Which Definition Correctly Describes a Haploid Cell During Meiosis

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

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

Meiosis
(; from Ancient Greek


μείωσις

(
meíōsis
) ‘lessening’, since it is a
reductional division)^[one]
[2]
is a special type of cell partition of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves ii rounds of division that ultimately result in four 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.^[iii]
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 cause of miscarriage and the about 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 one-half the number of chromosomes as the original parent cell.^[iii]
The two meiotic divisions are known every bit meiosis I and meiosis II. Before meiosis begins, during S phase of the prison cell bike, the Deoxyribonucleic acid of each chromosome is replicated and so that it consists of two identical sis chromatids, which remain held together through sister chromatid cohesion. This S-phase can exist referred to as “premeiotic S-stage” or “meiotic Southward-phase”. Immediately following Dna replication, meiotic cells enter a prolonged K₂-similar stage known every bit meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut then repaired, which allows them to commutation 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 alphabetic character Chi (Χ)) betwixt the homologous chromosomes. In most organisms, these links can assistance direct each pair of homologous chromosomes to segregate away from each other during Meiosis I, resulting in 2 haploid cells that accept one-half the number of chromosomes every bit the parent prison cell.

During meiosis 2, the cohesion betwixt sister chromatids is released and they segregate from 1 another, every bit during mitosis. In some cases, all four of the meiotic products class gametes such as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and but one 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 aforementioned number of chromosomes. For example, diploid human cells comprise 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 ane set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is one time again diploid, with the mother and father each contributing 23 chromosomes. This same blueprint, but not the same number of chromosomes, occurs in all organisms that utilise meiosis.

Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi.^{[5]
[6]
[seven]}
Information technology is an essential process for oogenesis and spermatogenesis.

Overview

[edit]

Although the process of meiosis is related to the more general cell division process of mitosis, it differs in 2 important respects:

recombination	meiosis	shuffles the genes betwixt 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 not issue in genetic changes
chromosome number (ploidy)	meiosis	produces four genetically unique cells, each with half the number of chromosomes as 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 two copies of each chromosome, termed homologs. First, the prison cell undergoes DNA replication, so each homolog now consists of 2 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 outset meiotic division, the homologs are segregated to separate girl cells by the spindle appliance. The cells so proceed to a 2d division without an intervening round of Deoxyribonucleic acid replication. The sister chromatids are segregated to divide daughter cells to produce a full of four haploid cells. Female person animals employ a slight variation on this blueprint and produce one big ovum and 2 pocket-size 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 assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection tin act.

Meiosis uses many of the same mechanisms equally mitosis, the blazon of jail cell division used past eukaryotes to divide i prison cell into two 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, similar bdelloid rotifers, do not take the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.

Meiosis does non occur in archaea or bacteria, which generally reproduce asexually via binary fission. However, a “sexual” process known equally horizontal gene transfer involves the transfer of Dna from one bacterium or archaeon to another and recombination of these DNA molecules of unlike parental origin.

History

[edit]

Meiosis was discovered and described for the outset time in sea urchin eggs in 1876 past the German biologist Oscar Hertwig. It was described again 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 only in 1890 by German 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 fly
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 biological science by J.B. Farmer and J.East.S. Moore in 1905, using the idiosyncratic rendering “maiosis”:

We propose to apply the terms Maiosis or Maiotic stage to comprehend the whole series of nuclear changes included in the two divisions that were designated every bit Heterotype and Homotype past Flemming.^[eight]

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

• Growth 1 (G_ane) stage: In this very active phase, the prison cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will demand for growth. In G₁, each of the chromosomes consists of a single linear molecule of Deoxyribonucleic acid.
• Synthesis (S) phase: The genetic material is replicated; each of the prison cell’southward chromosomes duplicates to go two identical sister chromatids attached at a centromere. This replication does not alter the ploidy of the jail cell since the centromere number remains the same. The identical sis chromatids accept non yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis.
• Growth two (G₂) phase: G₂
  phase as seen before mitosis is non present in meiosis. Meiotic prophase corresponds virtually closely to the G₂
  phase of the mitotic cell bicycle.

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

Meiosis I and Two are each divided into prophase, metaphase, anaphase, and telophase stages, similar 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 Two (prophase Two, metaphase Ii, anaphase Ii, telophase II).

During meiosis, specific genes are more highly transcribed.^[11]
[12]
In improver to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic phase-specific protein expression of genes during meiosis.^[13]
Thus, both transcriptional and translational controls determine 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 ii 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 Two is an
equational division
analogous to mitosis, in which the sister chromatids are segregated, creating 4 haploid girl cells (1n, 1c).^[xiv]

Prophase I

[edit]

Prophase I is by far the longest phase of meiosis (lasting thirteen out of fourteen days in mice^[15]). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and substitution genetic information (by homologous recombination), forming at least one crossover per chromosome.^[16]
These crossovers become visible as chiasmata (plural; singular chiasma).^[17]
This process facilitates stable pairing betwixt homologous chromosomes and hence enables accurate segregation of the chromosomes at the commencement meiotic division. The paired and replicated chromosomes are called bivalents (ii chromosomes) or tetrads (4 chromatids), with 1 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 start stage of prophase I is the
leptotene
stage, also known as
leptonema, from Greek words meaning “thin threads”.<sup>[18]

: 27</sup>

In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—go “individualized” to course visible strands within the nucleus.<sup>[18]

: 27

[nineteen]

: 353</sup>

The chromosomes each form a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal circuitous gather forming an “axial element” from which the loops emanate.^[20]
Recombination is initiated in this phase 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 past the
zygotene
stage, also known as
zygonema, from Greek words meaning “paired threads”,<sup>[18]

: 27</sup>

which in some organisms is as well called the bouquet stage because of the way the telomeres cluster at one 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 cardinal elements of the synaptonemal complex.^[twenty]
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
phase (

PAK-i-teen), besides known as
pachynema, from Greek words meaning “thick threads”.<sup>[18]

: 27</sup>

is the phase at which all autosomal chromosomes take synapsed. In this phase 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 cistron conversion.^[24]
Withal, a subset of breaks (at least one per chromosome) course crossovers between not-sis (homologous) chromosomes resulting in the exchange of genetic information.^[25]
Sexual activity chromosomes, even so, are not wholly identical, and only exchange information over a minor region of homology called the pseudoautosomal region.^[26]
The exchange of information between the homologous chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed equally a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal circuitous, the actual deed of crossing over is not perceivable through an ordinary light microscope, and chiasmata are non visible until the next stage.

Diplotene

[edit]

During the
diplotene
phase, also known equally
diplonema, from Greek words meaning “two threads”,<sup>[18]

: 30</sup>

the synaptonemal complex disassembles and homologous chromosomes separate from ane another a trivial. 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 let homologous chromosomes to move to opposite poles of the cell.

In man fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth.^[27]
This suspended country is referred to equally the
dictyotene stage
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”.<sup>[18]

: thirty</sup>

This is the starting time point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata conspicuously visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.

Meiotic spindle germination

[edit]

Unlike mitotic cells, homo and mouse oocytes do non 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 attain out towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until two poles have formed, generating a barrel shaped spindle.^[28]
In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that somewhen expands to environs the chromosomes.^[29]
Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores grade 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 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 aforementioned equatorial line.^[17]
The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the strength of kinetochore microtubules pulling in reverse directions creates tension. The jail cell senses this tension and does non progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in add-on to cohesin between sister chromatids (see Chromosome segregation).

Anaphase I

[edit]

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to contrary poles. Nonkinetochore microtubules lengthen, pushing the centrosomes further apart. The jail cell elongates in preparation for division down the middle.^[17]
Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a poly peptide named Shugoshin (Japanese for “guardian spirit”), what prevents the sister chromatids from separating.^[31]
This allows the sister chromatids to remain together while homologs are segregated.

Telophase I

[edit]

The first meiotic partition effectively ends when the chromosomes arrive at the poles. Each daughter prison cell at present 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 prepare. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. All the same, cytokinesis does not fully consummate resulting in “cytoplasmic bridges” which enable the cytoplasm to be shared betwixt daughter cells until the terminate of meiosis Two.^[32]
Sister chromatids remain attached during telophase I.

Cells may enter a catamenia of balance known equally interkinesis or interphase II. No DNA replication occurs during this phase.

Meiosis II

[edit]

Meiosis II is the 2d meiotic sectionalization, 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 stop 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 four primary steps of meiosis Ii are: prophase Two, metaphase 2, anaphase Two, and telophase II.

In
prophase II, nosotros see the disappearance of the nucleoli and the nuclear envelope once more equally well as the shortening and thickening of the chromatids. Centrosomes movement to the polar regions and suit spindle fibers for the second meiotic division.

In
metaphase II, the centromeres comprise two kinetochores that attach to spindle fibers from the centrosomes at reverse poles. The new equatorial metaphase plate is rotated by ninety degrees when compared to meiosis I, perpendicular to the previous plate.^[33]

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

The process ends with
telophase Two, 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-grade and cleavage or prison 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 up with four new girl cells.

Origin and function

[edit]

The new combinations of DNA created during meiosis are a significant source of genetic variation aslope mutation, resulting in new combinations of alleles, which may exist beneficial. Meiosis generates gamete genetic diversity in two ways: (i) Police of Contained Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sis 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);^[34]
and (2) Crossing Over. The concrete commutation of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic data within chromosomes.^[35]

Prophase I arrest

[edit]

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.^[36]
In humans, as an example, oocytes are formed between iii and 4 months of gestation within 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 four genome re-create stage was proposed to provide the informational redundancy needed to repair damage in the Deoxyribonucleic acid of the germline.^[36]
The repair process used appears to involve homologous recombinational repair^[36]
[37]
Prophase I arrested oocytes have a high capability for efficient repair of DNA amercement, particularly exogenously induced double-strand breaks.^[37]
DNA repair capability appears to be a central quality control machinery in the female germ line and a disquisitional determinant of fertility.^[37]

Occurrence

[edit]

In life cycles

[edit]

Meiosis occurs in eukaryotic life cycles involving sexual reproduction, consisting of the abiding cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell sectionalization. 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 alternating haploid and diploid states. The organism phase of the life cycle tin occur either during the diploid country (diplontic
life wheel), during the haploid state (haplontic
life cycle), or both (haplodiplontic
life cycle, in which in that location are ii distinct organism phases, ane during the haploid land and the other during the diploid state). In this sense there are iii types of life cycles that employ sexual reproduction, differentiated past the location of the organism phase(s).^{[
citation needed
]}

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

In the
haplontic life cycle
(with postal service-zygotic meiosis), the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid jail cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to course 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 cycle
(with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism’s germ-line cells undergo meiosis to produce spores. The spores proliferate past mitosis, growing into a haploid organism. The haploid organism’s gamete so combines with some other haploid organism’southward gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to go a diploid organism again. The haplodiplontic life bike can be considered a fusion of the diplontic and haplontic life cycles.^{[38]
[
citation needed
]}

In plants and animals

[edit]

Meiosis occurs in all animals and plants. The end consequence, the product of gametes with one-half the number of chromosomes equally the parent cell, is the same, simply the detailed procedure is different. In animals, meiosis produces gametes directly. 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 by mitosis, developing into the haploid gametophyte generation, which then gives rise to gametes directly (i.e. without further meiosis). In both animals and plants, the final phase is for the gametes to fuse, restoring the original number of chromosomes.^[39]

In mammals

[edit]

In females, meiosis occurs in cells known equally oocytes (atypical: oocyte). Each primary oocyte divides twice in meiosis, unequally in each case. The first division produces a daughter cell, and a much smaller polar body which may or may not undergo a second division. In meiosis 2, division of the daughter prison cell produces a second polar body, and a single haploid prison cell, which enlarges to become an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in one mature ovum and ane or 2 polar bodies.

Note that at that place are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and prevarication dormant within a protective shell of somatic cells called the follicle. At the beginning of each menstrual bicycle, FSH secretion from the inductive pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this procedure, the maturing oocytes resume meiosis and keep until metaphase II of meiosis Two, where they are once again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and consummate meiosis. During folliculogenesis in humans, usually 1 follicle becomes dominant while the others undergo atresia. The procedure of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the dictyate stage and lacks the assistance of centrosomes.^[xl]
[41]

In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell chosen spermatocytes, which will after mature to get spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male person testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules chosen Sertoli cells commencement making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL.^[42]
[43]
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 not required during the time when meiosis initiates.^[44]

In female person mammals, meiosis begins immediately after primordial germ cells drift 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 person testis suppress meiosis by degrading retinoic acid.^[45]
Even so, genetic loss-of-function studies on retinoic acid-generating enzymes take shown that retinoic acrid is not required for initiation of either female person meiosis which occurs during embryogenesis^[46]
or male meiosis which initiates postnatally.^[44]

Flagellates

[edit]

While the bulk of eukaryotes have a 2-bounded meiosis (though sometimes achiasmatic), a very rare form, i-divisional meiosis, occurs in some flagellates (parabasalids and oxymonads) from the gut of the wood-feeding cockroach
Cryptocercus.^[47]

Role in homo genetics and disease

[edit]

Recombination amid the 23 pairs of homo chromosomes is responsible for redistributing non just the actual chromosomes, but too pieces of each of them. In that location is also an estimated 1.6-fold more than recombination in females relative to males. In addition, boilerplate, female recombination is higher at the centromeres and male person recombination is college at the telomeres. On boilerplate, 1 million bp (1 Mb) correspond to 1 cMorgan (cm = one% recombination frequency).^[48]
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. Nevertheless, simply a subset of DSBs (~5–30% depending on the organism), become on to produce crossovers,^[49]
which would event in merely 1-2 cantankerous-overs per human chromosome.

Nondisjunction

[edit]

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis 2 is termed
disjunction. When the segregation is non normal, it is called
nondisjunction. This results in the production of gametes which have either too many or as well few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis Two, phases of cellular reproduction, or during mitosis.

Near monosomic and trisomic human embryos are non feasible, just 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 conditions include but are non limited to:

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

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

Comparison to mitosis

[edit]

In order to sympathize meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis.^[52]

	Meiosis	Mitosis
Finish result	Normally four cells, each with half the number of chromosomes as the parent	Ii cells, having the same number of chromosomes equally the parent
Part	Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle	Cellular reproduction, growth, repair, asexual reproduction
Where does information technology happen?	Almost all eukaryotes (animals, plants, fungi, and protists);[53] [47] In gonads, before gametes (in diplontic life cycles); After zygotes (in haplontic); Before spores (in haplodiplontic)	All proliferating cells in all eukaryotes
Steps	Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase 2, Anaphase Ii, Telophase II	Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically aforementioned as parent?	No	Yes
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 Ii	Occurs in Telophase
Centromeres split	Does non occur in Anaphase I, but occurs in Anaphase 2	Occurs in Anaphase

Molecular regulation

[edit]

This section needs expansion. You lot can assist by adding to information technology. (August 2020)

How a cell proceeds to meiotic partition in meiotic cell sectionalisation is not well known. Maturation promoting factor (MPF) seemingly accept role in frog Oocyte meiosis. In the fungus
S. pombe. in that location is a role of MeiRNA bounden poly peptide for entry to meiotic cell division.^[54]

It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I.^[55]

Meiotic recombination is mediated through double stranded break, which is catalyzed by Spo11 protein. Besides Mre11, Sae2 and Exo1 play function in breakage and recombination. Later the breakage happen, recombination take identify which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second one gives to noncrossover product).^[56]

Seemingly there are checkpoints for meiotic cell sectionalization besides. In South. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is idea to grade a checkpoint.^[57]

In vertebrate oogenesis, maintained by cytostatic factor (CSF) has office in switching into meiosis-Ii.^[55]

See also

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References

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Cited texts

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• Freeman Due south (2005).
  
  Biological science
  
  (third ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN9780131409415.
  

External links

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• Meiosis Flash Blitheness
• Animations from the U. of Arizona Biology Dept.
• Meiosis at Kimball’s Biology Pages
• Khan Academy, video lecture
• CCO The Cell-Wheel Ontology
• Stages of Meiosis animation
• *”Abby Dernburg Seminar: Chromosome Dynamics During Meiosis”