Meiosis (from Greek μειώσεις, meiosis, which means lessening)
/maɪˈoʊsɪs/ ( listen) is a specialized type of cell
division that reduces the chromosome number by half, creating four
haploid cells, each genetically distinct from the parent cell that
gave rise to them. This process occurs in all sexually reproducing
single-celled and multicellular eukaryotes, including animals, plants,
and fungi. Errors in meiosis resulting in aneuploidy are
the leading known cause of miscarriage and the most frequent genetic
cause of developmental disabilities.
DNA replication is followed by two rounds of cell division
to produce four daughter cells, each with half the number of
chromosomes 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 is 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
(X)) between the homologous chromosomes. In most organisms, these
links are essential to 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
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
3 Occurrence in eukaryotic life cycles
126.96.36.199 Synchronous processes
6 Origin and function
8 In plants and animals
9 In mammals
10 Compared to mitosis
11 See also
12.1 Cited texts
13 External links
Although the process of meiosis is related to the more general cell
division process of mitosis, it differs in two important respects:
shuffles the genes between the two chromosomes in each pair (one
received from each parent), producing recombinant chromosomes with
unique genetic combinations in every gamete
occurs only if needed to repair DNA damage;
usually occurs between identical sister chromatids and does not result
in genetic changes
chromosome number (ploidy)
produces four genetically unique cells, each with half the number of
chromosomes as in the parent
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 DNA by homologous recombination 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
Female animals employ a slight variation on this
pattern and produce one large ovum and two small polar bodies. Because
of recombination, an individual chromatid can consist of a new
combination of maternal and paternal DNA, 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 eukaryotes to divide one cell into two identical
daughter cells. In some plants, fungi, and protists meiosis results in
the formation of spores: 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 archaeon to another and recombination of these DNA
molecules of different parental origin.
Meiosis was discovered and described for the first time in sea urchin
eggs in 1876 by the German biologist Oscar Hertwig. It was described
again in 1883, at the level of chromosomes, by the Belgian zoologist
Edouard Van Beneden, in
Ascaris roundworm eggs. The significance of
meiosis for reproduction and inheritance, however, was described only
in 1890 by German biologist August Weismann, who noted that two cell
divisions were necessary to transform one diploid cell into four
haploid cells if the number of chromosomes had to be maintained. In
1911 the American geneticist
Thomas Hunt Morgan
Thomas Hunt Morgan detected crossovers in
meiosis in the fruit fly Drosophila melanogaster, which helped to
establish that genetic traits are transmitted on chromosomes.
The term meiosis (originally spelled "maiosis") was introduced to
biology by J.B. Farmer and J.E.S. Moore in 1905:
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.
It is derived from the Greek word μείωσις, meaning 'lessening'.
Occurrence in eukaryotic life cycles
Gametic life cycle.
Zygotic life cycle.
Main article: Biological life cycle
Meiosis occurs in eukaryotic life cycles involving sexual
reproduction, consisting of the constant cyclical process of meiosis
and fertilization. This takes place alongside normal mitotic cell
division. In multicellular organisms, there is an intermediary step
between the diploid and haploid transition where the organism grows.
At certain stages of the life cycle, germ cells produce gametes.
Somatic cells make up the body of the organism and are not involved in
Cycling meiosis and fertilization events produces a series of
transitions back and forth between alternating haploid and diploid
states. The organism phase of the life cycle can occur either during
the diploid state (gametic or diploid life cycle), during the haploid
state (zygotic or haploid life cycle), or both (sporic or haplodiploid
life cycle, in which there are two distinct organism phases, one
during the haploid state and the other during the diploid state). In
this sense there are three types of life cycles that utilize sexual
reproduction, differentiated by the location of the organism
In the gametic life cycle or " diplontic life cycle", of which humans
are a part, the organism is diploid, grown from a diploid cell called
the zygote. The organism's diploid germ-line stem cells undergo
meiosis to create haploid gametes (the spermatozoa for males and ova
for females), which fertilize to form the zygote. The diploid zygote
undergoes repeated cellular division by mitosis to grow into the
In the zygotic life cycle the organism is haploid instead, spawned 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 fungi and many protozoa utilize the
zygotic life cycle.
Finally, in the sporic life cycle, 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 become a diploid organism again. The sporic life
cycle can be considered a fusion of the gametic and zygotic life
The preparatory steps that lead up to meiosis are identical in pattern
and name to interphase of the mitotic cell cycle.
Interphase is 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 identical sister
chromatids attached at a centromere. This replication does not change
the ploidy of the cell since the centromere number remains the same.
The identical sister chromatids have not yet condensed into the
densely packaged chromosomes visible with the light microscope. This
will take place during prophase I in meiosis.
Growth 2 (G2) phase:
G2 phase as seen before mitosis is not present in
meiosis. Meiotic prophase corresponds most closely to the
G2 phase of
the mitotic cell cycle.
Interphase is followed by meiosis I and then meiosis II.
separates 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).
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 &
orientation of sister chromatids in metaphase II, this is the
subsequent separation of homologs and sister chromatids during
anaphase I & II, it allows a random and independent distribution
of chromosomes to each daughter cell (and ultimately to gametes);
and (2) Crossing Over. The physical exchange of homologous chromosomal
regions by homologous recombination during prophase I results in new
combinations of DNA within chromosomes.
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.
Diagram of the meiotic phases
Meiosis is divided into meiosis I and meiosis II which are further
divided into Karyokinesis I and
Cytokinesis I and Karyokinesis II and
Cytokinesis II respectively.
Meiosis I segregates homologous chromosomes, which are joined as
tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in
humans) which each contain chromatid pairs (1n, 2c). Because the
ploidy is reduced from diploid to haploid, meiosis I is referred to as
a reductional division.
Meiosis II is an equational division analogous
to mitosis, in which the sister chromatids are segregated, creating
four haploid daughter cells (1n, 1c).
Prophase I in mice. L:Leptotene, Z:Zygotene, P:Pachytene,
Prophase I is typically the longest phase of meiosis. During prophase
I, homologous chromosomes pair and exchange DNA (homologous
recombination). This often results in chromosomal crossover. This
process is critical for pairing between homologous chromosomes and
hence for accurate segregation of the chromosomes at the first meiosis
division. The new combinations of DNA created during crossover are a
significant source of genetic variation, and result in new
combinations of alleles, which may be beneficial. The paired and
replicated chromosomes are called bivalents or tetrads, which have two
chromosomes and four chromatids, with one chromosome coming from each
parent. The process of pairing the homologous chromosomes is called
synapsis. At this stage, non-sister chromatids may cross-over at
points called chiasmata (plural; singular chiasma).
Prophase I has
historically been divided into a series of substages which are named
according to the appearance of chromosomes.
The first stage of prophase I is the leptotene stage, also known as
leptonema, from Greek words meaning "thin threads".:27In this
stage of prophase I, individual chromosomes—each consisting of two
sister chromatids—become "individualized" to form visible strands
within the nucleus.:27:353 The two sister chromatids closely
associate and are visually indistinguishable from one another. During
leptotene, lateral elements of the synaptonemal complex assemble.
Leptotene is of very short duration and progressive condensation and
coiling of chromosome fibers takes place.
The zygotene stage, also known as zygonema, from Greek words meaning
"paired threads",:27 occurs as the chromosomes approximately line
up with each other into homologous chromosome pairs. In some
organisms, this is called the bouquet stage because of the way the
telomeres cluster at one end of the nucleus. At this stage, the
synapsis (pairing/coming together) of homologous chromosomes takes
place, facilitated by assembly of central element of the synaptonemal
complex. Pairing is brought about in a zipper-like fashion and may
start at the centromere (procentric), at the chromosome ends
(proterminal), or at any other portion (intermediate). Individuals of
a pair are equal in length and in position of the centromere. Thus
pairing is highly specific and exact. The paired chromosomes are
called bivalent or tetrad chromosomes.
The pachytene (pronounced /ˈpækɪtiːn/ PAK-ə-teen) stage, also
known as pachynema, from Greek words meaning "thick threads",.:27
At this point a tetrad of the chromosomes has formed known as a
bivalent. This is the stage when homologous recombination, including
chromosomal crossover (crossing over), occurs. Nonsister chromatids of
homologous chromosomes may exchange segments over regions of homology.
Sex chromosomes, however, are not wholly identical, and only exchange
information over a small region of homology. At the sites where
exchange happens, chiasmata form. The exchange of information between
the non-sister 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 the microscope,
and chiasmata are not visible until the next stage.
During the diplotene stage, also known as diplonema, from Greek words
meaning "two threads",:30 the synaptonemal complex degrades and
homologous chromosomes separate from one another a little. The
chromosomes themselves uncoil a bit, allowing some transcription of
DNA. 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.
In human fetal oogenesis, all developing oocytes develop to this stage
and are arrested in prophase I before birth. This suspended state
is referred to as the dictyotene stage or dictyate. It lasts until
meiosis is resumed to prepare the oocyte for ovulation, which happens
at puberty or even later.
Chromosomes condense further during the diakinesis stage, from Greek
words meaning "moving through".: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 resembles prometaphase of mitosis; the
nucleoli disappear, the nuclear membrane disintegrates into vesicles,
and the meiotic spindle begins to form.
During these stages, two centrosomes, containing a pair of centrioles
in animal cells, migrate to the two poles of the cell. These
centrosomes, which were duplicated during S-phase, function as
microtubule organizing centers nucleating microtubules, which are
essentially cellular ropes and poles. The microtubules invade the
nuclear region after the nuclear envelope disintegrates, attaching to
the chromosomes at the kinetochore. The kinetochore functions as a
motor, pulling the chromosome along the attached microtubule toward
the originating centrosome, like a train on a track. There are four
kinetochores on each tetrad, but the pair of kinetochores on each
sister chromatid fuses and functions as a unit during meiosis
Microtubules that attach to the kinetochores are known as kinetochore
microtubules. Other microtubules will interact with microtubules from
the opposite centrosome: these are called nonkinetochore microtubules
or polar microtubules. A third type of microtubules, the aster
microtubules, radiates from the centrosome into the cytoplasm or
contacts components of the membrane skeleton.
Homologous pairs move together along the metaphase plate: As
kinetochore microtubules from both centrosomes attach to their
respective kinetochores, the paired homologous chromosomes align along
an equatorial plane that bisects the spindle, due to continuous
counterbalancing forces exerted on the bivalents by the microtubules
emanating from the two kinetochores of homologous chromosomes. This
attachment is referred to as a bipolar attachment. The physical basis
of the independent assortment of chromosomes is the random orientation
of each bivalent along the metaphase plate, with respect to the
orientation of the other bivalents along the same equatorial line.
The protein complex cohesin holds 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 requires at least one crossover per
chromosome pair in addition to cohesin between sister chromatids.
Kinetochore microtubules shorten, pulling homologous chromosomes
(which 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. Unlike in mitosis, only the cohesin from the chromosome
arms is degraded while the cohesin surrounding the centromere remains
protected. This allows the sister chromatids to remain together while
homologs are segregated.
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. The chromosomes uncoil
back into chromatin. Cytokinesis, the pinching of the cell membrane in
animal cells or the formation of the cell wall in plant cells, occurs,
completing the creation of two daughter cells. Sister chromatids
remain attached during telophase I.
Cells may enter a period of rest known as interkinesis or interphase
DNA replication occurs during this stage.
Meiosis II is the second meiotic division, and usually involves
equational segregation, or separation of sister chromatids.
Mechanically, the process is similar to mitosis, though its genetic
results are fundamentally different. The end result is production of
four haploid cells (n chromosomes, 23 in humans) from the two haploid
cells (with n chromosomes, each consisting of two sister chromatids)
produced in meiosis I. The four main steps of meiosis II are: prophase
II, metaphase II, anaphase II, and telophase II.
In prophase II we see the disappearance of the nucleoli and the
nuclear envelope 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 kinetochores that attach
to spindle fibers from the centrosomes at opposite poles. The new
equatorial metaphase plate is rotated by 90 degrees when compared to
meiosis I, perpendicular to the previous plate. 
This is followed by anaphase II, in which the remaining centromeric
cohesin 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 reform 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
Main article: Origin and function of meiosis
The origin and function of meiosis are fundamental to understanding
the evolution of sexual reproduction in eukaryotes. There is no
current consensus among biologists on the questions of how sex in
Eukaryotes arose in evolution, what basic function sexual reproduction
serves, and why it is maintained, given the basic two-fold cost of
sex. It is clear that it evolved over 1.2 billion years ago, and that
almost all species which are descendants of the original sexually
reproducing species are still sexual reproducers, including plants,
fungi, and animals.
Meiosis is a key event of the sexual cycle in Eukaryotes. It is the
stage of the life cycle when a cell gives rise to two haploid cells
(gametes) each having half as many chromosomes. Two such haploid
gametes, arising from different individual organisms, fuse by the
process of fertilization, thus completing the sexual cycle.
Meiosis is ubiquitous among eukaryotes. It occurs in single-celled
organisms such as yeast, as well as in multicellular organisms, such
Eukaryotes arose from prokaryotes more than 2.2 billion
years ago and the earliest eukaryotes were likely single-celled
organisms. To understand sex in eukaryotes, it is necessary to
understand (1) how meiosis arose in single celled eukaryotes, and (2)
the function of meiosis.
Main article: Nondisjunction
The normal separation of chromosomes in meiosis I or sister chromatids
in meiosis II is termed disjunction. When the segregation is not
normal, it is called nondisjunction. This results in the production of
gametes which have either too many or too few of a particular
chromosome, and is a common mechanism for trisomy or monosomy.
Nondisjunction can occur in the meiosis I or meiosis II, phases of
cellular reproduction, or during mitosis.
Most monosomic and trisomic human 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:
Down syndrome – trisomy of chromosome 21
Patau syndrome – trisomy of chromosome 13
Edwards syndrome – trisomy of chromosome 18
Klinefelter syndrome – extra X chromosomes in males – i.e. XXY,
XXXY, XXXXY, etc.
Turner syndrome – lacking of one
X chromosome in females – i.e. X0
Triple X syndrome
Triple X syndrome – an extra
X chromosome in females
XYY syndrome – an extra
Y chromosome in males.
The probability of nondisjunction in human oocytes increases with
increasing maternal age, presumably due to loss of cohesin over
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 plants. The end 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 an alternation of generations such that meiosis in the
diploid sporophyte generation produces haploid spores. These spores
multiply by mitosis, developing into the haploid gametophyte
generation, which then gives rise to gametes directly (i.e. without
further meiosis). In both animals and plants, the final stage is for
the gametes to fuse, restoring the original number of chromosomes.
In females, meiosis occurs in cells known as oocytes (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 an ovum.
Therefore, in females each primary oocyte that undergoes meiosis
results in one mature ovum and one or two polar bodies.
Note that there are pauses during meiosis in females. Maturing oocytes
are arrested in prophase I of meiosis I and lie dormant within a
protective shell of somatic cells called the follicle. At the
beginning of each menstrual cycle, FSH secretion from the anterior
pituitary stimulates a few follicles to mature in a 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. 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 centrosomes.
In males, meiosis occurs during spermatogenesis in the seminiferous
tubules of the testicles.
Meiosis during spermatogenesis is specific
to a type of cell called spermatocytes, which will later mature to
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, 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
In female mammals, meiosis begins immediately after primordial germ
cells migrate to the ovary in the embryo. It is retinoic acid, derived
from the primitive kidney (mesonephros) that stimulates meiosis in
ovarian oogonia. Tissues of the male testis suppress meiosis by
degrading retinoic acid, a stimulator of meiosis. This is overcome at
puberty when cells within seminiferous tubules called Sertoli cells
start making their own retinoic acid.
Compared to mitosis
In order to understand meiosis, a comparison to mitosis is helpful.
The table below shows the differences between meiosis and mitosis.
Normally four cells, each with half the number of chromosomes as the
Two cells, having the same number of chromosomes as the parent
Production of gametes (sex cells) in sexually reproducing eukaryotes
Cellular reproduction, growth, repair, asexual reproduction
Where does it happen?
Reproductive cells of almost all eukaryotes (animals, plants, fungi,
All proliferating cells in all eukaryotes
Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same as parent?
Crossing over happens?
Yes, normally occurs between each pair of homologous chromosomes
Pairing of homologous chromosomes?
Telophase I and
Occurs in Telophase
Does not occur in
Anaphase I, but occurs in
Occurs in Anaphase
Coefficient of coincidence
Biological life cycle
Alternation of generations
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Wikimedia Commons has media related to Meiosis.
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Evolution of sexual reproduction
Sexual reproduction in animals
Cell cycle proteins
A (A1, A2)
B (B1, B2, B3)
D (D1, D2, D3)
E (E1, E2)
INK4a/ARF (p14arf/p16, p15, p18, p19)
cip/kip (p21, p27, p57)
P53 p63 p73 family
Cellular apoptosis susceptibility protein
Maturation promoting factor
Cell cycle checkpoints
Other cellular phases
Sex chromosome (or allosome or heterosome)
Circular chromosome/Linear chromosome
Extra chromosome (or accessory chromosome)
A chromosome/B chromosome
Telomere-binding protein (TINF2)
List of organisms by chromosome count
List of chromosome lengths for various organisms
List of sequenced genomes
International System for Human Cytoge