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  1. Maj A Hultén

Published Online: 15 SEP 2010

DOI: 10.1002/9780470015902.a0005772.pub2



How to Cite

Hultén, M. A. 2010. Meiosis. eLS. .

Author Information

  1. University of Warwick, Coventry, UK

Publication History

  1. Published Online: 15 SEP 2010


Meiosis is a specialised type of cell division, the principal function of which is to produce spores/gametes (sperm and eggs in mammals) that have the haploid number of chromosomes. In humans, this represents a reduction from 46 (23 pairs) to 23 chromosomes (one complete set) in sperm and eggs. The normal somatic number of 46 chromosomes is restored at fertilisation. The most complex part of meiosis (prophase I) involves intimate pairing and synapsis of the homologous chromosomes followed by reciprocal recombination/crossing-over/chiasma formation. In most organisms, chiasma formation is obligatory to allow the regular segregation of the (rearranged) homologues at the first (reductional) division (anaphase I). In mammals, there are substantial differences between the two sexes as regards the meiotic process. Mammalian female prophase I takes place during foetal development, and the second meiotic cell division is not completed until after fertilisation. In sharp contrast, mammalian male meiosis does not start until puberty and is continuous throughout life.

Key Concepts:

  • Meiosis is the term used for the cell divisions leading to gametes, sperm and eggs in mammals.

  • At meiosis, the chromosome number is halved compared to that in somatic cell nuclei.

  • Meiosis takes place in the germ line, testes and ovaries in mammals.

  • The meiotic process is very different in the two mammalian sexes.

  • Female mammalian meiosis is subdivided into three stages: the first taking place during foetal development, the second just before ovulation and the third after fertilisation.

  • Male mammalian meiosis starts at puberty and is ongoing throughout life.

  • Reciprocal recombination/crossing-over/chiasma formation (taking place during foetal development in mammalian females) is necessary for normal first meiotic cell division (taking place just before ovulation in mammalian females).

  • Many of the proteins involved in the meiotic recombination process, as well as some structural components of meiotic chromosomes, are conserved from yeast to man.

  • Human females produce only 300–400 mature eggs in a lifetime, in stark contrast to human males who normally produce around 300–400 million sperm daily.

  • An abnormal chromosome number (aneuploidy) is common in humans and is thought to arise due to abnormalities in meiotic cell divisions.


  • chromosome pairing;
  • synapsis;
  • crossing-over;
  • chiasma formation;
  • chromosome segregation;
  • non-disjunction;
  • synaptonemal complex;
  • MLH1;
  • recombination proteins;
  • origin of aneuploidy