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Meiosis (; , because it is a reductional division) is a special type of
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
of
germ cells A germ cell is any biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to a ...
in sexually-reproducing organisms used to produce the
gametes A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the foll ...
, such as
sperm Sperm 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 with a tail known as ...

sperm
or
egg cells Diagram of a chicken egg in its 9th day. Membranes: allantois, chorion, amnion, and vitellus/ yolk. An egg is the organic vessel containing the zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγο ...
. It involves two rounds of division that ultimately result in four cells with only one copy of each
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
(
haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is
crossed over Crossed may refer to: * Crossed (comics), ''Crossed'' (comics), a 2008 comic book series by Garth Ennis * Crossed (novel), ''Crossed'' (novel), a 2010 young adult novel by Ally Condie * Crossed (The Walking Dead), "Crossed" (''The Walking Dead''), ...
, creating new combinations of code on each chromosome. Later on, during
fertilisation Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilisation
, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
. Errors in meiosis resulting in
aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell (biology), cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more ploidy#Haploid and monoploid, c ...
(an abnormal number of chromosomes) are the leading known cause of
miscarriage Miscarriage, also known in medical terms as a spontaneous abortion and pregnancy loss, is the natural loss of an embryo or fetus before it is fetal viability, able to survive independently. Some use the cutoff of 20 weeks of gestation, after whi ...
and the most frequent genetic cause of
developmental disabilities Developmental disability is a diverse group of chronic conditions that are due to mental or physical impairments that arise before adulthood. Developmental disabilities cause individuals living with them many difficulties in certain areas of life, ...
. In meiosis,
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
is followed by two rounds of cell division to produce four daughter cells, each with half the number of
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during
S phase S phase (Synthesis Phase) is the phase of the cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may ...
of the
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication In , DNA replication is the of pro ...

cell cycle
, the DNA of each chromosome is replicated so that it consists of two identical
sister chromatids A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the dup ...
, 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 Prophase () is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the Cell (biology), cell enters prophase. The main occurrences in prophase are the condensation of the ...

prophase
. During this time,
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pen ...
s pair with each other and undergo
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...
, 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 bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monom ...
. 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 can help direct each pair of homologous chromosomes to segregate away from each other during Meiosis I, resulting in two
haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
cells that have half the number of chromosomes as the parent cell. During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
. In some cases, all four of the meiotic products form
gametes A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the foll ...
such as
sperm Sperm 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 with a tail known as ...

sperm
,
spores In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...
or
pollen Pollen is a powdery substance consisting of pollen grains which are microsporophytes of seed plants The spermatophytes (; ), also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plant Plant ...

pollen
. 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 ...

extrusion
into polar bodies, and only one cell develops to produce an
ovum The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduct ...

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.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
) to form a diploid
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
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 Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
, with successive generations maintaining the same number of chromosomes. For example,
diploid 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. Sets of chromosomes refer to the number of mate ...
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 chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular ...
organisms (which are all
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s), including
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s,
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s and
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
. It is an essential process for
oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...

oogenesis
and
spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the Mitosis, mitotic division of the stem cells located close to the basement membrane of the ...

spermatogenesis
.


Overview

Although the process of meiosis is related to the more general cell division process of
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
, it differs in two important respects: Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed . First, the cell undergoes
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
, so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by
homologous recombination 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 ...

homologous recombination
often leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the
spindle apparatus In cell biology, the spindle apparatus (or mitotic spindle) refers to 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 m ...

spindle apparatus
. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and two 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 right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
can act. Meiosis uses many of the same mechanisms as
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
, the type of cell division used by
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s to divide one cell into two identical daughter cells. In some plants, fungi, and
protist A protist () is any eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are c ...
s meiosis results in the formation of
spore )'', growing on a thinning, thinned hybrid black poplar ''(populus, Populus x canadensis)''. The last stage of the moss#Life cycle, moss lifecycle is shown, where the sporophytes are visible before dispersion of their spores: the calyptra (1) is ...
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 grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction Asexual reproduction is a typ ...
. Meiosis does not occur in
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
or
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
, which generally reproduce asexually via
binary fission Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineeri ...

binary fission
. 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 unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
involves the transfer of DNA from one bacterium or
archaeon Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

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 () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone — from the intertidal seashore down to . The spherical, ha ...

sea urchin
egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do not. eg ...
s in 1876 by the German biologist
Oscar Hertwig Illustration from O. Hertwig's book ''Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbeltiere'' (Textbook of developmental history of humans and vertebrates), 1906. Oscar Hertwig (21 April 1849 in Friedberg – 25 October 1922 in B ...
. It was described again in 1883, at the level of
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s, by the Belgian zoologist
Edouard Van Beneden Édouard Joseph Louis Marie Van Beneden (5 March 1846 in Leuven – 28 April 1910 in Liège), son of Pierre-Joseph Van Beneden, was a Belgian embryologist, cytologist and marine biologist. He was professor of zoology Zoology ()The pronunciat ...
, in ''
Ascaris ''Ascaris'' is a genus of parasite, parasitic nematode worms known as the "small intestinal roundworms", which is a type of parasitic worm. One species, ''Ascaris lumbricoides'', affects humans and causes the disease ascariasis. Another species, ...

Ascaris
'' roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist
August Weismann Prof August Friedrich Leopold Weismann FRS (For), HonFRSE, LLD (17 January 18345 November 1914) was a German evolutionary biologist. Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading ...

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 American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is ...

American
geneticist
Thomas Hunt Morgan Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role t ...

Thomas Hunt Morgan
detected crossovers in meiosis in the fruit 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 ''.
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 and Cytokinesis I and 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 Interphase is the portion of the cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA ...

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 chromatid 300px, The paternal (blue) chromosome and the maternal (pink) chromosome are homologous chromosomes. Following chromosomal DNA replication, the blue chromosome is composed of two identical sister chromatids and the pink chromosome is composed of t ...
s attached at a centromere. This replication does not change the
ploidy Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, wh ...
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. 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 Prophase () is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the Cell (biology), cell enters prophase. The main occurrences in prophase are the condensation of the ...

prophase
,
metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical s ...

metaphase
,
anaphase Anaphase (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

anaphase
, and
telophase Fluorescence micrograph of a human cell in telophase showing chromosomes (DNA) in blue, microtubules in green and kinetochores in pink Telophase (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el ...

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 i ...

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.


Meiosis I

Meiosis I segregates
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pen ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiol ...
, 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 ...

homologous recombination
), 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 In the diagram, (1) refers to a chromatid: 1-half of two identical threadlike strands of a replicated sister chromatid pair") are joined at the region called the centromere">ister_chromatids.html" ;"title="chromosome. During cell division, the ide ...
), 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 Cohesin is a protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnik ...

cohesin
, and the lateral elements of the
synaptonemal complex The synaptonemal complex (SC) is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

synaptonemal complex
assemble forming an "axial element" from which the loops emanate. Recombination is initiated in this stage by the enzyme
SPO11 Spo11 is a protein that in humans is encoded by the ''SPO11'' gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic genetic recombination, recombination. Its active site contains a tyrosine which ligates and diss ...
which creates programmed
double strand breaks DNA repair is a collection of processes by which a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...
(around 300 per meiosis in mice). This process generates single stranded DNA filaments coated by
RAD51 DNA repair protein RAD51 homolog 1 is a protein encoded by the gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language ...
and
DMC1 Meiotic recombination protein DMC1/LIM15 homolog is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one ...
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 The synaptonemal complex (SC) is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

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 conversionGene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces anot ...
. However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in the exchange of genetic information.
Sex chromosomes A sex chromosome, (also referred to as an allosome, heterotypical chromosome, gonosome, or heterochromosome, or idiochromosome) is a chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. M ...
, however, are not wholly identical, and only exchange information over a small region of homology called the
pseudoautosomal region Image:A region in the pseudoautosomal region of the short arms of the X- and Y-chromosome.jpg, Detail of a human metaphase spread. A region in the pseudoautosomal region of the short arms of the X chromosome (left) and the Y chromosome (top right) w ...

pseudoautosomal region
. 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 The synaptonemal complex (SC) is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

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 Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...

oogenesis
, all developing oocytes develop to this stage and are arrested in prophase I before birth. This suspended state is referred to as the 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 Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryote, eukaryotic Somatic (biology), somatic Cell (biology), cells. In prometaphase, the nuclear membrane breaks apart into numerous "membrane vesicles", a ...

prometaphase
of mitosis; the
nucleoli The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis. Nucleoli also participate in the formation of signal recognition particles and play a role ...

nucleoli
disappear, the
nuclear membrane The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a c ...

nuclear membrane
disintegrates into vesicles, and the begins to form.


=Meiotic spindle formation

= Unlike mitotic cells, human and mouse oocytes do not have
centrosome In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...

centrosome
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 A kinetochore (, ) is a disc-shaped protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to anoth ...

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 Mendelian inheritance is a type of biology, biological inheritance (biology), inheritance that follows the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 and popularized by William Bateson. These princi ...

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 Cohesin is a protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnik ...

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 ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see
Chromosome segregationChromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the Cell nucleus, nucleus. This ...
).


Anaphase I

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents the sister chromatids from separating.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. 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. 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 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, 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 The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electr ...

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, 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

The new combinations of DNA created during meiosis are a significant source of
genetic variation thumb File:Genetic Variation and Inheritance.svg, Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits t ...

genetic variation
alongside mutation, resulting in new combinations of
alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...

alleles
, which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1)
Law of Independent Assortment Mendelian inheritance is a type of biological Biology is the natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific me ...
. 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 during prophase I results in new combinations of genetic information within chromosomes.


Prophase I arrest

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these
oocyte An oocyte (, ), oöcyte, ovocyte, or rarely ocyte, is a female gametocyte A gametocyte is a eukaryotic germ cell that divides by mitosis into other gametocytes or by meiosis into gametids during gametogenesis. Male gametocytes are called ''sperm ...
s 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 Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking#Development, Filmmaking, development phase, including finance and budgeting *Development ...
within the fetus and are therefore present at birth. During this prophase I arrested stage (
dictyate The dictyate or dictyoteneMedical Physiology, Boron & Boulpaep, , Elsevier Saunders 2005. Updated edition. 1300 pages. is a prolonged resting phase in oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) in ...
), which may last for decades, four copies of the
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to of the
germline In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
. 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 ...

homologous recombination
al repair Prophase I arrested oocytes have a high capability for efficient repair of DNA damages, 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 is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified Demography, demographicall ...
.


Occurrence


In life cycles

Meiosis occurs in eukaryotic
life cycles Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...
involving
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal
mitotic In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In gene ...

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 cell A germ cell is any biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to a ...
s produce gametes.
Somatic cell A somatic cell (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following period ...
s make up the body of the organism and are not involved in gamete production. 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 (''diplontic'' life cycle), during the haploid state (''haplontic'' life cycle), or both (''haplodiplontic'' 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 phase(s). In the ''diplontic life cycle'' (with pre-gametic meiosis), of which humans are a part, the organism is diploid, grown from a diploid cell called the
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the
spermatozoa A spermatozoon (pronounced , alternate spelling spermatozoön; plural spermatozoa; from grc, σπέρμα ("seed") and grc, ζῷον ("living being")) is a motile Motility is the ability of an organism In biology, an organism (from An ...

spermatozoa
for males and
ova , abbreviated as OVA and sometimes as OAV (original animation video), are Japanese animated films and series made specially for release in home video Home video is prerecorded media sold or Video rental shop, rented for home viewing. The ter ...

ova
for females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
to grow into the organism. In the ''haplontic life cycle'' (with post-zygotic meiosis), the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
. 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 In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
to create the organism. Many
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
and many
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that ...

protozoa
utilize the haplontic life cycle. Finally, 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 Alternation of generations (also known as metagenesis or heterogenesis) is the type of that occurs in those s and in the and the that have distinct haploid sexual and diploid asexual stages. In these groups, a with ''n'' s alternates with ...

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 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 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 Alternation of generations (also known as metagenesis or heterogenesis) is the type of that occurs in those s and in the and the that have distinct haploid sexual and diploid asexual stages. In these groups, a with ''n'' s alternates with ...

alternation of generations
such that meiosis in the diploid
sporophyte A sporophyte () is the diploid 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. Sets of chr ...
generation produces haploid spores. These spores multiply by mitosis, developing into the haploid
gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all ...
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 mammals

In females, meiosis occurs in cells known as
oocyte An oocyte (, ), oöcyte, ovocyte, or rarely ocyte, is a female gametocyte A gametocyte is a eukaryotic germ cell that divides by mitosis into other gametocytes or by meiosis into gametids during gametogenesis. Male gametocytes are called ''sperm ...
s (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 The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduct ...
. 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 . At the beginning of each
menstrual cycle The menstrual cycle is a series of natural changes in hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country l ...

menstrual cycle
, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as
folliculogenesis :''Although the process is similar in many animals, this article will deal exclusively with human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, ...
. 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
atresiaAtresia is a condition in which an orifice or passage in the body is (usually abnormally) closed or absent. Examples of atresia include: * Aural atresia, a congenital deformity where the ear canal is underdeveloped. * Biliary atresia, a condition i ...
. The process of meiosis in females occurs during
oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...

oogenesis
, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the
dictyate The dictyate or dictyoteneMedical Physiology, Boron & Boulpaep, , Elsevier Saunders 2005. Updated edition. 1300 pages. is a prolonged resting phase in oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) in ...
stage and lacks the assistance of
centrosome In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...

centrosome
s. In males, meiosis occurs during
spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the Mitosis, mitotic division of the stem cells located close to the basement membrane of the ...

spermatogenesis
in the
seminiferous tubule Seminiferous tubules are located within the testes Testicle or testis (plural testes) is the or in all animals, including humans. It is to the female . The functions of the testes are to produce both and , primarily . Testosterone rele ...
s of the
testicle Testicle or testis (plural testes) is the male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male or ...

testicle
s. Meiosis during spermatogenesis is specific to a type of cell called
spermatocyte Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and se ...
s, which will later mature to become
spermatozoa A spermatozoon (pronounced , alternate spelling spermatozoön; plural spermatozoa; from grc, σπέρμα ("seed") and grc, ζῷον ("living being")) is a motile Motility is the ability of an organism In biology, an organism (from An ...
. 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 Female (symbol: ♀) is the sex of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a s ...
, 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 ( parabasalids and oxymonads) from the gut of the wood-feeding cockroach ''
Cryptocercus ''Cryptocercus'' is a genus of Dictyoptera (cockroaches and allies) and the sole member of its own family Cryptocercidae. Species are known as wood roaches or brown-hooded cockroaches. These roaches are subsocial, their young requiring considerabl ...
''.


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.


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 A trisomy is a type of polysomy in which there are three instances of a particular chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling mic ...
or
monosomy Monosomy is a form of aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell (biology), cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or mor ...
. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

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 Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome In the fields of molecular biology and genetics Gen ...
– trisomy of chromosome 21 *
Patau syndrome Patau syndrome is a syndrome caused by a chromosomal abnormality, in which some or all of the cells of the body contain extra genetic material from chromosome 13. The extra genetic material disrupts normal development, causing multiple and comp ...
– trisomy of chromosome 13 *
Edwards syndrome Edwards syndrome, also known as trisomy 18, is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene In biology, a gene (from ''geno ...
– trisomy of chromosome 18 *
Klinefelter syndrome Klinefelter syndrome (KS), also known as 47,XXY is a syndrome where a male has an additional copy of the X chromosome. The primary features are infertility and small poorly functioning testicles. Often, symptoms are subtle and subjects do not ...
– extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc. *
Turner syndrome Turner syndrome (TS), also known 45,X, or 45,X0, is a genetic condition in which a female is partly or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hairline at ...
– lacking of one X chromosome in females – i.e. X0 *
Triple X syndrome Trisomy X, also known as triple X syndrome and characterized by the karyotype 47,XXX, is a chromosome disorder in which a female has an extra copy of the X chromosome. It is relatively common and occurs in 1 in 1,000 women but it is rarely dia ...
– 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 Cohesin is a protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnik ...

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

How a cell proceeds to meiotic division in meiotic cell division is not well known. Maturation promoting factor (MPF) seemingly have role in frog Oocyte meiosis. In the fungus ''S. pombe''. there is a role of MeiRNA binding protein for entry to meiotic cell 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 Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

Fertilisation
* Coefficient of coincidence *
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene mol ...

DNA repair
*
Oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species Reactive oxygen species (ROS) are highly chemicals formed from O2. Examples of ROS include s, , , , and . The reduction of molecular oxygen ...
* Synizesis (biology) *Biological life cycle *Apomixis *Parthenogenesis *Alternation of generations *Brachymeiosis *Mitotic recombination *Dikaryon *Mating of yeast


References


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 Meiosis, Cellular processes Cell cycle Molecular genetics Plant sexuality Articles containing video clips 1876 in science