Meiosis

Abstract

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.

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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.

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