Meiosis is a differentiation programme used by sexually reproducing organisms for generating reproductive cells. Its purpose is to reduce the genetic complement by half to ensure restoration of the correct number of chromosomes upon union of two sex cells. The halving of the genome is achieved by a single round of genome duplication followed by two successive rounds of genome division. During the period between completion of genome duplication and the first division, homologous maternal and paternal chromosomes pair with one another and exchange genetic material, culminating in formation of crossovers, which physically link the homologous chromosomes until they are separated at anaphase I. During the second genome division, the duplicated sister chromatids, which remain connected during meiosis I, are separated. Following the second division, four separated haploid genomes undergo a series of developmental steps, which give rise to reproductive cells capable of forming a diploid organism upon fertilisation.

Key Concepts:

  • Meiosis is a specialized cell division programme for generating reproductive cells.

  • The aim of meiosis is to generate haploid reproductive cells from a diploid precursor.

  • Reduction of the genome content by half is achieved by a single round of genome duplication followed by two successive rounds of genome divisions.

  • During the first meiotic genome division (meiosis I), homologous maternal and paternal chromosomes separate; during the second genome division (meiosis II), the newly duplicated sister chromatids separate.

  • Physical connection between homologous maternal and paternal chromatids or a crossover is essential for homologue separation during meiosis I.

  • Crossover formation depends on meiosis‐specific pairing of the homologues and genetic recombination between the homologues.

  • Meiotic recombination is initiated by developmentally programmed formation of DNA double‐strand breaks.

  • Meiotic recombination leads to exchange of genetic information between homologous maternal and paternal DNA and contributes to genome variation and evolution.

  • Meiotic homologue pairing and recombination are evolutionarily conserved from yeast to humans.

  • Defects in meiotic homologue pairing and recombination contribute to infertility, birth defects and aneuploidy (e.g. Downs syndrome) in humans.

Keywords: DNA replication; sister chromatid cohesion; homologue pairing; meiotic recombination; synaptonemal complex; crossovers; chromosome segregation

Figure 1.

The eukaryotic life cycle. Sexually reproducing eukaryotes have a life cycle with alternating diploid and haploid phases. Diploid cells (2n) that have differentiated to undergo meiosis perform two divisions after a single round of DNA replication. The four resulting haploid cells carry the basic set of chromosomes characteristic for each species (n). The haploid products of meiosis differentiate into gametes directly, or an intervening phase of mitotic divisions may occur before gamete formation. Two gametes of different mating type (red versus blue) fuse to a diploid cell, the zygote, which may enter a phase of mitotic divisions, or undergo meiosis directly.

Figure 2.

Meiosis. (a) The aim of meiosis to reduce chromosome numbers by half (from a diploid cell [2n] to haploid sex cells [1n]) is achieved by a single round of genome duplication followed by two rounds of genome segregation. (i) During meiosis I (MI), the genetic complement is reduced by half (‘reductional’ division) by disjunction of homologous maternal and paternal chromosomes. (ii) During meiosis II (MII), the newly duplicated sister chromatids separate. (iii) Following two rounds of genome segregation, maturation of the gametes take place. (iv) During mitotic cell division like MII, duplicated sister chromatids separate, leading to two daughter cells carrying the same genetic complement as the starting parent; this mode of genome segregation is referred to as ‘equational’. (b) Left panel: (i) Upon completion of genome duplication, newly duplicated sister chromatids are held together by a multiprotein complex named ‘cohesin’. (ii) Meiotic recombination is initiated by developmentally programmed formation of DNA double‐strand breaks (DSBs). (iii) A resected break end invades a homologous nonsister chromatid to repair the DSB via single end invasion (SEI). (iv) Broken DNA ends are repaired, giving rise to a structure referred to as double Holliday junction (dHJ). (v) As the cells exit pachytene, dHJ is resolved as a CO. (vi) At anaphase I, sister cohesion distal to centromere is removed while centromeric cohesion is maintained. Right panel: Five stages of prophase I based on the extent of homologue synapsis. Meiotic DSB repair on the left panel and homologue pairing on the right panel are genetically and mechanistically coupled. (c) Schematic representation of the behaviour of chromosomes as visualised by light and electron microscopy during prophase I. (i) leptotene; (ii) zygotene; (iii) pachytene; (iv) diplotene. (d) The organisation of chromosomes in the nucleus after mitosis (Rabl) is compared with the chromosome configuration during chromosome pairing in early meiotic prophase (bouquet). (e) The SC. The two axial elements organise the sister chromatids, and the transverse filaments assure synapsis.

Figure 3.

Recombination. The four chromatids of a bivalent resulting from premeiotic DNA replication are drawn schematically as DNA double helices. Thus, there are eight DNA single strands. At the three genetic markers a, b and c different DNA sequences are present in the two homologous chromosomes: + is wild‐type sequence, − is mutant sequence. Three examples of recombination products are shown. The 5+: 3− tetrad for marker b is the result of half‐chromatid conversion leading to postmeiotic segregation (PMS). The 6+: 2− tetrad for marker b derives from full‐chromatid conversion. PMS and conversions are also named noncrossover (NCO) events. The third recombination event is again a 6+: 2− segregation, but in this case associated with a crossover. The flanking markers a and b have exchanged reciprocally, resulting in a+ c and a c+ recombinants.



Allers T and Lichten M (2001) Differential timing and control of noncrossover and crossover recombination during meiosis. Cell 106: 47–57.

Baudat F, Manova K, Yuen JP, Jasin M and Keeney S (2000) Chromosome synapsis defects and sexually dimorphic meiotic progression in mice lacking Spo11. Molecular Cell 6: 989–998.

Buard J and de Massy B (2007) Playing hide and seek with mammalian meiotic crossover hotspots. Trends in Genetics 23: 301–309.

Callan H (1974) DNA replication in the chromosomes of eukaryotes. Cold Spring Harbor Symposia on Quantitative Biology 38: 195–203.

Carballo JA, Panizza S, Serrentino ME et al. (2013) Budding yeast ATM/ATR control meiotic double‐strand break (DSB) levels by down‐regulating Rec114, an essential component of the DSB‐machinery. PLoS Genetics 9: e1003545.

Cha RS, Weiner BM, Keeney S, Dekker J and Kleckner N (2000) Progression of meiotic DNA replication is modulated by interchromosomal interaction proteins, negatively by Spo11p and positively by Rec8p. Genes & Development 14: 493–503.

Craig JM and Choo KHA (2005) Kiss and break up – a safe passage to anaphase in mitosis and meiosis. Chromosoma 114: 252–262.

Dernburg A, McDonald K, Moulder G et al. (1998) Meiotic recombination in C. elegans initiates by a conserved mechanism and is dispensable for homologous chromosome synapsis. Cell 94: 387–398.

Garcia‐Muse T and Boulton SJ (2007) Meiotic recombination in Caenorhabditis elegans. Chromosome Research 15: 607–621.

Gerton JL and Hawley RS (2005) Homologous chromosome interactions in meiosis: diversity amidst conservation. Nature Reviews Genetics 6: 477–487.

Gray S, Allison R, Garcia V, Goldman A and Neale M (2013) Positive regulation of meiotic DNA double‐strand break formation by activation of the DNA damage checkpoint kinase Mec1(ATR). Open Biology 3: 130019.

Grelon M, Vezon D, Gendrot G and Pelletier G (2001). AtSPO11‐1 is necessary for efficient meiotic recombination in plants. EMBO Journal 20: 589–600.

Hassold T and Hunt P (2001) To err (meiotically) is human: the genesis of human aneuploidy. Nature Reviews Genetics 2: 280–291.

Hedges DJ and Deininger PL (2006) Inviting instability: transposable elements, double‐strand breaks, and the maintenance of genome integrity. Mutation Research 616: 46–59.

Holm PB (1977) The premeiotic DNA replication of euchromatin and heterochromatin in lilium longiflorum (Thunb.). Carlsberg Research Communications 42: 249–281.

Joyce EF, Pedersen M, Tiong S et al. (2011) Drosophila ATM and ATR have distinct activities in the regulation of meiotic DNA damage and repair. Journal of Cell Biology 195: 359–367.

Keeney S (2001) Mechanism and control of meiotic recombination initiation. Current Topics in Developmental Biology 52: 1–53.

Lange J, Pan J, Cole F et al. (2011) ATM controls meiotic double‐strand‐break formation. Nature 479: 237–240.

Loidl J, Klein F and Scherthan H (1994) Homologous pairing is reduced but not abolished in asynaptic mutants of yeast. Journal of Cell Biology 125: 1191–1200.

Mahadevaiah SK, Turner JM, Baudat F et al. (2001) Recombinational DNA double‐strand breaks in mice precede synapsis. Nature Genetics 27: 271–276.

McKim KS, Green‐Marroquin BL, Sekelsky JJ et al. (1998) Meiotic synapsis in the absence of recombination. Science 279: 876–878.

McKim KS, Jang JK and Manheim EA (2002) Meiotic recombination and chromosome segregation in Drosophila females. Annual Review of Genetics 36: 205–232.

Mézard C, Vignard J, Drouaud J and Mercier R (2007) The road to crossovers: plants have their say. Trends in Genetics 23: 91–99.

Morelli MA and Cohen PE (2005) Not all germ cells are created equal: aspects of sexual dimorphism in mammalian meiosis. Reproduction 130: 761–781.

Page SL and Hawley RS (2003) Chromosome choreography: the meiotic ballet. Science 301: 785–789.

Pâques F and Haber JE (1999) Multiple pathways of recombination induced by double‐strand breaks in Saccharomyces cerevisiae. Microbiology and Molecular Biology Review 63: 349–404.

Petes TD and Hill CW (1988) Recombination between repeated genes in microorganisms. Annual Review of Genetics 22: 147–168.

Petronczki M, Siomos MF and Nasmyth K (2003) Un ménage a quatre: the molecular biology of chromosome segregation. Cell 112: 423–440.

Raji H and Hartsuiker E (2006) Double‐strand break repair and homologous recombination in Schizosaccharomyces pombe. Yeast 23: 963–976.

Rivera T and Losada A (2006) Shugoshin and PP2A, shared duties at the centromere. BioEssays 28: 775–779.

Romanienko P and Camerini‐Otero R (2000) The mouse Spo11 gene is required for meiotic chromosome synapsis. Molecular Cell 6: 975–987.

Smagulova F, Gregoretti I, Brick K et al. (2011) Genome‐wide analysis reveals novel molecular features of mouse recombination hotspots. Nature 472: 375 –378.

Strich R (2004) Meiotic DNA replication. Current Topics in Developmental Biology 61: 29–60.

Thomas SE, Soltani‐Bejnood M, Roth P et al. (2005) Identification of two proteins required for conjunction and regular segregation of achiasmate homologs in Drosophila male meiosis. Cell 123: 555–568.

Tripathi A, Kumar K and Chaube S (2010) Meiotic cell cycle arrest in mammalian oocytes. Journal of Cellular Physiology 223: 592–600.

Tsuchiya D, Gonzalez C and Lacefield S (2011) The spindle checkpoint protein Mad2 regulates APC/C activity during prometaphase and metaphase of meiosis I in Saccharomyces cerevisiae. Molecular Biology of the Cell 22: 2848–2861.

Zheng J, Khil P, Camerini‐Otero R and Przytycka T (2010) Detecting sequence polymorphisms associated with meiotic recombination hotspots in the human genome. Genome Biology 11: R103.

Zickler D, Moreau P, Huynh AD and Slezec A (1992) Correlation between pairing initiation sites, recombination nodules, and meiotic recombination in Sordaria macrospora. Genetics 132: 135–148.

Zickler D and Kleckner N (1999) Meiotic chromosomes: integrating structure and function. Annual Review of Genetics 33: 603–754.

Further Reading

Collection of review articles on cytological aspects of meiosis (2006) Chromosoma 115(3): 151–271.

Collection of 13 review articles on meiosis and recombination (2007) Chromosome Research 15(5): 517–586.

Monckton D (ed.) (2006) Meisosis and the causes and consequences of recombination. Biochemical Society Transactions 34(4): 519–586.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Cha, Rita S, and Hartsuiker, Edgar(Jul 2014) Meiosis. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001359.pub3]