
Meiosis () is a special type of
cell division of
germ cells in
sexually-reproducing organisms that produces the
gametes, the
sperm
Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
or
egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
(
haploid
Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...
). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is
crossed over, creating new combinations of code on each chromosome.
Later on, during
fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
, a cell with two copies of each chromosome.
Errors in meiosis resulting in
aneuploidy (an abnormal number of chromosomes) are the leading known cause of
miscarriage
Miscarriage, also known in medical terms as a spontaneous abortion, is an end to pregnancy resulting in the loss and expulsion of an embryo or fetus from the womb before it can fetal viability, survive independently. Miscarriage before 6 weeks ...
and the most frequent genetic cause of
developmental disabilities
Developmental disability is a diverse group of chronic conditions, comprising mental or physical impairments that arise before adulthood. Developmental disabilities cause individuals living with them many difficulties in certain areas of life, espe ...
.
In meiosis,
DNA replication
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all life, living organisms, acting as the most essential part of heredity, biolog ...
is followed by two rounds of cell division to produce four daughter cells, each with half the number of
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s 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 cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...
, 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 chromosome
Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...
s pair with each other and undergo
genetic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their
genetic information
A nucleic acid sequence is a succession of Nucleobase, bases within the nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. This succession is denoted by a series of a set of five different letters that indicate the orde ...
. A subset of recombination events results in
crossovers, which create physical links known as
chiasmata (singular: chiasma, for the Greek letter
Chi, Χ) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to
segregate away from each other during meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell.
During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
. In some cases, all four of the meiotic products form
gametes such as
sperm
Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
,
spores or
pollen
Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
. In female animals, three of the four meiotic products are typically eliminated by
extrusion
Extrusion is a process used to create objects of a fixed cross section (geometry), cross-sectional profile by pushing material through a Die (manufacturing), die of the desired cross-section. Its two main advantages over other manufacturing pro ...
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
Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
) to form a diploid
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
, with successive generations maintaining the same number of chromosomes. For example,
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...
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
Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...
gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.
Meiosis occurs in all sexually reproducing single-celled and
multicellular
A multicellular organism is an organism that consists of more than one cell (biology), cell, unlike unicellular organisms. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few organism ...
organisms (which are all
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s), including
animal
Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s,
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, and
fungi
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
.
It is an essential process for
oogenesis and
spermatogenesis.
Overview
Although the process of meiosis is related to the more general cell division process of
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
, it differs in two important respects:
Meiosis begins with a diploid cell, which contains two copies of each
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
, termed
homologs. First, the cell undergoes
DNA replication
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all life, living organisms, acting as the most essential part of heredity, biolog ...
, so each homolog now consists of two identical
sister chromatids. Then each set of homologs pair with each other and exchange
genetic information
A nucleic acid sequence is a succession of Nucleobase, bases within the nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. This succession is denoted by a series of a set of five different letters that indicate the orde ...
by
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
often leading to physical connections (
crossovers) between the homologs. In the first meiotic division, the homologs are
segregated to separate daughter cells by the
spindle apparatus
In cell biology, the spindle apparatus is the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter Cell (biology), cells. It is referred to as the mitotic spindle during mitos ...
. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and three small polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
can act.
Meiosis uses many of the same mechanisms as
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
, the type of cell division used by
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s to divide one cell into two identical daughter cells. In some plants, fungi, and
protist
A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...
s meiosis results in the formation of
spore
In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...
s: 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
Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
.
Meiosis does not occur in
archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
or
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
, which generally reproduce asexually via
binary fission
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two values (0 and 1) for each digit
* Binary function, a function that takes two arguments
* Binary operation, a mathematical o ...
. However, a "sexual" process known as
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
involves the transfer of DNA from one bacterium or
archaeon to another and recombination of these DNA molecules of different parental origin.
History
Meiosis was discovered and described for the first time in
sea urchin
Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...
eggs in 1876 by the German biologist
Oscar Hertwig. It was described again in 1883, at the level of
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s, 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
August Friedrich Leopold Weismann (; 17 January 18345 November 1914) was a German evolutionary biology, evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Charl ...
, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911, the
American geneticist
Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly ''
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'', which helped to establish that genetic traits are transmitted on chromosomes.
The term "meiosis" is derived from the Greek word , meaning 'lessening'. It was introduced to biology by
J.B. Farmer and
J.E.S. Moore in 1905, using the idiosyncratic rendering "maiosis":
''We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming''.
The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for
transliterating Greek.
Phases
Meiosis is divided into
meiosis I and
meiosis II which 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.
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 G
1, 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: G
2 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G
2 phase 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 as
interkinesis between meiosis I and meiosis II.
Meiosis I and II are each divided into
prophase,
metaphase,
anaphase, and
telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
During meiosis, specific genes are more highly
transcribed. In addition to strong meiotic stage-specific expression of
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
, 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.
Meiosis I
Meiosis I segregates
homologous chromosome
Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...
s, 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
Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice). During prophase I, homologous maternal and paternal chromosomes pair,
synapse
In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...
, and exchange genetic information (by
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
), forming at least one crossover per chromosome. These crossovers become visible as chiasmata (plural; singular
chiasma).
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 (four
chromatids), 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
=
The first stage of prophase I is the ''leptotene'' stage, also known as ''leptonema'', from Greek words meaning "thin threads".
In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus.
The chromosomes each form a linear array of loops mediated by
cohesin, and the lateral elements of the
synaptonemal complex assemble forming an "axial element" from which the loops emanate.
Recombination is initiated in this stage by the enzyme
SPO11 which creates programmed
double strand breaks (around 300 per meiosis in mice). This process generates single stranded DNA filaments coated by
RAD51 and
DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of ~400 nm in mice).
=Zygotene
=
Leptotene is followed by the ''zygotene'' stage, also known as ''zygonema'', from Greek words meaning "paired threads",
which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. 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 the
synaptonemal complex.
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
=
The ''pachytene'' stage ( ), also known as ''pachynema'', from Greek words meaning "thick threads".
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.
Most breaks are repaired without forming crossovers resulting in
gene conversion. However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in the exchange of genetic information. 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
=
During the ''diplotene'' stage, also known as ''diplonema'', from Greek words meaning "two threads",
the
synaptonemal complex disassembles 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 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.
=Diakinesis
=
Chromosomes condense further during the ''diakinesis'' stage, from Greek words meaning "moving through".
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.
=Meiotic spindle formation
=
Unlike mitotic cells, human and mouse oocytes do not have
centrosome
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progre ...
s to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the
kinetochore. Over time, the MTOCs merge until two poles have formed, generating a barrel shaped spindle. In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules.
Metaphase I
Homologous pairs move together along the
metaphase plate: As
kinetochore microtubules from both
spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of
homologous chromosomes. This attachment is referred to as a bipolar attachment. The physical basis of the
independent assortment of chromosomes is the random orientation of each bivalent along with the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.
The protein complex
cohesin holds
sister chromatids together from the time of their replication until anaphase I. In
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
, 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. ''(see
Chromosome segregation)''
Anaphase I
Kinetochore microtubules shorten, pulling
homologous chromosomes (which each consist of a pair of
sister chromatids) to opposite poles. Non-kinetochore microtubules lengthen, pushing the
centrosomes
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progre ...
farther apart. The cell elongates in preparation for division down the center.
Unlike in
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
, 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.
[Pierce, Benjamin (2009). «Chromosomes and Cell Reproduction». Genetics: A Conceptual Approach, Third Edition. W.H. FREEMAN AND CO. P. 32] This allows the sister chromatids to remain together while homologs are segregated.
Telophase I
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. Sister chromatids remain attached during telophase I.
Cells may enter a period of rest known as
interkinesis or interphase II. No
DNA replication
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all life, living organisms, acting as the most essential part of heredity, biolog ...
occurs during this stage.
Meiosis II
Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of
sister chromatids. Mechanically, the process is similar to
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
, 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) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II.
In prophase II, the disappearance of the
nucleoli and the
nuclear envelope is seen again as well as the shortening and thickening of the chromatids.
Centrosomes
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progre ...
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, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
The process ends with telophase II, which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle.
Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
Meiosis is now complete and ends up with four new daughter cells.
Origin and function
Origin of meiosis
Meiosis appears to be a fundamental characteristic of
eukaryotic organisms and 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. intestinalis'' has now been found to possess a core set of meiotic genes, including five meiosis specific genes. Also evidence for
meiotic recombination, indicative of
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
, was found in ''G. intestinalis''. Another example of organisms previously thought to be asexual are parasitic protozoa of the genus ''
Leishmania
''Leishmania'' () is a genus of parasitic protozoans, single-celled eukaryotic organisms of the trypanosomatid group that are responsible for the disease leishmaniasis. The parasites are transmitted by sandflies of the genus '' Phlebotomus'' ...
'', which cause human disease. However, these organisms were shown to have a sexual cycle consistent with a meiotic process. Although
amoeba were 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. Dacks and Rogers proposed, based on a phylogenetic analysis, that facultative sex was likely present in the common ancestor of eukaryotes.
Genetic variation
The new combinations of DNA created during meiosis are a significant source of
genetic variation alongside mutation, resulting in new combinations of
alleles, 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); and (2)
Crossing Over. The physical exchange of homologous chromosomal regions by homologous
recombination during prophase I results in new combinations of genetic information within chromosomes. However, such physical exchange does not always occur during meiosis. In the oocytes of the silkworm ''
Bombyx mori'', meiosis is completely
achiasmate (lacking crossovers). Although
synaptonemal complexes are present during the
pachytene stage of meiosis in ''B. mori'', crossing-over
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
is absent between the paired
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s.
Prophase I arrest
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these
oocytes are arrested at the prophase I stage of meiosis.
In humans, as an example, oocytes are formed between three and four months of
gestation
Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...
within the fetus and are therefore present at birth. During this prophase I arrested stage (
dictyate), which may last for decades, four copies of the
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to
repair damage in the DNA of the
germline.
[ The repair process used appears to involve ]homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
al repair Prophase I arrested oocytes have a high capability for efficient repair of DNA damage, particularly exogenously induced double-strand breaks.[ DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of ]fertility
Fertility in colloquial terms refers the ability to have offspring. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to reproduce, which is termed fecundity. The fertility rate ...
.[
]
Meiosis as an adaptation for repairing germline DNA
Genetic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
can be viewed as fundamentally a DNA repair
DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...
process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
that is passed on to progeny. Experimental findings indicate that a substantial benefit of meiosis is recombinational repair of DNA damage in the germline, as indicated by the following examples. Hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
is an agent that causes oxidative stress leading to oxidative DNA damage. Treatment of the yeast '' Schizosaccharomyces pombe'' with hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
increased the frequency of mating and the formation of meiotic spores by 4 to 18-fold. '' Volvox carteri'', a haploid multicellular, facultatively sexual green algae, can be induced by heat shock to reproduce by meiotic sex. This induction can be inhibited by antioxidant
Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...
s indicating that the induction of meiotic sex by heat shock is likely mediated by oxidative stress leading to increased DNA damage.
Occurrence
In life cycles
Meiosis occurs in eukaryotic life cycles involving sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
, consisting of the cyclical process of growth and development by mitotic cell division, production of gametes by meiosis and fertilization. At certain stages of the life cycle, germ cell
A germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they unde ...
s produce gametes. Somatic cells make 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 (''diplontic'' life cycle), during the haploid state (''haplontic'' life cycle), or both (''haplodiplontic'' life cycle), in which there are two distinct organism phases, one with haploid cells and the other with diploid cells.
In the ''diplontic life cycle'' (with pre-gametic meiosis), as in humans, the organism is multicellular and diploid, grown by mitosis from a diploid cell called the zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
. The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes (the spermatozoa in males and ova in females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
to grow into the organism.
In the ''haplontic life cycle'' (with post-zygotic meiosis), the organism is haploid, by the proliferation and differentiation of a single haploid cell called the gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
. 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
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
and many protozoa utilize the haplontic life cycle.
In the ''haplodiplontic life cycle'' (with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to produce a new diploid organism. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles.
In plants and animals
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 an alternation of generations such that meiosis in the diploid sporophyte
A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...
generation produces haploid spores instead of gametes. When they germinate, these spores undergo repeated cell division by mitosis, developing into a multicellular haploid gametophyte
A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...
generation, 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 a zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
in which the original number of chromosomes is restored.
In mammals
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 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 the follicle. At this stage, the oocyte nucleus is called the germinal vesicle. At the beginning of each menstrual cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible. The ovarian cycle controls the production and release of eg ...
, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. The breakdown of the germinal vesicle, condensation of chromosomes, and assembly of the bipolar metaphase I spindle are all clear indications that meiosis has resumed. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while the others undergo atresia. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the dictyate stage and lacks the assistance of centrosome
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progre ...
s.
In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicle
A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is Homology (biology), homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of Androgen, androgens, p ...
s. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes, which will later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL. Genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later in spermatocytes undergoing meiosis, however retinoic acid is not required during the time when meiosis initiates.
In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. Some studies suggest that retinoic acid derived from the primitive kidney (mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid. However, genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis or male meiosis which initiates postnatally.
Flagellates
While the majority of eukaryotes have a two-divisional meiosis (though sometimes achiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalid
The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiosis, symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and ...
s and oxymonad
The Oxymonads (or Oxymonadida) are a group of flagellated protists found exclusively in the intestines of animals, mostly termites and other Xylophagy, wood-eating insects. Along with the similar parabasalid flagellates, they harbor the Symbiosi ...
s) from the gut of the wood-feeding cockroach '' Cryptocercus''.
Role in human genetics and disease
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). 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, which would result in only 1-2 cross-overs per human chromosome.
In humans, recombination rates differ between maternal and paternal DNA:
* Maternal DNA: Recombines approximately 42 times on average.
* Paternal DNA: Recombines approximately 27 times on average.
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
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
.
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 – an extra X chromosome in females
* Jacobs 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 time.
Comparison to mitosis
In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis.
Molecular regulation
Maturation promoting factor (MPF) seems to have a role in meiosis based on experiments with ''Xenopus laevis'' oocytes. Mammalian oocyte MPF induced germinal vesicle breakdown (GVB) in starfish and ''Xenopus laevis'' oocytes. MPF is active prior to GVB but falls off toward the end of meiosis I. CDK1 and cyclin B levels are correlated with oocyte GVB competence and are likely under translational rather than transcriptional control. In meiosis II, MPF reappears ahead of metaphase II, and its activity remains high up to fertilization.
In mammals, meiotic arrest begins with natriuretic peptide type C (NPPC) from mural granulosa cells, which activates production of cyclic guanosine 3′,5′-monophosphate (cGMP) in concert with natriuretic peptide receptor 2 (NPR2) on cumulus cells. cGMP diffuses into oocytes and halts meiosis by inhibiting phosphodiesterase 3A (PDE3A) and cyclic adenosine 3′,5′-monophosphate (cAMP) hydrolysis. In the oocyte, G-protein-coupled receptor GPR3/12 activates adenylyl cyclase to generate cAMP. cAMP stimulates protein kinase A (PKA) to activate the nuclear kinase WEE2 by phosphorylation. PKA also assists in phosphorylation of the CDK1 phosphatase CDC25B to keep it in the cytoplasm; in its unphosphorylated form, CDC25B migrates to the nucleus. Protein kinase C (PKC) may also have a role in inhibiting meiotic progression to metaphase II. Overall, CDK1 activity is suppressed to prevent resumption of meiosis. Oocytes further promote expression of NPR2 and inosine monophosphate dehydrogenase (and thereby the production of cGMP) in cumulus cells. Follicle-stimulating hormone and estradiol likewise promote expression of NPPC and NPR2. Hypoxanthine, a purine apparently originating in the follicle, also inhibits in vitro oocyte meiosis. A spike in luteinizing hormone (LH) spurs oocyte maturation, in which oocytes are released from meiotic arrest and progress from prophase I through metaphase II. LH-induced epidermal growth factor-like factors like amphiregulin and epiregulin synthesized in mural granulosa cells reduce levels of cGMP in oocytes by restricting cGMP transport through cumulus cell-oocyte gap junctions and lowering NPPC levels and NPR2 activity. In fact, LH-induced epidermal growth factor-like factors may cause the destabilization and breakdown of gap junctions altogether. LH-induced epidermal growth factor-like factors may trigger production of additional oocyte maturation factors like steroids and follicular fluid-derived meiosis-activating sterol (FF-MAS) in cumulus cells. FF-MAS promotes progression from metaphase I to metaphase II, and it may help stabilize metaphase II arrest. Meiosis resumption is reinforced by the exit of WEE2 from the nucleus due to CDK1 activation. Phosphodiesterases (PDEs) metabolize cAMP and may be temporarily activated by PKA-mediated phosphorylation. Longer-term regulation of phosphodiesterases may require modulation of protein expression. For example, hypoxanthine is a PDE inhibitor that may stymie cAMP metabolism. Kinases like protein kinase B, Aurora kinase A, and polo-like kinase 1 contribute to the resumption of meiosis. There are similarities between the mechanisms of meiotic prophase I arrest and resumption and the mitotic G2 DNA damage checkpoint: CDC14B-based activation of APC-CDH1 in arrest and CDC25B-based resumption. Meiotic arrest requires inhibitory phosphorylation of CDK1 at amino acid residues Thr-14 and Tyr-15 by MYT1 and WEE1 as well as regulation of cyclin B levels facilitated by the anaphase-promoting complex (APC). CDK1 is regulated by cyclin B, whose synthesis peaks at the end of meiosis I. At anaphase I, cyclin B is degraded by an ubiquitin-dependent pathway. Cyclin B synthesis and CDK1 activation prompt oocytes to enter metaphase, while entry into anaphase follows ubiquitin-mediated cyclin B degradation, which brings down CDK1 activity. Proteolysis of adhesion proteins between homologous chromosomes is involved in anaphase I, while proteolysis of adhesion proteins between sister chromatids is involved in anaphase II. Meiosis II arrest is effected by cytostatic factor (CSF), whose elements include the MOS protein, mitogen-activated protein kinase kinase (MAPKK/MEK1), and MAPK. The protein kinase p90 (RSK) is one critical target of MAPK and may help block entry into S-phase between meiosis I and II by reactivating CDK1. There's evidence that RSK aids entry into meiosis I by inhibiting MYT1, which activates CDK1. CSF arrest might take place through regulation of the APC as part of the spindle assembly checkpoint.
In the budding yeast ''S. cerevisiae'', Clb1 is the main meiotic regulatory cyclin, though Clb3 and Clb4 are also expressed during meiosis and activate a p34cdc28-associated kinase immediately prior to the first meiotic division. The IME1 transcription factor drives entry into meiotic S-phase and is regulated according to inputs like nutrition. a1/α2 represses a repressor of ''IME1'', initiating meiosis. Numerous ''S. cerevisiae'' meiotic regulatory genes have been identified. A few are presented here. ''IME1'' enables sporulation of non-a/α diploids. ''IME2''/''SME1'' enables sporulation when nitrogen is present, supports recombination in a/α cells expressing ''RME1'', an inhibitor of meiosis, and encodes a protein kinase homolog. ''MCK1'' (meiosis and centromere regulatory kinase) also supports recombination in a/α cells expressing ''RME1'' and encodes a protein kinase homolog. ''SME2'' enables sporulation when ammonia or glucose are present. ''UME1-5'' enable expression of certain early meiotic genes in vegetative, non-a/α cells.
In the fission yeast ''S. pombe'', the Cdc2 kinase and Cig2 cyclin together initiate the premeiotic S phase, while cyclin Cdc13 and the CDK activator Cdc25 are necessary for both meiotic divisions. However, the Pat1-Mei2 system is at the heart of ''S. pombe'' meiotic regulation. Mei2 is the major meiotic regulator. It moves between the nucleus and cytoplasm and works with meiRNA to promote meiosis I. Moreover, Mei2 is implicated in exit from mitosis and induction of premeiotic S phase. Mei2 may inactivate the DSR-Mmi1 system through sequestration of Mmi1 to stabilize meiosis-specific transcript expression. Mei2 may stall growth and bring about G1 arrest. Pat1 is a Ser/Thr protein kinase that phosphorylates Mei2, an RNA-binding protein, on residues Ser438 and Thr527. This phosphorylation may decrease the half-life of Mei2 by making it more likely to be destroyed by a proteasome working with E2 Ubc2 and E3 Ubr1. The Mei4 transcription factor is necessary to transcriptionally activate ''cdc25'' in meiosis, and the ''mei4'' mutant experiences cell cycle arrest. Mes1 inhibits the APC/C activator Slp1 such that the Cdc2-Cdc13 MPF activity can drive the second meiotic division.
It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I.
Meiotic recombination is mediated through double stranded break, which is catalyzed by Spo11 protein. Also Mre11, Sae2 and Exo1 play role in breakage and recombination. After the breakage happen, recombination take place which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second one gives to noncrossover product).
Seemingly there are checkpoints for meiotic cell division too. In S. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is thought to form a checkpoint.
In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II.
See also
* Fertilisation
* Coefficient of coincidence
* DNA repair
DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...
* Oxidative stress
* Synizesis (biology)
* Biological life cycle
* Apomixis
* Parthenogenesis
Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
* Alternation of generations
* Brachymeiosis
* Mitotic recombination
* Dikaryon
The dikaryon (karyogamy) is a cell nucleus feature that is unique to certain fungi. (The green alga '' Derbesia'' had been long considered an exception, until the heterokaryotic hypothesis was challenged by later studies.) Compatible cell-types c ...
* Mating of yeast
The mating of yeast, also known as yeast sexual reproduction, is a biological process that promotes genetic diversity and adaptation in yeast species. Yeast species, such as ''Saccharomyces cerevisiae'' (baker's yeast), are single-celled eukaryot ...
References
Footnotes
Citations
Cited texts
*
External links
Meiosis Flash Animation
Khan Academy, video lecture
CCO
The Cell-Cycle Ontology
Stages of Meiosis animation
*
"Abby Dernburg Seminar: Chromosome Dynamics During Meiosis"
{{Authority control
Cellular processes
Cell cycle
Molecular genetics
Plant sexuality
Articles containing video clips
1876 in science