Sister chromatids are preferred over homologs as substrates for recombinational repair in Saccharomyces cerevisiae.

A diploid Saccharomyces cerevisiae strain was constructed in which the products of both homolog recombination and unequal sister chromatid recombination events could be selected. This strain was synchronized in G1 or in G2, irradiated with X-rays to induce DNA damage, and monitored for levels of recombination. Cells irradiated in G1 were found to repair recombinogenic damage primarily by homolog recombination, whereas those irradiated in G2 repaired such damage preferentially by sister chromatid recombination. We found, as have others, that G1 diploids were much more sensitive to the lethal effects of X-ray damage than were G2 diploids, especially at higher doses of irradiation. The following possible explanations for this observation were tested: G2 cells have more potential templates for repair than G1 cells; G2 cells are protected by the RAD9-mediated delay in G2 following DNA damage; sister chromatids may share more homology than homologous chromosomes. All these possibilities were ruled out by appropriate tests. We propose that, due to a special relationship they share, sister chromatids are not only preferred over homologous chromatids as substrates for recombinational repair, but have the capacity to repair more DNA damage than do homologs.