Repair of DNA double-strand breaks requires two homologous DNA duplexes.

DNA repair and cell survival in haploid and its diploid derivative strains of Saccharomyces cerevisiae were studied after 100 krad X-ray irradiation. The cells were in the G1 stage of the cell cycle, where haploid cells had only one copy of genetic material per genome and diploid had two copies. It was found that diploid could repair double-strand breaks in its DNA after 48 hr of liquid holding which was accompanied by a four-fold rise in survival. In contrast a haploid strain failed to repair its DNA and showed no increase in survival after liquid holding. It is concluded that (1) repair of DNA double-strand breaks requires the availability of two homologous DNA duplexes, (2) restoration of cell viability during liquid holding is connected with repair of DNA double-strand breaks and (3) this repair is a slow process possibly associated with slow finding and conjugation of homologous chromosomes.