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The Cells Resulting From Oogenesis That Either Disintegrate or Divide Again Are Called _ Bodies

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

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

Meiosis (; from Ancient Greek μείωσις ( meíōsis ) 'lessening', since it is a reductional division)[1] [2] is a special type of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. Information technology involves ii rounds of division that ultimately result in iv cells with only one copy 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 code on each chromosome.[3] After on, during fertilisation, the haploid cells produced by meiosis from a male person and female person will 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 most frequent genetic crusade of developmental disabilities.[4]

In meiosis, DNA replication is followed past 2 rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell.[iii] The two meiotic divisions are known equally meiosis I and meiosis Two. Earlier meiosis begins, during Due south phase of the cell cycle, the Deoxyribonucleic acid of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-stage can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following Dna replication, meiotic cells enter a prolonged One thousand2-similar stage known as meiotic prophase. During this fourth dimension, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which Deoxyribonucleic acid may be cut and 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 as chiasmata (singular: chiasma, for the Greek letter Chi (Χ)) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate abroad from each other during Meiosis I, resulting in 2 haploid cells that have half the number of chromosomes as the parent cell.

During meiosis Ii, the cohesion betwixt sister chromatids is released and they segregate from i another, every bit during mitosis. In some cases, all four of the meiotic products form gametes such equally sperm, spores or pollen. In female person animals, 3 of the four meiotic products are typically eliminated by extrusion into polar bodies, and only ane cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two 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 comprise 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 incorporate i fix of 23 chromosomes. When 2 gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, only not the same number of chromosomes, occurs in all organisms that utilize meiosis.

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

Overview [edit]

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

recombination meiosis shuffles the genes between the two chromosomes in each pair (1 received from each parent), producing recombinant chromosomes with unique genetic combinations in every gamete
mitosis occurs merely if needed to repair DNA damage;

unremarkably occurs between identical sister chromatids and does not consequence in genetic changes

chromosome number (ploidy) meiosis produces 4 genetically unique cells, each with half the number of chromosomes equally in the parent
mitosis produces two genetically identical cells, each with the aforementioned number of chromosomes as in the parent

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. Start, the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections (crossovers) between the homologs. In the beginning meiotic partition, the homologs are segregated to separate daughter cells past the spindle apparatus. The cells and so keep to a second partitioning without an intervening round of DNA replication. The sister chromatids are segregated to split up girl cells to produce a total of four haploid cells. Female animals utilise a slight variation on this pattern and produce one big ovum and ii modest polar bodies. Considering 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 private gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic variety resulting from sexual reproduction contributes to the variation in traits upon which natural pick can act.

Meiosis uses many of the same mechanisms as mitosis, the blazon of cell sectionalization used by eukaryotes to divide one jail cell into two identical girl cells. In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that tin can dissever vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, exercise not have the power to carry out meiosis and have caused the ability to reproduce by parthenogenesis.

Meiosis does non occur in archaea or bacteria, which generally reproduce asexually via binary fission. Nonetheless, a "sexual" process known every bit horizontal gene transfer involves the transfer of DNA from i bacterium or archaeon to another and recombination of these Deoxyribonucleic acid molecules of dissimilar parental origin.

History [edit]

Meiosis was discovered and described for the offset time 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, all the same, was described just in 1890 past German language biologist August Weismann, who noted that two 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 Hunt 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 give-and-take μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using the idiosyncratic rendering "maiosis":

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

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

Phases [edit]

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

  • Growth 1 (G1) phase: In this very active phase, the jail cell synthesizes its vast assortment of proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA.
  • Synthesis (S) phase: The genetic material is replicated; each of the jail cell's chromosomes duplicates to become two identical sister chromatids fastened at a centromere. This replication does non 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 take identify during prophase I in meiosis.
  • Growth 2 (G2) phase: 10002 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds virtually closely to the G2 phase of the mitotic cell wheel.

Interphase is followed by meiosis I and so meiosis II. Meiosis I separates replicated homologous chromosomes, each still made upwards of two sister chromatids, into ii daughter cells, thus reducing the chromosome number by one-half. During meiosis 2, sis chromatids decouple and the resultant daughter chromosomes are segregated into four girl cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only 1 copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis Two.

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

Diagram of the meiotic phases

During meiosis, specific genes are more highly transcribed.[eleven] [12] In add-on to strong meiotic stage-specific expression of mRNA, there are besides pervasive translational controls (due east.thou. selective usage of preformed mRNA), regulating the ultimate meiotic phase-specific protein expression of genes during meiosis.[13] Thus, both transcriptional and translational controls make up one's mind the broad restructuring of meiotic cells needed to comport 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 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 division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c).[14]

Meiosis Prophase I in mice. In Leptotene (50) the axial elements (stained by SYCP3) begin to form. In Zygotene (Z) the transverse elements (SYCP1) and cardinal elements of the synaptonemal circuitous are partially installed (appearing every bit xanthous 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 circuitous at unlike stages of prophase I and the chromosomes bundled every bit a linear assortment of loops.

Prophase I [edit]

Prophase I is by 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 exchange genetic information (past homologous recombination), forming at least one crossover per chromosome.[sixteen] These crossovers become visible as chiasmata (plural; atypical chiasma).[17] This process facilitates stable pairing between homologous chromosomes and hence enables authentic segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (2 chromosomes) or tetrads (iv chromatids), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named co-ordinate to the appearance of chromosomes.

Leptotene [edit]

The showtime stage of prophase I is the leptotene phase, also known as leptonema, from Greek words pregnant "sparse threads".[18] : 27 In this stage of prophase I, individual chromosomes—each consisting of 2 replicated sister chromatids—become "individualized" to form visible strands within the nucleus.[18] : 27 [19] : 353 The chromosomes each class a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "centric 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 unmarried stranded Deoxyribonucleic acid filaments coated by 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).[20] [22]

Zygotene [edit]

Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words meaning "paired threads",[18] : 27 which in some organisms is also called the boutonniere phase because of the way the telomeres cluster at one end of the nucleus.[23] In this stage the homologous chromosomes get much more than closely (~100 nm) and stably paired (a procedure called synapsis) mediated by the installation of the transverse and cardinal elements of the synaptonemal complex.[xx] Synapsis is idea 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 stage 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.[20] Well-nigh breaks are repaired without forming crossovers resulting in cistron conversion.[24] However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in the exchange of genetic information.[25] Sex chromosomes, nevertheless, are not wholly identical, and only commutation information over a small-scale region of homology called the pseudoautosomal region.[26] The exchange of information between the homologous chromatids results in a recombination of data; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the procedure. Because the chromosomes cannot be distinguished in the synaptonemal circuitous, the actual human action 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 phase, also known equally diplonema, from Greek words significant "ii threads",[eighteen] : thirty the synaptonemal complex disassembles and homologous chromosomes separate from 1 another a little. 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 permit homologous chromosomes to move to reverse poles of the jail cell.

In human fetal oogenesis, all developing oocytes develop to this phase and are arrested in prophase I before birth.[27] This suspended state is referred to as the dictyotene phase 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 farther during the diakinesis stage, from Greek words meaning "moving through".[xviii] : 30 This is the showtime betoken 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 rest 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]

Different mitotic cells, human being and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately eighty MicroTubule Organizing Centers (MTOCs) grade a sphere in the ooplasm and brainstorm to nucleate microtubules that achieve out towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until 2 poles take formed, generating a barrel shaped spindle.[28] In human being oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to environs 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: Every bit kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes marshal 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 attachment is referred to as a bipolar attachment. The physical 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 same equatorial line.[17] The protein complex cohesin holds sister chromatids together from the fourth dimension 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 unremarkably requires at least one crossover per chromosome pair in addition to cohesin betwixt sister chromatids (see Chromosome segregation).

Anaphase I [edit]

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes further apart. The jail cell elongates in preparation for sectionalisation downward the middle.[17] Unlike in mitosis, only 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 starting time meiotic division finer ends when the chromosomes arrive at the poles. Each girl prison cell now has half the number of chromosomes merely 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 ready. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the germination of the prison cell wall in establish cells, occurs, completing the creation of two daughter cells. However, cytokinesis does not fully consummate resulting in "cytoplasmic bridges" which enable the cytoplasm to exist shared between daughter cells until the end of meiosis Two.[32] Sister chromatids remain attached during telophase I.

Cells may enter a period of balance known every bit interkinesis or interphase II. No Deoxyribonucleic acid replication occurs during this stage.

Meiosis Ii [edit]

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

In prophase II, we encounter the disappearance of the nucleoli and the nuclear envelope once again as well as the shortening and thickening of the chromatids. Centrosomes movement to the polar regions and adapt spindle fibers for the 2d meiotic division.

In metaphase 2, the centromeres contain two 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 broken, allowing the sis chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.[31]

The process ends with telophase II, which is similar 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 germination somewhen produces a total of iv girl cells, each with a haploid set of chromosomes.

Meiosis is at present consummate and ends up with four new daughter cells.

Origin and role [edit]

The origin and function of meiosis are currently non well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. At that place 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 it is maintained, given the basic 2-fold cost of sexual activity. It is clear that it evolved over i.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are however 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 prison cell gives rise to haploid cells (gametes) each having half as many chromosomes equally the parental prison cell. Two such haploid gametes, usually arising from different private organisms, fuse by the process of fertilization, thus completing the sexual cycle.

Meiosis is ubiquitous amid eukaryotes. It occurs in single-celled organisms such as yeast, every bit well as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than 2.ii billion years ago[34] and the primeval eukaryotes were likely single-celled organisms. To sympathise sexual activity in eukaryotes, information technology is necessary to understand (1) how meiosis arose in single celled eukaryotes, and (two) the function of meiosis.

The new combinations of DNA created during meiosis are a meaning source of genetic variation alongside mutation, resulting in new combinations of alleles, which may be beneficial. Meiosis generates gamete genetic multifariousness in 2 means: (1) 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 2, this is the subsequent separation of homologs and sister chromatids during anaphase I and 2, information technology allows a random and independent 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 within chromosomes.[36]

Prophase I abort [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 four months of gestation inside the fetus and are therefore nowadays 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 re-create phase was proposed to provide the informational redundancy needed to repair harm 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 capability 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 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 jail cell division. In multicellular organisms, at that place is an intermediary pace between the diploid and haploid transition where the organism grows. At sure stages of the life bicycle, germ cells produce gametes. Somatic cells make upwards 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 bike can occur either during the diploid state (diplontic life cycle), during the haploid state (haplontic life wheel), or both (haplodiplontic life bike, in which at that place are two singled-out organism phases, ane during the haploid land and the other during the diploid state). In this sense at that place are three types of life cycles that apply sexual reproduction, differentiated by the location of the organism stage(s).[ citation needed ]

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

In the haplontic life bike (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 activity 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 employ the haplontic life bicycle.[ citation needed ]

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

In plants and animals [edit]

Overview of chromatides' and chromosomes' distribution within the mitotic and meiotic wheel of a male person human prison cell

Meiosis occurs in all animals and plants. The terminate outcome, the production of gametes with half the number of chromosomes every bit the parent prison cell, is the same, only the detailed process is dissimilar. In animals, meiosis produces gametes directly. 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 past mitosis, developing into the haploid gametophyte generation, which so gives rise to gametes direct (i.e. without further meiosis). In both animals and plants, the terminal stage is for the gametes to fuse, restoring the original number of chromosomes.[twoscore]

In mammals [edit]

In females, meiosis occurs in cells known as oocytes (atypical: oocyte). Each main oocyte divides twice in meiosis, unequally in each case. The commencement division produces a daughter cell, and a much smaller polar body which may or may not undergo a second division. In meiosis II, segmentation of the daughter cell produces a second polar body, and a unmarried haploid cell, which enlarges to go an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in one 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 beat of somatic cells called the follicle. At the beginning of each menstrual cycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a procedure known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase 2 of meiosis Ii, where they are once more arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one 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 period 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 prison cell called spermatocytes, which volition later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much after than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to exist 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 also adapted past proteins chosen nanos and DAZL.[43] [44] Genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later on in spermatocytes undergoing meiosis, however retinoic acid is not 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 archaic 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 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 have a 2-bounded meiosis (though sometimes achiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalids and oxymonads) from the gut of the woods-feeding cockroach Cryptocercus.[48]

Function in human genetics and disease [edit]

Recombination amid the 23 pairs of human chromosomes is responsible for redistributing non just the bodily chromosomes, but besides pieces of each of them. There is also an estimated 1.6-fold more recombination in females relative to males. In addition, boilerplate, female recombination is higher at the centromeres and male recombination is higher at the telomeres. On average, 1 million bp (1 Mb) correspond to 1 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. Notwithstanding, merely a subset of DSBs (~5–30% depending on the organism), go on to produce crossovers,[fifty] which would upshot in just 1-2 cantankerous-overs per human chromosome.

Nondisjunction [edit]

The normal separation of chromosomes in meiosis I or sis chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is called nondisjunction. This results in the production of gametes which have either too 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.

Most monosomic and trisomic human being embryos are not viable, but some aneuploidies can be tolerated, such equally trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from astringent developmental disorders to asymptomatic. Medical atmospheric condition include just are non limited to:

  • Down's syndrome – trisomy of chromosome 21
  • Patau syndrome – trisomy of chromosome 13
  • Edwards syndrome – trisomy of chromosome 18
  • Klinefelter syndrome – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc.
  • Turner syndrome – defective of one X chromosome in females – i.east. X0
  • Triple X syndrome – an extra X chromosome in females
  • Jacobs syndrome – an actress 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 social club to understand meiosis, a comparison to mitosis is helpful. The table beneath shows the differences between meiosis and mitosis.[53]

Meiosis Mitosis
End result Normally four cells, each with half the number of chromosomes as the parent Ii cells, having the aforementioned number of chromosomes every bit the parent
Role Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life wheel 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);
Afterward zygotes (in haplontic);
Before spores (in haplodiplontic)		 All proliferating cells in all eukaryotes
Steps Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase Two, Metaphase Ii, Anaphase II, Telophase II		 Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same equally parent? No Yeah
Crossing over happens? Yeah, 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, simply occurs in Anaphase Two Occurs in Anaphase

Molecular regulation [edit]

How a prison cell proceeds to meiotic division in meiotic cell division is not well known. Maturation promoting factor (MPF) seemingly accept role in frog Oocyte meiosis. In the fungus S. pombe. there is a function of MeiRNA binding protein for entry to meiotic cell sectionalization.[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 past Spo11 poly peptide. As well 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 2d one gives to noncrossover product).[57]

Seemingly at that place are checkpoints for meiotic cell partitioning too. In S. pombe, Rad proteins, Southward. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is thought to class a checkpoint.[58]

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

Run into 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 Science (tertiary 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"

gaymonpoer1987.blogspot.com

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

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