The nucleolus is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis. Nucleoli participate in the formation of signal recognition particles and play a role in the cell's response to stress. Nucleoli are made of proteins, DNA and RNA and form around specific chromosomal regions called nucleolar organizing regions. Malfunction of nucleoli can be the cause of several human conditions called "nucleolopathies" and the nucleolus is being investigated as a target for cancer chemotherapy; the nucleolus was identified by bright-field microscopy during the 1830s. Little was known about the function of the nucleolus until 1964, when 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 such eggs were not capable of life. Half of the eggs had one nucleolus and 25% had two, they concluded. In 1966 Max L. Birnstiel and collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli code for ribosomal RNA.
Three major components of the nucleolus are recognized: the fibrillar center, the dense fibrillar component, the granular component. Transcription of the rDNA occurs in the FC; the DFC contains the protein fibrillarin, important in rRNA processing. The GC contains the protein nucleophosmin, involved in ribosome biogenesis. However, it has been proposed that this particular organization is only observed in higher eukaryotes and that it evolved from a bipartite organization with the transition from anamniotes to amniotes. 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 a clear area in the center of the structure referred to as a nucleolar vacuole. Nucleoli of various plant species have been shown to have high concentrations of iron in contrast to human and animal cell nucleoli; the nucleolus ultrastructure can be seen through an electron microscope, while the organization and dynamics can be studied through fluorescent protein tagging and fluorescent recovery after photobleaching.
Antibodies against the PAF49 protein can be used as a marker for the nucleolus in immunofluorescence experiments. Although only one or two nucleoli can be seen, a diploid human cell has ten nucleolus organizer regions and could have more nucleoli. Most multiple NORs participate in each nucleolus. In ribosome biogenesis, two of the three eukaryotic RNA polymerases are required, 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 this transcription to occur, several pol I-associated factors and DNA-specific trans-acting factors are required. In yeast, the most important are: UAF, TBP, core binding factor ) which bind promoter elements and form the preinitiation complex, in turn recognized by RNA pol. In humans, a similar PIC is assembled with SL1, the promoter selectivity factor, transcription initiation factors, UBF. RNA polymerase I transcribes most rRNA transcripts 28S, 18S, 5.8S) 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. Further processing is needed to generate 5.8 S and 28S RNA molecules. In eukaryotes, the RNA-modifying enzymes are brought to their respective recognition sites by interaction with guide RNAs, which bind these specific sequences; these guide RNAs belong to the class of small nucleolar RNAs which are complexed with proteins and exist as small-nucleolar-ribonucleoproteins. Once the rRNA subunits are processed, they are ready to be assembled into larger ribosomal subunits. However, an additional rRNA molecule, the 5S rRNA, is necessary. In yeast, the 5S rDNA sequence is localized in the intergenic spacer and is transcribed in the nucleolus by RNA pol. In higher eukaryotes and plants, the situation is more complex, for the 5S DNA sequence lies outside the Nucleolus Organiser Region and is transcribed by RNA pol III in the nucleoplasm, after which it finds its way into the nucleolus to participate in the ribosome assembly.
This assembly not only ribosomal proteins as well. The genes encoding these r-proteins are transcribed by pol II in the nucleoplasm by a "conventional" pathway of protein synthesis; the mature r-proteins are "imported" back into the nucleus and the nucleolus. Association and maturation of rRNA and r-proteins result in the formation of the 40S and 60S subunits of the complete ribosome; these are exported through the nuclear pore complexes to the cytoplasm, where they remain free or become associated with the endoplasmic reticulum, forming rough endoplasmic reticulum. In human endometrial cells, a network of nucleolar channels is sometimes formed; the origin and function of this network has not yet been identified. In addition to its role in ribosomal biogenesis, the nucleolus is known to capture and immobilize proteins, a process known as nucleolar detention. Proteins that are detained in the nucle
Metabolism is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; these enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, respond to their environments.. Metabolic reactions may be categorized as catabolic - the breaking down of compounds. Catabolism releases energy, anabolism consumes energy; the chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts - they allow a reaction to proceed more - and they allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.
The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals; the basal metabolic rate of an organism is the measure of the amount of energy consumed by all of these chemical reactions. A striking feature of metabolism is the similarity of the basic metabolic pathways among vastly different species. For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms like elephants; these similarities in metabolic pathways are due to their early appearance in evolutionary history, their retention because of their efficacy. Most of the structures that make up animals and microbes are made from three basic classes of molecule: amino acids and lipids; as these molecules are vital for life, metabolic reactions either focus on making these molecules during the construction of cells and tissues, or by breaking them down and using them as a source of energy, by their digestion.
These biochemicals can be joined together to make polymers such as DNA and proteins, essential macromolecules of life. Proteins are made of amino acids arranged in a linear chain joined together by peptide bonds. Many proteins are enzymes. Other proteins have structural or mechanical functions, such as those that form the cytoskeleton, a system of scaffolding that maintains the cell shape. Proteins are important in cell signaling, immune responses, cell adhesion, active transport across membranes, the cell cycle. Amino acids contribute to cellular energy metabolism by providing a carbon source for entry into the citric acid cycle when a primary source of energy, such as glucose, is scarce, or when cells undergo metabolic stress. Lipids are the most diverse group of biochemicals, their main structural uses are as part of biological membranes both internal and external, such as the cell membrane, or as a source of energy. Lipids are defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as benzene or chloroform.
The fats are a large group of compounds that contain fatty glycerol. Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, hydrophilic groups such as phosphate as in phospholipids. Steroids such as cholesterol are another major class of lipids. Carbohydrates are aldehydes or ketones, with many hydroxyl groups attached, that can exist as straight chains or rings. Carbohydrates are the most abundant biological molecules, fill numerous roles, such as the storage and transport of energy and structural components; the basic carbohydrate units are called monosaccharides and include galactose and most glucose. Monosaccharides can be linked together to form polysaccharides in limitless ways; the two nucleic acids, DNA and RNA, are polymers of nucleotides. Each nucleotide is composed of a phosphate attached to a ribose or deoxyribose sugar group, attached to a nitrogenous base. Nucleic acids are critical for the storage and use of genetic information, its interpretation through the processes of transcription and protein biosynthesis.
This information is propagated through DNA replication. Many viruses have an RNA genome, such as HIV, which uses reverse transcription to create a DNA template from its viral RNA genome. RNA in ribozymes such as spliceosomes and ribosomes is similar to enzymes as it can catalyze chemical reactions. Individual nucleosides are made
Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It is a sign of genotoxic events and chromosomal instability. Micronuclei are seen in cancerous cells and may indicate genomic damage events that can increase the risk of developmental or degenerative diseases. Micronuclei form during anaphase from lagging acentric chromosome or chromatid fragments caused by incorrectly repaired or unrepaired DNA breaks or by nondisjunction of chromosomes; this incorrect segregation of chromosomes may result from hypomethylation of repeat sequences present in pericentromeric DNA, irregularities in kinetochore proteins or their assembly, dysfunctional spindle apparatus, or flawed anaphase checkpoint genes. Many micronucleus assays have been developed to test for the presence of these structures and determine their frequency in cells exposed to certain chemicals or subjected to stressful conditions.
The term micronucleus may refer to the smaller nucleus in ciliate protozoans, such as the Paramecium. In fission it divides by mitosis, in conjugation it furnishes the pairing of gamete nuclei, by whose reciprocal fusion a zygote nucleus is formed, which gives rise to the macronuclei and micronuclei of the individuals of the next cycle of fission. Micronuclei in newly formed red blood cells in humans are known as Howell-Jolly bodies because these structures were first identified and described in erythrocytes by hematologists William Howell and Justin Jolly; these structures were found to be associated with deficiencies in vitamins such as folate and B12. The relationship between formation of micronuclei and exposure to environmental factors was first reported in root tip cells exposed to ionizing radiation. Micronucleus induction by a chemical was first reported in Ehrlich ascites tumor cells treated with colchicine. Micronuclei result from acentric chromosome fragments or lagging whole chromosomes that are not included in the daughter nuclei produced by mitosis because they fail to attach to the spindle during the segregation of chromosomes in anaphase.
These full chromosomes or chromatid fragments are enclosed by a nuclear membranes and are structurally similar to conventional nuclei, albeit smaller in size. This small nucleus is referred to as a micronucleus; the formation of micronuclei can only be observed in cells undergoing nuclear division and can be seen using cytochalasin B to block cytokinesis to produce a binucleated cells. Acentric chromosome fragments may arise in a variety of ways. One way is that disrepair of DNA double-strand breaks can lead to symmetrical or asymmetrical chromatid and chromosome exchanges as well as chromatid and chromosome fragments. If DNA damage exceeds the repair capacity of the cell, unrepaired double-stranded DNA breaks may result in acentric chromosome fragments. Another way eccentric chromosome fragments may arise is when defects in genes related to homologous recombinational repair result in a dysfunctional error-free homologous recombinational DNA repair pathway and causes the cell to resort to the error-prone non-homologous end-joining repair pathway, increasing the likelihood of incorrect repair of DNA breaks, formation of dicentric chromosomes, acentric chromosome fragments.
If enzymes in the NHEJ repair pathway are defective as well, DNA breaks may not be repaired at all. Additionally, simultaneous excision repair of damaged or inappropriate bases incorporated in DNA that are in proximity and on opposite complementary DNA strands may lead to DNA double-stranded breaks and micronucleus formation if the gap-filling step of the repair pathway is not completed. Micronuclei can form from fragmented chromosomes when nucleoplasmic bridges are formed and broken during telophase. Micronuclei formation may result from chromosome malsegregation during anaphase. Hypomethylation of cytosine in centromeric and pericentromeric areas and higher-order repeats of satellite DNA in centromeric DNA can result in such chromosomal loss events. Classical satellite DNA is heavily methylated at cytosine residues but may become fully unmethylated due to ICF syndrome or after treatment by DNA methyl transferase inhibitors. Since assembly of kinetochore proteins at centromeres is affected by the methylation of cytosine and histone proteins, a reduction in heterochromatin integrity as a result of hypomethylation can interfere with microtubule attachment to chromosomes and with the sensing of tension from correct microtubule-kinetochore connections.
Other possible causes of chromosome loss that could lead to micronuclei formation are defects in kinetochore and microtubule interactions, defects in mitotic spindle assembly, mitosis check point defects, abnormal centrosome amplification, telomeric end fusions that result in dicentric chromosomes that detach from the spindle during anaphase. Micronuclei originating from chromosome loss events and acentric chromosome fragments can be distinguished using pancentromeric DNA probes; the number of micronuclei per cell can be predicted using the following formula: M N / c e l l = A F / c e l l ∗ F AF is the number of acentric fragments and F = 0.5 - 0.5P, where P equals the probability of fragments being included in the traditional nucleus and not forming a micronucleus. One study, which used Giemsa stain to stain nuclear material, established the following criteria for identifying micronuclei
In biology and genetics, the germline in a multicellular organism is the population of its bodily cells that are so differentiated or segregated that in the usual processes of reproduction they may pass on their genetic material to the progeny. As a rule this passing-on happens via a process of sexual reproduction. However, there are many exceptions, including processes and concepts such as various forms of apomixis, automixis, cloning, or parthenogenesis; the cells of the germline are called germ cells. For example, gametes such as the sperm or the egg are part of the germline. So are the cells that divide to produce the gametes, called gametocytes, the cells that produce those, called gametogonia, all the way back to the zygote, the cell from which the individual developed. In sexually reproducing organisms, cells that are not in the germline are called somatic cells. According to this view mutations and other genetic changes in the germline may be passed to offspring, but a change in a somatic cell will not be.
This need not apply to somatically reproducing organisms, such as many plants. For example, many varieties of citrus, plants in the Rosaceae and some in the Asteraceae, such as Taraxacum produce seeds apomictically when somatic diploid cells displace the ovule or early embryo. In an earlier stage of genetic thinking, the distinction between germline and somatic cell was clear cut. For example, August Weismann proposed and pointed out, a germline cell is immortal in the sense that it is part of a lineage that has reproduced indefinitely since the beginning of life and, barring accident could continue doing so indefinitely. However, it is now known in some detail that this distinction between somatic and germ cells is artificial and depends on particular circumstances and internal cellular mechanisms such as telomeres and controls such as the selective application of telomerase in germ cells, stem cells and the like. Not all multicellular organisms differentiate into somatic and germ lines, but in the absence of specialised technical human intervention all but the simplest multicellular structures do so.
In such organisms somatic cells tend to be totipotent, for over a century sponge cells have been known to reassemble into new sponges after having been separated by forcing them through a sieve. Germline can refer to a lineage of cells spanning many generations of individuals—for example, the germline that links any living individual to the hypothetical last universal common ancestor, from which all plants and animals descend. Plants and basal metazoans such as sponges and corals do not sequester a distinct germline, generating gametes from multipotent stem cell lineages that give rise to ordinary somatic tissues, it is therefore that germline sequestration first evolved in complex animals with sophisticated body plans, i.e. bilaterians. There are several theories on the origin of the strict germline-soma distinction. Setting aside an isolated germ cell population early in embryogenesis might promote cooperation between the somatic cells of a complex multicellular organism. Another recent theory suggests that early germline sequestration evolved to limit the accumulation of deleterious mutations in mitochondrial genes in complex organisms with high energy requirements and fast mitochondrial mutation rates.
Reactive oxygen species are produced as byproducts of metabolism. In germline cells, ROS are a significant cause of DNA damages that, upon DNA replication, lead to mutations. 8-Oxoguanine, an oxidized derivative of guanine, is produced by spontaneous oxidation in the germline cells of mice, during the cell’s DNA replication cause GC to TA transversion mutations. Such mutations occur throughout the mouse chromosomes as well as during different stages of gametogenesis; the mutation frequencies for cells in different stages of gametogenesis are about 5 to 10-fold lower than in somatic cells both for spermatogenesis and oogenesis. The lower frequencies of mutation in germline cells compared to somatic cells appears to be due to more efficient DNA repair of DNA damages homologous recombinational repair, during germline meiosis. Among humans, about five percent of live-born offspring have a genetic disorder, of these, about 20% are due to newly arisen germline mutations. August Weismann Epigenetics Germ line development Germinal choice technology Weismann barrier
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, attachment and sensation. Ciliates are an important group of protists, common anywhere there is water — in lakes, oceans and soils. About 3,500 species have been described, the potential number of extant species is estimated at 30,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm to as much as 4 mm in length, include some of the most morphologically complex protozoans. In most systems of taxonomy, "Ciliophora" is ranked as a phylum, under either the kingdom Protista or Protozoa. In some systems of classification, ciliated protozoa are placed within the class "Ciliata,".
In the taxonomic scheme proposed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata. Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus, a large, polyploid macronucleus; the latter is generated from the micronucleus by amplification of the heavy editing. The micronucleus does not express its genes; the macronucleus provides the nuclear RNA for vegetative growth. Division of the macronucleus occurs by amitosis, the segregation of the chromosomes occurs by a process whose mechanism is unknown; this process is not perfect, after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and fuse to form new micronuclei and macronuclei.
Food vacuoles are formed through phagocytosis and follow a particular path through the cell as their contents are digested and broken down by lysosomes so the substances the vacuole contains are small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches. Most ciliates have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. In some genera, such as Paramecium, these have a distinctive star shape, with each point being a collecting tube. Cilia are arranged in rows called kineties. In some forms there are body polykinetids, for instance, among the spirotrichs where they form bristles called cirri. More body cilia are arranged in mono- and dikinetids, which include one and two kinetosomes, each of which may support a cilium; these are arranged into rows called kineties. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, include various fibrils and microtubules involved in coordinating the cilia.
The infraciliature is one of the main components of the cell cortex. Others are the alveoli, small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are generally present; the presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates and dinoflagellates. These superficially dissimilar groups make up the alveolates. Most ciliates are heterotrophs, feeding on smaller organisms, such as bacteria and algae, detritus swept into the oral groove by modified oral cilia; this includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet. Feeding techniques vary however; some ciliates are mouthless and feed by absorption, while others are predatory and feed on other protozoa and in particular on other ciliates.
Some ciliates parasitize animals, although only one species, Balantidium coli, is known to cause disease in humans. Ciliates reproduce asexually, by various kinds of fission. During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis; the cell divides in two, each new cell obtains a copy of the micronucleus and the macronucleus. The cell is divided transversally, with the anterior half of the ciliate forming one new organism, the posterior half forming another. However, other types of fission occur in some ciliate groups; these include budding.
Protozoa is an informal term for single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. The protozoa were regarded as "one-celled animals", because they possess animal-like behaviors, such as motility and predation, lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy. In some systems of biological classification, Protozoa is a high-level taxonomic group; when first introduced in 1818, Protozoa was erected as a taxonomic class, but in classification schemes it was elevated to a variety of higher ranks, including phylum and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom; the seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Percolozoa and Sulcozoa.
Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates and the parasitic apicomplexans, all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a paraphyletic group or evolutionary grade, within which the members of Fungi and Chromista are thought to have evolved; the word "protozoa" was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals". Goldfuss created Protozoa as a class containing; the group included not only single-celled microorganisms but some "lower" multicellular animals, such as rotifers, sponges, jellyfish and polychaete worms. The term Protozoa is formed from the Greek words πρῶτος, meaning "first", ζῶα, plural of ζῶον, meaning "animal"; the use of Protozoa as a formal taxon has been discouraged by some researchers because the term implies kinship with animals and promotes an arbitrary separation of "animal-like" from "plant-like" organisms.
In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoans such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa. At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria, Rhizopoda; the definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use. As a phylum under Animalia, the Protozoa were rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants; as long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, studied in departments of Botany.
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, but can feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and some will continue so—whether many of these organisms, or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae, which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum. Six years Ernst Haeckel proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, remained so for more than a century. In the course of the 20th century, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, that most of the unicellular protozoa were no more related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants and Fungi. A variety of multi-kingdom systems were proposed, Kingdoms Protista and Protoctista became well est
Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parents". Reproduction is a fundamental feature of all known life. There are two forms of reproduction: sexual. In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms; the cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates a genetically identical copy of itself; the evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sexual reproduction is that only 50% of organisms reproduce and organisms only pass on 50% of their genes. Sexual reproduction requires the sexual interaction of two specialized organisms, called gametes, which contain half the number of chromosomes of normal cells and are created by meiosis, with a male fertilizing a female of the same species to create a fertilized zygote; this produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms.
Asexual reproduction is a process by which organisms create genetically similar or identical copies of themselves without the contribution of genetic material from another organism. Bacteria divide asexually via binary fission; these organisms do not possess different sexes, they are capable of "splitting" themselves into two or more copies of themselves. Most plants have the ability to reproduce asexually and the ant species Mycocepurus smithii is thought to reproduce by asexual means; some species that are capable of reproducing asexually, like hydra and jellyfish, may reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can reproduce sexually. Bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthenogenesis and spore formation that involves only mitosis. Parthenogenesis is the development of embryo or seed without fertilization by a male. Parthenogenesis occurs in some species, including lower plants and vertebrates.
It is sometimes used to describe reproduction modes in hermaphroditic species which can self-fertilize. Sexual reproduction is a biological process that creates a new organism by combining the genetic material of two organisms in a process that starts with meiosis, a specialized type of cell division; each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes. Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as female. In isogamous species, the gametes are similar or identical in form, but may have separable properties and may be given other different names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as many fungi and the ciliate Paramecium aurelia, have more than two "sexes", called syngens. Most animals and plants reproduce sexually. Sexually reproducing organisms have different sets of genes for every trait.
Offspring inherit one allele for each trait from each parent. Thus, offspring have a combination of the parents' genes, it is believed that "the masking of deleterious alleles favors the evolution of a dominant diploid phase in organisms that alternate between haploid and diploid phases" where recombination occurs freely. Bryophytes reproduce sexually, but the larger and commonly-seen organisms are haploid and produce gametes; the gametes fuse to form a zygote which develops into a sporangium, which in turn produces haploid spores. The diploid stage is small and short-lived compared to the haploid stage, i.e. haploid dominance. The advantage of diploidy, only exists in the diploid life generation. Bryophytes retain sexual reproduction despite the fact that the haploid stage does not benefit from heterosis; this may be an indication that the sexual reproduction has advantages other than heterosis, such as genetic recombination between members of the species, allowing the expression of a wider range of traits and thus making the population more able to survive environmental variation.
Allogamy is the fertilization of the combination of gametes from two parents the ovum from one individual with the spermatozoa of another. Self-fertilization known as autogamy, occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual, e.g. many vascular plants, some foraminiferans, some ciliates. The term "autogamy" is sometimes substituted for autogamous pollination and describes self-pollination within the same flower, distinguished from geitonogamous pollination, transfer of pollen to a different flower on the same flowering plant, or within a single monoecious Gymnosperm plant. Mitosis and meiosis are types of cell division. Mitosis occurs in somatic cells. Mitosis The resultant number of cells in mitosis is t