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Meiosis (; (since it is a reductional division)) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each 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. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome.
Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.
In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, 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 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 away from each other during meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell.
During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases, all four of the meiotic products form 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 only 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 same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two 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, but 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 ), including , , and fungi. It is an essential process for oogenesis and spermatogenesis.
| mitosis | occurs only if needed to repair DNA damage; usually occurs between identical sister chromatids and does not result in genetic changes | ||||||||||
| 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 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 first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and three small 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 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 selection can act.
Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by to divide one cell into two identical daughter cells. In some plants, fungi, and meiosis results in the formation of : haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.
Meiosis does not occur in archaea or bacteria, which generally reproduce asexually via binary fission. However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaea to another and recombination of these DNA molecules of different parental origin.
The term "meiosis" is derived from the Greek word 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 series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Walther Flemming. as quoted in the Oxford English Dictionary, Third Edition, June 2001, s.v.
The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek.
Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing the chromosome number by half. During meiosis II, sister chromatids decouple, and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy 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, 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 II (prophase II, metaphase II, anaphase II, telophase II).
During meiosis, specific genes are more highly transcribed. In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.
In human fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth. (2015). 9781437706963, Elsevier. ISBN 9781437706963 This suspended state is referred to as the dictyotene or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.
Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.
In prophase II, the disappearance of the nucleoli and the nuclear envelope is seen again as well 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 two 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.
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 called sister chromosomes as they move toward opposing poles.
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 cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
Meiosis is now complete and ends up with four new daughter cells.
Cycling meiosis and fertilization events results in alternation between haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state ( diplontic life cycle), during the haploid state ( haplontic life cycle), or both ( haplodiplontic life cycle), in which there are two distinct organism phases, one with haploid cells and the other with diploid cells.
In the diplontic life cycle (with pre-gametic meiosis), as in humans, the organism is multicellular and diploid, grown by mitosis from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes (the spermatozoa in males and ovum in females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.
In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid, by the proliferation and differentiation of a single haploid cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many Fungus and many protozoa utilize the haplontic life cycle.
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 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 produce a new diploid organism. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles.
In both animals and plants, the final stage is for the gametes to fuse to form a zygote in which the original number of chromosomes is restored. (2025). 9780073040523, McGraw Hill. ISBN 9780073040523
There are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie dormant within a protective shell of somatic cells called the ovarian follicle. At this stage, the oocyte nucleus is called the germinal vesicle. At the beginning of each menstrual cycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. The breakdown of the germinal vesicle, condensation of chromosomes, and assembly of the bipolar metaphase I spindle are all clear indications that meiosis has resumed. 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 as the dictyate stage and lacks the assistance of .
In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the . Meiosis during spermatogenesis is specific to a type of cell called , which will later mature to become spermatozoon. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL. 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 in spermatocytes undergoing meiosis, however retinoic acid is not required during the time when meiosis initiates.
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. 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 or male meiosis which initiates postnatally.
In humans, recombination rates differ between maternal and paternal DNA:
Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include but are not limited to:
The probability of nondisjunction in human oocytes increases with increasing maternal age, presumably due to loss of cohesin over time.
| Two cells, having the same number of chromosomes as the parent |
| Cellular reproduction, growth, repair, asexual reproduction |
| All proliferating cells in all eukaryotes |
| Prophase, Prometaphase, Metaphase, Anaphase, Telophase |
| Yes |
| Very rarely |
| No |
| Occurs in Telophase |
| Occurs in Anaphase |
In mammals, meiotic arrest begins with natriuretic peptide type C (NPPC) from mural granulosa cells, which activates production of cyclic guanosine 3′,5′-monophosphate (cGMP) in concert with natriuretic peptide receptor 2 (NPR2) on cumulus cells. cGMP diffuses into oocytes and halts meiosis by inhibiting phosphodiesterase 3A (PDE3A) and cyclic adenosine 3′,5′-monophosphate (cAMP) hydrolysis. In the oocyte, G-protein-coupled receptor GPR3/12 activates adenylyl cyclase to generate cAMP. cAMP stimulates protein kinase A (PKA) to activate the nuclear kinase WEE2 by phosphorylation. PKA also assists in phosphorylation of the CDK1 phosphatase CDC25B to keep it in the cytoplasm; in its unphosphorylated form, CDC25B migrates to the nucleus. Protein kinase C (PKC) may also have a role in inhibiting meiotic progression to metaphase II. Overall, CDK1 activity is suppressed to prevent resumption of meiosis. Oocytes further promote expression of NPR2 and inosine monophosphate dehydrogenase (and thereby the production of cGMP) in cumulus cells. Follicle-stimulating hormone and estradiol likewise promote expression of NPPC and NPR2. Hypoxanthine, a purine apparently originating in the follicle, also inhibits in vitro oocyte meiosis. A spike in luteinizing hormone (LH) spurs oocyte maturation, in which oocytes are released from meiotic arrest and progress from prophase I through metaphase II. LH-induced epidermal growth factor-like factors like amphiregulin and epiregulin synthesized in mural granulosa cells reduce levels of cGMP in oocytes by restricting cGMP transport through cumulus cell-oocyte gap junctions and lowering NPPC levels and NPR2 activity. In fact, LH-induced epidermal growth factor-like factors may cause the destabilization and breakdown of gap junctions altogether. LH-induced epidermal growth factor-like factors may trigger production of additional oocyte maturation factors like steroids and follicular fluid-derived meiosis-activating sterol (FF-MAS) in cumulus cells. FF-MAS promotes progression from metaphase I to metaphase II, and it may help stabilize metaphase II arrest. Meiosis resumption is reinforced by the exit of WEE2 from the nucleus due to CDK1 activation. Phosphodiesterases (PDEs) metabolize cAMP and may be temporarily activated by PKA-mediated phosphorylation. Longer-term regulation of phosphodiesterases may require modulation of protein expression. For example, hypoxanthine is a PDE inhibitor that may stymie cAMP metabolism. Kinases like protein kinase B, Aurora kinase A, and polo-like kinase 1 contribute to the resumption of meiosis. There are similarities between the mechanisms of meiotic prophase I arrest and resumption and the mitotic G2 DNA damage checkpoint: CDC14B-based activation of APC-CDH1 in arrest and CDC25B-based resumption. Meiotic arrest requires inhibitory phosphorylation of CDK1 at amino acid residues Thr-14 and Tyr-15 by MYT1 and WEE1 as well as regulation of cyclin B levels facilitated by the anaphase-promoting complex (APC). CDK1 is regulated by cyclin B, whose synthesis peaks at the end of meiosis I. At anaphase I, cyclin B is degraded by an ubiquitin-dependent pathway. Cyclin B synthesis and CDK1 activation prompt oocytes to enter metaphase, while entry into anaphase follows ubiquitin-mediated cyclin B degradation, which brings down CDK1 activity. Proteolysis of adhesion proteins between homologous chromosomes is involved in anaphase I, while proteolysis of adhesion proteins between sister chromatids is involved in anaphase II. Meiosis II arrest is effected by cytostatic factor (CSF), whose elements include the MOS protein, mitogen-activated protein kinase kinase (MAPKK/MEK1), and MAPK. The protein kinase p90 (RSK) is one critical target of MAPK and may help block entry into S-phase between meiosis I and II by reactivating CDK1. There's evidence that RSK aids entry into meiosis I by inhibiting MYT1, which activates CDK1. CSF arrest might take place through regulation of the APC as part of the spindle assembly checkpoint.
In the budding yeast S. cerevisiae, Clb1 is the main meiotic regulatory cyclin, though Clb3 and Clb4 are also expressed during meiosis and activate a p34cdc28-associated kinase immediately prior to the first meiotic division. The IME1 transcription factor drives entry into meiotic S-phase and is regulated according to inputs like nutrition. a1/α2 represses a repressor of IME1, initiating meiosis. Numerous S. cerevisiae meiotic regulatory genes have been identified. A few are presented here. IME1 enables sporulation of non-a/α diploids. IME2/ SME1 enables sporulation when nitrogen is present, supports recombination in a/α cells expressing RME1, an inhibitor of meiosis, and encodes a protein kinase homolog. MCK1 (meiosis and centromere regulatory kinase) also supports recombination in a/α cells expressing RME1 and encodes a protein kinase homolog. SME2 enables sporulation when ammonia or glucose are present. UME1-5 enable expression of certain early meiotic genes in vegetative, non-a/α cells.
In the fission yeast S. pombe, the Cdc2 kinase and Cig2 cyclin together initiate the premeiotic S phase, while cyclin Cdc13 and the CDK activator Cdc25 are necessary for both meiotic divisions. However, the Pat1-Mei2 system is at the heart of S. pombe meiotic regulation. Mei2 is the major meiotic regulator. It moves between the nucleus and cytoplasm and works with meiRNA to promote meiosis I. Moreover, Mei2 is implicated in exit from mitosis and induction of premeiotic S phase. Mei2 may inactivate the DSR-Mmi1 system through sequestration of Mmi1 to stabilize meiosis-specific transcript expression. Mei2 may stall growth and bring about G1 arrest. Pat1 is a Ser/Thr protein kinase that phosphorylates Mei2, an RNA-binding protein, on residues Ser438 and Thr527. This phosphorylation may decrease the half-life of Mei2 by making it more likely to be destroyed by a proteasome working with E2 Ubc2 and E3 Ubr1. The Mei4 transcription factor is necessary to transcriptionally activate cdc25 in meiosis, and the mei4 mutant experiences cell cycle arrest. Mes1 inhibits the APC/C activator Slp1 such that the Cdc2-Cdc13 MPF activity can drive the second meiotic division.
It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I.
Meiotic recombination is mediated through double stranded break, which is catalyzed by Spo11 protein. Also Mre11, Sae2 and Exo1 play role in breakage and recombination. After the breakage happen, recombination take place which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second one gives to noncrossover product).
Seemingly there are checkpoints for meiotic cell division too. In S. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is thought to form a checkpoint.
In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II.
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