiated. Cellular senescence occurs in response to DNA damage and external stress, some cells enter the G0 phase semi-permanently, and are considered post-mitotic e. g. some liver, kidney, stomach cells. Many cells do not enter G0 and continue to divide throughout an organisms life, before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done during interphase, interphase is a series of changes that takes place in a newly formed cell and its nucleus, before it becomes capable of division again. It is also called preparatory phase or intermitosis, previously it was called resting stage because there is no apparent activity related to cell division. Typically interphase lasts for at least 90% of the total time required for the cell cycle
Cell cycle
–
Onion (
Allium) cells in different phases of the cell cycle. Growth in an
organism is carefully controlled by regulating the cell cycle.
Cell cycle
–
Schematic of the cell cycle. outer ring: I =
Interphase, M =
Mitosis; inner ring: M =
Mitosis, G 1 =
Gap 1, G 2 =
Gap 2, S =
Synthesis; not in ring: G 0 =
Gap 0/Resting.
Cell cycle
–
Nobel Laureate Paul Nurse
9.
Chromosomes
–
A chromosome is a DNA molecule with part or all of the genetic material of an organism. Prokaryotes usually have one single circular chromosome, whereas most eukaryotes are diploid, chromosomes in eukaryotes are composed of chromatin fiber. Chromatin fiber is made of nucleosomes, a nucleosome is a histone octamer with part of a longer DNA strand attached to and wrapped around it. Chromatin fiber, together with associated proteins is known as chromatin, chromatin is present in most cells, with a few exceptions, for example, red blood cells. Occurring only in the nucleus of cells, chromatin contains the vast majority of DNA, except for a small amount inherited maternally. Chromosomes are normally visible under a microscope only when the cell is undergoing the metaphase of cell division. Before this happens every chromosome is copied once, and the copy is joined to the original by a centromere resulting in an X-shaped structure, the original chromosome and the copy are now called sister chromatids. During metaphase, when a chromosome is in its most condensed state, in this highly condensed form chromosomes are easiest to distinguish and study. In prokaryotic cells, chromatin occurs free-floating in cytoplasm, as these cells lack organelles, the main information-carrying macromolecule is a single piece of coiled double-helix DNA, containing many genes, regulatory elements and other noncoding DNA. The DNA-bound macromolecules are proteins that serve to package the DNA, chromosomes vary widely between different organisms. Some species such as certain bacteria also contain plasmids or other extrachromosomal DNA and these are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. Chromosomal recombination during meiosis and subsequent sexual reproduction plays a significant role in genetic diversity. In prokaryotes and viruses, the DNA is often densely packed and organized, in the case of archaea, by homologs to eukaryotic histones, small circular genomes called plasmids are often found in bacteria and also in mitochondria and chloroplasts, reflecting their bacterial origins. Some use the term chromosome in a sense, to refer to the individualized portions of chromatin in cells. However, others use the concept in a sense, to refer to the individualized portions of chromatin during cell division. The word chromosome comes from the Greek χρῶμα and σῶμα, describing their strong staining by particular dyes, schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now so familiar to everyone as chromosomes. The term was coined by von Waldeyer-Hartz, referring to the term chromatin, in a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles were the continuity of chromosomes and the individuality of chromosomes and it is the second of these principles that was so original
Chromosomes
–
Walter Sutton (left) and
Theodor Boveri (right) independently developed the chromosome theory of inheritance in 1902.
Chromosomes
–
Diagram of a replicated and condensed
metaphase eukaryotic chromosome. (1)
Chromatid – one of the two identical parts of the chromosome after
S phase. (2)
Centromere – the point where the two chromatids touch. (3) Short arm. (4) Long arm.
Chromosomes
Chromosomes
–
Human chromosomes during
metaphase
10.
Interphase
–
Interphase is the phase of the cell cycle in which a typical cell spends most of its life. During this phase, the cell copies its DNA in preparation for mitosis, interphase is the daily living or metabolic phase of the cell, in which the cell obtains nutrients and metabolizes them, grows, reads its DNA, and conducts other normal cell functions. The majority of eukaryotic cells spend most of their time in interphase and this phase was formerly called the resting phase. However, interphase does not describe a cell that is resting, rather, the cell is actively living. A common misconception is that interphase is the first stage of mitosis, however, since mitosis is the division of the nucleus, prophase is actually the first stage. In interphase, the cell gets itself ready for mitosis or meiosis, there are three stages of cellular interphase, with each phase ending when a cellular checkpoint checks the accuracy of the stages completion before proceeding to the next. The stages of interphase are, G1, in which the cell grows, during this time, a high amount of protein synthesis occurs and the cell grows - more organelles are produced and the volume of the cytoplasm increases. If the cell is not to again, it will enter G0. Synthesis, in which the cell duplicates its DNA, G2, in which the cell resumes its growth in preparation for division. The mitochondria divide and the continues to grow until mitosis begins. In plants, chloroplasts also divide during G2, in addition, some cells that do not divide often or ever, enter a stage called G0, which is either a stage separate from interphase or an extended G1. The duration of time spent in interphase and in stage of interphase is variable. Most cells of adult mammals spend about 20 hours in interphase, interphase includes G1, S, and G2 phases. Mitosis and cytokinesis, however, are separate from interphase, when G2 is completed, the cell enters a relatively brief period of nuclear and cellular division, composed of mitosis and cytokinesis, respectively. After the successful completion of mitosis and cytokinesis, both resulting daughter cells re-enter G1 of interphase, in the cell cycle, interphase is preceded by telophase and cytokinesis of the M phase. In alternative fashion, interphase is sometimes interrupted by G0 phase, after the successful completion of the G2 checkpoint, the final checkpoint in interphase, the cell proceeds to prophase, or in plants to preprophase, which is the first stage of mitosis. G0 phase is viewed as either an extended G1 phase where the cell is neither dividing nor preparing to divide, in gamete production interphase is succeeded by meiosis. In programmed cell death, interphase is followed or preempted by apoptosis
Interphase
–
An illustration of interphase. The
chromatin has not yet condensed, and the cell is undergoing its normal functions.
11.
Cytokinesis
–
Cytokinesis is that part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the stages of nuclear division in mitosis and meiosis. During cytokinesis the spindle apparatus partitions and transports duplicated chromatids into the cytoplasm of the daughter cells. After the completion of the telophase and cytokinesis each daughter cell enters the interphase of the cell cycle, particular functions demand various deviations from the process of symmetrical cytokinesis, for example in oogenesis in animals the ovum takes almost all the cytoplasm and organelles. This leaves very little for the polar bodies, which in most species die without function. Another form of mitosis occurs in such as liver and skeletal muscle, it omits cytokinesis. Plant cytokinesis differs from animal cytokinesis, partly because of the rigidity of plant cell walls, Cytokinesis largely resembles the prokaryotic process of binary fission, but because of differences between prokaryotic and eukaryotic cell structures and functions, the mechanisms differ. Animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis, the process can be divided to the following distinct steps, anaphase spindle reorganization, division plane specification, actin-myosin ring assembly and contraction, and abscission. Faithful partitioning of the genome to emerging daughter cells is ensured through the temporal coordination of the above individual events by molecular signaling pathways. Animal cell cytokinesis starts with the stabilization of microtubules and reorganization of the spindle to form the central spindle. Central spindle forms when non-kinetochore microtubule fibres are bundled between the spindle poles, the process of mitotic spindle reorganization and central spindle formation is caused by the decline of CDK1 activity during anaphase. The decline of CDK1 activity at the metaphase-anaphase transition leads to dephosphorylating of inhibitory sites on multiple central spindle components. First of all, the removal of a CDK1 phosphorylation from a subunit of the CPC allows its translocalization to the spindle from the centromeres. Besides being a component of the central spindle itself, CPC also plays a role in the phosphoregulation of other central spindle components, including PRC1. MKLP1, together with the Rho-family GTPase activating protein CYK-4, forms the centralspindlin complex, centralspindlin binds to the central spindle as higher-order clusters. The centralspindlin cluster formation is promoted by phosphorylation of MLKP1 by Aurora B, the second step of animal cell cytokinesis involves division plane specification and cytokinetic furrow formation. Precise positioning of the plane between the two masses of segregated chromosomes is essential to prevent chromosome loss. Meanwhile, the mechanism by which the spindle determines the division plane in animal cells is perhaps the most enduring mystery in cytokinesis, there exist three hypotheses of furrow induction
Cytokinesis
–
Cilliate undergoing cytokinesis, with the
cleavage furrow being clearly visible.
12.
Cytoplasm
–
In cell biology, the cytoplasm is the material within a living cell, excluding the cell nucleus. It comprises cytosol and the organelles – the cells internal sub-structures, all of the contents of the cells of prokaryotic organisms are contained within the cytoplasm. Within the cells of organisms the contents of the cell nucleus are separated from the cytoplasm. The cytoplasm is about 80% water and usually colorless, the submicroscopic ground cell substance or cytoplasmatic matrix which remains after exclusion the cell organelles and particles is groundplasm. It is within the cytoplasm that most cellular activities occur, such as many metabolic pathways including glycolysis, the concentrated inner area is called the endoplasm and the outer layer is called the cell cortex or the ectoplasm. Movement of calcium ions in and out of the cytoplasm is an activity for metabolic processes. In plants, movement of the cytoplasm around vacuoles is known as cytoplasmic streaming, the term was introduced by Rudolf von Kölliker in 1863, originally as a synonym for protoplasm, but later it has come to mean the cell substance and organelles outside the nucleus. There has been disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles. The physical properties of the cytoplasm have been contested in recent years and it remains uncertain how the varied components of the cytoplasm interact to allow movement of particles and organelles while maintaining the cell’s structure. The flow of cytoplasmic components plays an important role in cellular functions which are dependent on the permeability of the cytoplasm. An example of function is cell signalling, a process which is dependent on the manner in which signaling molecules are allowed to diffuse across the cell. While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules, the irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm. There has long been evidence that the cytoplasm behaves like a sol-gel and it is thought that the component molecules and structures of the cytoplasm behave at times like a disordered colloidal solution and at other times like an integrated network, forming a solid mass. Recently it has proposed that the cytoplasm behaves like a glass-forming liquid approaching the glass transition. A cells ability to vitrify in the absence of activity, as in dormant periods. There has been examining the motion of cytoplasmic particles independent of the nature of the cytoplasm. In such an approach, the aggregate random forces within the cell caused by motor proteins explain the non-Brownian motion of cytoplasmic constituents. The three major elements of the cytoplasm are the cytosol, organelles and inclusions, the cytosol is the portion of the cytoplasm not contained within membrane-bound organelles
Cytoplasm
–
Proteins in different
cellular compartments and structures tagged with
green fluorescent protein
Cytoplasm
–
Nucleolus
13.
Organelle
–
In cell biology, an organelle is a specialized subunit within a cell that has a specific function. Individual organelles are usually enclosed within their own lipid bilayers. The name organelle comes from the idea that these structures are to cells what an organ is to the body, organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells, while prokaryotes do not possess organelles per se, some do contain protein-based bacterial microcompartments, which are thought to act as primitive organelles. In biology organs are defined as confined functional units within an organism, the analogy of bodily organs to microscopic cellular substructures is obvious, as from even early works, authors of respective textbooks rarely elaborate on the distinction between the two. In the 1830s, Félix Dujardin refuted Ehrenberg theory which said that microorganims have the organs of multicellular animals. Credited as the first to use a diminutive of organ for cellular structures was German zoologist Karl August Möbius, under this definition, there would only be two broad classes of organelles, mitochondria plastids. Other organelles are also suggested to have endosymbiotic origins, but do not contain their own DNA, under the more restricted definition of membrane-bound structures, some parts of the cell do not qualify as organelles. Nevertheless, the use of organelle to refer to non-membrane bound structures such as ribosomes is common and this has led some texts to delineate between membrane-bound and non-membrane bound organelles. The larger organelles, such as the nucleus and vacuoles, are visible with the light microscope. They were among the first biological discoveries made after the invention of the microscope, not all eukaryotic cells have each of the organelles listed below. Exceptional organisms have cells that do not include some organelles that might otherwise be considered universal to eukaryotes, there are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below. In addition, the number of organelles of each type found in a given cell varies depending upon the function of that cell. This idea is supported in the Endosymbiotic theory, in the past, they were often viewed as having little internal organization, but slowly, details are emerging about prokaryotic internal structures. However, more recent research has revealed that at least some prokaryotes have microcompartments such as carboxysomes and these subcellular compartments are 100–200 nm in diameter and are enclosed by a shell of proteins. The function of a protein is correlated with the organelle in which it resides. CoRR Hypothesis Ejectosome Endosymbiotic theory Organelle biogenesis Membrane vesicle trafficking Host-pathogen interface Tree of Life project, Eukaryotes
Organelle
–
(A) Electron micrograph of
Halothiobacillus neapolitanus cells, arrows highlight
carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.
Organelle
–
Nucleolus
14.
Cell membrane
–
The cell membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is permeable to ions and organic molecules and controls the movement of substances in. The basic function of the membrane is to protect the cell from its surroundings. It consists of the bilayer with embedded proteins. Cell membranes can be artificially reassembled, Some authors that did not believe that there was a functional permeable boundary at the surface of the cell preferred to use the term plasmalemma to the extern region of the cell. The cell membrane surrounds the cytoplasm of living cells, physically separating the components from the extracellular environment. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, fungi, bacteria, most archaea, and plants also have a cell wall, which provides a mechanical support to the cell and precludes the passage of larger molecules. The cell membrane is permeable and able to regulate what enters and exits the cell. The movement of substances across the membrane can be passive, occurring without the input of cellular energy, or active. The membrane also maintains the cell potential, the cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. The cell employs a number of mechanisms that involve biological membranes,1. Passive osmosis and diffusion, Some substances such as carbon dioxide and oxygen, can move across the membrane by diffusion. Because the membrane acts as a barrier for certain molecules and ions, such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water. Transmembrane protein channels and transporters, Nutrients, such as sugars or amino acids, must enter the cell, such molecules diffuse passively through protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by transmembrane transporters. Protein channel proteins, also called permeases, are quite specific, recognizing and transporting only a limited food group of chemical substances. Endocytosis, Endocytosis is the process in which cells absorb molecules by engulfing them, the plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, Endocytosis is a pathway for internalizing solid particles, small molecules and ions, and macromolecules. Endocytosis requires energy and is thus a form of active transport and this is the process of exocytosis
Cell membrane
–
Illustration of a
Eukaryotic cell membrane
15.
Cell (biology)
–
The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, the study of cells is called cell biology. Cells consist of cytoplasm enclosed within a membrane, which contains many such as proteins. Organisms can be classified as unicellular or multicellular, while the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion cells. Most plant and animal cells are only under a microscope. The cell was discovered by Robert Hooke in 1665, who named the unit for its resemblance to cells inhabited by Christian monks in a monastery. Cells emerged on Earth at least 3.5 billion years ago, Cells are of two types, eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular, prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling and being self-sustaining. They are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as the nucleus, prokaryotes include two of the domains of life, bacteria and archaea. The DNA of a prokaryotic cell consists of a chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid, most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. Though most prokaryotes have both a cell membrane and a wall, there are exceptions such as Mycoplasma and Thermoplasma which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, the cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment, some eukaryotic cells also have a cell wall. Inside the cell is the region that contains the genome, ribosomes. The genetic material is found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular, linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid, plasmids encode additional genes, such as antibiotic resistance genes
Cell (biology)
–
Onion (
Allium) cells in different phases of the cell cycle
Cell (biology)
–
Structure of an animal cell
Cell (biology)
–
A fluorescent image of an endothelial cell. Nuclei are stained blue,
mitochondria are stained red, and microfilaments are stained green.
Cell (biology)
–
Human cancer cells with nuclei (specifically the DNA) stained blue. The central and rightmost cell are in
interphase, so the entire nuclei are labeled. The cell on the left is going through
mitosis and its DNA has condensed.
16.
Cell division
–
Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a cell cycle. Meiosis results in four daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated in the first division, and sister chromatids are separated in the second division, both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor, prokaryotes undergo a vegetative cell division known as binary fission, where their genetic material is segregated equally into two daughter cells. All cell divisions, regardless of organism, are preceded by a round of DNA replication. For simple unicellular organisms such as the amoeba, one cell division is equivalent to reproduction – an entire new organism is created, on a larger scale, mitotic cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Mitotic cell division enables sexually reproducing organisms to develop from the one-celled zygote, after growth, cell division by mitosis allows for continual construction and repair of the organism. The human body experiences about 10 quadrillion cell divisions in a lifetime, the primary concern of cell division is the maintenance of the original cells genome. Before division can occur, the information that is stored in chromosomes must be replicated. A great deal of infrastructure is involved in keeping genomic information consistent between generations. Interphase is the process a cell must go through mitosis, meiosis. Interphase consists of four stages, G1, S, G2. If the cell does not progress through G1, the cell enters a stage called G0. In G0, cells are living but they are put on hold. The cells may later be called back into interphase if needed at a later time, there are checkpoints during interphase that allow the cell to be either progressed or denied further development. In S phase, the chromosomes are replicated in order for the content to be maintained. During G2, the cell undergoes the final stages of growth before it enters the M phase, the M phase, can be either mitosis or meiosis depending on the type of cell
Cell division
–
Image of the
mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red.
Cell division
–
Three types of cell division
17.
Prophase
–
Prophase, is a stage of Mitosis. This is when the chromatin condenses into two rod-shaped structures called chromosomes in which the chromatin becomes visible and this process, known as chromatin condensation, is involved with the condensin complex. Since the genetic material has been replicated in the interphase of the cell cycle. Those copies are called sister chromatids and they are attached to each other at a DNA element present on every chromosome called the centromere, also during prophase, giemsa staining can be applied to elicit G-banding in chromosomes. An important organelle in mitosis is the centrosome, the organizing center in metazoans. During prophase, the two centrosomes, which replicate independently of mitosis, have their microtubule-activity increased due to the recruitment of γ-tubulin, the centrosomes will be pushed apart to opposite ends of the cell nucleus by the action of molecular motors acting on the microtubules. At the end of prophase, the tiny nucleolus within the nucleus dissolves, the nucleolus starts moving towards the nuclear membrane at the beginning of prophase, and its disintegration occurs near the nuclear envelope. Its contents are dispersed in masses, basically, the chromatin in the nucleus coils into chromosomes. The centrosomes move to opposite poles of the cell, forming a bridge of spindle fibers, at the end of prophase, the nucleolus disperses. In prometaphase, the step of mitosis, the nuclear membrane breaks apart. Interphase Metaphase Anaphase Telophase Farhan Phase Media related to Prophase at Wikimedia Commons
Prophase
Prophase
–
Fluorescence microscope image of two mouse cell nuclei in prophase (scale bar is 5 µm).
18.
Prometaphase
–
Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells. Prometaphase is sometimes included as part of the end of prophase. In prometaphase, the nuclear membrane breaks apart into numerous membrane vesicles, kinetochore microtubules emerging from the centrosomes at the poles of the spindle reach the chromosomes and attach to the kinetochores, throwing the chromosomes into agitated motion. Other spindle microtubules make contact with microtubules coming from the opposite pole, forces exerted by protein motors associated with spindle microtubules move the chromosomes toward the center of the cell. The microtubules are composed of two types, kinetochore microtubules and non-kinetochore microtubules, kinetochore microtubules begin searching for kinetochores to attach to. A number of microtubules or polar microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle. The role of prometaphase is completed all of the kinetochore microtubules have attached to their kinetochores. An unattached kinetochore, and thus a non-aligned chromosome, even when most of the chromosomes have lined up. This prevents premature progression into anaphase by inhibiting the complex until all kinetochores are attached. Media related to Prometaphase at Wikimedia Commons
Prometaphase
Prometaphase
–
The 23 human chromosome territories during prometaphase in
fibroblast cells.
19.
Metaphase
–
Metaphase is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage. These chromosomes, carrying information, align in the equator of the cell before being separated into each of the two daughter cells. Metaphase accounts for approximately 4% of the cell cycles duration, preceded by events in prometaphase and followed by anaphase, microtubules formed in prophase have already found and attached themselves to kinetochores in metaphase. In metaphase, the centromeres of the chromosomes convene themselves on the metaphase plate, in certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the middleline. One of the cell cycle checkpoints occurs during prometaphase and metaphase, only after all chromosomes have become aligned at the metaphase plate, when every kinetochore is properly attached to a bundle of microtubules, does the cell enter anaphase. Such a signal creates the mitotic spindle checkpoint and this would be accomplished by regulation of the anaphase-promoting complex, securin, and separase. The analysis of chromosomes is one of the main tools of classical cytogenetics. Chromosomes are condensed and highly coiled in metaphase, which makes them most suitable for visual analysis, Metaphase chromosomes make the classical picture of chromosomes. For classical cytogenetic analyses, cells are grown in short term culture, further they are used for slide preparation and banding of chromosomes to be visualised under microscope to study structure and number of chromosomes. Staining of the slides, often with Giemsa or Quinacrine, produces a pattern of in total up to several hundred bands, normal metaphase spreads are used in methods like FISH and as a hybridization matrix for comparative genomic hybridization experiments. Malignant cells from solid tumors or leukemia samples can also be used for analysis to generate metaphase preparations. Interphase Prophase Anaphase Telophase Media related to Metaphase at Wikimedia Commons
Metaphase
–
Chromosomes lined up on the metaphase plate. Two views with the metaphase plate rotated 90°.
20.
Anaphase
–
Anaphase, is the stage of mitosis after the metaphase when replicated chromosomes are split and the daughter chromatids are moved to opposite poles of the cell. Chromosomes also reach their overall maximum condensation in late anaphase, to chromosome segregation. Anaphase starts when the anaphase promoting complex marks an inhibitory chaperone called securin with ubiquitin for destruction, securin is a protein which inhibits a protease known as separase. The destruction of securin unleashes separase which then breaks down cohesin, the centromeres are split, and the new daughter chromosomes are pulled toward the poles. They take on a V-shape or Y-shape as they are pulled back, while the chromosomes are drawn to each side of the cell, the non-kinetochore spindle fibers push against each other, in a ratcheting action, that stretches the cell into an oval. Once anaphase is complete, the moves into telophase. Anaphase accounts for approximately 1% of the cell cycles duration and it begins with the regulated triggering of the metaphase-to-anaphase transition. Metaphase ends with the destruction of B cyclin, B cyclin is marked with ubiquitin which flags it for destruction by proteasomes, which is required for the function of metaphase cyclin-dependent kinases. Interphase Prophase Metaphase Telophase Anaphase I Anaphase II Cdc20 Media related to Anaphase at Wikimedia Commons
Anaphase
Anaphase
–
A cell during anaphase.
21.
Telophase
–
Telophase is the final stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase are reversed, the chromosomes reach the poles as the nuclear membranes re-form around each set of chromatids, the nucleoli also reappear. The remaining spindle microtubules begin to depolymerize, the chromosomes also unwind back into the expanded chromatin that is present during interphase. Telophase accounts for approximately 2% of the cell cycles duration, cytokinesis typically begins before late telophase. In land plant cells, vesicles derived from the Golgi apparatus move to the middle of the cell along a microtubule scaffold called the phragmoplast and this structure directs packets of cell wall materials which coalesce into a disk-shaped structure called a cell plate. The cell plate grows out centrifugally and eventually develops into a cell wall. Interphase Prophase Metaphase Anaphase Media related to Telophase at Wikimedia Commons
Telophase
–
Reappearance of the nuclear membrane and nucleolus: the telophase
Telophase
–
Vesicle fusion – When fragments of the nuclear membrane fuse to rebuild the nuclear membrane
22.
Spindle apparatus
–
In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. Besides chromosomes, the apparatus is composed of hundreds of proteins. Microtubules comprise the most abundant components of the machinery, attachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset. Microtubule polymerization and depolymerization dynamics drive chromosome congression, once every chromosome is bi-oriented, anaphase commences and cohesin, which couples sister chromatids, is severed, permitting the transit of the sister chromatids to opposite poles. The spindle apparatus is vaguely ellipsoid in cross section and tapers at the ends, in the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells, acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during female meiosis in most animals. In this instance, a Ran GTP gradient is the regulator of spindle microtubule organization. In fungi, spindles form between spindle pole bodies embedded in the envelope, which does not break down during mitosis. Microtubule-associated proteins associate with microtubules at the midzone and the poles to regulate their dynamics. γ-tubulin is a specialized tubulin variant that assembles into a complex called γ-TuRC which nucleates polymerization of α/β tubulin heterodimers into microtubules. The growing ends of microtubules are protected against catastrophe by the action of plus-end microtubule tracking proteins to promote their association with kinetochores at the midzone, cLIP170 was shown to localize near microtubule plus-ends in HeLa cells and to accumulate in kinetochores during prometaphase. CLIP-associated proteins like CLASP1 in humans have also shown to localize to plus-ends. Plus-end polymerization may be moderated by the EB1 protein, which directly binds the growing ends of microtubules and coordinates the binding of other +TIPs. MCAK localizes to the tips of microtubules at kinetochores where it can trigger catastrophe in direct competition with stabilizing +TIP activity. These proteins harness the energy of ATP hydrolysis to induce destabilizing conformational changes in protofilament structure that cause kinesin release, loss of their activity results in numerous mitotic defects. Additional microtubule destabilizing proteins include Op18/stathmin and katanin which have roles in remodeling the mitotic spindle as well as promoting chromosome segregation during anaphase. The activities of these MAPs are carefully regulated to maintain proper microtubule dynamics during spindle assembly, with many of these serving as Aurora. The precise orientation of this complex is required to ensure accurate chromosome segregation, however, it remains unclear how the spindle becomes organized
Spindle apparatus
–
Micrograph showing condensed
chromosomes in blue,
kinetochores in pink, and microtubules in green during
metaphase of
mitosis
23.
Apoptosis
–
Apoptosis is a process of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes and death and these changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA fragmentation, and global mRNA decay. Between 50 and 70 billion cells die each day due to apoptosis in the human adult. For an average child between the ages of 8 and 14, approximately 20 billion to 30 billion cells die a day, for example, the separation of fingers and toes in a developing human embryo occurs because cells between the digits undergo apoptosis. Because apoptosis cannot stop once it has begun, it is a regulated process. Apoptosis can be initiated through one of two pathways, in the intrinsic pathway the cell kills itself because it senses cell stress, while in the extrinsic pathway the cell kills itself because of signals from other cells. Both pathways induce cell death by activating caspases, which are proteases, the two pathways both activate initiator caspases, which then activate executioner caspases, which then kill the cell by degrading proteins indiscriminately. Research on apoptosis has increased substantially since the early 1990s, in addition to its importance as a biological phenomenon, defective apoptotic processes have been implicated in a wide variety of diseases. Excessive apoptosis causes atrophy, whereas an insufficient amount results in uncontrolled cell proliferation, some factors like Fas receptors and caspases promote apoptosis, while some members of the Bcl-2 family of proteins inhibit apoptosis. German scientist Karl Vogt was first to describe the principle of apoptosis in 1842, in 1885, anatomist Walther Flemming delivered a more precise description of the process of programmed cell death. However, it was not until 1965 that the topic was resurrected, while studying tissues using electron microscopy, John Foxton Ross Kerr at University of Queensland was able to distinguish apoptosis from traumatic cell death. Following the publication of a paper describing the phenomenon, Kerr was invited to join Alastair R Currie, as well as Andrew Wyllie, in 1972, the trio published a seminal article in the British Journal of Cancer. Kerr had initially used the term programmed cell necrosis, but in the article, Kerr, Wyllie and Currie credited James Cormack, a professor of Greek language at University of Aberdeen, with suggesting the term apoptosis. Kerr received the Paul Ehrlich and Ludwig Darmstaedter Prize on March 14,2000 and he shared the prize with Boston biologist H. Robert Horvitz. For many years, the apoptosis and programmed cell death were not highly cited. The 2002 Nobel Prize in Medicine was awarded to Sydney Brenner, Horvitz, the genes were identified by studies in the nematode C. elegans and homologues of these genes function in humans to regulate apoptosis. In Greek, apoptosis translates to the falling off of leaves from a tree, Cormack, professor of Greek language, reintroduced the term for medical use as it had a medical meaning for the Greeks over two thousand years before. Hippocrates used the term to mean the falling off of the bones, galen extended its meaning to the dropping of the scabs
Apoptosis
–
An
etoposide -treated
DU145 prostate cancer cell exploding into a cascade of apoptotic bodies. The sub images were extracted from a 61 hour
time-lapse microscopy video, created using
quantitative phase-contrast microscopy. The optical thickness is color-coded. With increasing thickness, color changes from gray to yellow, red, purple and finally black. See the video at The Cell: An Image Library
Apoptosis
–
John E. Sulston won the Nobel Prize in Medicine in 2002, for his pioneering research on apoptosis.
Apoptosis
–
Control Of The Apoptotic Mechanisms
Apoptosis
–
A section of mouse liver showing several apoptotic cells, indicated by arrows
24.
Mutation
–
In biology, a mutation is the permanent alteration of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA or other genetic elements. Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements, mutations may or may not produce discernible changes in the observable characteristics of an organism. Mutations play a part in normal and abnormal biological processes including, evolution, cancer, and the development of the immune system. The genomes of RNA viruses are based on RNA rather than DNA, the RNA viral genome can be double stranded or single stranded. In some of these viruses replication occurs quickly and there are no mechanisms to check the genome for accuracy and this error-prone process often results in mutations. Mutation can result in different types of change in sequences. Mutations in genes can either have no effect, alter the product of a gene, mutations can also occur in nongenic regions. Mutations can involve the duplication of large sections of DNA, usually through genetic recombination and these duplications are a major source of raw material for evolving new genes, with tens to hundreds of genes duplicated in animal genomes every million years. Most genes belong to larger families of shared ancestry, known as homology. Here, protein domains act as modules, each with a particular and independent function, that can be mixed together to produce genes encoding new proteins with novel properties. For example, the eye uses four genes to make structures that sense light. Another advantage of duplicating a gene is that this increases engineering redundancy, other types of mutation occasionally create new genes from previously noncoding DNA. Changes in chromosome number may involve even larger mutations, where segments of the DNA within chromosomes break and then rearrange. For example, in the Homininae, two fused to produce human chromosome 2, this fusion did not occur in the lineage of the other apes. Sequences of DNA that can move about the genome, such as transposons, make up a fraction of the genetic material of plants and animals. For example, more than a million copies of the Alu sequence are present in the genome. Another effect of these mobile DNA sequences is that when they move within a genome, they can mutate or delete existing genes, nonlethal mutations accumulate within the gene pool and increase the amount of genetic variation. The abundance of some changes within the gene pool can be reduced by natural selection, while other more favorable mutations may accumulate
Mutation
–
A mutation has caused this garden
moss rose to produce flowers of different colors. This is a
somatic mutation that may also be passed on in the
germline.
Mutation
–
Diagrammatic representation of the
divergence of modern
taxonomic groups from their
common ancestor
25.
Eukaryote
–
A eukaryote is any organism whose cells contain a nucleus and other organelles enclosed within membranes. Eukaryotes belong to the taxon Eukarya or Eukaryota, the presence of a nucleus gives eukaryotes their name, which comes from the Greek εὖ and κάρυον. Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, plants and algae contain chloroplasts. Eukaryotic organisms may be unicellular or multicellular, only eukaryotes form multicellular organisms consisting of many kinds of tissue made up of different cell types. Eukaryotes can reproduce asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two identical cells. In meiosis, DNA replication is followed by two rounds of division to produce four daughter cells each with half the number of chromosomes as the original parent cell. These act as sex cells resulting from genetic recombination during meiosis, the domain Eukaryota appears to be monophyletic, and so makes up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features, eukaryotes represent a tiny minority of all living things. However, due to their larger size, eukaryotes collective worldwide biomass is estimated at about equal to that of prokaryotes. Eukaryotes first developed approximately 1. 6–2.1 billion years ago, in 1905 and 1910, the Russian biologist Konstantin Mereschkowsky argued three things about the origin of nucleated cells. Firstly, plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, secondly, the host had earlier in evolution formed by symbiosis between an amoeba-like host and a bacteria-like cell that formed the nucleus. Thirdly, plants inherited photosynthesis from cyanobacteria, the split between the prokaryotes and eukaryotes was introduced in the 1960s. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton, the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1938 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes. However he mentioned this in one paragraph, and the idea was effectively ignored until Chattons statement was rediscovered by Stanier. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in cells in her paper. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA and this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles, mitochondria and chloroplasts
Eukaryote
–
Had'n
Eukaryote
–
Longitudinal section through the flagellum of
Chlamydomonas reinhardtii
26.
Prokaryote
–
A prokaryote is a unicellular organism that lacks a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle. The word prokaryote comes from the Greek πρό before and καρυόν nut or kernel, prokaryotes can be divided into two domains, Archaea and bacteria. In contrast, species with nuclei and organelles are placed in the domain Eukaryota, in the prokaryotes, all the intracellular water-soluble components are located together in the cytoplasm enclosed by the cell membrane, rather than in separate cellular compartments. Bacteria, however, do possess protein-based bacterial microcompartments, which are thought to act as primitive organelles enclosed in protein shells, some prokaryotes, such as cyanobacteria may form large colonies. Others, such as myxobacteria, have multicellular stages in their life cycles, molecular studies have provided insight into the evolution and interrelationships of the three domains of biological species. Eukaryotes are organisms, including humans, whose cells have a well defined membrane-bound nucleus, the division between prokaryotes and eukaryotes reflects the existence of two very different levels of cellular organization. Distinctive types of prokaryotes include extremophiles and methanogens, these are common in extreme environments. Prokaryotes have a cytoskeleton, albeit more primitive than that of the eukaryotes. At least some also contain intracellular structures that can be seen as primitive organelles. Membranous organelles are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, in addition, some species also contain carbohydrate-enclosed microcompartments, which have distinct physiological roles. Most prokaryotes are between 1 µm and 10 µm, but they can vary in size from 0.2 µm to 750 µm, Bacteria and archaea reproduce through asexual reproduction, usually by binary fission. DNA transfer between prokaryotic cells occurs in bacteria and archaea, although it has mainly studied in bacteria. In bacteria, gene transfer occurs by three processes and these are bacterial virus -mediated transduction, plasmid-mediated conjugation, and natural transformation. Transduction of bacterial genes by bacteriophage appears to reflect an occasional error during intracellular assembly of virus particles, the transfer of bacterial DNA is under the control of the bacteriophage’s genes rather than bacterial genes. Conjugation in the well-studied E. coli system is controlled by plasmid genes, infrequently during this process, a plasmid may integrate into the host bacterial chromosome, and subsequently transfer part of the host bacterial DNA to another bacterium. Plasmid mediated transfer of host bacterial DNA also appears to be a process rather than a bacterial adaptation. Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the intervening medium, for a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state called competence. About 40 genes are required in Bacillus subtilis for the development of competence, the length of DNA transferred during B. subtilis transformation can be as much as a third to the whole chromosome
Prokaryote
–
Cell structure of a
bacterium, a member of one of the two domains of prokaryotic life
27.
Binary fission
–
In biology, fission is the division of a single entity into two or more parts and the regeneration of those parts into separate entities resembling the original. The object experiencing fission is usually a cell, but the term may refer to how organisms, bodies, populations. The fission may be binary fission, in which a single entity produces two parts, or multiple fission, in which a single entity produces multiple parts, organisms in the domains of Archaea and Bacteria reproduce with binary fission. This form of reproduction and cell division is also used by some organelles within eukaryotic organisms. Binary fission results in the reproduction of a prokaryotic cell by dividing the cell into two parts, each with the potential to grow to the size of the original. Unlike the process of mitosis used by cells, binary fission takes place without the formation of a spindle apparatus in the cell. The single DNA molecule first replicates, then attaches each copy to a different part of the cell membrane, when the cell begins to pull apart, the replicated and original chromosomes are separated. The consequence of this method of reproduction is that all the cells are genetically identical. The process of fission in bacteria involves the following steps. First, the cells DNA is replicated, the replicated DNA copies then move to opposite poles of the cell in an energy-dependent process. Then, the plate of the cell constricts and separates the plasma membrane such that each new cell has exactly the same genetic material. More specifically, the following occur, The bacterium before binary fission is when the DNA is tightly coiled. The DNA of the bacterium has uncoiled and duplicated, the DNA is pulled to the separate poles of the bacterium as it increases size to prepare for splitting. The growth of a new cell wall begins to separate the bacterium, the new cell wall fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes, and plasmids, binary fission is generally rapid though its speed varies between species. For E. coli, cells typically divide about every 20 minutes at 37 °C and this time period can therefore be referred to as the doubling time. Some species other than E. coli may have faster or slower doubling times, some organelles in eukaryotic cells reproduce using binary fission. Mitochondrial fission occurs frequently within the cell, even when the cell is not actively undergoing mitosis, binary fission in organisms can occur in four ways, irregular, longitudinal, transverse, oblique
Binary fission
28.
Animal
–
Animals are multicellular, eukaryotic organisms of the kingdom Animalia. The animal kingdom emerged as a clade within Apoikozoa as the group to the choanoflagellates. Animals are motile, meaning they can move spontaneously and independently at some point in their lives and their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later in their lives. All animals are heterotrophs, they must ingest other organisms or their products for sustenance, most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals can be divided broadly into vertebrates and invertebrates, vertebrates have a backbone or spine, and amount to less than five percent of all described animal species. They include fish, amphibians, reptiles, birds and mammals, the remaining animals are the invertebrates, which lack a backbone. These include molluscs, arthropods, annelids, nematodes, flatworms, cnidarians, ctenophores, the study of animals is called zoology. The word animal comes from the Latin animalis, meaning having breath, the biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects, and humans. Aristotle divided the world between animals and plants, and this was followed by Carl Linnaeus, in the first hierarchical classification. In Linnaeuss original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, in 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms, Metazoa and Protozoa. The protozoa were later moved to the kingdom Protista, leaving only the metazoa, thus Metazoa is now considered a synonym of Animalia. Animals have several characteristics that set apart from other living things. Animals are eukaryotic and multicellular, which separates them from bacteria and they are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile, if only at life stages. In most animals, embryos pass through a stage, which is a characteristic exclusive to animals. With a few exceptions, most notably the sponges and Placozoa and these include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals
Animal
–
Carl Linnaeus, known as the father of modern
taxonomy
Animal
–
Had'n
Animal
–
The use of
love darts by the land snail
Monachoides vicinus is a form of
sexual selection
Animal
–
A
newt lung cell stained with
fluorescent dyes undergoing the early
anaphase stage of
mitosis
29.
Nuclear envelope
–
A nuclear membrane, also known as the nucleolemma or karyotheca, is the phospho lipid bilayer membrane which surrounds the genetic material and nucleolus in eukaryotic cells. The nuclear membrane consists of two lipid bilayers—the inner nuclear membrane, and the nuclear membrane. The space between the membranes is called the space, a region contiguous with the lumen of the endoplasmic reticulum. It is usually about 20–40 nm wide, the nuclear membrane also has many small holes called nuclear pores that allow material to move in and out of the nucleus. The outer nuclear membrane also shares a border with the endoplasmic reticulum. While it is linked, the outer nuclear membrane contains proteins found in far higher concentrations than the endoplasmic reticulum. All 4 Nesprin proteins present in mammals are expressed in the nuclear membrane. Nesprin proteins connect cytoskeletal filaments to the nucleoskeleton, nesprin-mediated connections to the cytoskeleton contribute to nuclear positioning and to the cell’s mechanosensory function. Nesprin-3 and-4 may play a role in unloading cargo, Nesprin-3 proteins bind plectin. Nesprin-4 proteins bind the end directed motor kinesin-1. The outer nuclear membrane is also involved in development, as it fuses with the nuclear membrane to form nuclear pores. It is connected to the membrane by nuclear pores which penetrate the membranes. While the two membranes and the reticulum are linked, proteins embedded in the membranes tend to stay put rather than dispersing across the continuum. The nuclear membrane is punctured by thousands of nuclear pore complexes—large hollow proteins about 100 nm across and they link the inner and outer nuclear membranes. During the G2 phase of interphase, the membrane increases its surface area. In lower eukaryotes, such as yeast, which undergo closed mitosis, the spindle fibers either form within the membrane, or penetrate it without tearing it apart. In higher eukaryotes, the membrane must break down during the prometaphase state of mitosis to allow the mitotic spindle fibers to access the chromosomes inside. The breakdown and reformation processes are not well understood, in mammals, the nuclear membrane can break down within minutes, following a set of steps during the early stages of Mitosis
Nuclear envelope
–
Human cell nucleus
30.
Fungi
–
A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from the other eukaryotic life kingdoms of plants, a characteristic that places fungi in a different kingdom from plants, bacteria and some protists, is chitin in their cell walls. Similar to animals, fungi are heterotrophs, they acquire their food by absorbing dissolved molecules, growth is their means of mobility, except for spores, which may travel through the air or water. Fungi are the principal decomposers in ecological systems and this fungal group is distinct from the structurally similar myxomycetes and oomycetes. The discipline of biology devoted to the study of fungi is known as mycology, in the past, mycology was regarded as a branch of botany, although it is now known fungi are genetically more closely related to animals than to plants. Abundant worldwide, most fungi are inconspicuous because of the size of their structures. Fungi include symbionts of plants, animals, or other fungi and they may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform a role in the decomposition of organic matter and have fundamental roles in nutrient cycling. Since the 1940s, fungi have been used for the production of antibiotics, Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, the fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans, losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and local economies. The fungus kingdom encompasses a diversity of taxa with varied ecologies, life cycle strategies. However, little is known of the biodiversity of Kingdom Fungi. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, phylogenetic studies published in the last decade have helped reshape the classification within Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla. The English word fungus is directly adopted from the Latin fungus, used in the writings of Horace, a group of all the fungi present in a particular area or geographic region is known as mycobiota, e. g. the mycobiota of Ireland. Like plants, fungi grow in soil and, in the case of mushrooms, form conspicuous fruit bodies. The fungi are now considered a kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago. Fungi have membrane-bound cytoplasmic organelles such as mitochondria, sterol-containing membranes and they have a characteristic range of soluble carbohydrates and storage compounds, including sugar alcohols, disaccharides, and polysaccharides
Fungi
Fungi
–
Omphalotus nidiformis, a bioluminescent mushroom
Fungi
–
Bracket fungi on a tree stump
Fungi
–
Two types of edible fungi
31.
Mitotic cell rounding
–
Mitotic cell rounding is a shape change that occurs in most animal cells that undergo mitosis. Cells abandon the spread or elongated shape characteristic of interphase and contract into a spherical morphology during mitosis, the phenomena is seen both in artificial cultures in vitro and naturally forming tissue in vivo. In 1935, one of the first published accounts of mitotic rounding in live tissue described cell rounding in the epithelium of the mammalian neural tube. Sauer noticed that cells in mitosis rounded up to the apical, or luminal, for a long time it was not clear why cells became round in mitosis. Recent studies in the epithelia and epidermis of various organisms, however, failure to achieve this may result in mislocalization of one daughter cell to the basal region on the tissue layer and clearance via apoptotic cell death. Secondly, mitotic rounding has been proposed to be a driver for morphological events during tissue development, examples include epithelial invagination of the Drosophila melanogaster tracheal placode and the anisotropic shape and growth of the inner ear lumen in Zebrafish. Thus mitotic cell rounding is involved in organization and homeostasis. To understand the mechanisms of how cells round up in mitosis researchers have conducted mechanical measurements with cultured cells in vitro. More than 90% of the forces are generated by the activity of myosin II molecular motors in the actin cortex. As a result, the tension and effective stiffness of the actin cortex increase as has been consistently observed in mitotic cells. In HeLa cells in vitro, the force generated by a mitotic cell is on the order of 50 to 100 nanonewtons. Internal hydrostatic pressure has been measured to increase from below 100 pascals in interphase to 3 to 10 fold that in mitosis, in similar in vitro experiments, it was found that the threshold forces required to prevent mitosis are in excess of 100 nN. At threshold forces the cell suffers a loss of cortical F-actin uniformity and these effects potentiate distortion of cell dimensions and subsequent perturbation of mitotic progression via spindle defects. Release of stable focal adhesions is another important aspect of mitotic rounding, overall, the biochemical events governing the morphological and mechanical changes in mitotic cells are orchestrated by the mitotic master regulator Cdk1. Qucosa, The Mechanics of Mitotic Cell Rounding, ETH ETH E-Collection, The Mechanism of Mitotic Rounding, Role of the Actomyosin Cortex — ETH E-Collection. Two Minute Talk, Mitotic Cell Rounding — YouTube
Mitotic cell rounding
–
Cell shape changes as a function of
mitotic phase. Shown is an example of a
HeLa cell cultured on a glass surface. For visualization of
DNA and
mitotic phase assignment, the cell expresses
Histone H2B -
GFP to provide
fluorescent labeling of
chromosomes. Transmitted light (
DIC),
fluorescence (
GFP), and merged images are shown every 4 minutes as the cell transitions from
G2 phase through
mitosis to
telophase /
G1 phase.
32.
Sperm
–
Sperm is the male reproductive cell and is derived from the Greek word sperma. In the types of reproduction known as anisogamy and its subtype oogamy. A uniflagellar sperm cell that is motile is referred to as a spermatozoon, Sperm cells cannot divide and have a limited life span, but after fusion with egg cells during fertilization, a new organism begins developing, starting as a totipotent zygote. The human sperm cell is haploid, so that its 23 chromosomes can join the 23 chromosomes of the egg to form a diploid cell. In mammals, sperm develops in the testicles and is released from the penis, the mammalian sperm cell consists of a head, neck, a midpiece and a tail. The head contains the nucleus with densely coiled chromatin fibres, surrounded anteriorly by an acrosome, the neck contains the sperm centriole. The midpiece has a central core with many mitochondria spiralled around it, used for ATP production for the journey through the female cervix, uterus. The tail or flagellum executes the lashing movements that propel the spermatocyte, although semen contains millions of sperm, the egg will admit only one. The other ones will die and be absorbed. The spermatozoa of animals are produced through spermatogenesis inside the gonads via meiotic division. The initial spermatozoon process takes around 70 days to complete, the spermatid stage is where the sperm develops the familiar tail. The next stage where it becomes fully mature takes around 60 days when it is called a spermatozoan, Sperm cells are carried out of the male body in a fluid known as semen. Human sperm cells can survive within the reproductive tract for more than 5 days post coitus. Semen is produced in the vesicles, prostate gland and urethral glands. In 2016 scientists at Nanjing Medical University claimed they had produced cells resembling mouse spermatids artificially from stem cells and they injected these spermatids into mouse eggs and produced pups. Sperm quantity and quality are the parameters in semen quality. Thus, in humans, it is a measure of fertility in a man, the genetic quality of sperm, as well as its volume and motility, all typically decrease with age. Human sperm cells are vulnerable to free radical attack and the generation of oxidative DNA damage
Sperm
–
Sperm and egg fusing
Sperm
–
Diagram of a human sperm cell
Sperm
–
Human sperm stained for semen quality testing.
33.
Ovum
–
The egg cell, or ovum, is the female reproductive cell in oogamous organisms. The egg cell is not capable of active movement. When egg and sperm fuse, a cell is formed. Karl Ernst von Baer discovered the mammalian ovum in 1827, the fusion of spermatozoa with ova was observed by Oskar Hertwig in 1876. In animals, egg cells are known as ova. The term ovule is used for the young ovum of an animal, in vertebrates, ova are produced by female gonads called ovaries a number of ova are present at birth in mammals and mature via oogenesis. White et al. disproved the longstanding dogma that all of the ova are produced before birth, the team from the Vincent Center for Reproductive Biology, Massachusetts, Boston could prove that oocyte fromation takes place in ovaries of reproductive-age women. This report challenged a fundamental belief, held since the 1950s, in all mammals the ovum is fertilized inside the female body. The human ova grow from primitive germ cells that are embedded in the substance of the ovaries, each of them divides repeatedly to give secretions of the uterine glands, ultimately forming a blastocyst. The ovum is one of the largest cells in the human body, the human ovum measures approximately 0.12 mm in diameter. Ooplasm is the yolk of the ovum, a substance at its center, which contains its nucleus, named the germinal vesicle. Mammalian ova contain only an amount of the nutritive yolk. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the period of incubation. In the oviparous animals the ova develop protective layers and pass through the oviduct to the outside of the body and they are fertilized by male sperm either inside the female body, or outside. After fertilization, an embryo develops, nourished by nutrients contained in the egg and it then hatches from the egg, outside the mothers body. See egg for a discussion of eggs of oviparous animals, the egg cells cytoplasm and mitochondria are the sole means the egg is able to reproduce by mitosis and eventually form a blastocyst after fertilization. Some fish, reptiles and many use this technique. Nearly all land plants have alternating diploid and haploid generations, gametes are produced by the gametophyte, which is the haploid generation
Ovum
–
Ovum and sperm fusing together
Ovum
–
A human ovum with
corona radiata surrounding it
Ovum
34.
Walther Flemming
–
Walther Flemming was a German biologist and a founder of cytogenetics. He was born in Sachsenberg as the child and only son of the psychiatrist Carl Friedrich Flemming and his second wife. He graduated from the Gymnasium der Residenzstadt, where one of his colleagues, Flemming trained in medicine at University of Rostock, graduating in 1868. Afterwards, he served in 1870–71 as a physician in the Franco-Prussian War. From 1873 to 1876 he worked as a teacher at the University of Prague, in 1876 he accepted a post as a professor of anatomy at the University of Kiel. He became the director of the Anatomical Institute and stayed there until his death, with the use of aniline dyes he was able to find a structure which strongly absorbed basophilic dyes, which he named chromatin. He identified that chromatin was correlated to threadlike structures in the cell nucleus – the chromosomes, the Belgian scientist Edouard Van Beneden had also observed them, independently. Flemming investigated the process of division and the distribution of chromosomes to the daughter nuclei. However, he did not see the splitting into identical halves and he studied mitosis both in vivo and in stained preparations, using as the source of biological material the fins and gills of salamanders. These results were published first in 1878 and in 1882 in the seminal book Zellsubstanz, on the basis of his discoveries, Flemming surmised for the first time that all cell nuclei came from another predecessor nucleus. Flemming is famously known for the work outside of his work that he did and he weekly feed those who were homeless, donating every year, 20% of his salary to homeless shelters. He taught especially young children who were too poor to attend school about mathematics, Flemming was unaware of the work of Gregor Mendel on heredity, so he did not make the connection between his observations and genetic inheritance. Two decades would pass before the significance of Flemmings work was realized with the rediscovery of Mendels rules. Flemmings name is honoured by a medal awarded by the German Society for Cell Biology, walter Flemming, discoverer of chromatin and mitotic cell division. Walther Flemming, pioneer of mitosis research, beitrage zur Kenntniss der Zelle und ihrer Lebenserscheinungen. Beiträge zur Kenntniss der Zelle und Ihrer Lebenserscheinungen, Flemming, W. Zur Kenntniss der Zelle und ihrer Theilungs-Erscheinungen. In, Schriften des Naturwissenschaftlichen Vereins für Schleswig-Holstein 3, 23–27, the Analysis of Mitosis Shifts Attention to the Chromosomes. Lasker Labs Zellsubstanz, Kern und Zelltheilung, original text of the book, as PDF
Walther Flemming
–
this picture shows Walther Flemming
Walther Flemming
–
Polytene chromosomes in a Chironimus salivary gland cell, one of over 100 drawings from Flemming's book Zellsubstanz, Kern und Zelltheilung, 1885
Walther Flemming
–
Illustrations of cells with chromosomes and mitosis, from the book Zellsubstanz, Kern und Zelltheilung, 1882
35.
Histologist
–
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is commonly performed by examining cells and tissues under a microscope or electron microscope, the specimen having been sectioned, stained. Histological studies may be conducted using tissue culture, where human or animal cells are isolated and maintained in an artificial environment for various research projects. The ability to visualize or differentially identify microscopic structures is frequently enhanced through the use of histological stains, histology is an essential tool of biology and medicine. Trained physicians, frequently licensed pathologists, are the personnel who perform histopathological examination and their field of study is called histotechnology. In the 17th century, Italian Marcello Malpighi invented one of the first microscopes for studying tiny biological entities, Malpighi analysed several parts of the organs of bats, frogs and other animals under the microscope. Malpighi, while studying the structure of the lung, noticed its membranous alveoli and his discovery established how the oxygen we breathe enters the blood stream and serves the body. In the 19th century, histology was a discipline in its own right. The French anatomist Bichat introduced the concept of tissue in anatomy in 1801, the 1906 Nobel Prize in Physiology or Medicine was awarded to histologists Camillo Golgi and Santiago Ramon y Cajal. They had dueling interpretations of the structure of the brain based in differing interpretations of the same images. Cajal won the prize for his theory and Golgi for the staining technique he invented to make it possible. There are four types of animal tissues, muscle tissue, nervous tissue, connective tissue. All tissue types are subtypes of these four basic tissue types and their structure is very different from animal tissues. The most common fixative for light microscopy is 10% neutral buffered formalin, for electron microscopy, the most commonly used fixative is glutaraldehyde, usually as a 2. 5% solution in phosphate buffered saline. These fixatives preserve tissues or cells mainly by irreversibly cross-linking proteins and this can be detrimental to certain histological techniques. Further fixatives are often used for electron microscopy such as osmium tetroxide or uranyl acetate Formalin fixation leads to degradation of mRNA, miRNA and DNA in tissues, however, extraction, amplification and analysis of these nucleic acids from formalin-fixed, paraffin-embedded tissues is possible using appropriate protocols. Frozen section procedure is a way to fix and mount histology sections using a refrigeration device called a cryostat. It is often used after surgical removal of tumors to allow determination of margin
Histologist
–
A stained histologic specimen, sandwiched between a glass
microscope slide and
coverslip, mounted on the stage of a light microscope.
Histologist
–
Histologic specimen for light microscopy. 1 microscope slide, 2 cover slip, 3 stained tissue section between 1 and 2
Histologist
–
Microscopic view of a histologic specimen of human
lung tissue stained with
hematoxylin and
eosin.
Histologist
–
Medical students (EMIS) getting samples from a human stomach for future histological studies in 2012 at Instituto Nacional de Cardiologia in Mexico
36.
Greek language
–
Greek is an independent branch of the Indo-European family of languages, native to Greece and other parts of the Eastern Mediterranean. It has the longest documented history of any living language, spanning 34 centuries of written records and its writing system has been the Greek alphabet for the major part of its history, other systems, such as Linear B and the Cypriot syllabary, were used previously. The alphabet arose from the Phoenician script and was in turn the basis of the Latin, Cyrillic, Armenian, Coptic, Gothic and many other writing systems. Together with the Latin texts and traditions of the Roman world, during antiquity, Greek was a widely spoken lingua franca in the Mediterranean world and many places beyond. It would eventually become the official parlance of the Byzantine Empire, the language is spoken by at least 13.2 million people today in Greece, Cyprus, Italy, Albania, Turkey, and the Greek diaspora. Greek roots are used to coin new words for other languages, Greek. Greek has been spoken in the Balkan peninsula since around the 3rd millennium BC, the earliest written evidence is a Linear B clay tablet found in Messenia that dates to between 1450 and 1350 BC, making Greek the worlds oldest recorded living language. Among the Indo-European languages, its date of earliest written attestation is matched only by the now extinct Anatolian languages, the Greek language is conventionally divided into the following periods, Proto-Greek, the unrecorded but assumed last ancestor of all known varieties of Greek. The unity of Proto-Greek would have ended as Hellenic migrants entered the Greek peninsula sometime in the Neolithic era or the Bronze Age, Mycenaean Greek, the language of the Mycenaean civilisation. It is recorded in the Linear B script on tablets dating from the 15th century BC onwards, Ancient Greek, in its various dialects, the language of the Archaic and Classical periods of the ancient Greek civilisation. It was widely known throughout the Roman Empire, after the Roman conquest of Greece, an unofficial bilingualism of Greek and Latin was established in the city of Rome and Koine Greek became a first or second language in the Roman Empire. The origin of Christianity can also be traced through Koine Greek, Medieval Greek, also known as Byzantine Greek, the continuation of Koine Greek in Byzantine Greece, up to the demise of the Byzantine Empire in the 15th century. Much of the written Greek that was used as the language of the Byzantine Empire was an eclectic middle-ground variety based on the tradition of written Koine. Modern Greek, Stemming from Medieval Greek, Modern Greek usages can be traced in the Byzantine period and it is the language used by the modern Greeks, and, apart from Standard Modern Greek, there are several dialects of it. In the modern era, the Greek language entered a state of diglossia, the historical unity and continuing identity between the various stages of the Greek language is often emphasised. Greek speakers today still tend to regard literary works of ancient Greek as part of their own rather than a foreign language and it is also often stated that the historical changes have been relatively slight compared with some other languages. According to one estimation, Homeric Greek is probably closer to demotic than 12-century Middle English is to modern spoken English, Greek is spoken by about 13 million people, mainly in Greece, Albania and Cyprus, but also worldwide by the large Greek diaspora. Greek is the language of Greece, where it is spoken by almost the entire population
Greek language
–
Idealized portrayal of
Homer
Greek language
–
regions where Greek is the official language
Greek language
–
Greek language road sign, A27 Motorway, Greece
37.
Drosophila melanogaster
–
Drosophila melanogaster is a species of fly in the family Drosophilidae. The species is generally as the common fruit fly or vinegar fly. It is typically used because it is a species that is easy to care for, has four pairs of chromosomes, breeds quickly. D. melanogaster is a common pest in homes, restaurants, flies belonging to the family Tephritidae are also called fruit flies. This can cause confusion, especially in Australia and South Africa, the D. melanogaster lifespan is about 30 days at 29 °C. The developmental period for D. melanogaster varies with temperature, as with many ectothermic species, the shortest development time,7 days, is achieved at 28 °C. Development times increase at higher temperatures due to heat stress, under ideal conditions, the development time at 25 °C is 8.5 days, at 18 °C it takes 19 days and at 12 °C it takes over 50 days. Under crowded conditions, development time increases, while the emerging flies are smaller, females lay some 400 eggs, about five at a time, into rotting fruit or other suitable material such as decaying mushrooms and sap fluxes. The eggs, which are about 0.5 mm long, the resulting larvae grow for about 4 days while molting twice, at about 24 and 48 h after hatching. During this time, they feed on the microorganisms that decompose the fruit, the mother puts feces on the egg sacs to establish the same microbial composition in the larvaes guts which has worked positively for herself. Then the larvae encapsulate in the puparium and undergo a four-day-long metamorphosis, the female fruit fly prefers a shorter duration when it comes to sex. Males, on the hand, prefer it to last longer. Males perform a sequence of five behavioral patterns to court females, first, males orient themselves while playing a courtship song by horizontally extending and vibrating their wings. Soon after, the positions itself at the rear of the females abdomen in a low posture to tap. Finally, the male curls its abdomen and attempts copulation, females can reject males by moving away, kicking, and extruding their ovipositor. Copulation lasts around 15–20 minutes, during which males transfer a few hundred, females store the sperm in a tubular receptacle and in two mushroom-shaped spermathecae, sperm from multiple matings compete for fertilization. A last male precedence is believed to exist in which the last male to mate with a female sires about 80% of her offspring and this precedence was found to occur through both displacement and incapacitation. The displacement is attributed to sperm handling by the fly as multiple matings are conducted and is most significant during the first 1–2 days after copulation
Drosophila melanogaster
–
Drosophila melanogaster
Drosophila melanogaster
–
View from above
Drosophila melanogaster
–
Front view
Drosophila melanogaster
–
Mating fruit flies: Note the sex combs on the forelegs of the male (insert)
38.
Embryo
–
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the zygote possesses half the DNA of each of its two parents. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism, the result of this process is an embryo. First attested in English in the mid-14c, the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον, lit. Young one, which is the neuter of ἔμβρυος, lit, growing in, from ἐν, in and βρύω, swell, be full, the proper Latinized form of the Greek term would be embryum. In animals, the development of the zygote into an embryo proceeds through specific stages of blastula, gastrula. The blastula stage typically features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, in a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at an advanced stage of development. In humans, this is from the week of gestation. During gastrulation the cells of the blastula undergo coordinated processes of cell division, invasion, in triploblastic organisms, the three germ layers are called endoderm, ectoderm, and mesoderm. The position and arrangement of the layers are highly species-specific, however. For example, in neurogenesis, a subpopulation of cells is set aside to become the brain, spinal cord. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis, chondrogenesis, myogenesis, osteogenesis, and many others. In botany, a plant embryo is part of a seed, consisting of precursor tissues for the leaves, stem. Once the embryo begins to germinate — grow out from the seed — it is called a seedling, bryophytes and ferns also produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated, the structure and development of the rest of the embryo varies by group of plants. Once the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo, embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the embryo may be frozen for later use
Embryo
–
A human embryo. This embryo is about 7 weeks old (or from the 9th week of pregnancy).
Embryo
Embryo
–
An embryo 8 weeks after fertilization (i.e. 10 weeks LMP)
Embryo
–
A complete spontaneous abortion at about 6 weeks from
conception, i.e. 8 weeks from
LMP
39.
Genome
–
In modern molecular biology and genetics, a genome is the genetic material of an organism. The genome includes both the genes, the noncoding DNA and the material of the mitochondria and chloroplasts. The term genome was created in 1920 by Hans Winkler, professor of botany at the University of Hamburg, the Oxford Dictionary suggests the name is a blend of the words gene and chromosome. However, see omics for a thorough discussion. A few related -ome words already existed—such as biome, rhizome, Some organisms have multiple copies of chromosomes, diploid, triploid, tetraploid and so on. In classical genetics, in a sexually reproducing organism the gamete has half the number of chromosomes of the somatic cell, the halving of the genetic material in gametes is accomplished by the segregation of homologous chromosomes during meiosis. Additionally, the genome can comprise non-chromosomal genetic elements such as viruses, plasmids, even in species that exist in only one sex, what is described as a genome sequence may be a composite read from the chromosomes of various individuals. Colloquially, the genetic makeup is sometimes used to signify the genome of a particular individual or organism. Both the number of pairs and the number of genes vary widely from one species to another. The only exception in humans is found in red blood cells which become enucleated during development. In 1976, Walter Fiers at the University of Ghent was the first to establish the complete sequence of a viral RNA-genome. The next year Fred Sanger completed the first DNA-genome sequence, Phage Φ-X174, the first genome sequence for an archaeon, Methanococcus jannaschii, was completed in 1996, again by The Institute for Genomic Research. The development of new technologies has made it easier and cheaper to do sequencing. The US National Institutes of Health maintains one of several databases of genomic information. Among the thousands of completed genome sequencing projects include those for rice, a mouse, the plant Arabidopsis thaliana, the fish. In December 2013, scientists first sequenced the genome of a Neanderthal. The genome was extracted from the toe bone of a 130, new sequencing technologies, such as massive parallel sequencing have also opened up the prospect of personal genome sequencing as a diagnostic tool, as pioneered by Manteia Predictive Medicine. A major step toward that goal was the completion in 2007 of the genome of James D. Watson
Genome
–
An image of the 46 chromosomes making up the diploid genome of a human male. (The
mitochondrial chromosome is not shown.)
40.
DNA
–
Deoxyribonucleic acid is a molecule that carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses. DNA and RNA are nucleic acids, alongside proteins, lipids and complex carbohydrates, most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are termed polynucleotides since they are composed of simpler units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases—cytosine, guanine, adenine, or thymine —a sugar called deoxyribose, and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA. The total amount of related DNA base pairs on Earth is estimated at 5.0 x 1037, in comparison the total mass of the biosphere has been estimated to be as much as 4 trillion tons of carbon. The DNA backbone is resistant to cleavage, and both strands of the double-stranded structure store the same biological information and this information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences, the two strands of DNA run in opposite directions to each other and are thus antiparallel. Attached to each sugar is one of four types of nucleobases and it is the sequence of these four nucleobases along the backbone that encodes biological information. RNA strands are created using DNA strands as a template in a process called transcription, under the genetic code, these RNA strands are translated to specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells DNA is organized into structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, eukaryotic organisms store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts. In contrast prokaryotes store their DNA only in the cytoplasm, within the eukaryotic chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed, DNA was first isolated by Friedrich Miescher in 1869. DNA is used by researchers as a tool to explore physical laws and theories, such as the ergodic theorem. The unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro-, among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a polymer made from repeating units called nucleotides
DNA
–
T7 RNA polymerase (blue) producing a mRNA (green) from a DNA template (orange).
DNA
–
The structure of the DNA
double helix. The
atoms in the structure are colour-coded by
element and the detailed structure of two base pairs are shown in the bottom right.
DNA
–
Sculpture of DNA made out of shopping carts
DNA
–
James Watson and
Francis Crick (right), co-originators of the double-helix model, with Maclyn McCarty (left).
41.
DNA sequence
–
A nucleic acid sequence is a succession of letters that indicate the order of nucleotides within a DNA or RNA molecule. By convention, sequences are presented from the 5 end to the 3 end. For DNA, the strand is used. Because nucleic acids are linear polymers, specifying the sequence is equivalent to defining the covalent structure of the entire molecule. For this reason, the nucleic acid sequence is also termed the primary structure, the sequence has capacity to represent information. Biological deoxyribonucleic acid represents the information which directs the functions of a living thing, nucleic acids also have a secondary structure and tertiary structure. Primary structure is sometimes referred to as primary sequence. Conversely, there is no concept of secondary or tertiary sequence. Nucleic acids consist of a chain of linked units called nucleotides, each nucleotide consists of three subunits, a phosphate group and a sugar make up the backbone of the nucleic acid strand, and attached to the sugar is one of a set of nucleobases. The nucleobases are important in base pairing of strands to form secondary and tertiary structure such as the famed double helix. The possible letters are A, C, G, and T, in the typical case, the sequences are printed abutting one another without gaps, as in the sequence AAAGTCTGAC, read left to right in the 5 to 3 direction. With regards to transcription, a sequence is on the strand if it has the same order as the transcribed RNA. One sequence can be complementary to sequence, meaning that they have the base on each position in the complementary. For example, the sequence to TTAC is GTAA. If one strand of the double-stranded DNA is considered the sense strand, then the other strand, considered the antisense strand, will have the complementary sequence to the sense strand. Apart from adenine, cytosine, guanine, thymine and uracil, in DNA, the most common modified base is 5-methylcytidine. In RNA, there are many modified bases, including pseudouridine, dihydrouridine, inosine, ribothymidine and 7-methylguanosine, hypoxanthine and xanthine are two of the many bases created through mutagen presence, both of them through deamination. Hypoxanthine is produced from adenine, xanthine from guanine, similarly, deamination of cytosine results in uracil
DNA sequence
–
A series of codons in part of a
mRNA molecule. Each codon consists of three
nucleotides, usually representing a single
amino acid.
42.
Clone (genetics)
–
Molecular cloning is a set of experimental methods in molecular biology that are used to assemble recombinant DNA molecules and to direct their replication within host organisms. The use of the word cloning refers to the fact that the method involves the replication of one molecule to produce a population of cells with identical DNA molecules, molecular cloning methods are central to many contemporary areas of modern biology and medicine. In a conventional molecular cloning experiment, the DNA to be cloned is obtained from an organism of interest, subsequently, these fragments are then combined with vector DNA to generate recombinant DNA molecules. The recombinant DNA is then introduced into a host organism and this will generate a population of organisms in which recombinant DNA molecules are replicated along with the host DNA. Because they contain foreign DNA fragments, these are transgenic or genetically modified microorganisms and this process takes advantage of the fact that a single bacterial cell can be induced to take up and replicate a single recombinant DNA molecule. This single cell can then be expanded exponentially to generate an amount of bacteria. Thus, both the bacterial population, and the recombinant DNA molecule, are commonly referred to as clones. Strictly speaking, recombinant DNA refers to DNA molecules, while molecular cloning refers to the methods used to assemble them. Prior to the 1970s, our understanding of genetics and molecular biology was severely hampered by an inability to isolate and this changed dramatically with the advent of molecular cloning methods. They showed that restriction enzymes cleaved chromosome-length DNA molecules at specific locations, using a second enzyme, DNA ligase, fragments generated by restriction enzymes could be joined in new combinations, termed recombinant DNA. The first recombinant DNA molecules were generated and studied in 1972, molecular cloning takes advantage of the fact that the chemical structure of DNA is fundamentally the same in all living organisms. Molecular cloning is similar to polymerase chain reaction in that it permits the replication of DNA sequence. Although the detailed planning of the cloning can be done in any text editor, together with online utilities for e. g. PCR primer design, software for the purpose include for example ApE, DNAStrider, Serial Cloner and Collagene. Notably, the capacity and fidelity of DNA synthesis platforms allows for increasingly intricate designs in molecular engineering. These projects may include very long strands of novel DNA sequence and/or test entire libraries simultaneously and these shifts introduce complexity require design to move away from the flat nucleotide based representation and towards a higher level of abstraction. Examples of such tools are GenoCAD GenoCAD, Teselagen or GeneticConstructor, E. coli and plasmid vectors are in common use because they are technically sophisticated, versatile, widely available, and offer rapid growth of recombinant organisms with minimal equipment. If the DNA to be cloned is exceptionally large, then an artificial chromosome or yeast artificial chromosome vector is often chosen. Specialized applications may call for specialized host-vector systems, alternatively, if replication of the DNA in different species is desired, then a multiple host range vector may be selected
Clone (genetics)
–
Diagram of molecular cloning using bacteria and plasmids.
43.
DNA replication
–
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. This process occurs in all living organisms and is the basis for biological inheritance, DNA is made up of a double helix of two complementary strands. During replication, these strands are separated, each strand of the original DNA molecule then serves as a template for the production of its counterpart, a process referred to as semiconservative replication. Cellular proofreading and error-checking mechanisms ensure near perfect fidelity for DNA replication, in a cell, DNA replication begins at specific locations, or origins of replication, in the genome. Unwinding of DNA at the origin and synthesis of new results in replication forks growing bi-directionally from the origin. A number of proteins are associated with the fork to help in the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new strands by adding nucleotides that complement each strand, DNA replication occurs during the S-stage of interphase. DNA replication can also be performed in vitro, DNA polymerases isolated from cells and artificial DNA primers can be used to initiate DNA synthesis at known sequences in a template DNA molecule. The polymerase chain reaction, a laboratory technique, cyclically applies such artificial synthesis to amplify a specific target DNA fragment from a pool of DNA. DNA usually exists as a structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotides, nucleotides in DNA contain a deoxyribose sugar, a phosphate, and a nucleobase. These nucleotides form phosphodiester bonds, creating the phosphate-deoxyribose backbone of the DNA double helix with the bases pointing inward. Nucleotides are matched between strands through hydrogen bonds to form base pairs, adenine pairs with thymine, and guanine pairs with cytosine. DNA strands have a directionality, and the different ends of a single strand are called the 3 end and the 5 end. By convention, if the sequence of a single strand of DNA is given. The strands of the helix are anti-parallel with one being 5 to 3. These terms refer to the atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3 end of a DNA strand, the pairing of complementary bases in DNA means that the information contained within each strand is redundant
DNA replication
–
Formation of pre-replication complex.
DNA replication
44.
Sister chromatids
–
A sister chromatid refers to either of the two identical copies formed by the replication of a single chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be one-half of the duplicated chromosome, a full set of sister chromatids is created during the synthesis phase of interphase, when all the chromosomes in a cell are replicated. The two sister chromatids are separated from each other into two different cells during mitosis or during the division of meiosis. Compare sister chromatids to homologous chromosomes, which are the two different copies of a chromosome that diploid organisms inherit, one from each parent, sister chromatids are by and large identical because they derive from one original chromosome. Homologous chromosomes might or might not be the same as each other because they derive from different parents, there is evidence that, in some species, sister chromatids are the preferred template for DNA repair. Sister chromatid cohesion is essential for the distribution of genetic information between daughter cells and the repair of damaged chromosomes. Defects in this process may lead to aneuploidy and cancer, especially when checkpoints fail to detect DNA damage or when incorrectly attached mitotic spindles dont function properly, mitotic recombination is primarily a result of DNA repair processes responding to spontaneous or induced damages
Sister chromatids
45.
Cohesin
–
Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis. Cohesins hold sister chromatids together after DNA replication until anaphase when removal of cohesin leads to separation of sister chromatids, Cohesin is a multi-subunit protein complex, made up of four core subunits, two SMC proteins, an alpha-kleisin, and an orthologue of the yeast Scc3 protein. Smc1 and Smc3 are members of the Structural Maintenance of Chromosomes family, SMC proteins have two main structural characteristics, an ATP-binding cassette-like head domain with ATPase activity and a hinge domain that allows dimerization of SMCs. The head and the domains are connected to each other via long anti-parallel coiled coils. The dimer is present in a V-shaped form, connected by the hinges, upon ATP binding, the two head domains in the dimer bind to each other, forming a ring structure. ATP hydrolysis can therefore trigger opening and closing of the ring, Scc1 and Scc3 bind the ATPase domains of Smc1 and Smc3 stabilizing the ring structure. The amino and carboxy terminus of Scc1 bind Smc1 and Smc3, once Scc1 binds the SMC proteins, Scc3 can also associate by binding with the C-terminal region of Scc1. When Scc1 binds on both Smc1 and Smc3, the complex forms a closed ring structure. When it binds to one of the SMC proteins, the complex forms an open ring. While some studies support the idea of a model, the model is inconsistent with a number of experimental observations. The cohesin ring has many functions,1 and it is used to keep the sister chromatids connected with each other during metaphase ensuring that during mitosis, each sister chromatid segregates to opposite poles. It facilitates spindle attachment onto chromosomes and it facilitates DNA repair by recombination. Recently many novel functions of cohesin have been discovered in different cellular processes. The anaphase promoting complex associated to Cdc20 cleaves the securin, which holds the sister chromatids together, securin is cleaved at anaphase, following APC/C-cdc20 mediated degradation, and it renders separase to cleave the kleisin subunit. Dissociation of sister chromatids cohesion defines anaphase onset, which two sets of identical chromosomes at each pole of the cell. Then the two daughter cells separate, and a new round of the cell cycle starts in each one. Therefore, when the division process will end, each daughter cell will receive a complete set of organelles. At the same time, during S phase all cells must duplicate their DNA very precisely and it is not clear how the cohesion ring links sister chromatids together
Cohesin
–
Cohesin complex composed of SCC1 and SMC1 proteins.
46.
Centromere
–
The centromere is the part of a chromosome that links sister chromatids or a dyad. During mitosis, spindle fibers attach to the centromere via the kinetochore, centromeres were first thought to be genetic loci that direct the behavior of chromosomes. e. There are, broadly speaking, two types of centromeres, point centromeres bind to specific proteins that recognise particular DNA sequences with high efficiency. Any piece of DNA with the point centromere DNA sequence on it will form a centromere if present in the appropriate species. The best characterised point centromeres are those of the budding yeast, regional centromeres is the term coined to describe most centromeres, which typically form on regions of preferred DNA sequence, but which can form on other DNA sequences as well. The signal for formation of a regional centromere appears to be epigenetic, most organisms, ranging from the fission yeast Schizosaccharomyces pombe to humans, have regional centromeres. Regarding mitotic chromosome structure, centromeres represent a region of the chromosome where two identical sister chromatids are most closely in contact. When cells enter mitosis, the chromatids are linked along their length by the action of the cohesin complex. Each chromosome has two arms, labeled p and q, many remember that the short arm p is named for the French word petit meaning small, although this explanation was shown to be apocryphal. They can be connected in either metacentric, submetacentric, acrocentric or telocentric manner and these are X-shaped chromosomes, with the centromere in the middle so that the two arms of the chromosomes are almost equal. A chromosome is metacentric if its two arms are equal in length. In a normal karyotype, five chromosomes are considered metacentric, chromosomes 1,3,16,19. In some cases, a chromosome is formed by balanced translocation. If arms lengths are unequal, the chromosome is said to be submetacentric, if the p arm is so short that it is hard to observe, but still present, then the chromosome is acrocentric. The human genome includes six acrocentric chromosomes,13,14,15,21,22, the domestic horse genome includes one metacentric chromosome that is homologous to two acrocentric chromosomes in the conspecific but undomesticated Przewalskis horse. A telocentric chromosomes centromere is located at the end of the chromosome. Telomeres may extend from both ends of the chromosome, for example, the standard house mouse karyotype has only telocentric chromosomes. Humans do not possess telocentric chromosomes, if the chromosomes centromere is located closer to its end than to its center, it may be described as subtelocentric
Centromere
–
In this diagram of a duplicated chromosome, (2) identifies the centromere—the region that joins the two
sister chromatids, or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called
kinetochores attach chromosomes to spindle fibers.
47.
Nucleolus
–
The nucleolus is the largest structure in the nucleus of eukaryotic cells, where it is primarily responsible for the creation of ribosomes via a process known as ribosome biogenesis. Nucleoli also have important functions like assembly of signal recognition particles. Nucleoli are made of proteins, DNA and RNA and form around specific chromosomal regions, malfunction of nucleoli can be the cause of several human conditions which include diseases, disorders, syndromes and illnesses. Until 1964, little was known about the function of the nucleolus, during that year, a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopus laevis generated increasing interest in the function and detailed structure of the nucleolus. They found that 25% of the eggs had no nucleolus. Half of the eggs had one nucleolus and 25% had two and they concluded that the nucleolus had a function necessary for life. In 1966 Max L. Birnstiel and Hugh Wallace showed via hybridization experiments that genes within nucleoli code for ribosomal RNA, three major components of the nucleolus are recognized, the fibrillar center, the dense fibrillar component, and the granular component. The DFC consists of newly transcribed rRNA bound to ribosomal proteins, reflecting the substantial increase in the DNA intergenic region, an original fibrillar component would have separated into the FC and the DFC. Another structure identified within many nucleoli is an area in the center of the structure referred to as a nucleolar vacuole. Nucleoli of various plant species have shown to have very high concentrations of iron in contrast to human. Antibodies against the PAF49 protein can also be used as a marker for the nucleolus in immunofluorescence experiments, nucleoli are formed around specific genetic loci called nucleolar organizing regions, first described by Barbara McClintock. Because of this organization, the nucleolus is defined as a genetically determined element. An NOR is composed of repeats of rDNA, which can be found on several different chromosomes. The human genome, for example, contains more than 200 clustered copies of rDNA on five different chromosomes, in a typical eukaryote rDNA consists of a promoter, internal and external transcribed spacers, rRNA coding sequences and an intergenic spacer. In ribosome biogenesis, two of the three eukaryotic RNA polymerases are required, and these function in a coordinated manner, in an initial stage, the rRNA genes are transcribed as a single unit within the nucleolus by RNA polymerase I. In order for transcription to occur, several pol I-associated factors. In yeast, the most important are, UAF, TBP, and CF, which bind promoter elements and form the preinitiation complex, in humans, a similar PIC is assembled with SL1, the promoter selectivity factor, transcription initiation factors, and UBF. RNA polymerase I transcribes most rRNA transcripts but the 5S rRNA subunit is transcribed by RNA polymerase III, transcription of rRNA yields a long precursor molecule which still contains the ITS and ETS
Nucleolus
–
The nucleolus is contained within the
cell nucleus.
48.
Microtubule
–
Microtubules are a component of the cytoskeleton, found throughout the cytoplasm. These tubular polymers of tubulin can grow as long as 50 micrometres and are highly dynamic, the outer diameter of a microtubule is about 24 nm while the inner diameter is about 12 nm. They are found in cells, as well as some bacteria. Microtubules are very important in a number of cellular processes and they are involved in maintaining the structure of the cell and, together with microfilaments and intermediate filaments, they form the cytoskeleton. They also make up the structure of cilia and flagella. They are also involved in chromosome separation, and are the constituents of mitotic spindles. There are many proteins bind to microtubules, including the motor proteins kinesin and dynein, severing proteins like katanin. Recently an actin-like protein has found in a gram-positive bacterium Bacillus thuringiensis. Tubulin and microtubule-mediated processes, like cell locomotion, were seen by early microscopists, in eukaryotes, microtubules are long, hollow cylinders made up of polymerised α- and β-tubulin dimers. The inner space of the hollow microtubule cylinders is referred to as the lumen, the α and β-tubulin subunits are approximately 50% identical at the amino acid level, and each have a molecular weight of approximately 50 kDa. These α/β-tubulin dimers polymerize end-to-end into linear protofilaments that associate laterally to form a single microtubule, typically, microtubules are formed by the parallel association of thirteen protofilaments, although microtubules composed of fewer or more protofilaments have been observed in vitro. Microtubules have a polarity that is critical for their biological function. Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer, therefore, in a protofilament, one end will have the α-subunits exposed while the other end will have the β-subunits exposed. These ends are designated the and ends, respectively, while microtubule elongation can occur at both the and ends, it is significantly more rapid at the end. The lateral association of the protofilaments generates a pseudo-helical structure, with one turn of the helix containing 13 tubulin dimers, each from a different protofilament. In the most common 13-3 architecture, the 13th tubulin dimer interacts with the tubulin dimer with a vertical offset of 3 tubulin monomers due to the helicity of the turn. There are other alternative architectures, such as 11-3, 12-3, 14-3, 15-4, or 16-4, Microtubules can also morph into other forms such as helical filaments, which are observed in protist organisms like foraminifera. There are two types of interactions that can occur between the subunits of lateral protofilaments within the microtubule called the A-type and B-type lattices
Microtubule
–
Just as our bodies rely on bones for structural support, our cells rely on a cellular skeleton. In addition to helping cells keep their shape, this cytoskeleton transports material within cells and coordinates cell division. Microtubules are shown here as thin strands
Microtubule
–
Structure of a microtubule. The ring shape depicts a microtubule in cross-section, showing the 13 protofilaments surrounding a hollow center.
Microtubule
–
Components of the
eukaryotic cytoskeleton.
Actin filaments are shown in red,
microtubules are in green, and the
nuclei are in blue. The cystoskeleton provides the cell with an inner framework and enables it to move and change shape.
Microtubule
–
Image of a fibroblast cell containing fluorescently labeled actin (red) and microtubules (green).
49.
Cleavage furrow
–
The same proteins responsible for muscle contraction, actin and myosin, begin the process of forming the cleavage furrow. Other cytoskeletal proteins and actin binding proteins are involved in the procedure, plant cells do not perform cytokinesis through this exact method but the two procedures are not totally different. Animal cells form a contractile ring within the equatorial region of the cell membrane that constricts to form the cleavage furrow. In plant cells, Golgi vesicle secretions form a plate or septum on the equatorial plane of the cell wall by the action of microtubules of the phragmoplast. The cell cycle begins with interphase when the DNA replicates, the cell grows, mitosis includes four phases, prophase, metaphase, anaphase, and telophase. Prophase is the phase when spindle fibers appear that function to move the chromosomes toward opposite poles. This spindle apparatus consists of microtubules, microfilaments and a network of various proteins. During metaphase, the line up using the spindle apparatus in the middle of the cell along the equatorial plate. The chromosomes move to opposite poles during anaphase and remain attached to the fibers by their centromeres. Animal cell cleavage furrow formation is caused by a ring of actin microfilaments called the contractile ring, myosin is present in the region of the contractile ring as concentrated microfilaments and actin filaments are predominant in this region. The actin filaments here are both pre-existing and new, cleavage is driven by these motor proteins, actin and myosin, which are the same proteins involved with muscle contraction. During cellular cleavage, the contractile ring tightens around the cytoplasm of the cell until the cytoplasm is pinched into two daughter cells, during the final phase of mitosis, telophase, the furrow forms an intercellular bridge using mitotic spindle fibers. Phosphatidylethanolamine has been shown to be present during this time indicates that is may play a role in movement between the plasma membrane and contractile ring. The bridge is broken and resealed to form two identical daughter cells during cytokinesis. The breakage is formed by microtubules and the resealing is negated by Calcium dependent exocytosis using Golgi vesicles, in comparison, the plant cell septum and the animal cell mid-zone are analogous. Both require vesicular secretions by the Golgi apparatus for resealing and formation of the network in addition to microtubules and microfilaments for division
Cleavage furrow
–
Figure 1. In this
electron micrograph of a cell, the cleavage furrow has nearly completely divided the cell.
Cleavage furrow
–
Figure 2. Cilliate undergoing the last processes of binary fission, the cleavage furrow being clearly visible
50.
Plant cell
–
Plant cells are eukaryotic cells that differ in several key aspects from the cells of other eukaryotic organisms. A cell wall composed of cellulose and hemicelluloses, pectin and in many cases lignin, is secreted by the protoplast on the outside of the cell membrane. This contrasts with the walls of fungi, which are made of chitin, and of bacteria. Other types of plastids are the amyloplasts, specialized for storage, elaioplasts specialized for fat storage. Parenchyma cells are living cells that have functions ranging from storage and support to photosynthesis, apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells. Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are called chlorenchyma cells, others, such as the majority of the parenchyma cells in potato tubers and the seed cotyledons of legumes, have a storage function. Collenchyma cells – collenchyma cells are alive at maturity and have only a primary wall and these cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become apparent. Plastids do not develop, and the secretory apparatus proliferates to secrete additional primary wall, pectin and hemicellulose are the dominant constituents of collenchyma cell walls of dicotyledon angiosperms, which may contain as little as 20% of cellulose in Petasites. Collenchyma cells are quite elongated, and may divide transversely to give a septate appearance. The role of cell type is to support the plant in axes still growing in length. Stretchable support is a way to describe what collenchyma does. Parts of the strings in celery are collenchyma, Sclerenchyma cells – Sclerenchyma cells are hard and tough cells with a function in mechanical support. They are of two broad types – sclereids or stone cells and fibres, the cells develop an extensive secondary cell wall that is laid down on the inside of the primary cell wall. The secondary wall is impregnated with lignin, making it hard, thus, these cells cannot survive for long as they cannot exchange sufficient material to maintain active metabolism. Sclerenchyma cells are dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity. Functions for sclereid cells include discouraging herbivory, by damaging digestive passages in small insect larval stages, functions of fibres include provision of load-bearing support and tensile strength to the leaves and stems of herbaceous plants. The major classes of cells differentiate from undifferentiated meristematic cells to form the structures of roots, stems, leaves, flowers. Xylem cells are elongated cells with lignified secondary thickening of the cell walls, xylem cells are specialised for conduction of water, and first appeared in plants during their transition to land in the Silurian period more than 425 million years ago
Plant cell
–
cells of
Arabidopsis thaliana epidermis
Plant cell
–
Plant cell structure
Plant cell
–
This is an electron micrograph of the epidermal cells of a Brassica chinensis leaf. The stomates are also visible.
. PMID 17660363. doi:10.1128/EC.00178-07.
