Meiosis is the process leading to formation of the haploid gametes from diploid cells.
Keywords: DNA replication; sister chromatid cohesion; synaptonemal complex; chromosome segregation; meiotic recombination
Meiosis
Jürg Kohli, University of Berne, Berne, Switzerland
Edgar Hartsuiker, University of Sussex, Brighton, UK
Published online: March 2008
DOI: 10.1002/9780470015902.a0001359.pub2
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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.
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Figure 2. Meiosis. The schematic drawings represent the crucial features of meiosis. In (a) the emphasis is on the fate of the two sets of chromatids resulting from premeiotic DNA replication. Each red and blue line represents a double helix of DNA. (b) Schematic representation of the behaviour of chromosomes as visualized by light and electron microscopy. In (c) the organization of chromosomes in the nucleus after mitosis (Rabl) is compared with the chromosome configuration during chromosome pairing in early meiotic prophase (bouquet). (d) The synaptonemal complex. The two axial elements organize the sister chromatids, and the transverse filaments assure synapsis.
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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
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| References | |
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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. | |
| other Monckton D (ed.) (2006) Meisosis and the causes and consequences of recombination. Biochemical Society Transactions 34(4): 519–586. | |
