Cell-stage dependence of the formation of SCEs and chromosomal aberrations.

Sister chromatid exchanges (SCEs) and homologous chromatid exchanges (chromatid exchanges between homologous chromosomes at homologous sites) are similar, not only in the type of chromosomal alteration but also in their high yields in Bloom syndrome cells or in cells treated with mitomycin C (MMC). However, the similarity between SCEs and homologous exchanges seems superficial as evidenced by the present experiments. In human lymphocytes, when caffeine was added during the S phase under conditions of concurrent exposure to MMC, both SCEs and homologous exchanges increased. However, when caffeine was added during the G2 phase, the frequency of SCEs remained unaffected whereas the frequency of homologous and nonhomologous chromatid exchanges decreased and that of other types of chromatid aberrations increased. We conclude that the process of formation of SCEs is, at least partly, different from that of chromatid exchanges or other types of chromatid aberrations, the former presumably occurring during the S phase and the latter during the G2 phase.