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.

Keywords: chromosome pairing; synapsis; crossing‐over; chiasma formation; chromosome segregation; non‐disjunction; synaptonemal complex; MLH1; recombination proteins; origin of aneuploidy

Figure 1.

Chromosome behaviour in the later stages of meiosis. After diakinesis, the bivalents attach to spindle microtubules and orient in a monopolar fashion at metaphase I. When orientation is complete, the cohesin complex proteins that bind sister chromatids are removed (except at the kinetochores), releasing the chiasmata and allowing the chromosomes to be pulled to the spindle poles at anaphase I. The chromosomes reorient on the metaphase II spindle, and the chromatids separate and move to opposite spindle poles. The net result is the formation of cells containing a haploid, unreplicated genome. See also Figure a as regards the reciprocal recombination/crossing‐over/chiasma formation taking place at prophase I of meiosis.

Figure 2.

(a) (EM) image of a single synaptonemal complex (SC). The tripartite structure of the SC is evident. The structure comprises two (LEs) each of 50 nm and a central region of 100 nm. A thin (CE) can be seen within the central region. (b) Cartoon to show the relationship of the chromatin to the axial cores formed by the cohesin complex proteins and the LEs of the SC. Two SC proteins (SYCP2 and SYCP3) are present in the LEs. These are large proteins that associate with the cohesin complex holding sister chromatids together. The drawing shows the cohesin constitution of the LE separately, but it is to be noted that there is no indication that there are in fact two layers making up the LE. Only one protein, SYCP1, has so far been isolated from the central region. The C‐terminal domain of this protein has been shown to bind to the chromosomal axes. The central part of the protein is a coiled coil and stretches out into the central region of the SC. The protein does not traverse the whole central region but overlaps and associates, at the N‐terminal domain, with SYCP1 fibrils stretching out from the opposite lateral element. This overlapped region appears to be largely responsible for the central element. (c) EM of an SC stained to show recombination nodules (arrows). (a) and (c) courtesy of N. Sadaallah.

Figure 3.

Electron microscopy (EM) pictures of synaptonemal complexes (SCs) at the pachytene stage showing so‐called excrescences of unpaired chromosome segments. (a) Unpaired chromosome segments within inverted segment of chromosome 13 show excrescences. The (NOR) has been broken into two separate parts (N) in the formation of the inversion. (b) Interlocked SC where the unpaired segments show the same excrescences as the unpaired segments of the XY bivalent, marked X and Y. Only a very small part of the short arms of the X and Y are synapsed. Courtesy of N. Sadaallah.

Figure 4.

(IM) picture of human spermatocyte at the zygotene stage illustrating the RAD51 foci. (a) Labelling with anti‐hRAD51 (white) and sera GS (centromeres, red) show numerous RAD51 foci, some indicated by arrows. (b) Same picture as in (a) but with anti‐A1 (anti‐lateral element, white) superimposed, illustrating how the RAD51 foci have disappeared along the segments of two bivalents, where synapsis has been successful, but remain within the interstitial unpaired segment (green and white arrows in (a) and (b) and white arrow in insert). Reproduced from Barlow et al. , with permission from Nature Publishing Group.

Figure 5.

Double‐strand break (DSB) model of recombination (see Kirkpatrick, ). Cutting of strands in opposite orientation at each junction (arrowhead) produces a crossover.

Figure 6.

Detecting the positions of crossovers at pachytene using the DNA mismatch repair protein, MLH1. The cells have been stained using antibodies against SYCP3 (red), MLH1 (yellow) and, in the spermatocyte, the kinetochore (blue). (a) Normal pachytene spermatocyte. (b) Normal pachytene oocyte. There are obvious differences between the spermatocyte and the oocyte: the SCs are much longer in the oocyte; there are more MLH1 foci in the oocyte; MLH1 foci tend to be positioned closer to the telomeres in the spermatocyte. (c) Pachytene oocyte showing failure of pairing affecting some chromosomes (arrows). MLH1 foci are present on synapsed chromosome pairs (arrowheads).

Figure 7.

(a) Cartoon illustrating how the configurations in (b) are achieved by bivalents with one or two chiasmata. Two chromosome pairs are shown, in each case one homologue is coloured blue, the other red. A reciprocal recombination/crossover between a blue and a red chromatid generates an exchange that is visible as a cross‐configuration/chiasma in the metaphase I bivalent. The acrocentric chromosome pair has a single reciprocal recombination/crossover/chiasma and produces a rod bivalent at metaphase I; the metacentric has two reciprocal recombinations/crossovers/chiasmata and produces a ring bivalent. (b) Metaphase I spermatocyte. Pairs of homologous chromosomes are held together by variable numbers of chiasmata. Bivalents with 1, 2 or 3 chiasmata are indicated, as is the XY‐chromosome pair. (c) Metaphase II spermatocyte. The chromatids of each chromosome (arrowheads) are held together only at the centromeres (arrow). (d) Metaphase I spermatocyte from a man with an abnormally low number of chiasmata. In contrast to (b), bivalents (arrows) have only one or two chiasmata; additionally, a number of chromosome pairs have failed to form crossovers and are present as univalents (arrowheads).

Figure 8.

Human spermatocyte at the metaphase I stages (a) and (b), where the chiasmata have been highlighted (middle) and oocyte at the metaphase I stage, revised from Yuncken . Note the difficulty in identifying the chiasmata in the oocyte in comparison to the spermatocyte. (a) Reproduced from Hultén et al. , with permission and (b) from Yuncken , with permission from S. Kargel AG, Basel.



Armstrong SJ and Hultén MA (1998) Meiotic segregation analysis by FISH investigations in sperm and spermatocytes of translocation heterozygotes. European Journal of Human Genetics 6: 430–431.

Armstrong SJ, Kirkham AJ and Hultén MA (1994) XY chromosome behaviour in the germ‐line of the human male: a FISH analysis of spatial orientation, chromatin condensation and pairing. Chromosome Research 2(6): 445–452.

Barlow AL, Benson FE, West SC and Hultén MA (1997) Distribution of the Rad51 recombinase in human and mouse spermatocytes. EMBO Journal 16(17): 5207–5215.

Barlow AL and Hultén MA (1997) Sequential immunocytogenetics, molecular cytogenetics and transmission electron microscopy of microspread meiosis I oocytes from a human fetal carrier of an unbalanced translocation. Chromosoma 106(5): 293–303.

Barlow AL and Hultén MA (1998) Crossing over analysis at pachytene in man. European Journal of Human Genetics 6(4): 350–358.

Borde V (2007) The multiple roles of the Mre11 complex for meiotic recombination. Chromosome Research 15: 551–563.

Cheng EY, Hunt PA, Naluai‐Cecchini TA et al. (2009) Meiotic recombination in human oocytes. PLoS Genetics 5(9): e1000661 Epub September 18.

Goldman ASH and Hultén MA (1993) Analysis of chiasma frequency and first meiotic segregation in a human male reciprocal translocation heterozygote, t(1;11) (p36.3;q13.1), using fluorescence in situ hybridisation. Cytogenetics and Cell Genetics 63: 16–23.

Hastings PJ, Lupski JR, Rosenberg SM and Ira G (2009) Mechanisms of change in gene copy number. Nature Reviews. Genetics 10: 551–564.

Holloway JK, Morelli MA, Borst PL and Cohen PE (2010) Mammalian BLM helicase is critical for integrating multiple pathways of meiotic recombination. Journal of Cell Biology 188(6): 779–789.

Hultén M (1974) Chiasma distribution at diakinesis in the normal human male. Hereditas 76(1): 55–78.

Hultén M (1994) Chiasma formation, crossing‐over and recombination in meiosis. Trends in Genetics 10(4): 112–113.

Hultén M, Eliasson R and Tillinger KG (1970) Low chiasma count and other meiotic irregularities in two infertile 46, XY men with spermatogenic arrest. Hereditas 65(2): 285–290.

Hultén MA, Patel S, Jonasson J and Iwarsson E (2010b) On the origin of the maternal age effect in trisomy 21 Down syndrome: the Oocyte Mosaicism Selection (OMS) model. Reproduction 139: 1–9.

Hultén MA, Patel SD, Tankimanova M et al. (2008) On the origin of trisomy 21 Down syndrome. Molecular Cytogenetics 1: 21.

Hultén MA, Smith E and Delhanty JDA (2010a) Errors in chromosome segregation during oogenesis and early embryogenesis. In: Carrell DT and Peterson CM (eds) Reproductive Endocrinology and Infertility: Integrating Modern Clinical and Laboratory Practice. New York: Springer.

Hultén MA, Tease C and Lawrie NM (1995) Chiasma‐based genetic map of the mouse X chromosome. Chromosoma 104: 223–227.

Kauppi L, May CA and Jeffreys AJ (2009) Analysis of meiotic recombination products from human sperm. Methods in Molecular Biology 557: 323–355.

Keeney S and Neale MJ (2006) Initiation of meiotic recombination by formation of DNA double‐strand breaks: mechanism and regulation. Biochemical Society Transactions 34(part 4): 523–525.

Kirkpatrick DT (1999) Roles of the DNA mismatch repair and nucleotide excision repair proteins during meiosis. Cellular and Molecular Life Sciences 55(3): 437–449.

Laurie DA and Hultén MA (1985) Further studies on bivalent chiasma frequency in human males with normal karyotypes. Annals of Human Genetics 49(part 3): 189–201.

Lawrie NM, Tease C and Hultén MA (1995) Chiasma frequency, distribution and interference maps of mouse autosomes. Chromosoma 104(4): 308–314.

Lenzi ML, Smith J, Snowden T et al. (2005) Extreme heterogeneity in the molecular events leading to the establishment of chiasmata during meiosis I in human oocytes. American Journal of Human Genetics 76(1): 112–127.

Lynn A, Ashley T and Hassold T (2004) Variation in human meiotic recombination. Annual Review of Genomics and Human Genetics 5: 317–349.

Lynn A, Koehler KE, Judis L et al. (2002) Covariation of synaptonemal complex length and mammalian meiotic exchange rates. Science 296(5576): 2222–2225.

Lynn A, Soucek R and Börner GV (2007) ZMM proteins during meiosis: crossover artists at work. Chromosome Research 15: 591–605.

Lyrakou S, Hultén MA and Hartshorne GM (2002) Growth factors promote meiosis in mouse fetal ovaries in vitro. Molecular Human Reproduction 8(10): 906–911.

Martin RH (2008) Cytogenetic determinants of male fertility. Human Reproduction Update 14(4): 379–390.

Martinez‐Perez E and Colaiacovo MP (2009) Distribution of meiotic recombination events: talking to you neighbors. Current Opinion in Genetics & Development 19(2): 105–112.

Moens PB, Marcon E, Shore JS, Kochakpour N and Spyropoulos B (2007) Initiation and resolution of interhomolog connections: crossover and non‐crossover sites along mouse synaptonemal complexes. Journal of Cell Science 120(part 6): 1017–1027.

Moses MJ (1956) Chromosome structures in crayfish spermatocytes. Journal of Biophysical and Biochemical Cytology 2: 215–218.

Nicklas RB (1997) How cells get the right chromosomes. Science (New York) 275(5300): 632–637.

Page SL and Hawley RS (2004) The genetics and molecular biology of the synaptonemal complex. Annual Review of Cell and Developmental Biology 20: 525–558.

Perheentupa A and Huhtaniemi I (2009) Aging of the human ovary and testis. Molecular and Cellular Endocrinology 299(1): 2–13.

Prieto I, Tease C, Pezzi N et al. (2004) Cohesin component dynamics during meiotic prophase I in mammalian oocytes. Chromosome Research 12: 197–213.

Redon C, Pilch D, Rogakou E et al. (2002) Histone H2A variants H2AX and H2AZ. Current Opinion in Genetics & Development 12(2): 162–169.

Rupnik A, Lowndes NF and Grenon M (2010) MRN and the race to the break. Chromosoma 119: 115–135.

Saadallah N and Hultén MA (1986) EM investigations of surface spread synaptonemal complexes in human male carrier of a pericentric inversion inv(13)(p12q14): the role of heterosynapsis for spermatocyte survival. Annals of Human Genetics 4: 369–383.

Sharp AJ (2009) Engineering themes and new challenges in defining the role of structural variation in human disease. Human Mutation 2: 135–144.

Stahl FW and Housworth EA (2009) Methods for analysis of crossover interference in Saccharomyces cerevisiae. Methods in Molecular Biology 557: 35–53.

Stankiewics P and Lupski JR (2010) Structural variation in the human genome and its role in disease. Annual Review of Medicine 61: 437–455.

Suja JA and Barbero JL (2009) Cohesin complexes and sister chromatid cohesion in mammalian meiosis. Genome Dynamics 5: 94–116.

Sun F, Oliver‐Bonet M, Liehr T et al. (2004) Human male recombination maps for individual chromosomes. American Journal of Human Genetics 74: 521–531.

Sung P and Klein H (2006) Mechanism of homologous recombination: mediators and helicases take on regulatory functions. Nature Reviews. Molecular Cell Biology 7: 739–750.

Tease C, Hartshorne G and Hultén M (2006) Altered patterns of meiotic recombination in human fetal oocytes with asynapsis and/or synaptonemal complex fragmentation at pachytene. Reproductive Biomedicine Online 13(1): 88–95.

Tease C, Hartshorne GM and Hultén MA (2002) Patterns of meiotic recombination in human fetal oocytes. American Journal of Human Genetics 70(6): 1469–1479.

Tease C and Hultén MA (2004) Inter‐sex variation in synaptonemal complex lengths largely determine the different recombination rates in male and female germ cells. Cytogenetic and Genome Research 107(3–4): 208–215.

Vanneste E, Voet T, Caignec C et al. (2009) Chromosome instability is common in human cleavage‐stage embryos. Nature Medicine 15(5): 577–583.

Wallace BM and Hultén MA (1985) Meiotic chromosome pairing in the normal human female. Annals of Human Genetics 49(3): 215–226.

West SC (2009) The search for a human Holliday junction resolvase. Biochemical Society Transactions 37: 519–526.

Yuncken C (1968) Meiosis in the human female. Cytogenetics 7(3): 234–238.

Zickler D (2006) From early homologue recognition to synaptonemal complex formation. Chromosoma 115: 158–174.

Further Reading

Alsheimer M (2009) The dance floor of meiosis: evolutionary conservation of nuclear envelope attachment and dynamics of meiotic telomeres. Genome Dynamics 5: 81–93.

de Boer E, Lhuissier FG and Heyting C (2009) Cytological analysis of interference in mouse meiosis. Methods in Molecular Biology 558: 355–382.

Burgoyne PS, Mahadevaiah SK and Turner JM (2009) The consequences of asynapsis for mammalian meiosis. Nature Reviews. Genetics 10(3): 207–216.

Handel MA and Schimenti JC (2010) Genetics of mammalian meiosis: regulation, dynamics and impact on fertility. Nature Reviews of Genetics 11: 124–136.

Harrison CJ, Alvey E and Henderson IR (2010) Meiosis in flowering plants and other green organisms. Journal of Experimental Botany 61: 2863–2875.

Inagaki A, Schoenmakers S and Baarends WM (2010) DNA double strand break repair, chromosome synapsis and transcriptional silencing in meiosis. Epigenetics 5: 255–266.

Khil PP and Camerini Otero RD (2009) Variation in patterns of human meiotic recombination. Genome Dynamics 5: 117–127.

Neale MJ and Keeney S (2006) Clarifying the mechanics of DNA strand exchange in meiotic recombination. Nature 442: 153–158.

Sasaki M, Lange J and Keeney S (2010) Genome destabilization by homologous recombination in the germ line. Nature Reviews in Molecular Cell Biology 11: 182–195.

Sherthan H (2009) Analysis of telomere dynamics in mouse spermatogenesis. Methods in Molecular Biology 558: 383–399.

Susiarjo M, Rubio C and Hunt P (2009) Analyzing mammalian female meiosis. Methods in Molecular Biology 558: 339–335.

Yang F and Wang PJ (2009) The mammalian synaptonemal complex: a scaffold and beyond. Genome Dynamics 5: 69–80.

Yin S, Sun XF, Schatten H and Sun QY (2008) Molecular insights into mechanisms regulating faithful chromosome separation in female meiosis. Cell Cycle 7(19): 2997–3005.

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Hultén, Maj A(Sep 2010) Meiosis. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005772.pub2]