Abstract
Recent studies led to the proposal that meiotic gene conversion can result after transient engagement of the donor chromatid and subsequent DNA synthesis-dependent strand annealing (SDSA). Double Holliday junction (dHJ) intermediates were previously proposed to form both reciprocal crossover recombinants (COs) and noncrossover recombinants (NCOs); however, dHJs are now thought to give rise mainly to COs, with SDSA forming most or all NCOs. To test this model in Saccharomyces cerevisiae, we constructed a random spore system in which it is possible to identify a subset of NCO recombinants that can readily be accounted for by SDSA, but not by dHJ-mediated recombination. The diagnostic class of recombinants is one in which two markers on opposite sides of a double-strand break site are converted, without conversion of an intervening heterologous insertion located on the donor chromatid. This diagnostic class represents 26% of selected NCO recombinants. Tetrad analysis using the same markers provided additional evidence that SDSA is a major pathway for NCO gene conversion in meiosis.
Highlights
Homologous recombination is essential for meiosis, the cellular division process specific to gametogenesis
In organisms that reproduce sexually, sex cells are produced by the specialized cell division called meiosis, which halves the number of chromosomes from two sets to one
A regulatory process selects a subset of breaks to be healed by a mechanism that forms crossover recombinants
Summary
Homologous recombination is essential for meiosis, the cellular division process specific to gametogenesis. If the original configuration of chromosome arms is retained, the event is designated a noncrossover (NCO) Both CO and NCO events can result in a type of non-Mendelian segregation, called gene conversion, of heterozygous markers near the recombination initiation site. Tetrad analysis in fungi showed that gene conversion of a marker is frequently associated with reciprocal exchange of flanking markers [3,4] This association was neatly accounted for by Robin Holliday’s proposal that recombination involved an intermediate in which only two of the four single DNA strands were exchanged [5]. This feature was retained even when Szostak et al proposed the Double-strand Break Repair model [6] (Figure 1A), in which two HJs form and are resolved during each recombination event Hereafter this model will be referred to as the ‘‘dHJ model’’ (double Holliday Junction model).
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