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What type of cells are used to create gametes

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Type of jail cell sectionalization in sexually-reproducing organisms used to produce gametes

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

Meiosis (; from Ancient Greek μείωσις ( meíōsis ) 'lessening', since it is a reductional division)[1] [2] is a special type of prison cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately effect in iv cells with merely i copy of each chromosome (haploid). Additionally, prior to the sectionalisation, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome.[3] After on, during fecundation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with ii copies of each chromosome again, the zygote.

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

In meiosis, DNA replication is followed by two rounds of cell division to produce four girl cells, each with half the number of chromosomes as the original parent cell.[3] The two meiotic divisions are known as meiosis I and meiosis Two. Before meiosis begins, during S stage of the prison cell cycle, the DNA of each chromosome is replicated so that information technology consists of ii identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to every bit "premeiotic Southward-phase" or "meiotic S-stage". Immediately following Dna replication, meiotic cells enter a prolonged Gtwo-like phase known as meiotic prophase. During this fourth dimension, homologous chromosomes pair with each other and undergo genetic recombination, a programmed procedure in which DNA may be cut and then repaired, which allows them to commutation some of their genetic data. A subset of recombination events results in crossovers, which create physical links known as chiasmata (atypical: chiasma, for the Greek letter Chi (Χ)) betwixt the homologous chromosomes. In nearly organisms, these links can help directly each pair of homologous chromosomes to segregate abroad from each other during Meiosis I, resulting in two haploid cells that have one-half the number of chromosomes as the parent prison cell.

During meiosis II, the cohesion between sister chromatids is released and they segregate from 1 another, as during mitosis. In some cases, all 4 of the meiotic products class gametes such as sperm, spores or pollen. In female person animals, iii of the four meiotic products are typically eliminated by extrusion into polar bodies, and just 1 cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to class a diploid zygote that contains two copies of each chromosome, i from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the aforementioned number of chromosomes. For example, diploid human being cells incorporate 23 pairs of chromosomes including 1 pair of sexual activity chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set up of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is in one case once more diploid, with the female parent and father each contributing 23 chromosomes. This same design, just not 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.[five] [vi] [7] It is an essential process for oogenesis and spermatogenesis.

Overview [edit]

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

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

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

chromosome number (ploidy) meiosis produces four genetically unique cells, each with one-half the number of chromosomes equally in the parent
mitosis produces 2 genetically identical cells, each with the same number of chromosomes as in the parent

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the prison cell undergoes DNA replication, so each homolog at present consists of two identical sis chromatids. And then each set of homologs pair with each other and commutation genetic information by homologous recombination ofttimes leading to physical connections (crossovers) between the homologs. In the kickoff meiotic division, the homologs are segregated to separate daughter cells past the spindle apparatus. The cells then 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 four haploid cells. Female animals utilise a slight variation on this blueprint and produce one large ovum and 2 small-scale polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an private gamete can include an array of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection tin can act.

Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by eukaryotes to dissever one cell into 2 identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that tin can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, do not have the power to carry out meiosis and have acquired the power to reproduce by parthenogenesis.

Meiosis does not occur in archaea or bacteria, which generally reproduce asexually via binary fission. However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from ane bacterium or archaeon to some other and recombination of these DNA molecules of dissimilar parental origin.

History [edit]

Meiosis was discovered and described for the showtime time in sea urchin eggs in 1876 by the High german biologist Oscar Hertwig. It was described once more in 1883, at the level of chromosomes, past the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described merely in 1890 by German language biologist August Weismann, who noted that two cell divisions were necessary to transform one diploid jail cell into 4 haploid cells if the number of chromosomes had to exist maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster, which helped to establish that genetic traits are transmitted on chromosomes.

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

Nosotros propose to employ the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming.[8]

The spelling was inverse 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 2 which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis Two respectively. The preparatory steps that lead up to meiosis are identical in blueprint and proper name to interphase of the mitotic prison cell cycle.[ten] Interphase is divided into three phases:

  • Growth one (Thou1) phase: In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In Gane, each of the chromosomes consists of a single linear molecule of Dna.
  • Synthesis (Due south) phase: The genetic material is replicated; each of the cell's chromosomes duplicates to become ii identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will accept place during prophase I in meiosis.
  • Growth 2 (G2) phase: K2 phase every bit seen earlier mitosis is not present in meiosis. Meiotic prophase corresponds virtually closely to the Gii stage of the mitotic cell bike.

Interphase is followed by meiosis I and and so meiosis II. Meiosis I separates replicated homologous chromosomes, each notwithstanding fabricated upward of two sister chromatids, into two daughter cells, thus reducing the chromosome number by one-half. During meiosis 2, sister chromatids decouple and the resultant daughter chromosomes are segregated into iv daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and comprise only i copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II.

Meiosis I and Two 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 highly transcribed.[xi] [12] In add-on to strong meiotic stage-specific expression of mRNA, there are too pervasive translational controls (e.thousand. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific poly peptide expression of genes during meiosis.[xiii] Thus, both transcriptional and translational controls make up one's mind the wide 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 (n chromosomes, 23 in humans) which each incorporate chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional sectionalization. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c).[xiv]

Meiosis Prophase I in mice. In Leptotene (50) the centric elements (stained past SYCP3) begin to form. In Zygotene (Z) the transverse elements (SYCP1) and cardinal elements of the synaptonemal complex are partially installed (appearing every bit yellow as they overlap with SYCP3). In Pachytene (P) information technology's fully installed except on the sex 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 arranged as a linear array of loops.

Prophase I [edit]

Prophase I is by far the longest stage of meiosis (lasting thirteen out of xiv days in mice[15]). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and exchange genetic information (by homologous recombination), forming at least one crossover per chromosome.[xvi] These crossovers become visible as chiasmata (plural; atypical chiasma).[17] This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (ii chromosomes) or tetrads (four chromatids), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the advent of chromosomes.

Leptotene [edit]

The offset stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".[18] : 27 In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus.[18] : 27 [19] : 353 The chromosomes each form a linear array of loops mediated past cohesin, and the lateral elements of the synaptonemal complex assemble forming an "centric chemical element" from which the loops emanate.[20] Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (effectually 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-centrality 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 phase, also known as zygonema, from Greek words pregnant "paired threads",[xviii] : 27 which in some organisms is besides chosen the bouquet stage because of the way the telomeres cluster at one finish of the nucleus.[23] In this stage the homologous chromosomes become much more than closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex.[20] Synapsis is thought to occur in a attachment-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

Pachytene [edit]

The pachytene phase ( PAK-i-teen), also known as pachynema, from Greek words meaning "thick threads".[18] : 27 is the phase at which all autosomal chromosomes accept synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene.[twenty] Most breaks are repaired without forming crossovers resulting in gene conversion.[24] All the same, a subset of breaks (at least one per chromosome) grade crossovers betwixt non-sister (homologous) chromosomes resulting in the exchange of genetic data.[25] Sex activity chromosomes, even so, are not wholly identical, and only exchange information over a small-scale region of homology called the pseudoautosomal region.[26] The commutation of information between the homologous chromatids results in a recombination of information; each chromosome has the complete set of data information technology had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the side by side stage.

Diplotene [edit]

During the diplotene phase, also known as diplonema, from Greek words significant "two threads",[18] : 30 the synaptonemal complex disassembles and homologous chromosomes separate from ane some other a fiddling. However, the homologous chromosomes of each bivalent remain tightly jump 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 opposite poles of the cell.

In human being fetal oogenesis, all developing oocytes develop to this phase and are arrested in prophase I earlier nascency.[27] This suspended state is referred to equally the dictyotene phase or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.

Diakinesis [edit]

Chromosomes condense further during the diakinesis stage, from Greek words significant "moving through".[eighteen] : 30 This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the remainder of the phase 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, man and mouse oocytes do non accept centrosomes to produce the meiotic spindle. In mice, approximately eighty MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and brainstorm 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 man oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes.[29] Chromosomes so slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules.[30]

Metaphase I [edit]

Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align forth an equatorial aeroplane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the 2 kinetochores of homologous chromosomes. This zipper is referred to as a bipolar attachment. The physical basis of the independent array of chromosomes is the random orientation of each bivalent forth with the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.[17] The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the forcefulness of kinetochore microtubules pulling in opposite directions creates tension. The 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 least ane crossover per chromosome pair in improver to cohesin between sister chromatids (see Chromosome segregation).

Anaphase I [edit]

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sis chromatids) to contrary poles. Nonkinetochore microtubules lengthen, pushing the centrosomes further apart. The cell elongates in preparation for division downward the heart.[17] Unlike in mitosis, but the cohesin from the chromosome artillery is degraded while the cohesin surrounding the centromere remains protected past 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 commencement meiotic division effectively ends when the chromosomes make it at the poles. Each daughter jail cell now has one-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. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the jail cell membrane in animal cells or the formation of the jail cell wall in plant cells, occurs, completing the creation of 2 girl cells. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the terminate of meiosis Two.[32] Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known every bit interkinesis or interphase Two. No Deoxyribonucleic acid replication occurs during this phase.

Meiosis Ii [edit]

Meiosis II is the second meiotic sectionalisation, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the procedure is similar to mitosis, though its genetic results are fundamentally different. The finish result is production of four haploid cells (north chromosomes, 23 in humans) from the two haploid cells (with north chromosomes, each consisting of 2 sister chromatids) produced in meiosis I. The 4 principal steps of meiosis II are: prophase II, metaphase Ii, anaphase Two, and telophase Ii.

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

In metaphase II, the centromeres contain 2 kinetochores that adhere to spindle fibers from the centrosomes at reverse poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.[33]

This is followed past anaphase 2, in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sis chromosomes as they move 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 somewhen produces a total of iv daughter cells, each with a haploid set of chromosomes.

Meiosis is now complete and ends upwardly with iv new daughter cells.

Origin and function [edit]

The origin and role of meiosis are currently not well understood scientifically, and would provide cardinal insight into the evolution of sexual reproduction in eukaryotes. At that place is no current consensus amid biologists on the questions of how sex in eukaryotes arose in evolution, what basic role sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.

Meiosis is a key event of the sexual bike in eukaryotes. It is the stage of the life cycle when a jail cell gives ascension to haploid cells (gametes) each having one-half as many chromosomes every bit the parental jail cell. Two such haploid gametes, normally arising from dissimilar individual organisms, fuse by the process of fertilization, thus completing the sexual cycle.

Meiosis is ubiquitous among eukaryotes. It occurs in single-celled organisms such as yeast, besides as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than than two.2 billion years ago[34] and the primeval eukaryotes were probable single-celled organisms. To understand sex in eukaryotes, information technology is necessary to understand (1) how meiosis arose in single celled eukaryotes, and (2) the function 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 be benign. Meiosis generates gamete genetic diversity in two ways: (one) Constabulary of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and Two, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes);[35] and (2) Crossing Over. The physical commutation of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes.[36]

Prophase I arrest [edit]

Female person mammals and birds are born possessing all the oocytes needed for hereafter ovulations, and these oocytes are arrested at the prophase I stage of meiosis.[37] In humans, equally an example, oocytes are formed between 3 and four months of gestation inside the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may terminal for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the 4 genome copy phase was proposed to provide the advisory 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 capability for efficient repair of DNA damages, especially exogenously induced double-strand breaks.[38] DNA repair capability appears to exist a key quality command mechanism in the female germ line and a disquisitional determinant of fertility.[38]

Occurrence [edit]

In life cycles [edit]

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

Cycling meiosis and fertilization events produces a series of transitions back and forth between alternate haploid and diploid states. The organism phase of the life wheel can occur either during the diploid state (diplontic life cycle), during the haploid state (haplontic life bike), or both (haplodiplontic life cycle, in which at that place are two singled-out organism phases, ane during the haploid state and the other during the diploid land). In this sense there are 3 types of life cycles that use sexual reproduction, differentiated by the location of the organism stage(s).[ citation 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 abound into the organism.

In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Ii organisms of opposing sex contribute their haploid gametes to form 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 bicycle.[ commendation needed ]

Finally, in the haplodiplontic life bike (with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this wheel is as well known equally 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'south gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle 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 person man cell

Meiosis occurs in all animals and plants. The cease effect, the production of gametes with half the number of chromosomes as the parent cell, is the aforementioned, but the detailed process is unlike. In animals, meiosis produces gametes direct. In land plants and some algae, at that place 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 and so gives rise to gametes directly (i.due east. without further meiosis). In both animals and plants, the terminal 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 non undergo a second partitioning. In meiosis II, division of the girl cell produces a second polar body, and a single haploid cell, which enlarges to go an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in i mature ovum and one or two polar bodies.

Note 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 kickoff of each menstrual cycle, FSH secretion from the inductive pituitary stimulates a few follicles to mature in a process known equally folliculogenesis. During this procedure, the maturing oocytes resume meiosis and continue until metaphase 2 of meiosis Ii, where they are again arrested just earlier ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, unremarkably 1 follicle becomes dominant while the others undergo atresia. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that information technology features a long period of meiotic arrest known as 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 called spermatocytes, which volition later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis past degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their ain retinoic acid. Sensitivity to retinoic acid is besides adjusted by proteins called nanos and DAZL.[43] [44] Genetic loss-of-function studies on retinoic acrid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later in spermatocytes undergoing meiosis, however retinoic acrid is not required during the time when meiosis initiates.[45]

In female person mammals, meiosis begins immediately later on primordial germ cells drift 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 testis suppress meiosis by degrading retinoic acid.[46] Still, genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis[47] or male person meiosis which initiates postnatally.[45]

Flagellates [edit]

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

Role in human genetics and disease [edit]

Recombination among the 23 pairs of human chromosomes is responsible for redistributing not merely the bodily chromosomes, but also pieces of each of them. There is also an estimated ane.half dozen-fold more recombination in females relative to males. In addition, average, female person recombination is higher at the centromeres and male recombination is college at the telomeres. On average, 1 one thousand thousand bp (1 Mb) stand for to ane cMorgan (cm = 1% 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 jail cell. However, only a subset of DSBs (~5–30% depending on the organism), proceed to produce crossovers,[50] which would result in just 1-ii cross-overs per human being 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 called nondisjunction. This results in the product of gametes which have either likewise many or too few of a particular chromosome, and is a common machinery for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

Nearly monosomic and trisomic human embryos are not viable, merely 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 but are not limited to:

  • Down's syndrome – trisomy of chromosome 21
  • Patau syndrome – trisomy of chromosome 13
  • Edwards syndrome – trisomy of chromosome eighteen
  • Klinefelter syndrome – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc.
  • Turner syndrome – lacking of ane Ten chromosome in females – i.e. X0
  • Triple X syndrome – an extra X chromosome in females
  • Jacobs syndrome – an extra Y chromosome in males.

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

Comparison to mitosis [edit]

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

Meiosis Mitosis
End result Normally 4 cells, each with one-half the number of chromosomes equally the parent Two cells, having the same number of chromosomes as the parent
Function Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life bike Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Almost all eukaryotes (animals, plants, fungi, and protists);[54] [48]
In gonads, before gametes (in diplontic life cycles);
Afterwards zygotes (in haplontic);
Before spores (in haplodiplontic)		 All proliferating cells in all eukaryotes
Steps Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase 2, Metaphase 2, Anaphase II, Telophase Two		 Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same equally parent? No Yes
Crossing over happens? Aye, usually occurs between each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Aye 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 Two Occurs in Anaphase

Molecular regulation [edit]

How a jail cell proceeds to meiotic sectionalisation in meiotic cell division is non well known. Maturation promoting factor (MPF) seemingly take role in frog Oocyte meiosis. In the fungus S. pombe. there is a role of MeiRNA binding protein for entry to meiotic cell partitioning.[55]

It 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 break, which is catalyzed by Spo11 protein. Besides Mre11, Sae2 and Exo1 play part in breakage and recombination. Later on the breakage happen, recombination accept 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 1 gives to noncrossover product).[57]

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

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

See 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 Due south (2005). Biological Scientific discipline (3rd ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN9780131409415.

External links [edit]

  • Meiosis Wink Animation
  • Animations from the U. of Arizona Biology Dept.
  • Meiosis at Kimball'due south Biology Pages
  • Khan Academy, video lecture
  • CCO The Jail cell-Cycle Ontology
  • Stages of Meiosis animation
  • *"Abby Dernburg Seminar: Chromosome Dynamics During Meiosis"

agnewbeene1967.blogspot.com

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

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