Jump to content

Meiosis

Checked
Page protected with pending changes
From Wikipedia, the free encyclopedia
For the figure of speech, seeMeiosis (figure of speech).For the process whereby cell nuclei divide to produce two copies of themselves, seeMitosis.For excessive constriction of the pupils, seeMiosis.For the parasitic infestation, seeMyiasis.For muscle inflammation, seeMyositis.

In meiosis, thechromosomesduplicate (duringinterphase) andhomologous chromosomesexchange genetic information (chromosomal crossover) during the first division, calledmeiosis I.The daughter cells divide again inmeiosis II,splitting upsister chromatidsto form haploidgametes.Two gametes fuse duringfertilization,forming a diploid cell (zygote) with a complete set of paired chromosomes.
A video of meiosis I in acrane flyspermatocyte,played back at 120× the recorded speed.

Meiosis(/maɪˈoʊsɪs/ⓘ;fromAncient Greekμείωσις(meíōsis)'lessening', (since it is a reductional division)[1][2]is a special type ofcell divisionofgerm cellsinsexually-reproducingorganisms that produces thegametes,thespermoregg cells.It involves two rounds of division that ultimately result in four cells, each with only one copy of eachchromosome(haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome iscrossed over,creating new combinations of code on each chromosome.[3]Later on, duringfertilisation,the haploid cells produced by meiosis from a male and a female will fuse to create azygote,a cell with two copies of each chromosome again.

Errors in meiosis resulting inaneuploidy(an abnormal number of chromosomes) are the leading known cause ofmiscarriageand the most frequent genetic cause ofdevelopmental disabilities.[4]

In meiosis,DNA replicationis followed by two rounds of cell division to produce four daughter cells, each with half the number ofchromosomesas the original parent cell.[3]The two meiotic divisions are known asmeiosis Iandmeiosis II.Before meiosis begins, duringS phaseof thecell cycle,the DNA of each chromosome is replicated so that it consists of two identicalsister 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 prolongedG2-like stage known as meioticprophase.During this time,homologous chromosomespair with each other and undergogenetic recombination,a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of theirgenetic information.A subset of recombination events results incrossovers,which create physical links known aschiasmata(singular: chiasma, for the Greek letterChi,Χ) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes tosegregateaway from each other during meiosis I, resulting in twohaploidcells 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 duringmitosis.In some cases, all four of the meiotic products formgametessuch assperm,sporesorpollen.In female animals, three of the four meiotic products are typically eliminated byextrusionintopolar bodies,and only one cell develops to produce anovum.Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e.fertilization) to form a diploidzygotethat contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enablesexual reproduction,with successive generations maintaining the same number of chromosomes. For example,diploidhuman cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produceshaploidgametes (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 andmulticellularorganisms (which are alleukaryotes), includinganimals,plantsandfungi.[5][6][7]It is an essential process foroogenesisandspermatogenesis.

Overview

[edit]

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

recombination meiosis shuffles the genesbetween the two chromosomes in each pair (one received from each parent), producing lots of 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 result in genetic changes

chromosome number (ploidy) meiosis produces four genetically unique cells, each withhalfthe number of chromosomes as in the parent
mitosis produces two genetically identical cells, each withthe same numberof chromosomes as in the parent

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termedhomologs.First, the cell undergoesDNA replication,so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information byhomologous recombinationoften leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by thespindle 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 whichnatural selectioncan act.

Meiosis uses many of the same mechanisms asmitosis,the type of cell division used byeukaryotesto divide one cell into two identical daughter cells. In some plants, fungi, andprotistsmeiosis results in the formation ofspores:haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, likebdelloid rotifers,do not have the ability to carry out meiosis and have acquired the ability to reproduce byparthenogenesis.

Meiosis does not occur inarchaeaorbacteria,which generally reproduce asexually viabinary fission.However, a "sexual" process known ashorizontal gene transferinvolves the transfer of DNA from one bacterium orarchaeonto another and recombination of these DNA molecules of different parental origin.

History

[edit]

Meiosis was discovered and described for the first time insea urchineggsin 1876 by the German biologistOscar Hertwig.It was described again in 1883, at the level ofchromosomes,by the Belgian zoologistEdouard Van Beneden,inAscarisroundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologistAugust 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, theAmericangeneticistThomas Hunt Morgandetected crossovers in meiosis in the fruit flyDrosophila melanogaster,which helped to establish that genetic traits are transmitted on chromosomes.

The term "meiosis" is derived from the Greek wordμείωσις,meaning 'lessening'. It was introduced to biology byJ.B. FarmerandJ.E.S. Moorein 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 byFlemming.[8]

The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions fortransliterating Greek.[9]

Phases

[edit]

Meiosis is divided intomeiosis Iandmeiosis IIwhich are further divided into Karyokinesis I, Cytokinesis I, Karyokinesis II, and Cytokinesis II, respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle.[10]Interphaseis divided into three phases:

  • Growth 1 (G1) phase:In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1,each of the chromosomes consists of a single linear molecule of DNA.
  • Synthesis (S) phase:The genetic material is replicated; each of the cell's chromosomes duplicates to become two identicalsister chromatidsattached at a centromere. This replication does not change theploidyof 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 place during prophase I in meiosis.
  • Growth 2 (G2) phase:G2phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G2phase of the mitotic cell cycle.

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 asinterkinesisbetween meiosis I and meiosis II.

Meiosis I and II are each divided intoprophase,metaphase,anaphase,andtelophasestages, 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).

Diagram of the meiotic phases

During meiosis, specific genes are more highlytranscribed.[11][12]In addition to strong meiotic stage-specific expression ofmRNA,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.[13]Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.

Meiosis I

[edit]

Meiosis I segregateshomologous 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 areductional division.Meiosis II is anequational divisionanalogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c).[14]

Meiosis Prophase I in mice. In Leptotene (L), the axial elements (stained by SYCP3) begin to form. In Zygotene (Z), the transverse elements (SYCP1) and central elements of the synaptonemal complex are partially installed (appearing as yellow as they overlap with SYCP3). In Pachytene (P), it is fully installed except on the sex chromosomes. In Diplotene (D), it disassembles revealing chiasmata. CREST marks the centromeres.
Schematic of the synaptonemal complex at different stages of prophase I and the chromosomes arranged as a linear array of loops.

Prophase I

[edit]

Prophase I is by far the longest 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 (byhomologous recombination), forming at least one crossover per chromosome.[16]These crossovers become visible as chiasmata (plural; singularchiasma).[17]This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (fourchromatids), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes.

Leptotene
[edit]
Main article:Leptotene stage

The first stage of prophase I is theleptotenestage, also known asleptonema,from Greek words meaning "thin threads".[18]: 27 In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus.[18]: 27 [19]: 353 The chromosomes each form a linear array of loops mediated bycohesin,and the lateral elements of thesynaptonemal complexassemble forming an "axial element" from which the loops emanate.[20]Recombination is initiated in this stage by the enzymeSPO11which creates programmeddouble strand breaks(around 300 per meiosis in mice).[21]This process generates single stranded DNA filaments coated byRAD51andDMC1which 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]
Main article:Zygotene

Leptotene is followed by thezygotenestage, also known aszygonema,from Greek words meaning "paired threads",[18]: 27 which in some organisms is also 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 central elements of thesynaptonemal complex.[20]Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

Pachytene
[edit]
Main article:Pachytene

Thepachytenestage (/ˈpækɪtiːn/PAK-i-teen), also known aspachynema,from Greek words meaning "thick threads".[18]: 27 is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene.[20]Most breaks are repaired without forming crossovers resulting ingene 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]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 as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the next stage.

Diplotene
[edit]

During thediplotenestage, also known asdiplonema,from Greek words meaning "two threads",[18]: 30 thesynaptonemal complexdisassembles and homologous chromosomes separate from one 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 allow homologous chromosomes to move to opposite poles of the cell.

In human fetaloogenesis,all developing oocytes develop to this stage and are arrested in prophase I before birth.[26]This suspended state is referred to as thedictyotene stageor 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 thediakinesisstage, from Greek words meaning "moving through".[18]: 30 This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resemblesprometaphaseof mitosis; thenucleolidisappear, thenuclear membranedisintegrates into vesicles, and themeiotic spindlebegins to form.

Meiotic spindle formation
[edit]

Unlike mitotic cells, human and mouse oocytes do not havecentrosomesto produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at thekinetochore.Over time, the MTOCs merge until two poles have formed, generating a barrel shaped spindle.[27]In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes.[28]Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules.[29]

Metaphase I

[edit]

Homologous pairs move together along the metaphase plate: Askinetochore microtubulesfrom 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 theindependent assortmentof 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 complexcohesinholds sister chromatids together from the time of their replication until anaphase. In mitosis, the force 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 ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (seeChromosome 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 farther apart. The cell elongates in preparation for division down the center.[17]Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit" ), what prevents the sister chromatids from separating.[30]This allows the sister chromatids to remain together while homologs are segregated.

Telophase I

[edit]

The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. 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. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II.[31]Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known asinterkinesisor interphase II. No DNA 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 result is the production of four haploid cells (n chromosomes; 23 in humans) from the two haploid cells (with n chromosomes, each consisting of two sister chromatids)[clarification needed]produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II.

Inprophase II,we see the disappearance of the nucleoli and thenuclear envelopeagain 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.

Inmetaphase II,the centromeres contain twokinetochoresthat 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.[32]

This is followed byanaphase 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.[30]

The process ends withtelophase 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.

Origin and function

[edit]

Origin of meiosis

[edit]

Meiosis appears to be a fundamental characteristic ofeukaryotic organismsand to have been present early in eukaryotic evolution. Eukaryotes that were once thought to lack meiotic sex have recently been shown to likely have, or once have had, this capability. As one example,Giardia intestinalis,a common intestinal parasite, was previously considered to have descended from a lineage that predated the emergence of meiosis and sex. However,G. intestinalishas now been found to possess a core set of meiotic genes, including five meiosis specific genes.[33]Also evidence formeiotic recombination,indicative ofsexual reproduction,was found inG. intestinalis.[34]Another example of organisms previously thought to be asexual are parasitic protozoa of the genusLeishmania,which cause human disease. However, these organisms were shown to have a sexual cycle consistent with a meiotic process.[35]Althoughamoebawere once generally regarded as asexual, evidence has been presented that most lineages are anciently sexual and that the majority of asexual groups probably arose recently and independently.[36]Dacks and Rogers[37]proposed, based on a phylogenetic analysis, that facultative sex was likely present in the common ancestor of eukaryotes.

Genetic variation

[edit]

The new combinations of DNA created during meiosis are a significant source ofgenetic variationalongside mutation, resulting in new combinations ofalleles,which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1)Law of Independent Assortment.The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes);[38]and (2)Crossing Over.The physical exchange of homologous chromosomal regions by homologousrecombinationduring prophase I results in new combinations of genetic information within chromosomes.[39]However, such physical exchange does not always occur during meiosis. In the oocytes of the silkwormBombyx mori,meiosis is completelyachiasmate(lacking crossovers).[40]Althoughsynaptonemal complexesare present during thepachytenestage of meiosis inB. mori,crossing-overhomologous recombinationis absent between the pairedchromosomes.[41]

Prophase I arrest

[edit]

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and theseoocytesare arrested at the prophase I stage of meiosis.[42]In humans, as an example, oocytes are formed between three and four months ofgestationwithin the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of thegenomeare present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed torepair damage in the DNAof thegermline.[42]The repair process used appears to involvehomologous recombinationalrepair[42][43]Prophase I arrested oocytes have a high capability for efficient repair ofDNA damage,particularly exogenously induced double-strand breaks.[43]DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant offertility.[43]

Meiosis as an adaptation for repairing germline DNA

[edit]

Genetic recombinationcan be viewed as fundamentally aDNA repairprocess, and that when it occurs during meiosis it is an adaptation for repairing thegenomicDNAthat is passed on to progeny.[44][45]Experimental findings indicate that a substantial benefit of meiosis isrecombinational repairofDNA damagein thegermline,as indicated by the following examples.Hydrogen peroxideis an agent that causesoxidative stressleading to oxidative DNA damage.[46]Treatment of the yeastSchizosaccharomyces pombewithhydrogen peroxideincreased the frequency of mating and the formation of meiotic spores by 4 to 18-fold.[47]Volvox carteri,a haploid multicellular, facultatively sexual green algae, can be induced by heat shock to reproduce by meiotic sex.[48]This induction can be inhibited byantioxidantsindicating that the induction of meiotic sex by heat shock is likely mediated byoxidative stressleading to increased DNA damage.[49]

Occurrence

[edit]

In life cycles

[edit]
Diplontic life cycle
Haplontic life cycle.
Main article:Biological life cycle
See also:Alternation of generations

Meiosis occurs in eukaryoticlife cyclesinvolvingsexual reproduction,consisting of the cyclical process of growth and development bymitoticcell division, production of gametes by meiosis and fertilization. At certain stages of the life cycle,germ cellsproduce gametes.Somatic cellsmake up the body of the organism and are not involved in gamete production.

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 (diplonticlife cycle), during the haploid state (haplonticlife cycle), or both (haplodiplonticlife cycle), in which there are two distinct organism phases, one with haploid cells and the other with diploid cells.

In thediplontic life cycle(with pre-gametic meiosis), as in humans, the organism is multicellular and diploid, grown by mitosis from a diploid cell called thezygote.The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes (thespermatozoain males andovain females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division bymitosisto grow into the organism.

In thehaplontic life cycle(with post-zygotic meiosis), the organism is haploid, by the proliferation and differentiation of a single haploid cell called thegamete.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. Manyfungiand manyprotozoautilize the haplontic life cycle.[citation needed]

In thehaplodiplontic life cycle(with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as thealternation 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.[50][citation needed]

In plants and animals

[edit]
Overview of chromatides' and chromosomes' distribution within the mitotic and meiotic cycle of a male human cell

Meiosis occurs in all animals and plants. The result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there is analternation of generationssuch that meiosis in the diploidsporophytegeneration produces haploid spores instead of gametes. When they germinate, these spores undergo repeated cell division by mitosis, developing into a multicellular haploidgametophytegeneration, which then produces gametes directly (i.e. without further meiosis).

In both animals and plants, the final stage is for the gametes to fuse to form azygotein which the original number of chromosomes is restored.[51]

In mammals

[edit]

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

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 thefollicle.At the beginning of eachmenstrual cycle,FSHsecretion from the anterior pituitary stimulates a few follicles to mature in a process known asfolliculogenesis.During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again 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 undergoatresia.The process of meiosis in females occurs duringoogenesis,and differs from the typical meiosis in that it features a long period of meiotic arrest known as thedictyatestage and lacks the assistance ofcentrosomes.[52][53]

In males, meiosis occurs duringspermatogenesisin theseminiferous tubulesof thetesticles.Meiosis during spermatogenesis is specific to a type of cell calledspermatocytes,which will later mature to becomespermatozoa.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.[54][55]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.[56]

Infemale 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.[57]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[58]or male meiosis which initiates postnatally.[56]

Flagellates

[edit]

While the majority of eukaryotes have a two-divisional meiosis (though sometimesachiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalidsandoxymonads) from the gut of the wood-feeding cockroachCryptocercus.[59]

Role in human genetics and disease

[edit]

Recombination among the 23 pairs of human chromosomes is responsible for redistributing not just the actual chromosomes, but also pieces of each of them. There is also an estimated 1.6-fold more recombination in females relative to males. In addition, average, 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).[60]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 cell. However, only a subset of DSBs (~5–30% depending on the organism), go on to produce crossovers,[61]which would result in only 1-2 cross-overs per human chromosome.

Nondisjunction

[edit]
Main article:Nondisjunction

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termeddisjunction.When the segregation is not normal, it is callednondisjunction.This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism fortrisomyormonosomy.Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or duringmitosis.

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,[62]presumably due to loss ofcohesinover time.[63]

Comparison to mitosis

[edit]

In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis.[64]

Meiosis Mitosis
End result Normally four cells, each with half the number of chromosomes as the parent Two cells, having the same number of chromosomes as the parent
Function Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Almost all eukaryotes (animals, plants, fungi, andprotists);[65][59]
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 II, Anaphase II, Telophase II		 Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same as parent? No Yes
Crossing over happens? Yes, normally occurs between each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Yes No
Cytokinesis Occurs in Telophase I and Telophase II Occurs in Telophase
Centromeres split Does not occur in Anaphase I, but occurs in Anaphase II Occurs in Anaphase

Molecular regulation

[edit]

How a cell proceeds to meiotic division in meiotic cell division is not well known.Maturation promoting factor (MPF)seemingly have role in frog Oocyte meiosis. In the fungusS. pombe.there is a role of MeiRNA binding protein for entry to meiotic cell division.[66]

It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I.[67]

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).[68]

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.[69]

In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II.[67]

See also

[edit]

References

[edit]
  1. ^"4.1: Meiosis".Biology LibreTexts.2019-10-01.Retrieved2021-05-29.
  2. ^"Definition of Reduction division".MedicineNet.Retrieved2021-05-29.
  3. ^abFreeman S (2011).Biological Science(6th ed.). Hoboken, NY: Pearson. p. 210.
  4. ^Hassold T, Hunt P (April 2001). "To err (meiotically) is human: the genesis of human aneuploidy".Nature Reviews Genetics.2(4): 280–91.doi:10.1038/35066065.PMID11283700.S2CID22264575.
  5. ^Letunic I, Bork P (2006)."Interactive Tree of Life".Archivedfrom the original on 29 January 2018.Retrieved23 July2011.
  6. ^Bernstein H, Bernstein C (2010). "Evolutionary origin of recombination during meiosis".BioScience.60(7): 498–505.doi:10.1525/bio.2010.60.7.5.S2CID86663600.
  7. ^Lodé T (June 2011)."Sex is not a solution for reproduction: the libertine bubble theory".BioEssays.33(6): 419–22.doi:10.1002/bies.201000125.PMID21472739.
  8. ^Farmer, J.B.; Moore, J.E.S. (February 1904)."On the Maiotic Phase (Reduction Divisions) in Animals and Plants".Quarterly Journal of Microscopical Science.48(192): 489–558.as quoted in theOxford English Dictionary,Third Edition, June 2001,s.v.
  9. ^Battaglia E (1985)."Meiosis and mitosis: a terminological criticism".Annali di Botanica.43:101–140.
  10. ^Carter JS (2012-10-27)."Mitosis".University of Cincinnati. Archived fromthe originalon 2012-10-27.Retrieved2018-02-09.
  11. ^Zhou A, Pawlowski WP (August 2014)."Regulation of meiotic gene expression in plants".Frontiers in Plant Science.5:413.doi:10.3389/fpls.2014.00413.PMC4142721.PMID25202317.
  12. ^Jung M, Wells D, Rusch J, Ahmad S, Marchini J, Myers SR, Conrad DF (June 2019)."Unified single-cell analysis of testis gene regulation and pathology in five mouse strains".eLife.8:e43966.doi:10.7554/eLife.43966.PMC6615865.PMID31237565.
  13. ^Brar GA, Yassour M, Friedman N, Regev A, Ingolia NT, Weissman JS (February 2012)."High-resolution view of the yeast meiotic program revealed by ribosome profiling".Science.335(6068): 552–7.Bibcode:2012Sci...335..552B.doi:10.1126/science.1215110.PMC3414261.PMID22194413.
  14. ^Freeman 2005,pp. 244–45
  15. ^Cohen PE, Pollack SE, Pollard JW (June 2006)."Genetic analysis of chromosome pairing, recombination, and cell cycle control during first meiotic prophase in mammals".Endocrine Reviews.27(4): 398–426.doi:10.1210/er.2005-0017.PMID16543383.
  16. ^Hunter N (October 2015)."Meiotic Recombination: The Essence of Heredity".Cold Spring Harbor Perspectives in Biology.7(12): a016618.doi:10.1101/cshperspect.a016618.PMC4665078.PMID26511629.
  17. ^abcFreeman 2005,pp. 249–250
  18. ^abcdefSnustad DP, Simmons MJ (December 2008).Principles of Genetics(5th ed.). Wiley.ISBN978-0-470-38825-9.
  19. ^Krebs JE, Goldstein ES, Kilpatrick ST (November 2009).Lewin's Genes X(10th ed.). Jones & Barlett Learning.ISBN978-0-7637-6632-0.
  20. ^abcdZickler D, Kleckner N (May 2015)."Recombination, Pairing, and Synapsis of Homologs during Meiosis".Cold Spring Harbor Perspectives in Biology.7(6): a016626.doi:10.1101/cshperspect.a016626.PMC4448610.PMID25986558.
  21. ^Baudat F, de Massy B (July 2007)."Regulating double-stranded DNA break repair towards crossover or non-crossover during mammalian meiosis".Chromosome Research.15(5): 565–77.doi:10.1007/s10577-007-1140-3.PMID17674146.S2CID26696085.
  22. ^O'Connor C (2008)."Meiosis, genetic recombination, and sexual reproduction".Nature Education.1(1): 174.
  23. ^Link J, Jantsch V (September 2019)."Meiotic chromosomes in motion: a perspective from Mus musculus and Caenorhabditis elegans".Chromosoma.128(3): 317–330.doi:10.1007/s00412-019-00698-5.PMC6823321.PMID30877366.
  24. ^Chen JM, Cooper DN, Chuzhanova N, Férec C, Patrinos GP (October 2007). "Gene conversion: mechanisms, evolution and human disease".Nature Reviews. Genetics.8(10): 762–75.doi:10.1038/nrg2193.PMID17846636.S2CID205484180.
  25. ^Bolcun-Filas E, Handel MA (July 2018)."Meiosis: the chromosomal foundation of reproduction".Biology of Reproduction.99(1): 112–126.doi:10.1093/biolre/ioy021.PMID29385397.S2CID38589675.
  26. ^Nussbaum RL, McInnes RR, Willard HF, Hamosh A (2015-05-21).Thompson & Thompson genetics in medicine(8th ed.). Elsevier. p. 19.ISBN978-1437706963.OCLC908336124.
  27. ^Schuh M, Ellenberg J (August 2007)."Self-organization of MTOCs replaces centrosome function during acentrosomal spindle assembly in live mouse oocytes".Cell.130(3): 484–98.doi:10.1016/j.cell.2007.06.025.PMID17693257.S2CID5219323.
  28. ^Holubcová Z, Blayney M, Elder K, Schuh M (June 2015)."Human oocytes. Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes".Science.348(6239): 1143–7.Bibcode:2015Sci...348.1143H.doi:10.1126/science.aaa9529.PMC4477045.PMID26045437.
  29. ^Kitajima TS, Ohsugi M, Ellenberg J (August 2011)."Complete kinetochore tracking reveals error-prone homologous chromosome biorientation in mammalian oocytes".Cell.146(4): 568–81.doi:10.1016/j.cell.2011.07.031.PMID21854982.S2CID5637615.
  30. ^abPierce, Benjamin (2009). «Chromosomes and Cell Reproduction». Genetics: A Conceptual Approach, Third Edition. W.H. FREEMAN AND CO.ISBN9780716779285P. 32
  31. ^Haglund K, Nezis IP, Stenmark H (January 2011)."Structure and functions of stable intercellular bridges formed by incomplete cytokinesis during development".Communicative & Integrative Biology.4(1): 1–9.doi:10.4161/cib.13550.PMC3073259.PMID21509167.
  32. ^"BioCoach Activity: Concept 11: Meiosis II: Metaphase II".The Biology Place.Pearson.Archivedfrom the original on 2018-02-28.Retrieved2018-02-10.
  33. ^Ramesh MA, Malik SB, Logsdon JM (January 2005)."A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis".Curr Biol.15(2): 185–91.Bibcode:2005CBio...15..185R.doi:10.1016/j.cub.2005.01.003.PMID15668177.
  34. ^Cooper MA, Adam RD, Worobey M, Sterling CR (November 2007)."Population genetics provides evidence for recombination in Giardia".Curr Biol.17(22): 1984–8.Bibcode:2007CBio...17.1984C.doi:10.1016/j.cub.2007.10.020.PMID17980591.
  35. ^Akopyants NS, Kimblin N, Secundino N, Patrick R, Peters N, Lawyer P, Dobson DE, Beverley SM, Sacks DL (April 2009)."Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector".Science.324(5924): 265–8.Bibcode:2009Sci...324..265A.doi:10.1126/science.1169464.PMC2729066.PMID19359589.
  36. ^Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E (July 2011)."The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms".Proc Biol Sci.278(1715): 2081–90.doi:10.1098/rspb.2011.0289.PMC3107637.PMID21429931.
  37. ^Dacks J, Roger AJ (June 1999). "The first sexual lineage and the relevance of facultative sex".J Mol Evol.48(6): 779–83.Bibcode:1999JMolE..48..779D.doi:10.1007/pl00013156.PMID10229582.
  38. ^Monaghan F, Corcos A (1984-01-01). "On the origins of the Mendelian laws".The Journal of Heredity.75(1): 67–9.doi:10.1093/oxfordjournals.jhered.a109868.PMID6368675.
  39. ^Saleem M, Lamb BC, Nevo E (December 2001)."Inherited differences in crossing over and gene conversion frequencies between wild strains of Sordaria fimicola from" Evolution Canyon "".Genetics.159(4): 1573–93.doi:10.1093/genetics/159.4.1573.PMC1461899.PMID11779798.
  40. ^Xiang Y, Tsuchiya D, Guo F, Gardner J, McCroskey S, Price A, Tromer EC, Walters JR, Lake CM, Hawley RS (May 2023)."A molecular cell biology toolkit for the study of meiosis in the silkworm Bombyx mori".G3 (Bethesda).13(5).doi:10.1093/g3journal/jkad058.PMC10151401.PMID36911915.
  41. ^Rasmussen SW (April 1977). "The transformation of the Synaptonemal Complex into the 'elimination chromatin' in Bombyx mori oocytes".Chromosoma.60(3): 205–21.doi:10.1007/BF00329771.PMID870294.
  42. ^abcMira A (September 1998). "Why is meiosis arrested?".Journal of Theoretical Biology.194(2): 275–87.Bibcode:1998JThBi.194..275M.doi:10.1006/jtbi.1998.0761.PMID9778439.
  43. ^abcStringer JM, Winship A, Zerafa N, Wakefield M, Hutt K (May 2020)."Oocytes can efficiently repair DNA double-strand breaks to restore genetic integrity and protect offspring health".Proceedings of the National Academy of Sciences of the United States of America.117(21): 11513–11522.Bibcode:2020PNAS..11711513S.doi:10.1073/pnas.2001124117.PMC7260990.PMID32381741.
  44. ^Bernstein H, Byerly HC, Hopf FA, Michod RE (September 1985). "Genetic damage, mutation, and the evolution of sex".Science.229(4719): 1277–81.Bibcode:1985Sci...229.1277B.doi:10.1126/science.3898363.PMID3898363.
  45. ^Bernstein H, Hopf FA, Michod RE (1987). "The Molecular Basis of the Evolution of Sex".Molecular Genetics of Development.Advances in Genetics. Vol. 24. pp. 323–70.doi:10.1016/s0065-2660(08)60012-7.ISBN978-0-12-017624-3.PMID3324702.
  46. ^Slupphaug G, Kavli B, Krokan HE (October 2003). "The interacting pathways for prevention and repair of oxidative DNA damage".Mutat Res.531(1–2): 231–51.Bibcode:2003MRFMM.531..231S.doi:10.1016/j.mrfmmm.2003.06.002.PMID14637258.
  47. ^Bernstein C, Johns V (April 1989)."Sexual reproduction as a response to H2O2 damage in Schizosaccharomyces pombe".J Bacteriol.171(4): 1893–7.doi:10.1128/jb.171.4.1893-1897.1989.PMC209837.PMID2703462.
  48. ^Kirk DL, Kirk MM (January 1986). "Heat shock elicits production of sexual inducer in Volvox".Science.231(4733): 51–4.Bibcode:1986Sci...231...51K.doi:10.1126/science.3941891.PMID3941891.
  49. ^Nedelcu AM, Marcu O, Michod RE (August 2004)."Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes".Proc Biol Sci.271(1548): 1591–6.doi:10.1098/rspb.2004.2747.PMC1691771.PMID15306305.
  50. ^South GR, Whittick A (2009-07-08).An Introduction to Phycology.Wiley.ISBN978-1-4443-1420-5.
  51. ^Bidlack JE, Jansky S, Stern KR (2011).Stern's Introductory Plant Biology.McGraw Hill. pp. 214–29.ISBN978-0-07-304052-3.OCLC320895017.
  52. ^Brunet S, Verlhac MH (2010)."Positioning to get out of meiosis: the asymmetry of division".Human Reproduction Update.17(1): 68–75.doi:10.1093/humupd/dmq044.PMID20833637.S2CID13988521.
  53. ^Rosenbusch B (November 2006). "The contradictory information on the distribution of non-disjunction and pre-division in female gametes".Human Reproduction.21(11): 2739–42.doi:10.1093/humrep/del122.PMID16982661.
  54. ^Lin Y, Gill ME, Koubova J, Page DC (December 2008). "Germ cell-intrinsic and -extrinsic factors govern meiotic initiation in mouse embryos".Science.322(5908): 1685–7.Bibcode:2008Sci...322.1685L.doi:10.1126/science.1166340.PMID19074348.S2CID11261341.
  55. ^Suzuki A, Saga Y (February 2008)."Nanos2 suppresses meiosis and promotes male germ cell differentiation".Genes & Development.22(4): 430–5.doi:10.1101/gad.1612708.PMC2238665.PMID18281459.
  56. ^abTeletin M, Vernet N, Yu J, Klopfenstein M, Jones JW, Féret B, Kane MA, Ghyselinck NB, Mark M (January 2019)."Two functionally redundant sources of retinoic acid secure spermatogonia differentiation in the seminiferous epithelium".Development.146(1): dev170225.doi:10.1242/dev.170225.PMC6340151.PMID30487180.
  57. ^Bowles J, Knight D, Smith C, Wilhelm D, Richman J, Mamiya S, Yashiro K, Chawengsaksophak K, Wilson MJ, Rossant J, Hamada H, Koopman P (April 2006). "Retinoid signaling determines germ cell fate in mice".Science.312(5773): 596–600.Bibcode:2006Sci...312..596B.doi:10.1126/science.1125691.PMID16574820.S2CID2514848.
  58. ^Kumar S, Chatzi C, Brade T, Cunningham TJ, Zhao X, Duester G (January 2011)."Sex-specific timing of meiotic initiation is regulated by Cyp26b1 independent of retinoic acid signalling".Nature Communications.2(1): 151.Bibcode:2011NatCo...2..151K.doi:10.1038/ncomms1136.PMC3034736.PMID21224842.
  59. ^abRaikov IB (1995). "Meiosis in protists: recent advances and persisting problems".European Journal of Protistology.31:1–7.doi:10.1016/s0932-4739(11)80349-4.
  60. ^Hegde MR, Crowley MR (2019-01-01). "Genome and Gene Structure".Emery and Rimoin's Principles and Practice of Medical Genetics and Genomics:53–77.doi:10.1016/B978-0-12-812537-3.00004-4.ISBN978-0-12-812537-3.S2CID92480716.
  61. ^Hunter N (2013-01-01). "Meiosis".Encyclopedia of Biological Chemistry(2nd ed.): 17–23.doi:10.1016/B978-0-12-378630-2.00474-6.ISBN978-0-12-378631-9.
  62. ^Hassold T, Jacobs P, Kline J, Stein Z, Warburton D (July 1980). "Effect of maternal age on autosomal trisomies".Annals of Human Genetics.44(1): 29–36.doi:10.1111/j.1469-1809.1980.tb00943.x.PMID7198887.S2CID24469567.
  63. ^Tsutsumi M, Fujiwara R, Nishizawa H, Ito M, Kogo H, Inagaki H, Ohye T, Kato T, Fujii T, Kurahashi H (May 2014)."Age-related decrease of meiotic cohesins in human oocytes".PLOS ONE.9(5): e96710.Bibcode:2014PLoSO...996710T.doi:10.1371/journal.pone.0096710.PMC4013030.PMID24806359.
  64. ^"How Cells Divide".PBS.Public Broadcasting Service.Archivedfrom the original on 1 November 2012.Retrieved6 December2012.
  65. ^Heywood P, Magee PT (March 1976)."Meiosis in protists. Some structural and physiological aspects of meiosis in algae, fungi, and protozoa".Bacteriological Reviews.40(1): 190–240.doi:10.1128/mmbr.40.1.190-240.1976.PMC413949.PMID773364.
  66. ^Kimble J (August 2011)."Molecular regulation of the mitosis/meiosis decision in multicellular organisms".Cold Spring Harbor Perspectives in Biology.3(8): a002683.doi:10.1101/cshperspect.a002683.PMC3140684.PMID21646377.
  67. ^abHonigberg SM, McCarroll RM, Esposito RE (April 1993). "Regulatory mechanisms in meiosis".Current Opinion in Cell Biology.5(2): 219–25.doi:10.1016/0955-0674(93)90106-z.PMID8389567.
  68. ^Lam I, Keeney S (October 2014)."Mechanism and regulation of meiotic recombination initiation".Cold Spring Harbor Perspectives in Biology.7(1): a016634.doi:10.1101/cshperspect.a016634.PMC4292169.PMID25324213.
  69. ^Pérez-Hidalgo L, Moreno S, San-Segundo PA (January 2003). "Regulation of meiotic progression by the meiosis-specific checkpoint kinase Mek1 in fission yeast".Journal of Cell Science.116(Pt 2): 259–71.doi:10.1242/jcs.00232.hdl:10261/62904.PMID12482912.S2CID14608163.

Cited texts

[edit]
[edit]
Wikimedia Commons has media related toMeiosis.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Meiosis&oldid=1241751550"