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During Meiosis Ii

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Blazon of cell segmentation in sexually-reproducing organisms used to produce gametes

For the figure of oral communication, encounter Meiosis (figure of speech). For the procedure whereby cell nuclei divide to produce two copies of themselves, see Mitosis. For excessive constriction of the pupils, run across Miosis. For the parasitic infestation, meet Myiasis. For muscle inflammation, see Myositis.

Meiosis (; from Ancient Greek μείωσις ( meíōsis ) 'lessening', since it is a reductional partition)[one] [two] is a special blazon of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. Information technology involves 2 rounds of division that ultimately upshot in iv cells with only i copy of each chromosome (haploid). Additionally, prior to the sectionalisation, genetic textile from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome.[iii] Later, during fertilisation, the haploid cells produced by meiosis from a male person and female volition fuse to create a jail cell with two copies of each chromosome again, the zygote.

Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the virtually frequent genetic crusade of developmental disabilities.[4]

In meiosis, Dna replication is followed past 2 rounds of cell partition to produce four girl cells, each with half the number of chromosomes every bit the original parent prison cell.[3] The two meiotic divisions are known every bit meiosis I and meiosis II. Before meiosis begins, during Southward phase of the cell cycle, the DNA of each chromosome is replicated so that information technology consists of two identical sis chromatids, which remain held together through sis chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic Southward-stage". Immediately following Deoxyribonucleic acid replication, meiotic cells enter a prolonged G2-like phase known equally meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed procedure in which Deoxyribonucleic acid may be cut and and so repaired, which allows them to exchange some of their genetic data. A subset of recombination events results in crossovers, which create concrete links known every bit chiasmata (atypical: chiasma, for the Greek letter Chi (Χ)) between the homologous chromosomes. In most organisms, these links tin can help direct each pair of homologous chromosomes to segregate away from each other during Meiosis I, resulting in ii haploid cells that take half the number of chromosomes as the parent cell.

During meiosis Two, the cohesion between sis chromatids is released and they segregate from one another, every bit during mitosis. In some cases, all four of the meiotic products grade gametes such equally sperm, spores or pollen. In female person animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and merely one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.east. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternate cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When ii gametes (an egg and a sperm) fuse, the resulting zygote is once once more diploid, with the mother and male parent each contributing 23 chromosomes. This aforementioned design, but non the same number of chromosomes, occurs in all organisms that utilize meiosis.

Meiosis occurs in all sexually-reproducing unmarried-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi.[5] [6] [vii] It is an essential process for oogenesis and spermatogenesis.

Overview [edit]

Although the procedure of meiosis is related to the more general cell division process of mitosis, it differs in two important respects:

recombination meiosis shuffles the genes betwixt the 2 chromosomes in each pair (one received from each parent), producing recombinant chromosomes with unique genetic combinations in every gamete
mitosis occurs but if needed to repair Dna damage;

usually occurs between identical sister chromatids and does non event in genetic changes

chromosome number (ploidy) meiosis produces four genetically unique cells, each with half the number of chromosomes every bit in the parent
mitosis produces two genetically identical cells, each with the same number of chromosomes equally in the parent

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the cell undergoes Deoxyribonucleic acid replication, so each homolog now consists of 2 identical sister chromatids. Then each set up of homologs pair with each other and exchange genetic information by homologous recombination oft leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to carve up daughter cells by the spindle appliance. The cells so proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to split up daughter cells to produce a full of iv haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and ii small-scale polar bodies. Because of recombination, an individual chromatid tin consist of a new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural pick can act.

Meiosis uses many of the aforementioned mechanisms every bit mitosis, the blazon of cell division used by eukaryotes to split one cell into two identical girl cells. In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that can separate vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, practice not take the ability to behave out meiosis and have acquired the power to reproduce past parthenogenesis.

Meiosis does not occur in archaea or leaner, which generally reproduce asexually via binary fission. Notwithstanding, a "sexual" process known as horizontal factor transfer involves the transfer of Deoxyribonucleic acid from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin.

History [edit]

Meiosis was discovered and described for the first fourth dimension in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. Information technology was described again in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, even so, was described only in 1890 by German biologist Baronial Weismann, who noted that ii cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Chase Morgan detected crossovers in meiosis in the fruit wing Drosophila melanogaster, which helped to establish that genetic traits are transmitted on chromosomes.

The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology past J.B. Farmer and J.E.S. Moore in 1905, using the idiosyncratic rendering "maiosis":

We advise to apply the terms Maiosis or Maiotic stage to cover the whole serial of nuclear changes included in the ii divisions that were designated every bit Heterotype and Homotype by Flemming.[8]

The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek.[nine]

Phases [edit]

Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis 2 respectively. The preparatory steps that lead up to meiosis are identical in blueprint and name to interphase of the mitotic cell bike.[10] Interphase is divided into iii phases:

  • Growth i (G1) stage: In this very agile phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it volition need for growth. In One thousandane, each of the chromosomes consists of a single linear molecule of Dna.
  • Synthesis (S) stage: The genetic textile is replicated; each of the cell's chromosomes duplicates to get two identical sister chromatids attached at a centromere. This replication does non alter the ploidy of the cell since the centromere number remains the aforementioned. The identical sis chromatids have not still condensed into the densely packaged chromosomes visible with the lite microscope. This will accept identify during prophase I in meiosis.
  • Growth 2 (Chiliad2) stage: Thouii phase as seen earlier mitosis is non present in meiosis. Meiotic prophase corresponds most closely to the Gii phase of the mitotic cell cycle.

Interphase is followed by meiosis I and then meiosis Two. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two girl cells, thus reducing the chromosome number past half. During meiosis Two, sis chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain but ane re-create of each chromosome. In some species, cells enter a resting phase known as interkinesis betwixt meiosis I and meiosis 2.

Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, like in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis Ii (prophase Two, metaphase Two, anaphase 2, telophase II).

Diagram of the meiotic phases

During meiosis, specific genes are more than highly transcribed.[11] [12] In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis.[13] Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.

Meiosis I [edit]

Meiosis I segregates homologous chromosomes, which are joined as tetrads (2n, 4c), producing two haploid cells (due north chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. Meiosis Ii is an equational partitioning analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c).[fourteen]

Meiosis Prophase I in mice. In Leptotene (L) the axial elements (stained by SYCP3) begin to class. In Zygotene (Z) the transverse elements (SYCP1) and central elements of the synaptonemal circuitous are partially installed (appearing as yellowish as they overlap with SYCP3). In Pachytene (P) it's fully installed except on the sexual practice chromosomes. In Diplotene (D) it disassembles revealing chiasmata. CREST marks the centromeres.

Schematic of the synaptonemal complex at different stages of prophase I and the chromosomes bundled as a linear array of loops.

Prophase I [edit]

Prophase I is by far the longest phase of meiosis (lasting xiii out of xiv days in mice[15]). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and exchange genetic data (by homologous recombination), forming at least one crossover per chromosome.[16] These crossovers get visible as chiasmata (plural; singular chiasma).[17] This process facilitates stable pairing between homologous chromosomes and hence enables authentic segregation of the chromosomes at the beginning meiotic partition. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids), with i chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes.

Leptotene [edit]

The first stage of prophase I is the leptotene stage, likewise known as leptonema, from Greek words meaning "thin threads".[18] : 27 In this stage of prophase I, private chromosomes—each consisting of two replicated sis chromatids—become "individualized" to form visible strands inside the nucleus.[18] : 27 [19] : 353 The chromosomes each form a linear assortment of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate.[twenty] Recombination is initiated in this phase past the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice).[21] This process generates single stranded Deoxyribonucleic acid filaments coated past RAD51 and DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of ~400 nm in mice).[xx] [22]

Zygotene [edit]

Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words significant "paired threads",[18] : 27 which in some organisms is also called the bouquet stage because of the style the telomeres cluster at one finish of the nucleus.[23] In this stage the homologous chromosomes get much more closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and key elements of the synaptonemal circuitous.[20] Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

Pachytene [edit]

The pachytene stage ( PAK-i-teen), also known as pachynema, from Greek words meaning "thick threads".[eighteen] : 27 is the phase at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene.[xx] Most breaks are repaired without forming crossovers resulting in cistron conversion.[24] Notwithstanding, a subset of breaks (at least one per chromosome) form crossovers betwixt non-sister (homologous) chromosomes resulting in the commutation of genetic information.[25] Sex chromosomes, however, are non wholly identical, and only commutation information over a small region of homology chosen the pseudoautosomal region.[26] The substitution of information between the homologous chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a upshot of the procedure. Considering the chromosomes cannot exist distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are non visible until the next stage.

Diplotene [edit]

During the diplotene phase, also known as diplonema, from Greek words pregnant "two threads",[18] : thirty the synaptonemal circuitous disassembles and homologous chromosomes dissever from one another a piddling. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I to allow homologous chromosomes to move to contrary poles of the cell.

In homo fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth.[27] This suspended state is referred to equally the dictyotene phase or dictyate. It lasts until meiosis is resumed to gear up the oocyte for ovulation, which happens at puberty or fifty-fifty later.

Diakinesis [edit]

Chromosomes condense further during the diakinesis phase, from Greek words meaning "moving through".[18] : 30 This is the first point in meiosis where the four parts of the tetrads are really visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata conspicuously visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.

Meiotic spindle formation [edit]

Different mitotic cells, human being and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until two poles have formed, generating a barrel shaped spindle.[28] In human being oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that somewhen expands to surroundings the chromosomes.[29] Chromosomes so slide along the microtubules towards the equator of the spindle, at which signal the chromosome kinetochores form stop-on attachments to microtubules.[xxx]

Metaphase I [edit]

Homologous pairs movement together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial airplane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This zipper is referred to equally a bipolar zipper. The concrete basis of the independent assortment of chromosomes is the random orientation of each bivalent along with the metaphase plate, with respect to the orientation of the other bivalents along the aforementioned equatorial line.[17] The protein complex cohesin holds sister chromatids together from the fourth dimension of their replication until anaphase. In mitosis, the strength of kinetochore microtubules pulling in reverse directions creates tension. The prison cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension normally requires at to the lowest degree one crossover per chromosome pair in addition to cohesin between sister chromatids (run into Chromosome segregation).

Anaphase I [edit]

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sis chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes further apart. The prison cell elongates in preparation for partition downwards the center.[17] Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents the sister chromatids from separating.[31] This allows the sister chromatids to remain together while homologs are segregated.

Telophase I [edit]

The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell at present has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set up. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in beast cells or the germination of the cell wall in constitute cells, occurs, completing the creation of two daughter cells. Notwithstanding, cytokinesis does non fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared betwixt daughter cells until the end of meiosis II.[32] Sister chromatids remain attached during telophase I.

Cells may enter a menses of rest known as interkinesis or interphase Two. No DNA replication occurs during this stage.

Meiosis 2 [edit]

Meiosis Two is the second meiotic partitioning, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the procedure is like to mitosis, though its genetic results are fundamentally different. The end result is production of four haploid cells (northward chromosomes, 23 in humans) from the ii haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase Two, metaphase Ii, anaphase Two, and telophase II.

In prophase II, we see the disappearance of the nucleoli and the nuclear envelope again as well equally the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic sectionalization.

In metaphase Two, the centromeres contain 2 kinetochores that adhere to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by ninety degrees when compared to meiosis I, perpendicular to the previous plate.[33]

This is followed by anaphase II, in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, assuasive the sis chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they movement toward opposing poles.[31]

The procedure ends with telophase II, which is like to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or jail cell plate formation eventually produces a full of four daughter cells, each with a haploid set of chromosomes.

Meiosis is now complete and ends up with iv new girl cells.

Origin and function [edit]

The origin and function of meiosis are currently not well understood scientifically, and would provide cardinal insight into the evolution of sexual reproduction in eukaryotes. In that location is no electric current consensus among biologists on the questions of how sex activity in eukaryotes arose in evolution, what bones function sexual reproduction serves, and why information technology is maintained, given the basic ii-fold cost of sex. It is clear that it evolved over ane.2 billion years ago, and that nearly all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.

Meiosis is a central event of the sexual bike in eukaryotes. It is the phase of the life wheel when a prison cell gives ascent to haploid cells (gametes) each having half as many chromosomes as the parental cell. Two such haploid gametes, normally arising from different private organisms, fuse past the process of fertilization, thus completing the sexual bike.

Meiosis is ubiquitous among eukaryotes. It occurs in unmarried-celled organisms such every bit yeast, likewise as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more 2.2 billion years agone[34] and the earliest eukaryotes were likely unmarried-celled organisms. To understand sex in eukaryotes, it is necessary to understand (ane) how meiosis arose in single celled eukaryotes, and (2) the role of meiosis.

The new combinations of Dna created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles, which may exist beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Police force of Contained Array. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase 2, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and contained distribution of chromosomes to each girl cell (and ultimately to gametes);[35] and (2) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information inside chromosomes.[36]

Prophase I arrest [edit]

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.[37] In humans, as an example, oocytes are formed between three and iv months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the iv genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline.[37] The repair process used appears to involve homologous recombinational repair[37] [38] Prophase I arrested oocytes have a high adequacy for efficient repair of DNA damages, particularly exogenously induced double-strand breaks.[38] Dna repair capability appears to be a key quality control mechanism in the female person germ line and a critical determinant of fertility.[38]

Occurrence [edit]

In life cycles [edit]

Meiosis occurs in eukaryotic life cycles involving sexual reproduction, consisting of the abiding cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell partitioning. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. At sure stages of the life wheel, germ cells produce gametes. Somatic cells make upwardly the trunk of the organism and are not involved in gamete production.

Cycling meiosis and fertilization events produces a series of transitions dorsum and forth betwixt alternating haploid and diploid states. The organism phase of the life cycle tin occur either during the diploid country (diplontic life bicycle), during the haploid state (haplontic life cycle), or both (haplodiplontic life cycle, in which there are 2 distinct organism phases, one during the haploid country and the other during the diploid country). In this sense there are three types of life cycles that utilize sexual reproduction, differentiated past the location of the organism phase(south).[ commendation needed ]

In the diplontic life bicycle (with pre-gametic meiosis), of which humans are a part, the organism is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to grade the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.

In the haplontic life wheel (with post-zygotic meiosis), the organism is haploid instead, spawned past the proliferation and differentiation of a single haploid cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to class a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa utilize the haplontic life cycle.[ citation needed ]

Finally, in the haplodiplontic life wheel (with sporic or intermediate meiosis), the living organism alternates betwixt haploid and diploid states. Consequently, this wheel is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism'south gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle tin can be considered a fusion of the diplontic and haplontic life cycles.[39] [ citation needed ]

In plants and animals [edit]

Overview of chromatides' and chromosomes' distribution inside the mitotic and meiotic cycle of a male homo cell

Meiosis occurs in all animals and plants. The finish result, the production of gametes with half the number of chromosomes equally the parent prison cell, is the same, but the detailed procedure is different. In animals, meiosis produces gametes direct. In land plants and some algae, there is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply by mitosis, developing into the haploid gametophyte generation, which then gives ascent to gametes straight (i.east. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes.[40]

In mammals [edit]

In females, meiosis occurs in cells known as oocytes (singular: oocyte). Each primary oocyte divides twice in meiosis, unequally in each case. The first division produces a daughter cell, and a much smaller polar body which may or may not undergo a second sectionalisation. In meiosis Two, division of the daughter cell produces a second polar body, and a single haploid prison cell, which enlarges to become an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in 1 mature ovum and one or ii polar bodies.

Annotation that there are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie fallow within a protective shell of somatic cells called the follicle. At the showtime of each menstrual bicycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known every bit folliculogenesis. During this process, the maturing oocytes resume meiosis and go on until metaphase II of meiosis II, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, commonly one follicle becomes dominant while the others undergo atresia. The procedure of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long flow of meiotic abort known every bit the dictyate stage and lacks the assistance of centrosomes.[41] [42]

In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell chosen spermatocytes, which will subsequently mature to go spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules chosen Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is too adjusted past proteins called nanos and DAZL.[43] [44] Genetic loss-of-office studies on retinoic acid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later in spermatocytes undergoing meiosis, all the same retinoic acid is not required during the time when meiosis initiates.[45]

In female person mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. Some studies suggest that retinoic acid derived from the archaic kidney (mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male person testis suppress meiosis by degrading retinoic acid.[46] However, genetic loss-of-function studies on retinoic acrid-generating enzymes have shown that retinoic acid is non required for initiation of either female person meiosis which occurs during embryogenesis[47] or male meiosis which initiates postnatally.[45]

Flagellates [edit]

While the majority of eukaryotes take a ii-divisional meiosis (though sometimes achiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalids and oxymonads) from the gut of the forest-feeding cockroach Cryptocercus.[48]

Part in homo genetics and illness [edit]

Recombination among the 23 pairs of man chromosomes is responsible for redistributing non but the bodily chromosomes, simply too pieces of each of them. There is as well an estimated i.half-dozen-fold more recombination in females relative to males. In add-on, average, female recombination is higher at the centromeres and male person recombination is higher at the telomeres. On average, one million bp (1 Mb) correspond to 1 cMorgan (cm = one% recombination frequency).[49] The frequency of cross-overs remain uncertain. In yeast, mouse and human, it has been estimated that ≥200 double-strand breaks (DSBs) are formed per meiotic cell. Even so, simply a subset of DSBs (~five–30% depending on the organism), go on to produce crossovers,[50] which would result in only 1-ii cross-overs per human chromosome.

Nondisjunction [edit]

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is chosen nondisjunction. This results in the product of gametes which accept either too many or as well few of a particular chromosome, and is a mutual mechanism for trisomy or monosomy. Nondisjunction tin occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

Well-nigh monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include just are not express to:

  • Down syndrome – trisomy of chromosome 21
  • Patau syndrome – trisomy of chromosome thirteen
  • Edwards syndrome – trisomy of chromosome 18
  • Klinefelter syndrome – actress Ten chromosomes in males – i.e. XXY, XXXY, XXXXY, etc.
  • Turner syndrome – defective of one Ten chromosome in females – i.due east. X0
  • Triple X syndrome – an extra Ten chromosome in females
  • Jacobs syndrome – an extra Y chromosome in males.

The probability of nondisjunction in human oocytes increases with increasing maternal age,[51] presumably due to loss of cohesin over time.[52]

Comparison to mitosis [edit]

In gild to empathize meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis.[53]

Meiosis Mitosis
End result Normally four cells, each with half the number of chromosomes equally the parent 2 cells, having the same number of chromosomes equally the parent
Role Production of gametes (sex activity cells) in sexually reproducing eukaryotes with diplont life bike Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Most all eukaryotes (animals, plants, fungi, and protists);[54] [48]
In gonads, before gametes (in diplontic life cycles);
After zygotes (in haplontic);
Before spores (in haplodiplontic)		 All proliferating cells in all eukaryotes
Steps Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase II, Metaphase II, Anaphase II, Telophase Ii		 Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same as parent? No Yes
Crossing over happens? Yes, normally occurs betwixt each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Yes No
Cytokinesis Occurs in Telophase I and Telophase II Occurs in Telophase
Centromeres split up Does non occur in Anaphase I, but occurs in Anaphase 2 Occurs in Anaphase

Molecular regulation [edit]

How a cell proceeds to meiotic partition in meiotic cell partitioning is not well known. Maturation promoting factor (MPF) seemingly have role in frog Oocyte meiosis. In the fungus South. pombe. there is a role of MeiRNA binding poly peptide for entry to meiotic cell partitioning.[55]

Information technology has been suggested that Yeast CEP1 gene production, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I.[56]

Meiotic recombination is mediated through double stranded suspension, which is catalyzed by Spo11 protein. Also Mre11, Sae2 and Exo1 play role in breakage and recombination. After the breakage happen, recombination take place which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second i gives to noncrossover product).[57]

Seemingly there are checkpoints for meiotic cell division too. In S. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown cistron is thought to grade a checkpoint.[58]

In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II.[56]

Meet also [edit]

  • Fertilisation
  • Coefficient of coincidence
  • Dna repair
  • Oxidative stress
  • Synizesis (biology)
  • Biological life cycle
  • Apomixis
  • Parthenogenesis
  • Alternation of generations
  • Brachymeiosis
  • Mitotic recombination
  • Dikaryon
  • Mating of yeast

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Cited texts [edit]

  • Freeman S (2005). Biology (3rd ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN9780131409415.

External links [edit]

  • Meiosis Flash Animation
  • Animations from the U. of Arizona Biology Dept.
  • Meiosis at Kimball's Biology Pages
  • Khan Academy, video lecture
  • CCO The Cell-Cycle Ontology
  • Stages of Meiosis blitheness
  • *"Abby Dernburg Seminar: Chromosome Dynamics During Meiosis"

johnswoperand1967.blogspot.com

Source: https://en.wikipedia.org/wiki/Meiosis

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