1.
Meiosis
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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
2.
Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
3.
Mitosis
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In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Mitosis and cytokinesis together define the phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other. The process of mitosis is divided into stages corresponding to the completion of one set of activities and these stages are prophase, prometaphase, metaphase, anaphase, and telophase. During mitosis, the chromosomes, which have already duplicated, condense, the result is two genetically identical daughter nuclei. The rest of the cell may then continue to divide by cytokinesis to produce two daughter cells, producing three or more daughter cells instead of normal two is a mitotic error called tripolar mitosis or multipolar mitosis. Other errors during mitosis can induce apoptosis or cause mutations, certain types of cancer can arise from such mutations. Mitosis occurs only in eukaryotic cells, prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Most animal cells undergo a change, known as mitotic cell rounding. Most human cells are produced by cell division. Important exceptions include the gametes – sperm and egg cells – which are produced by meiosis, german zoologist Otto Bütschli might have claimed the discovery of the process presently known as mitosis, a term coined by Walther Flemming in 1882. Mitosis was discovered in frog, rabbit, and cat cornea cells in 1873, the term is derived from the Greek word μίτος mitos warp thread. The primary result of mitosis and cytokinesis is the transfer of a parent cells genome into two daughter cells, the genome is composed of a number of chromosomes—complexes of tightly coiled DNA that contain genetic information vital for proper cell function. Because each resultant daughter cell should be identical to the parent cell. This occurs during the S phase of interphase, chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere. When mitosis begins, the chromosomes condense and become visible, in some eukaryotes, for example animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell, also disappears, microtubules project from opposite ends of the cell, attach to the centromeres, and align the chromosomes centrally within the cell. The microtubules then contract to pull the chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes, as the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase
4.
Metaphase
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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
5.
Chromosome
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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
6.
Securin
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Securin is a protein involved in control of the metaphase-anaphase transition and anaphase onset. Securin is initially present in the cytoplasm and binds to separase, separase is vital for onset of anaphase. This securin-separase complex is maintained when securin is phosphorylated by Cdk1, when bound to securin, separase is not functional. In addition, both securin and separase are well-conserved proteins, note that separase cannot function without initially forming the securin-separase complex. This is because securin helps properly fold separase into the functional conformation, however, yeast does not appear to require securin to form functional separase as anaphase occurs in yeast with a securin deletion mutation. Securin has 5 known phosphorylation sites that are targets of Cdk1,2 sites at the N-terminal in the Ken-Box and D-box region are known to affect APC recognition and ubiquitination, to initiate the onset of anaphase, securin is dephosphorylated by Cdc14 and other phosphatases. Dephosphorylated securin is recognized by the Anaphase-Promoting Complex bound primarily to Cdc20, the APCCdc20 complex ubiquitinates securin and targets it for degradation. This results in free separase that is able to destroy cohesin and it is thought that securin integrates multiple regulatory inputs to make separase activation switch-like, resulting in sudden, coordinated anaphase. This likely involves a network with several loops, including positive feedback which leads to switch-like behavior. One proposed signaling pathway generating switch-like behavior contains a positive feedback loop for activation of Cdc14 by separase, in addition, these mutant strains exhibited very high rates of mis-segregation compared to normal behavior. Switch-like characteristics are necessary to trigger quick, coordinated chromosomal segregation in anaphase and this means that strong inactivation of separase by securin followed by sudden, rapid destruction of securin and activation of separase is vital for proper anaphase. Overall, securin and separase act in an anaphase-regulating network, figure 4 depicts a potential network diagram
7.
Protease
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A protease is any enzyme that performs proteolysis, protein catabolism by hydrolysis of peptide bonds. Proteases have evolved multiple times, and different classes of protease can perform the reaction by completely different catalytic mechanisms. Proteases can be found in Animalia, Plantae, Fungi, Bacteria, Archaea, the threonine and glutamic-acid proteases were not described until 1995 and 2004 respectively. One way to make a nucleophile is by a catalytic triad, a seventh catalytic type of proteolytic enzymes, asparagine peptide lyase, was described in 2011. Its proteolytic mechanism is unusual since, rather than hydrolysis, it performs an elimination reaction, during this reaction, the catalytic asparagine forms a cyclic chemical structure that cleaves itself at asparagine residues in proteins under the right conditions. Given its fundamentally different mechanism, its inclusion as a peptidase may be debatable, an up-to-date classification of protease evolutionary superfamilies is found in the MEROPS database. In this database, proteases are classified firstly by clan based on structure, mechanism, within each clan, proteases are classified into families based on sequence similarity. Each family may contain hundreds of related proteases. Currently more than 50 clans are known, each indicating an independent evolutionary origin of proteolysis, alternatively, proteases may be classified by the optimal pH in which they are active, Acid proteases Neutral proteases involved in type 1 hypersensitivity. Here, it is released by mast cells and causes activation of complement, basic proteases Proteases are involved in digesting long protein chains into shorter fragments by splitting the peptide bonds that link amino acid residues. Some detach the terminal amino acids from the chain, others attack internal peptide bonds of a protein. Catalysis is achieved by one of two mechanisms, Aspartic, glutamic and metallo- proteases activate a water molecule performs a nucleophilic attack on the peptide bond to hydrolyse it. Serine, threonine and cysteine proteases use a nucleophilic residue and that residue performs a nucleophilic attack to covalently link the protease to the substrate protein, releasing the first half of the product. This covalent acyl-enzyme intermediate is hydrolysed by activated water to complete catalysis by releasing the second half of the product. Proteolysis can be highly promiscuous such that a range of protein substrates are hydrolysed. This is the case for digestive enzymes such as trypsin which have to be able to cleave the array of proteins ingested into smaller peptide fragments, promiscuous proteases typically bind to a single amino acid on the substrate and so only have specificity for that residue. For example, trypsin is specific for the sequences. K\, conversely some proteases are highly specific and only cleave substrates with a certain sequence. Blood clotting and viral polyprotein processing requires this level of specificity in order to achieve precise cleavage events and this is achieved by proteases having a long binding cleft or tunnel with several pockets along it which bind the specified residues
8.
Separase
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In humans, separin is encoded by the ESPL1 gene. In S. cerevisiae, separase is encoded by the esp1 gene, Esp1 was discovered by Kim Nasmyth and coworkers in 1998. Stable cohesion between sister chromatids before anaphase and their separation during anaphase are critical for cell division. In vertebrates, sister chromatid cohesion is released in 2 steps via distinct mechanisms, the first step involves phosphorylation of STAG1 or STAG2 in the cohesin complex. The second step involves cleavage of the cohesin subunit SCC1 by separase, in S. cerevisiae, Esp1 is coded by ESP1 and is regulated by the securin Pds1. The proteins that bind the two chromatids, disallowing any premature sister chromatid separation, are a part of the cohesin protein family. One of these cohesin proteins crucial for sister chromatid cohesion is Scc1, Esp1 is a separase protein that cleaves the cohesin subunit Scc1, allowing sister chromatids to separate at the onset of anaphase during mitosis. When the cell is not dividing, separase is prevented from cleaving cohesin through its association with another protein, securin and this provides two layers of negative regulation to prevent inappropriate cohesin cleavage. Note that separase cannot function without initially forming the complex in most organisms. This is because securin helps properly fold separase into the functional conformation, however, yeast does not appear to require securin to form functional separase because anaphase occurs in yeast even with a securin deletion. On the signal for anaphase, securin is ubiquitinated and hydrolysed, active separase can then cleave Scc1 for release of the sister chromatids. Separase initiates the activation of Cdc14 in early anaphase and Cdc14 has been found to dephosphorylate securin, the presence of this positive feedback loop offers a potential mechanism for giving anaphase a more switch-like behavior
9.
Cohesin
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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
. PMID 16488869.
Media related to Anaphase at Wikimedia Commons