The first step in homology-directed repair of DSBs is resection of the DSB ends ( Figure 1) (reviewed in Heyer et al., 2010). This change in outcome is achieved through the addition of numerous meiosis-specific elaborations to DSB repair. In contrast, in meiotic cells DSBs are induced enzymatically and repaired to ensure that some become crossovers, which lead to chiasmata that promote accurate segregation of homologous chromosomes ( Hawley, 1988). To avoid crossovers, non-crossover outcomes are promoted through the actions of helicases that disassemble recombination intermediates (reviewed in Huselid and Bunting, 2020). In this review we discuss the history of studies of crossover patterning, developments in methods used in the field, and our current understanding of the interplay between patterning phenomena.Ĭrossovers are generally avoided during repair of DNA double-strand breaks (DSBs) in mitotically proliferating cells, presumably because they can lead to loss of heterozygosity or to chromosome rearrangement (when occurring between non-allelic repetitive sequences) (reviewed in Andersen and Sekelsky, 2010). There is also considerable overlap between the known players that drive each patterning phenomenon. The mechanisms behind crossover patterning remain largely unknown, and key players appear to act at all scales, from the DNA level to inter-chromosome interactions. Suppression, first observed by Beadle in 1932, dictates that crossovers do not occur in regions surrounding the centromere and telomeres. Assurance, initially identified by Owen in 1949, describes the phenomenon in which a minimum of one crossover is formed per chromosome pair. Interference, first described by Sturtevant in 1915, is a phenomenon in which crossovers on the same chromosome do not occur near one another. Three types of patterning events have been identified. Although DSBs occur throughout the genome, crossover placement is intricately patterned, as observed first in early genetic studies by Muller and Sturtevant. Meiotic crossovers are formed via homologous repair of programmed double-strand breaks (DSBs). Proper number and placement of meiotic crossovers is vital to chromosome segregation, with failures in normal crossover distribution often resulting in aneuploidy and infertility. 3Integrative Program for Biological and Genome Sciences, The University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.2Department of Biology, The University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.1Curriculum in Genetics and Molecular Biology, The University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
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