Meiosis

Meiosis is a nuclear division that reduces chromosome-set number. In animals, it occurs in germ cells and contributes to the formation of haploid gametes such as eggs and sperm. A haploid cell has one set of chromosomes; a diploid cell has two. Fertilization joins haploid gametes to form a diploid zygote, helping maintain the chromosome number across generations. In plants and some other life cycles, meiosis instead produces haploid spores, which can develop into a haploid generation; the details depend on the organism.

Before meiosis begins, chromosomes are replicated once during interphase. Meiosis then proceeds through two nuclear divisions without an intervening round of DNA replication. During meiosis I, homologous chromosomes—one of each pair inherited from each genetic parent—pair and then separate into different cells, reducing the number of chromosome sets. During meiosis II, sister chromatids of each chromosome separate, in a process broadly comparable to chromosome separation in mitosis. The resulting nuclei are haploid. The exact cellular products vary among organisms; for example, animal egg formation is not always an equal division into four functional gametes.

During prophase I, homologous chromosomes pair. Nonsister chromatids can exchange corresponding DNA segments in crossing over, also called recombination. The homologous pairs also orient independently of one another at the cell's middle before separating in meiosis I. Recombination and this independent assortment produce new combinations of genetic material in the resulting haploid cells. These are important sources of variation, alongside other processes such as random fertilization in organisms that reproduce sexually.

Mitosis usually involves one nuclear division and maintains chromosome-set number, producing nuclei genetically similar to the starting nucleus. Meiosis has two divisions after a single replication and reduces chromosome-set number. Mitosis is commonly used for growth and cell replacement; meiosis is part of sexual life cycles, producing haploid cells directly or indirectly. Both rely on organized chromosome movement, but homologous pairing and recombination in prophase I are characteristic features of meiosis.