In humans, the average number of crossover recombination events per hotspot is one crossover per 1,300 meioses, and the most extreme hotspot has a crossover frequency of one per 110 meioses.
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They were able to work with 12 meioses and seven marker loci.
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The derivation of our model centers on tetraploid species in which multivalent pairing is only one mechanism for chromosomal pairings during meioses.
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In: Kendrew John, Lawrence Eleanor (eds.). The Encyclopedia of Molecular Biology. Oxford: Blackwell Science, 945–950. where the 4 products of individual meioses can be conveniently observed.
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A. thaliana is a predominantly self-pollinating plant with an outcrossing rate estimated at less than 0.3%. An analysis of the genome-wide pattern of linkage disequilibrium suggested that self-pollination evolved roughly a million years ago or more. Meioses that lead to self-pollination are unlikely to produce significant beneficial genetic variability. However, these meioses can provide the adaptive benefit of recombinational repair of DNA damages during formation of germ cells at each generation.
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In humans, the average number of crossover recombination events per hotspot is one per 1,300 meioses, and the most extreme hotspot has a crossover frequency of one per 110 meioses. These hotspots are binding sites for the PRDM9 Zinc Finger array. Upon binding to DNA, PRDM9 catalyzes trimethylation of Histone 3 at lysine 4 and lysine 36. As a result, local nucleosomes are reorganized and through an unknown mechanism the recombination machinery is recruited to form double strand breaks.
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The typical unit of genetic linkage is the centimorgan (cM). A distance of 1 cM between two markers means that the markers are separated to different chromosomes on average once per 100 meiotic product, thus once per 50 meioses...
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A study of the frequency of outcrossing in natural populations showed that selfing is the predominant mode of reproduction in C. elegans, but that infrequent outcrossing events occur at a rate around 1%. Meioses that result in selfing are unlikely to contribute significantly to beneficial genetic variability, but these meioses may provide the adaptive benefit of recombinational repair of DNA damages that arise, especially under stressful conditions. Nicotine dependence can also be studied using C. elegans because it exhibits behavioral responses to nicotine that parallel those of mammals. These responses include acute response, tolerance, withdrawal, and sensitization.
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Such a benefit may have been sufficient to allow the long-term persistence of meioses even when followed by self- fertilization. A physical mechanism for self-pollination in A. thaliana is through pre-anthesis autogamy, such that fertilisation takes place largely before flower opening.
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An MSH4 hypomorphic (partially functional) mutant of S. cerevisiae showed a 30% genome wide reduction in crossover numbers, and a large number of meioses with non exchange chromosomes. Nevertheless, this mutant gave rise to spore viability patterns suggesting that segregation of non-exchange chromosomes occurred efficiently. Thus in S. cerevisiae proper segregation apparently does not entirely depend on crossovers between homologous pairs.
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The capacity for selfing in these fishes has apparently persisted for at least several hundred thousand years. Meioses that lead to self-fertilization can reduce genetic fitness by causing inbreeding depression. However, self- fertilization does provide the benefit of “fertilization assurance” (reproductive assurance) at each generation. Meiosis can also provide the adaptive benefit of efficient recombinational repair of DNA damages during formation of germ cells at each generation.
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The MSH4 and MSH5 proteins form a hetero-oligomeric structure (heterodimer) in yeast and humans. In the yeast Saccharomyces cerevisiae MSH4 and MSH5 act specifically to facilitate crossovers between homologous chromosomes during meiosis. The MSH4/MSH5 complex binds and stabilizes double Holliday junctions and promotes their resolution into crossover products. An MSH4 hypomorphic (partially functional) mutant of S. cerevisiae showed a 30% genome wide reduction in crossover numbers, and a large number of meioses with non exchange chromosomes.
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In S. cerevisiae, MSH4 and MSH5 act specifically to facilitate crossovers between homologous chromosomes during meiosis. The MSH4/MSH5 complex binds and stabilizes double Holliday junctions and promotes their resolution into crossover products. An MSH4 hypomorphic (partially functional) mutant of S. cerevisiae showed a 30% genome-wide reduction in crossover numbers, and a large number of meioses with non-exchange chromosomes. Nevertheless, this mutant gave rise to spore viability patterns suggesting that segregation of non-exchange chromosomes occurred efficiently.
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The MSH4 and MSH5 proteins form a hetero-oligomeric structure (heterodimer) in yeast and humans. In the yeast Saccharomyces cerevisiae MSH4 and MSH5 act specifically to facilitate crossovers between homologous chromosomes during meiosis. The MSH4/MSH5 complex binds and stabilizes double Holliday junctions and promotes their resolution into crossover products. An MSH4 hypomorphic (partially functional) mutant of S. cerevisiae showed a 30% genome wide reduction in crossover numbers, and a large number of meioses with non exchange chromosomes.
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Studies of gene conversion have contributed to our understanding of the adaptive function of meiotic recombination. The ordinary segregation pattern of an allele pair (Aa) among the 4 products of meiosis is 2A:2a. Detection of infrequent gene conversion events (e.g. 3:1 or 1:3 segregation patterns during individual meioses) provides insight into the alternate pathways of recombination leading either to crossover or non-crossover chromosomes. Gene conversion events are thought to arise where the “A” and “a” alleles happen to be near the exact location of a molecular recombination event.
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