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During Meiosis Replication of Dna Occurs Before the Stasrt of Meiosis I and Again Before Meiosis Ii

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

For the figure of spoken language, see Meiosis (effigy of speech). For the process whereby cell nuclei separate to produce two copies of themselves, come across Mitosis. For excessive constriction of the pupils, see Miosis. For the parasitic infestation, see Myiasis. For musculus inflammation, see Myositis.

Meiosis (; from Aboriginal Greek μείωσις ( meíōsis ) 'lessening', since it is a reductional partition)[ane] [2] 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. It involves 2 rounds of segmentation that ultimately consequence in four cells with only one re-create of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of lawmaking on each chromosome.[3] Afterwards on, during fertilisation, 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 nearly frequent genetic cause of developmental disabilities.[4]

In meiosis, Dna replication is followed by two rounds of cell sectionalisation to produce four girl cells, each with half the number of chromosomes as the original parent cell.[iii] The two meiotic divisions are known as meiosis I and meiosis Two. Before meiosis begins, during Southward phase of the cell cycle, the Deoxyribonucleic acid of each chromosome is replicated and so that information technology consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to every bit "premeiotic S-stage" or "meiotic Southward-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this fourth dimension, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which Dna may be cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical 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 two haploid cells that take half the number of chromosomes as the parent jail cell.

During meiosis 2, the cohesion betwixt sister chromatids is released and they segregate from one another, as during mitosis. In some cases, all iv of the meiotic products course gametes such as sperm, spores or pollen. In female animals, iii of the four meiotic products are typically eliminated by extrusion into polar bodies, and just one 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 ii copies of each chromosome, one from each parent. Thus, alternating 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), one-half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one fix of 23 chromosomes. When ii gametes (an egg and a sperm) fuse, the resulting zygote is in one case again diploid, with the mother and father each contributing 23 chromosomes. This same design, but non the same number of chromosomes, occurs in all organisms that use meiosis.

Meiosis occurs in all sexually-reproducing unmarried-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi.[five] [6] [7] Information technology is an essential process for oogenesis and spermatogenesis.

Overview [edit]

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

recombination meiosis shuffles the genes between the two chromosomes in each pair (one 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 between identical sister chromatids and does non result 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 two genetically identical cells, each with the same number of chromosomes as in the parent

Meiosis begins with a diploid cell, which contains 2 copies of each chromosome, termed homologs. First, the cell undergoes Dna replication, so each homolog now consists of ii identical sis chromatids. Then each set of homologs pair with each other and substitution genetic information by homologous recombination ofttimes leading to physical connections (crossovers) betwixt the homologs. In the first meiotic division, the homologs are segregated to split up daughter cells by the spindle apparatus. The cells then proceed to a second partition without an intervening round of DNA replication. The sis chromatids are segregated to separate girl cells to produce a total of 4 haploid cells. Female animals employ a slight variation on this pattern and produce 1 large ovum and two minor polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal genetic data, 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 choice can human action.

Meiosis uses many of the same mechanisms as mitosis, the type of cell partitioning used by eukaryotes to divide 1 cell into ii 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, practice not have the ability to deport out meiosis and have acquired the ability to reproduce by parthenogenesis.

Meiosis does not occur in archaea or leaner, which generally reproduce asexually via binary fission. However, a "sexual" procedure known equally horizontal cistron transfer involves the transfer of Dna from one bacterium or archaeon to another and recombination of these Dna molecules of dissimilar parental origin.

History [edit]

Meiosis was discovered and described for the outset time in body of water urchin eggs in 1876 by the German biologist Oscar Hertwig. It 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, withal, was described only in 1890 past High german biologist August Weismann, who noted that two jail cell divisions were necessary to transform i 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 fly 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 by J.B. Farmer and J.E.South. Moore in 1905, using the idiosyncratic rendering "maiosis":

Nosotros propose to apply the terms Maiosis or Maiotic stage to cover the whole series of nuclear changes included in the two divisions that were designated equally Heterotype and Homotype past 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.[9]

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 Ii respectively. The preparatory steps that atomic number 82 upward to meiosis are identical in pattern and name to interphase of the mitotic cell bike.[ten] Interphase is divided into three phases:

  • Growth 1 (Yard1) phase: In this very agile phase, the jail cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In 10001, each of the chromosomes consists of a single linear molecule of Dna.
  • Synthesis (Southward) phase: The genetic textile is replicated; each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the aforementioned. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the low-cal microscope. This volition have place during prophase I in meiosis.
  • Growth 2 (Chiliad2) phase: Thou2 phase as seen before mitosis is not nowadays in meiosis. Meiotic prophase corresponds most closely to the Grandtwo phase of the mitotic cell bicycle.

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

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 bicycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase 2, metaphase II, anaphase Two, telophase 2).

Diagram of the meiotic phases

During meiosis, specific genes are more than highly transcribed.[11] [12] In addition to strong meiotic phase-specific expression of mRNA, there are also pervasive translational controls (e.1000. 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 determine the broad restructuring of meiotic cells needed to carry out meiosis.

Meiosis I [edit]

Meiosis I segregates homologous chromosomes, which are joined every bit tetrads (2n, 4c), producing 2 haploid cells (n 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 partitioning. Meiosis 2 is an equational partitioning analogous to mitosis, in which the sister chromatids are segregated, creating 4 haploid daughter cells (1n, 1c).[fourteen]

Meiosis Prophase I in mice. In Leptotene (Fifty) the centric elements (stained by SYCP3) begin to course. In Zygotene (Z) the transverse elements (SYCP1) and central elements of the synaptonemal complex are partially installed (appearing as yellow as they overlap with SYCP3). In Pachytene (P) it'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 bundled as a linear assortment of loops.

Prophase I [edit]

Prophase I is past far the longest phase of meiosis (lasting 13 out of 14 days in mice[15]). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and substitution genetic data (by homologous recombination), forming at least ane crossover per chromosome.[16] These crossovers become visible as chiasmata (plural; singular chiasma).[17] This procedure 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 (two chromosomes) or tetrads (4 chromatids), with ane 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 beginning stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".[18] : 27 In this phase 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 by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "axial chemical element" from which the loops emanate.[20] Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice).[21] This process generates single stranded Dna filaments coated by 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).[20] [22]

Zygotene [edit]

Leptotene is followed by the zygotene phase, also known as zygonema, from Greek words meaning "paired threads",[18] : 27 which in some organisms is besides called the boutonniere stage because of the way the telomeres cluster at i end of the nucleus.[23] In this stage the homologous chromosomes become much more closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex.[twenty] 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 stage ( PAK-i-teen), also known every bit pachynema, from Greek words meaning "thick threads".[xviii] : 27 is the stage 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.[20] Most breaks are repaired without forming crossovers resulting in gene conversion.[24] Even so, a subset of breaks (at least one per chromosome) form crossovers betwixt non-sister (homologous) chromosomes resulting in the exchange of genetic information.[25] Sex chromosomes, however, are not wholly identical, and merely substitution information over a pocket-sized region of homology chosen the pseudoautosomal region.[26] The exchange of information between the homologous chromatids results in a recombination of information; each chromosome has the complete set up of information it had before, and there are no gaps formed as a outcome of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the bodily human activity of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the next stage.

Diplotene [edit]

During the diplotene stage, also known every bit diplonema, from Greek words meaning "ii threads",[eighteen] : xxx the synaptonemal circuitous disassembles and homologous chromosomes carve up from one another a little. Notwithstanding, the homologous chromosomes of each bivalent remain tightly leap 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 reverse poles of the cell.

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

Diakinesis [edit]

Chromosomes condense further during the diakinesis phase, from Greek words significant "moving through".[eighteen] : xxx This is the showtime point in meiosis where the iv parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the residual of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to class.

Meiotic spindle formation [edit]

Unlike mitotic cells, human 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 accomplish out towards chromosomes, attaching to the chromosomes at the kinetochore. Over fourth dimension the MTOCs merge until two poles have formed, generating a barrel shaped spindle.[28] In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes.[29] Chromosomes then slide along the microtubules towards the equator of the spindle, at which signal the chromosome kinetochores class cease-on attachments to microtubules.[thirty]

Metaphase I [edit]

Homologous pairs motility 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 plane 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 as a bipolar attachment. The physical ground 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 time of their replication until anaphase. In mitosis, the forcefulness of kinetochore microtubules pulling in reverse 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 unremarkably requires at least one crossover per chromosome pair in improver to cohesin betwixt sister chromatids (see 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 division down the heart.[17] Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected past a protein named Shugoshin (Japanese for "guardian spirit"), what prevents the sis chromatids from separating.[31] This allows the sister chromatids to remain together while homologs are segregated.

Telophase I [edit]

The first meiotic partition finer 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 upward the spindle network disappear, and a new nuclear membrane surrounds each haploid fix. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the jail cell membrane in animal cells or the formation of the prison cell wall in found cells, occurs, completing the creation of two daughter cells. All the same, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis Ii.[32] Sister chromatids remain attached during telophase I.

Cells may enter a period of residual known as interkinesis or interphase 2. No DNA replication occurs during this stage.

Meiosis Ii [edit]

Meiosis 2 is the second meiotic division, and ordinarily involves equational segregation, or separation of sister chromatids. Mechanically, the process 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 four principal steps of meiosis II are: prophase Two, metaphase II, anaphase II, and telophase II.

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

In metaphase II, the centromeres contain 2 kinetochores that attach to spindle fibers from the centrosomes at opposite 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 by anaphase II, 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 chosen sis chromosomes equally they movement toward opposing poles.[31]

The process ends with telophase 2, which is similar to telophase I, and is marked past decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate germination eventually produces a total of four daughter cells, each with a haploid set of chromosomes.

Meiosis is at present complete and ends upwards with four new daughter cells.

Origin and role [edit]

The origin and role of meiosis are currently not well understood scientifically, and would provide fundamental 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 in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why information technology is maintained, given the basic two-fold toll 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 result of the sexual cycle in eukaryotes. It is the phase of the life bicycle when a prison cell gives rise to haploid cells (gametes) each having half as many chromosomes equally the parental prison cell. Two such haploid gametes, normally arising from different individual organisms, fuse by the process of fertilization, thus completing the sexual cycle.

Meiosis is ubiquitous amongst eukaryotes. It occurs in single-celled organisms such equally yeast, as well every bit in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than than two.2 billion years ago[34] and the earliest eukaryotes were probable unmarried-celled organisms. To understand sex in eukaryotes, it is necessary to understand (1) how meiosis arose in single celled eukaryotes, and (2) the part 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 multifariousness in 2 ways: (1) Law of Contained Assortment. The independent orientation of homologous chromosome pairs forth 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 Ii, information technology allows a random and independent distribution of chromosomes to each girl cell (and ultimately to gametes);[35] and (two) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic data within chromosomes.[36]

Prophase I arrest [edit]

Female 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, as an case, oocytes are formed betwixt three and four 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 abort of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair impairment 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 loftier 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 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 constant cyclical procedure of meiosis and fertilization. This takes identify alongside normal mitotic cell division. In multicellular organisms, there is an intermediary stride betwixt the diploid and haploid transition where the organism grows. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make upwards the body of the organism and are non involved in gamete production.

Cycling meiosis and fertilization events produces a serial of transitions dorsum and forth between alternate haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (diplontic life wheel), during the haploid state (haplontic life bike), or both (haplodiplontic life cycle, in which there are two singled-out organism phases, one during the haploid state and the other during the diploid land). In this sense there are three types of life cycles that employ sexual reproduction, differentiated by the location of the organism phase(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 partitioning by mitosis to grow 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 prison cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to grade 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 utilise the haplontic life cycle.[ citation needed ]

Finally, in the haplodiplontic life cycle (with sporic or intermediate meiosis), the living organism alternates between 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's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle tin 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 wheel of a male human cell

Meiosis occurs in all animals and plants. The stop issue, the production of gametes with one-half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes direct. In land plants and some algae, in that location is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply past mitosis, developing into the haploid gametophyte generation, which then gives ascent to gametes direct (i.e. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes.[xl]

In mammals [edit]

In females, meiosis occurs in cells known as oocytes (atypical: oocyte). Each primary oocyte divides twice in meiosis, unequally in each instance. The first sectionalisation produces a daughter cell, and a much smaller polar body which may or may non undergo a 2nd division. In meiosis Ii, sectionalisation of the girl cell produces a second polar trunk, and a unmarried haploid jail cell, which enlarges to go an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in i mature ovum and one or 2 polar bodies.

Note that there are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie dormant inside a protective shell of somatic cells chosen the follicle. At the beginning of each menstrual cycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known equally folliculogenesis. During this procedure, the maturing oocytes resume meiosis and keep until metaphase Two of meiosis Two, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and consummate meiosis. During folliculogenesis in humans, normally 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 it features a long menstruum of meiotic arrest known every bit the dictyate stage and lacks the help 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 will 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 inside seminiferous tubules called Sertoli cells first making their own retinoic acid. Sensitivity to retinoic acid is also adjusted past proteins called nanos and DAZL.[43] [44] Genetic loss-of-part 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, notwithstanding retinoic acid is non required during the time when meiosis initiates.[45]

In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. Some studies suggest that retinoic acid derived from the primitive kidney (mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid.[46] However, genetic loss-of-function studies on retinoic acid-generating enzymes take shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis[47] or male meiosis which initiates postnatally.[45]

Flagellates [edit]

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

Part in human genetics and disease [edit]

Recombination amid the 23 pairs of human chromosomes is responsible for redistributing not just the bodily chromosomes, but likewise pieces of each of them. In that location is also an estimated 1.half-dozen-fold more recombination in females relative to males. In addition, average, female recombination is college at the centromeres and male person recombination is higher at the telomeres. On average, ane million bp (1 Mb) correspond to 1 cMorgan (cm = ane% recombination frequency).[49] The frequency of cross-overs remain uncertain. In yeast, mouse and man, it has been estimated that ≥200 double-strand breaks (DSBs) are formed per meiotic cell. However, only a subset of DSBs (~5–30% depending on the organism), get on to produce crossovers,[50] which would result in but 1-2 cross-overs per human chromosome.

Nondisjunction [edit]

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

About monosomic and trisomic man embryos are not viable, simply some aneuploidies can exist tolerated, such every bit trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical weather condition include merely are not limited to:

  • Down syndrome – trisomy of chromosome 21
  • Patau syndrome – trisomy of chromosome thirteen
  • Edwards syndrome – trisomy of chromosome 18
  • Klinefelter syndrome – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc.
  • Turner syndrome – lacking of i X chromosome in females – i.e. X0
  • Triple Ten 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 order to understand meiosis, a comparison to mitosis is helpful. The tabular array below shows the differences between meiosis and mitosis.[53]

Meiosis Mitosis
End outcome Unremarkably four cells, each with one-half the number of chromosomes as the parent Two cells, having the same number of chromosomes equally the parent
Function Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Almost all eukaryotes (animals, plants, fungi, and protists);[54] [48]
In gonads, earlier 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 aforementioned as parent? No Yes
Crossing over happens? Yes, normally occurs between each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Yes No
Cytokinesis Occurs in Telophase I and Telophase Two Occurs in Telophase
Centromeres split Does not occur in Anaphase I, but occurs in Anaphase II Occurs in Anaphase

Molecular regulation [edit]

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

It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a office in chromosome pairing during meiosis-I.[56]

Meiotic recombination is mediated through double stranded break, which is catalyzed past Spo11 protein. As well Mre11, Sae2 and Exo1 play role in breakage and recombination. Subsequently 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 i gives to noncrossover product).[57]

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

In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-2.[56]

See too [edit]

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

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Cited texts [edit]

  • Freeman S (2005). Biological Science (tertiary ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN9780131409415.

External links [edit]

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

farmerimmakep.blogspot.com

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

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