Meiosis
i/maɪˈoʊsɨs/ is a special type of cell division necessary for sexual
reproduction which occurs or has occurred in all eukaryotes, including
animals, plants and fungi, including both multi-celled and single-celled
organisms.[1][2][3][4] The number of sets of chromosomes in the cell
undergoing meiosis is reduced to half the original number, typically
from two sets (diploid) to one set (haploid). The cells produced by
meiosis are either gametes (the usual case in animals) or otherwise
usually spores from which gametes are ultimately produced (the case in
land plants). In many organisms, including all animals and land plants
(but not some other groups such as fungi), gametes are called sperm in
males and egg cells or ova in females. Since meiosis has halved the
number of sets of chromosomes, when two gametes fuse during
fertilisation, the number of sets of chromosomes in the resulting zygote
is restored to the original number.
Meiotic division occurs in two stages, meiosis I and meiosis II, dividing the cells once at each stage. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated, so that each chromosome has two sister chromatids; a diploid organism now has a tetraploid DNA amount in the cell.
The first stage of meiosis begins with a cell that has (if it is from a diploid organism) two copies of each type of chromosome, one from each of the mother and father, called homologous chromosomes, each of which has two sister chromatids. The homologous chromosomes pair up and may exchange genetic material with each other in a process called crossing-over. Each pair then separates as two cells are formed, each with one chromosome (two chromatids) from every homologous pair. The chromatids composing a chromosome may differ from one another if crossing-over occurred. The chromosomes present in each of the two cells will be complementary subsets from the original set, some originally from the mother and some originally from the father.
In the second stage, each chromosome splits into two; each half, each sister chromatid, is separated into two new cells, which are haploid. (Note: The instant the “sister chromatids”
Meiotic division occurs in two stages, meiosis I and meiosis II, dividing the cells once at each stage. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated, so that each chromosome has two sister chromatids; a diploid organism now has a tetraploid DNA amount in the cell.
The first stage of meiosis begins with a cell that has (if it is from a diploid organism) two copies of each type of chromosome, one from each of the mother and father, called homologous chromosomes, each of which has two sister chromatids. The homologous chromosomes pair up and may exchange genetic material with each other in a process called crossing-over. Each pair then separates as two cells are formed, each with one chromosome (two chromatids) from every homologous pair. The chromatids composing a chromosome may differ from one another if crossing-over occurred. The chromosomes present in each of the two cells will be complementary subsets from the original set, some originally from the mother and some originally from the father.
In the second stage, each chromosome splits into two; each half, each sister chromatid, is separated into two new cells, which are haploid. (Note: The instant the “sister chromatids”
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