meiosis

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In biology, a process of cell division in which the number of chromosomes in the cell is halved. It only occurs in eukaryotic cells, and is part of a life cycle that involves sexual reproduction because it allows the genes of two parents to be combined without the total number of chromosomes increasing. Cells in reproductive organs – testes and ovaries in humans – divide to form sex cells (gametes) by meiosis.

In meiosis, the nucleus of a cell divides twice. A single cell produces four cells by the end of meiosis. In sexually reproducing diploid animals (having two sets of chromosomes per cell), meiosis occurs during formation of the gametes (sex cells – see sperm and ovum), so that the gametes are haploid (having only one set of chromosomes – half the number of chromosomes of the parent cell). When the gametes unite during fertilization, the diploid condition is restored.

In plants, meiosis occurs just before spore formation. Thus the spores are haploid and in lower plants such as mosses they develop into a haploid plant called a gametophyte which produces the gametes (see alternation of generations). See also mitosis.

Meiosis gives rise to variation. This is an important part of sexual reproduction. The variation produced is inherited, which means that evolution can take place as a result of the natural selection of certain variants to suit a changing environment. The way that meiosis gives rise to variation is by recombining genes from chromosomes in new ways. When the number of chromosomes is halved, there is some randomness in the way parts of chromosomes are selected to go into the gametes.