In biology, meiosis is the process by which one diploid eukaryotic cell divides to generate four haploid cells often called gametes.
Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually.
A few eukaryotes, notably the 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 mitosis or binary fission.
During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes.
Each gamete contains one complete set of chromosomes, or half of the genetic content of the original cell.
These resultant haploid cells can fuse with other haploid cells of the opposite sex or mating type during fertilization to create a new diploid cell, or zygote.
Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization.
Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA.
In other words, meiosis and sexual reproduction produce genetic variation.
Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes.
There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes.