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Meiosis (pronounced /maɪˈoʊsɨs/  ( listen)) is a special type of cell division necessary for sexual reproduction. In animals, meiosis produces gametes like sperm and egg cells, while in other organisms like fungi it generates spores. In many organisms, including humans, meiosis begins with one cell containing two copies of each chromosome—one from the organism's mother and one from its father—and produces four gamete 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 enables them to adapt during the course of evolution.

Meiosis begins when a cell's chromosomes are duplicated by a round of DNA replication. This leaves the maternal and paternal versions of each chromosome, called homologs, with an exact copy known as a sister chromatid attached at the center of the new chromosome pair. The maternal and paternal chromosome pairs then become interwoven by homologous recombination, which often leads to crossovers of DNA from the maternal version of the chromosome to the paternal version and vice versa. A spindle fiber binds to the center of each pair of homologs, and pulls the recombined maternal and paternal homolog pairs to different poles of the cell.

The cell then divides into two daughter cells as the chromosomes move away from the center. 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 half to 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.


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