Meiosis (pronounced /maɪˈoʊsɨs/ ( listen)) is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells.
Whilst the process of meiosis bears a number of similarities with the 'life-cycle' cell division process of mitosis, it differs in two important respects:
Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's mother and one from its father—and produces four haploid cells containing one copy of each chromosome. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, ensuring that offspring are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations, which provides the variation of physical and behavioural attributes (phenotypes) upon which natural selection acts, at a population level, leading to adaptation within the population, resulting in evolution.
Prior to the meiosis process the cell's chromosomes are duplicated by a round of DNA replication, creating a maternal and paternal version of each chromosome (homologs) composed of two exact copies, sister chromatids, attached at the centromere region. In the beginning of meiosis the maternal and paternal homologs pair to each other. Then they typically exchange parts by homologous recombination leading to crossovers of DNA between the maternal and paternal versions of the chromosome. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the center the cell divides into two daughter cells, each containing a haploid number of chromosomes composed of two chromatids. After the recombined maternal and paternal homologs have separated into the two daughter cells, a second round of cell division occurs. There meiosis ends as the two sister chromatids making up each homolog are separated and move into one of the four resulting gamete cells. Upon fertilization, for example when a sperm enters an egg cell, two gamete cells produced by meiosis fuse. The gamete from the mother and the gamete from the father each contribute one half of the set of chromosomes that make up the new offsping's genome.
Meiosis uses many of the same mechanisms as mitosis, a type of cell division used by eukaryotes like plants and animals to split one cell into two identical daughter cells. In all plants and in many protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.