Meiosis
Meiosis (/maɪˈoʊsɪs/ ( listen); from Ancient Greek μείωσις (meíōsis) 'lessening', since it is a
reductional division)[1][2] is a special t ype of cell division of germ cells in sexually-reproducing
organisms t hat produces t he gamet es, such as sperm or egg cells. It involves t wo rounds of
division t hat ult imat ely result in four cells wit h only one copy of each chromosome (haploid).
Addit ionally, prior t o t he division, genet ic mat erial from t he pat ernal and mat ernal copies of each
chromosome is crossed over, creat ing new combinat ions of code on each chromosome.[3] Lat er
on, during fert ilisat ion, t he haploid cells produced by meiosis from a male and female will fuse t o
creat e a cell wit h t wo copies of each chromosome again, t he zygot e.
In meiosis, the chromosome or chromosomes duplicate (during interphase) and homologous chromosomes exchange
genetic information (chromosomal crossover) during the first division, called meiosis I. The daughter cells divide again in
meiosis II, splitting up sister chromatids to form haploid gametes. Two gametes fuse during fertilization, forming a diploid
cell with a complete set of paired chromosomes.
0:18
A video of meiosis I in a crane fly spermatocyte, played back at 120× the recorded speed
Errors in meiosis result ing in aneuploidy (an abnormal number of chromosomes) are t he leading
known cause of miscarriage and t he most frequent genet ic cause of development al
disabilit ies.[4]
In meiosis, DNA replicat ion is followed by t wo rounds of cell division t o produce four daught er
cells, each wit h half t he number of chromosomes as t he original parent cell.[3] The t wo meiot ic
divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of t he cell
cycle, t he DNA of each chromosome is replicat ed so t hat it consist s of t wo ident ical sist er
chromat ids, which remain held t oget her t hrough sist er chromat id cohesion. This S-phase can be
referred t o as "premeiot ic S-phase" or "meiot ic S-phase". Immediat ely following DNA replicat ion,
meiot ic cells ent er a prolonged G2-like st age known as meiot ic prophase. During t his t ime,
homologous chromosomes pair wit h each ot her and undergo genet ic recombinat ion, a
programmed process in which DNA may be cut and t hen repaired, which allows t hem t o exchange
some of t heir genet ic informat ion. A subset of recombinat ion event s result s in crossovers, which
creat e physical links known as chiasmat a (singular: chiasma, for t he Greek let t er Chi (Χ)) bet ween
t he homologous chromosomes. In most organisms, t hese links can help direct each pair of
homologous chromosomes t o segregat e away from each ot her during Meiosis I, result ing in t wo
haploid cells t hat have half t he number of chromosomes as t he parent cell.
During meiosis II, t he cohesion bet ween sist er chromat ids is released and t hey segregat e from
one anot her, as during mit osis. In some cases, all four of t he meiot ic product s form gamet es
such as sperm, spores or pollen. In female animals, t hree of t he four meiot ic product s are
t ypically eliminat ed by ext rusion int o polar bodies, and only one cell develops t o produce an
ovum. Because t he number of chromosomes is halved during meiosis, gamet es can fuse (i.e.
fert ilizat ion) t o form a diploid zygot e t hat cont ains t wo copies of each chromosome, one from
each parent . Thus, alt ernat ing cycles of meiosis and fert ilizat ion enable sexual reproduct ion, wit h
successive generat ions maint aining t he same number of chromosomes. For example, diploid
human cells cont ain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 t ot al),
half of mat ernal origin and half of pat ernal origin. Meiosis produces haploid gamet es (ova or
sperm) t hat cont ain one set of 23 chromosomes. When t wo gamet es (an egg and a sperm) fuse,
t he result ing zygot e is once again diploid, wit h t he mot her and fat her each cont ribut ing 23
chromosomes. This same pat t ern, but not t he same number of chromosomes, occurs in all
organisms t hat ut ilize meiosis.
Meiosis occurs in all sexually-reproducing single-celled and mult icellular organisms (which are all
eukaryot es), including animals, plant s and fungi.[5][6][7] It is an essent ial process for oogenesis and
spermat ogenesis.
Overview
Alt hough t he process of meiosis is relat ed t o t he more general cell division process of mit osis, it
differs in t wo import ant respect s:
shuffles t he genes bet ween t he t wo chromosomes in each pair (one
meiosis
received from each parent ), producing recombinant chromosomes
wit h unique genet ic combinat ions in every gamet e
recombination
occurs only if needed t o repair DNA damage;
mit osis
usually occurs bet ween ident ical sist er chromat ids and does not
result in genet ic changes
chromosome meiosis
number
(ploidy)
mit osis
produces four genet ically unique cells, each wit h half t he number of
chromosomes as in t he parent
produces t wo genet ically ident ical cells, each wit h t he same number
of chromosomes as in t he parent
Meiosis begins wit h a diploid cell, which cont ains t wo copies of each chromosome, t ermed
homologs. First , t he cell undergoes DNA replicat ion, so each homolog now consist s of t wo
ident ical sist er chromat ids. Then each set of homologs pair wit h each ot her and exchange
genet ic informat ion by homologous recombinat ion oft en leading t o physical connect ions
(crossovers) bet ween t he homologs. In t he first meiot ic division, t he homologs are segregat ed
t o separat e daught er cells by t he spindle apparat us. The cells t hen proceed t o a second division
wit hout an int ervening round of DNA replicat ion. The sist er chromat ids are segregat ed t o
separat e daught er cells t o produce a t ot al of four haploid cells. Female animals employ a slight
variat ion on t his pat t ern and produce one large ovum and t wo small polar bodies. Because of
recombinat ion, an individual chromat id can consist of a new combinat ion of mat ernal and pat ernal
genet ic informat ion, result ing in offspring t hat are genet ically dist inct from eit her parent .
Furt hermore, an individual gamet e can include an assort ment of mat ernal, pat ernal, and
recombinant chromat ids. This genet ic diversit y result ing from sexual reproduct ion cont ribut es t o
t he variat ion in t rait s upon which nat ural select ion can act .
Meiosis uses many of t he same mechanisms as mit osis, t he t ype of cell division used by
eukaryot es t o divide one cell int o t wo ident ical daught er cells. In some plant s, fungi, and prot ist s
meiosis result s in t he format ion of spores: haploid cells t hat can divide veget at ively wit hout
undergoing fert ilizat ion. Some eukaryot es, like bdelloid rot ifers, do not have t he abilit y t o carry
out meiosis and have acquired t he abilit y t o reproduce by part henogenesis.
Meiosis does not occur in archaea or bact eria, which generally reproduce asexually via binary
fission. However, a "sexual" process known as horizont al gene t ransfer involves t he t ransfer of
DNA from one bact erium or archaeon t o anot her and recombinat ion of t hese DNA molecules of
different parent al origin.
History
Meiosis was discovered and described for t he first t ime in sea urchin eggs in 1876 by t he German
biologist Oscar Hert wig. It was described again in 1883, at t he level of chromosomes, by t he
Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis
for reproduct ion and inherit ance, however, was described only in 1890 by German biologist
August Weismann, who not ed t hat t wo cell divisions were necessary t o t ransform one diploid
cell int o four haploid cells if t he number of chromosomes had t o be maint ained. In 1911, t he
American genet icist Thomas Hunt Morgan det ect ed crossovers in meiosis in t he fruit fly
Drosophila melanogaster, which helped t o est ablish t hat genet ic t rait s are t ransmit t ed on
chromosomes.
The t erm "meiosis" is derived from t he Greek word μείωσις, meaning 'lessening'. It was
int roduced t o biology by J.B. Farmer and J.E.S. Moore in 1905, using t he idiosyncrat ic 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 Flemming.[8]
The spelling was changed t o "meiosis" by Koernicke (1905) and by Pant el and De Sinet y (1906)
t o follow t he usual convent ions for t ranslit erat ing Greek.[9]
Phases
Meiosis is divided int o meiosis I and meiosis II which are furt her divided int o Karyokinesis I and
Cyt okinesis I and Karyokinesis II and Cyt okinesis II respect ively. The preparat ory st eps t hat lead
up t o meiosis are ident ical in pat t ern and name t o int erphase of t he mit ot ic cell cycle.[10]
Int erphase is divided int o t hree phases:
Growt h 1 (G1) phase: In t his very act ive phase, t he cell synt hesizes it s vast array of prot eins,
including t he enzymes and st ruct ural prot eins it will need for growt h. In G1, each of t he
chromosomes consist s of a single linear molecule of DNA.
Synt hesis (S) phase: The genet ic mat erial is replicat ed; each of t he cell's chromosomes
duplicat es t o become t wo ident ical sist er chromat ids at t ached at a cent romere. This
replicat ion does not change t he ploidy of t he cell since t he cent romere number remains t he
same. The ident ical sist er chromat ids have not yet condensed int o t he densely packaged
chromosomes visible wit h t he light microscope. This will t ake place during prophase I in
meiosis.
Growt h 2 (G2) phase: G2 phase as seen before mit osis is not present in meiosis. Meiot ic
prophase corresponds most closely t o t he G2 phase of t he mit ot ic cell cycle.
Int erphase is followed by meiosis I and t hen meiosis II. Meiosis I separat es replicat ed
homologous chromosomes, each st ill made up of t wo sist er chromat ids, int o t wo daught er cells,
t hus reducing t he chromosome number by half. During meiosis II, sist er chromat ids decouple and
t he result ant daught er chromosomes are segregat ed int o four daught er cells. For diploid
organisms, t he daught er cells result ing from meiosis are haploid and cont ain only one copy of
each chromosome. In some species, cells ent er a rest ing phase known as int erkinesis bet ween
meiosis I and meiosis II.
Meiosis I and II are each divided int o prophase, met aphase, anaphase, and t elophase st ages,
similar in purpose t o t heir analogous subphases in t he mit ot ic cell cycle. Therefore, meiosis
includes t he st ages of meiosis I (prophase I, met aphase I, anaphase I, t elophase I) and meiosis II
(prophase II, met aphase II, anaphase II, t elophase II).
Diagram of the meiotic phases
During meiosis, specific genes are more highly t ranscribed.[11][12] In addit ion t o st rong meiot ic
st age-specific expression of mRNA, t here are also pervasive t ranslat ional cont rols (e.g. select ive
usage of preformed mRNA), regulat ing t he ult imat e meiot ic st age-specific prot ein expression of
genes during meiosis.[13] Thus, bot h t ranscript ional and t ranslat ional cont rols det ermine t he broad
rest ruct uring of meiot ic cells needed t o carry out meiosis.
Meiosis I
Meiosis I segregat es homologous chromosomes, which are joined as t et rads (2n, 4c), producing
t wo haploid cells (n chromosomes, 23 in humans) which each cont ain chromat id pairs (1n, 2c).
Because t he ploidy is reduced from diploid t o haploid, meiosis I is referred t o as a reductional
division. Meiosis II is an equational division analogous t o mit osis, in which t he sist er chromat ids
are segregat ed, creat ing four haploid daught er 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's fully installed except on the sex chromosomes. In Diplotene (D) it
disassembles revealing chiasmata. CREST marks the centromeres.
Schematic of the synaptonemal complex at different stages of prophase I and the chromosomes arranged as a linear
array of loops.
Prophase I
Prophase I is by far t he longest phase of meiosis (last ing 13 out of 14 days in mice [15]). During
prophase I, homologous mat ernal and pat ernal chromosomes pair, synapse, and exchange genet ic
informat ion (by homologous recombinat ion), forming at least one crossover per chromosome.[16]
These crossovers become visible as chiasmat a (plural; singular chiasma).[17] This process
facilit at es st able pairing bet ween homologous chromosomes and hence enables accurat e
segregat ion of t he chromosomes at t he first meiot ic division. The paired and replicat ed
chromosomes are called bivalent s (t wo chromosomes) or t et rads (four chromat ids), wit h one
chromosome coming from each parent . Prophase I is divided int o a series of subst ages which are
named according t o t he appearance of chromosomes.
Lept ot ene
The first st age of prophase I is t he leptotene st age, also known as leptonema, from Greek words
meaning "t hin t hreads".[18]: 27 In t his st age of prophase I, individual chromosomes—each consist ing
of t wo replicat ed sist er chromat ids—become "individualized" t o form visible st rands wit hin t he
nucleus.[18]: 27 [19]: 353 The chromosomes each form a linear array of loops mediat ed by cohesin,
and t he lat eral element s of t he synapt onemal complex assemble forming an "axial element " from
which t he loops emanat e.[20] Recombinat ion is init iat ed in t his st age by t he enzyme SPO11 which
creat es programmed double st rand breaks (around 300 per meiosis in mice).[21] This process
generat es single st randed DNA filament s coat ed by RAD51 and DMC1 which invade t he
homologous chromosomes, forming int er-axis bridges, and result ing in t he pairing/co-alignment
of homologues (t o a dist ance of ~400 nm in mice).[20][22]
Zygot ene
Lept ot ene is followed by t he zygotene st age, also known as zygonema, from Greek words
meaning "paired t hreads",[18]: 27 which in some organisms is also called t he bouquet st age
because of t he way t he t elomeres clust er at one end of t he nucleus.[23] In t his st age t he
homologous chromosomes become much more closely (~100 nm) and st ably paired (a process
called synapsis) mediat ed by t he inst allat ion of t he t ransverse and cent ral element s of t he
synapt onemal complex.[20] Synapsis is t hought t o occur in a zipper-like fashion st art ing from a
recombinat ion nodule. The paired chromosomes are called bivalent or t et rad chromosomes.
Pachyt ene
The pachytene st age (/ˈpækɪt iːn/ PAK-i-teen), also known as pachynema, from Greek words
meaning "t hick t hreads".[18]: 27 is t he st age at which all aut osomal chromosomes have synapsed.
In t his st age homologous recombinat ion, including chromosomal crossover (crossing over), is
complet ed t hrough t he repair of t he double st rand breaks formed in lept ot ene.[20] Most breaks
are repaired wit hout forming crossovers result ing in gene conversion.[24] However, a subset of
breaks (at least one per chromosome) form crossovers bet ween non-sist er (homologous)
chromosomes result ing in t he exchange of genet ic informat ion.[25] Sex chromosomes, however,
are not wholly ident ical, and only exchange informat ion over a small region of homology called
t he pseudoaut osomal region.[26] The exchange of informat ion bet ween t he homologous
chromat ids result s in a recombinat ion of informat ion; each chromosome has t he complet e set of
informat ion it had before, and t here are no gaps formed as a result of t he process. Because t he
chromosomes cannot be dist inguished in t he synapt onemal complex, t he act ual act of crossing
over is not perceivable t hrough an ordinary light microscope, and chiasmat a are not visible unt il
t he next st age.
Diplot ene
During t he diplotene st age, also known as diplonema, from Greek words meaning "t wo
t hreads",[18]: 30 t he synapt onemal complex disassembles and homologous chromosomes
separat e from one anot her a lit t le. However, t he homologous chromosomes of each bivalent
remain t ight ly bound at chiasmat a, t he regions where crossing-over occurred. The chiasmat a
remain on t he chromosomes unt il t hey are severed at t he t ransit ion t o anaphase I t o allow
homologous chromosomes t o move t o opposit e poles of t he cell.
In human fet al oogenesis, all developing oocyt es develop t o t his st age and are arrest ed in
prophase I before birt h.[27] This suspended st at e is referred t o as t he dictyotene stage or
dict yat e. It last s unt il meiosis is resumed t o prepare t he oocyt e for ovulat ion, which happens at
pubert y or even lat er.
Diakinesis
Chromosomes condense furt her during t he diakinesis st age, from Greek words meaning "moving
t hrough".[18]: 30 This is t he first point in meiosis where t he four part s of t he t et rads are act ually
visible. Sit es of crossing over ent angle t oget her, effect ively overlapping, making chiasmat a
clearly visible. Ot her t han t his observat ion, t he rest of t he st age closely resembles
promet aphase of mit osis; t he nucleoli disappear, t he nuclear membrane disint egrat es int o
vesicles, and t he meiot ic spindle begins t o form.
Meiot ic spindle format ion
Unlike mit ot ic cells, human and mouse oocyt es do not have cent rosomes t o produce t he meiot ic
spindle. In mice, approximat ely 80 MicroTubule Organizing Cent ers (MTOCs) form a sphere in t he
ooplasm and begin t o nucleat e microt ubules t hat reach out t owards chromosomes, at t aching t o
t he chromosomes at t he kinet ochore. Over t ime t he MTOCs merge unt il t wo poles have formed,
generat ing a barrel shaped spindle.[28] In human oocyt es spindle microt ubule nucleat ion begins on
t he chromosomes, forming an ast er t hat event ually expands t o surround t he chromosomes.[29]
Chromosomes t hen slide along t he microt ubules t owards t he equat or of t he spindle, at which
point t he chromosome kinet ochores form end-on at t achment s t o microt ubules.[30]
Metaphase I
Homologous pairs move t oget her along t he met aphase plat e: As kinetochore microtubules from
bot h spindle poles at t ach t o t heir respect ive kinet ochores, t he paired homologous chromosomes
align along an equat orial plane t hat bisect s t he spindle, due t o cont inuous count erbalancing
forces exert ed on t he bivalent s by t he microt ubules emanat ing from t he t wo kinet ochores of
homologous chromosomes. This at t achment is referred t o as a bipolar at t achment . The physical
basis of t he independent assort ment of chromosomes is t he random orient at ion of each bivalent
along wit h t he met aphase plat e, wit h respect t o t he orient at ion of t he ot her bivalent s along t he
same equat orial line.[17] The prot ein complex cohesin holds sist er chromat ids t oget her from t he
t ime of t heir replicat ion unt il anaphase. In mit osis, t he force of kinet ochore microt ubules pulling
in opposit e direct ions creat es t ension. The cell senses t his t ension and does not progress wit h
anaphase unt il all t he chromosomes are properly bi-orient ed. In meiosis, est ablishing t ension
ordinarily requires at least one crossover per chromosome pair in addit ion t o cohesin bet ween
sist er chromat ids (see Chromosome segregat ion).
Anaphase I
Kinet ochore microt ubules short en, pulling homologous chromosomes (which each consist of a
pair of sist er chromat ids) t o opposit e poles. Nonkinet ochore microt ubules lengt hen, pushing t he
cent rosomes fart her apart . The cell elongat es in preparat ion for division down t he cent er.[17]
Unlike in mit osis, only t he cohesin from t he chromosome arms is degraded while t he cohesin
surrounding t he cent romere remains prot ect ed by a prot ein named Shugoshin (Japanese for
"guardian spirit "), what prevent s t he sist er chromat ids from separat ing.[31] This allows t he sist er
chromat ids t o remain t oget her while homologs are segregat ed.
Telophase I
The first meiot ic division effect ively ends when t he chromosomes arrive at t he poles. Each
daught er cell now has half t he number of chromosomes but each chromosome consist s of a pair
of chromat ids. The microt ubules t hat make up t he spindle net work disappear, and a new nuclear
membrane surrounds each haploid set . The chromosomes uncoil back int o chromat in. Cyt okinesis,
t he pinching of t he cell membrane in animal cells or t he format ion of t he cell wall in plant cells,
occurs, complet ing t he creat ion of t wo daught er cells. However, cyt okinesis does not fully
complet e result ing in "cyt oplasmic bridges" which enable t he cyt oplasm t o be shared bet ween
daught er cells unt il t he end of meiosis II.[32] Sist er chromat ids remain at t ached during t elophase
I.
Cells may ent er a period of rest known as int erkinesis or int erphase II. No DNA replicat ion occurs
during t his st age.
Meiosis II
Meiosis II is t he second meiot ic division, and usually involves equat ional segregat ion, or
separat ion of sist er chromat ids. Mechanically, t he process is similar t o mit osis, t hough it s
genet ic result s are fundament ally different . The end result is product ion of four haploid cells (n
chromosomes, 23 in humans) from t he t wo haploid cells (wit h n chromosomes, each consist ing of
t wo sist er chromat ids) produced in meiosis I. The four main st eps of meiosis II are: prophase II,
met aphase II, anaphase II, and t elophase II.
In prophase II, we see t he disappearance of t he nucleoli and t he nuclear envelope again as well
as t he short ening and t hickening of t he chromat ids. Cent rosomes move t o t he polar regions and
arrange spindle fibers for t he second meiot ic division.
In metaphase II, t he cent romeres cont ain t wo kinet ochores t hat at t ach t o spindle fibers from
t he cent rosomes at opposit e poles. The new equat orial met aphase plat e is rot at ed by 90
degrees when compared t o meiosis I, perpendicular t o t he previous plat e.[33]
This is followed by anaphase II, in which t he remaining cent romeric cohesin, not prot ect ed by
Shugoshin anymore, is cleaved, allowing t he sist er chromat ids t o segregat e. The sist er
chromat ids by convent ion are now called sist er chromosomes as t hey move t oward opposing
poles.[31]
The process ends wit h telophase II, which is similar t o t elophase I, and is marked by
decondensat ion and lengt hening of t he chromosomes and t he disassembly of t he spindle.
Nuclear envelopes re-form and cleavage or cell plat e format ion event ually produces a t ot al of
four daught er cells, each wit h a haploid set of chromosomes.
Meiosis is now complet e and ends up wit h four new daught er cells.
Origin and function
The new combinat ions of DNA creat ed during meiosis are a significant source of genet ic
variat ion alongside mut at ion, result ing in new combinat ions of alleles, which may be beneficial.
Meiosis generat es gamet e genet ic diversit y in t wo ways: (1) Law of Independent Assort ment .
The independent orient at ion of homologous chromosome pairs along t he met aphase plat e during
met aphase I and orient at ion of sist er chromat ids in met aphase II, t his is t he subsequent
separat ion of homologs and sist er chromat ids during anaphase I and II, it allows a random and
independent dist ribut ion of chromosomes t o each daught er cell (and ult imat ely t o gamet es);[34]
and (2) Crossing Over. The physical exchange of homologous chromosomal regions by
homologous recombinat ion during prophase I result s in new combinat ions of genet ic informat ion
wit hin chromosomes.[35]
Prophase I arrest
Female mammals and birds are born possessing all t he oocyt es needed for fut ure ovulat ions, and
t hese oocyt es are arrest ed at t he prophase I st age of meiosis.[36] In humans, as an example,
oocyt es are formed bet ween t hree and four mont hs of gest at ion wit hin t he fet us and are
t herefore present at birt h. During t his prophase I arrest ed st age (dict yat e), which may last for
decades, four copies of t he genome are present in t he oocyt es. The arrest of ooct yes at t he
four genome copy st age was proposed t o provide t he informat ional redundancy needed t o repair
damage in t he DNA of t he germline.[36] The repair process used appears t o involve homologous
recombinat ional repair[36][37] Prophase I arrest ed oocyt es have a high capabilit y for efficient
repair of DNA damages, part icularly exogenously induced double-st rand breaks.[37] DNA repair
capabilit y appears t o be a key qualit y cont rol mechanism in t he female germ line and a crit ical
det erminant of fert ilit y.[37]
Occurrence
In life cycles
Diplontic life cycle
Haplontic life cycle.
Meiosis occurs in eukaryot ic life cycles involving sexual reproduct ion, consist ing of t he const ant
cyclical process of meiosis and fert ilizat ion. This t akes place alongside normal mit ot ic cell
division. In mult icellular organisms, t here is an int ermediary st ep bet ween t he diploid and haploid
t ransit ion where t he organism grows. At cert ain st ages of t he life cycle, germ cells produce
gamet es. Somat ic cells make up t he body of t he organism and are not involved in gamet e
product ion.
Cycling meiosis and fert ilizat ion event s produces a series of t ransit ions back and fort h bet ween
alt ernat ing haploid and diploid st at es. The organism phase of t he life cycle can occur eit her
during t he diploid st at e (diplontic life cycle), during t he haploid st at e (haplontic life cycle), or
bot h (haplodiplontic life cycle, in which t here are t wo dist inct organism phases, one during t he
haploid st at e and t he ot her during t he diploid st at e). In t his sense t here are t hree t ypes of life
cycles t hat ut ilize sexual reproduct ion, different iat ed by t he locat ion of t he organism phase(s).
In t he diplontic life cycle (wit h pre-gamet ic meiosis), of which humans are a part , t he organism is
diploid, grown from a diploid cell called t he zygot e. The organism's diploid germ-line st em cells
undergo meiosis t o creat e haploid gamet es (t he spermat ozoa for males and ova for females),
which fert ilize t o form t he zygot e. The diploid zygot e undergoes repeat ed cellular division by
mit osis t o grow int o t he organism.
In t he haplontic life cycle (wit h post -zygot ic meiosis), t he organism is haploid inst ead, spawned
by t he proliferat ion and different iat ion of a single haploid cell called t he gamet e. Two organisms
of opposing sex cont ribut e t heir haploid gamet es t o form a diploid zygot e. The zygot e
undergoes meiosis immediat ely, creat ing four haploid cells. These cells undergo mit osis t o
creat e t he organism. Many fungi and many prot ozoa ut ilize t he haplont ic life cycle.
Finally, in t he haplodiplontic life cycle (wit h sporic or int ermediat e meiosis), t he living organism
alt ernat es bet ween haploid and diploid st at es. Consequent ly, t his cycle is also known as t he
alt ernat ion of generat ions. The diploid organism's germ-line cells undergo meiosis t o produce
spores. The spores proliferat e by mit osis, growing int o a haploid organism. The haploid organism's
gamet e t hen combines wit h anot her haploid organism's gamet e, creat ing t he zygot e. The zygot e
undergoes repeat ed mit osis and different iat ion t o become a diploid organism again. The
haplodiplont ic life cycle can be considered a fusion of t he diplont ic and haplont ic life cycles.[38]
In plants and animals
Overview of chromatides' and chromosomes' distribution within the mitotic and meiotic cycle of a male human cell
Meiosis occurs in all animals and plant s. The end result , t he product ion of gamet es wit h half t he
number of chromosomes as t he parent cell, is t he same, but t he det ailed process is different . In
animals, meiosis produces gamet es direct ly. In land plant s and some algae, t here is an alt ernat ion
of generat ions such t hat meiosis in t he diploid sporophyt e generat ion produces haploid spores.
These spores mult iply by mit osis, developing int o t he haploid gamet ophyt e generat ion, which
t hen gives rise t o gamet es direct ly (i.e. wit hout furt her meiosis). In bot h animals and plant s, t he
final st age is for t he gamet es t o fuse, rest oring t he original number of chromosomes.[39]
In mammals
In females, meiosis occurs in cells known as oocyt es (singular: oocyt e). Each primary oocyt e
divides t wice in meiosis, unequally in each case. The first division produces a daught er cell, and a
much smaller polar body which may or may not undergo a second division. In meiosis II, division of
t he daught er cell produces a second polar body, and a single haploid cell, which enlarges t o
become an ovum. Therefore, in females each primary oocyt e t hat undergoes meiosis result s in
one mat ure ovum and one or t wo polar bodies.
Not e t hat t here are pauses during meiosis in females. Mat uring oocyt es are arrest ed in prophase
I of meiosis I and lie dormant wit hin a prot ect ive shell of somat ic cells called t he follicle. At t he
beginning of each menst rual cycle, FSH secret ion from t he ant erior pit uit ary st imulat es a few
follicles t o mat ure in a process known as folliculogenesis. During t his process, t he mat uring
oocyt es resume meiosis and cont inue unt il met aphase II of meiosis II, where t hey are again
arrest ed just before ovulat ion. If t hese oocyt es are fert ilized by sperm, t hey will resume and
complet e meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while
t he ot hers undergo at resia. The process of meiosis in females occurs during oogenesis, and
differs from t he t ypical meiosis in t hat it feat ures a long period of meiot ic arrest known as t he
dict yat e st age and lacks t he assist ance of cent rosomes.[40][41]
In males, meiosis occurs during spermat ogenesis in t he seminiferous t ubules of t he t est icles.
Meiosis during spermat ogenesis is specific t o a t ype of cell called spermat ocyt es, which will
lat er mat ure t o become spermat ozoa. Meiosis of primordial germ cells happens at t he t ime of
pubert y, much lat er t han in females. Tissues of t he male t est is suppress meiosis by degrading
ret inoic acid, proposed t o be a st imulat or of meiosis. This is overcome at pubert y when cells
wit hin seminiferous t ubules called Sert oli cells st art making t heir own ret inoic acid. Sensit ivit y t o
ret inoic acid is also adjust ed by prot eins called nanos and DAZL.[42][43] Genet ic loss-of-funct ion
st udies on ret inoic acid-generat ing enzymes have shown t hat ret inoic acid is required post nat ally
t o st imulat e spermat ogonia different iat ion which result s several days lat er in spermat ocyt es
undergoing meiosis, however ret inoic acid is not required during t he t ime when meiosis
init iat es.[44]
In female mammals, meiosis begins immediat ely aft er primordial germ cells migrat e t o t he ovary
in t he embryo. Some st udies suggest t hat ret inoic acid derived from t he primit ive kidney
(mesonephros) st imulat es meiosis in embryonic ovarian oogonia and t hat t issues of t he
embryonic male t est is suppress meiosis by degrading ret inoic acid.[45] However, genet ic loss-offunct ion st udies on ret inoic acid-generat ing enzymes have shown t hat ret inoic acid is not
required for init iat ion of eit her female meiosis which occurs during embryogenesis[46] or male
meiosis which init iat es post nat ally.[44]
Flagellates
While t he majorit y of eukaryot es have a t wo-divisional meiosis (t hough somet imes achiasmat ic),
a very rare form, one-divisional meiosis, occurs in some flagellat es (parabasalids and oxymonads)
from t he gut of t he wood-feeding cockroach Cryptocercus.[47]
Role in human genetics and disease
Recombinat ion among t he 23 pairs of human chromosomes is responsible for redist ribut ing not
just t he act ual chromosomes, but also pieces of each of t hem. There is also an est imat ed 1.6fold more recombinat ion in females relat ive t o males. In addit ion, average, female recombinat ion
is higher at t he cent romeres and male recombinat ion is higher at t he t elomeres. On average, 1
million bp (1 Mb) correspond t o 1 cMorgan (cm = 1% recombinat ion frequency).[48] The frequency
of cross-overs remain uncert ain. In yeast , mouse and human, it has been est imat ed t hat ≥200
double-st rand breaks (DSBs) are formed per meiot ic cell. However, only a subset of DSBs (~5–
30% depending on t he organism), go on t o produce crossovers,[49] which would result in only 1-2
cross-overs per human chromosome.
Nondisjunction
The normal separat ion of chromosomes in meiosis I or sist er chromat ids in meiosis II is t ermed
disjunction. When t he segregat ion is not normal, it is called nondisjunction. This result s in t he
product ion of gamet es which have eit her t oo many or t oo few of a part icular chromosome, and is
a common mechanism for t risomy or monosomy. Nondisjunct ion can occur in t he meiosis I or
meiosis II, phases of cellular reproduct ion, or during mit osis.
Most monosomic and t risomic human embryos are not viable, but some aneuploidies can be
t olerat ed, such as t risomy for t he smallest chromosome, chromosome 21. Phenot ypes of t hese
aneuploidies range from severe development al disorders t o asympt omat ic. Medical condit ions
include but are not limit ed t o:
Down syndrome – t risomy of chromosome 21
Pat au syndrome – t risomy of chromosome 13
Edwards syndrome – t risomy of chromosome 18
Klinefelt er syndrome – ext ra X chromosomes in males – i.e. XXY, XXXY, XXXXY, et c.
Turner syndrome – lacking of one X chromosome in females – i.e. X0
Triple X syndrome – an ext ra X chromosome in females
Jacobs syndrome – an ext ra Y chromosome in males.
The probabilit y of nondisjunct ion in human oocyt es increases wit h increasing mat ernal age,[50]
presumably due t o loss of cohesin over t ime.[51]
Comparison to mitosis
In order t o underst and meiosis, a comparison t o mit osis is helpful. The t able below shows t he
differences bet ween meiosis and mit osis.[52]
Meiosis
End result
Normally four cells, each wit h half t he
number of chromosomes as t he parent
Product ion of gamet es (sex cells) in
Funct ion
sexually reproducing eukaryot es wit h
diplont life cycle
Mitosis
Two cells, having t he same
number of chromosomes as t he
parent
Cellular reproduct ion, growt h,
repair, asexual reproduct ion
Almost all eukaryot es (animals, plant s,
fungi, and prot ist s);[53][47]
Where does it
In gonads, before gamet es (in diplont ic life All proliferat ing cells in all
happen?
cycles);
eukaryot es
Aft er zygot es (in haplont ic);
Before spores (in haplodiplont ic)
Prophase I, Met aphase I, Anaphase I,
St eps
Telophase I,
Prophase II, Met aphase II, Anaphase II,
Telophase II
Genet ically same
as parent ?
No
Crossing over
Yes, normally occurs bet ween each pair of
happens?
homologous chromosomes
Prophase, Promet aphase,
Met aphase, Anaphase,
Telophase
Yes
Very rarely
Pairing of
homologous
Yes
No
Cyt okinesis
Occurs in Telophase I and Telophase II
Occurs in Telophase
Cent romeres
Does not occur in Anaphase I, but occurs in
split
Anaphase II
chromosomes?
Occurs in Anaphase
Molecular regulation
How a cell proceeds t o meiot ic division in meiot ic cell division is not well known. Mat urat ion
promot ing fact or (MPF) seemingly have role in frog Oocyt e meiosis. In t he fungus S. pombe.
t here is a role of MeiRNA binding prot ein for ent ry t o meiot ic cell division.[54]
It has been suggest ed t hat Yeast CEP1 gene product , t hat binds cent romeric region CDE1, may
play a role in chromosome pairing during meiosis-I.[55]
Meiot ic recombinat ion is mediat ed t hrough double st randed break, which is cat alyzed by Spo11
prot ein. Also Mre11, Sae2 and Exo1 play role in breakage and recombinat ion. Aft er t he breakage
happen, recombinat ion t ake place which is t ypically homologous. The recombinat ion may go
t hrough eit her a double Holliday junct ion (dHJ) pat hway or synt hesis-dependent st rand annealing
(SDSA). (The second one gives t o noncrossover product ).[56]
Seemingly t here are checkpoint s for meiot ic cell division t oo. In S. pombe, Rad prot eins, S.
pombe Mek1 (wit h FHA kinase domain), Cdc25, Cdc2 and unknown fact or is t hought t o form a
checkpoint .[57]
In vert ebrat e oogenesis, maint ained by cyt ost at ic fact or (CSF) has role in swit ching int o meiosisII.[55]
See also
Fert ilisat ion
Coefficient of coincidence
DNA repair
Oxidat ive st ress
Synizesis (biology)
Biological life cycle
Apomixis
Part henogenesis
Alt ernat ion of generat ions
Brachymeiosis
Mit ot ic recombinat ion
Dikaryon
Mat ing of yeast
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Cited texts
Freeman S (2005). Biological Science (ht t ps://archive.org/det ails/biologicalscienc00scot )
(3rd ed.). Upper Saddle River, NJ: Pearson Prent ice Hall. ISBN 9780131409415.
External links
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CCO (ht t ps://web.archive.org/web/20190215050532/ht t p://www.cellcycleont ology.org/)
The Cell-Cycle Ont ology
St ages of Meiosis animat ion (ht t p://highered.mcgraw-hill.com/olc/dl/120074/bio19.swf)
*"Abby Dernburg Seminar: Chromosome Dynamics During Meiosis" (ht t ps://www.ibiology.org/cel
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