Nonrandom distribution of interhomolog recombination events induced by breakage of a dicentric chromosome in Saccharomyces cerevisiae.

Dicentric chromosomes undergo breakage in mitosis, resulting in chromosome deletions, duplications, and translocations. In this study, we map chromosome break sites of dicentrics in Saccharomyces cerevisiae by a mitotic recombination assay. The assay uses a diploid strain in which one homolog has a conditional centromere in addition to a wild-type centromere, and the other homolog has only the wild-type centromere; the conditional centromere is inactive when cells are grown in galactose and is activated when the cells are switched to glucose. In addition, the two homologs are distinguishable by multiple single-nucleotide polymorphisms (SNPs). Under conditions in which the conditional centromere is activated, the functionally dicentric chromosome undergoes double-stranded DNA breaks (DSBs) that can be repaired by mitotic recombination with the homolog. Such recombination events often lead to loss of heterozygosity (LOH) of SNPs that are centromere distal to the crossover. Using a PCR-based assay, we determined the position of LOH in multiple independent recombination events to a resolution of ∼4 kb. This analysis shows that dicentric chromosomes have recombination breakpoints that are broadly distributed between the two centromeres, although there is a clustering of breakpoints within 10 kb of the conditional centromere.