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) Electron microscopy (EM) image of a single synaptonemal complex (SC). The tripartite structure of the SC is evident. The structure comprises two lateral elements (LEs) each of 50 nm and a central region of 100 nm. A thin central element (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 nucleolar organising region (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.

Immunofluorescence microscopy (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.



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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.

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