Meiosis
Meiosis (; from Greek μείωσις, "meiosis", meaning "lessening", because it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). 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 female will fuse to create a cell with two copies of each chromosome again, the zygote.

Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.

In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi (X)) between the homologous chromosomes. In most organisms, these links 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. In some cases, all four of the meiotic products form gametes such as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human
cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.

Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi. It is an essential process for oogenesis and spermatogenesis.
Q9129
Greek
Concept
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cell division
Concept
Q281579
germ cells
Concept
Q182353
sexually-reproducing
Concept
Q211050
gametes
Concept
Q17145
sperm
Concept
Q1321695
egg cells
Concept
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chromosome
Concept
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haploid
Concept
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crossed over
Concept
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fertilisation
Concept
Q170145
zygote
Concept
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aneuploidy
Concept
Q28693
miscarriage
Concept
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developmental disabilities
Concept
Q130996
DNA replication
Concept
Q37748
chromosome
Concept
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S phase
Concept
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cell cycle
Concept
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sister chromatids
UNK
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G2
Concept
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prophase
Concept
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homologous chromosome
Concept
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genetic recombination
Concept
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genetic information
Concept
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crossovers
Concept
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chiasmata
UNK
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Chi (X)
Concept
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segregate
UNK
Q118406
haploid
Concept
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mitosis
Concept
Q211050
gametes
Concept
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sperm
Concept
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spores
Concept
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pollen
Concept
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extrusion
Concept
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polar bodies
UNK
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ovum
Concept
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fertilization
Concept
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zygote
Concept
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sexual reproduction
Concept
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diploid
Concept
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haploid
Concept
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multicellular
Concept
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eukaryote
Concept
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animal
Concept
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plant
Concept
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fungi
Concept
Q507032
oogenesis
UNK
Q36961
spermatogenesis
UNK
Q21100485
Meiosis
Concept
Q21100485
Meiosis
Concept
Q21100485
Meiosis
Concept
Q21100485
Meiosis
Concept
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