Kinetics of induction of sister-chromatid exchanges by X-rays through two cell cycles

V79 Chinese hamster cells were exposed to X-rays at various times through the two cell cycles required to obtain harlequin-stained chromosomes. A two-fold SCE enhancement was found between the first and the second G1 phase when BrdUrd was incorporated during the first S phase only. This BrdUrd effect was not found when MNNG was used. Furthermore, the kinetics of SCE and aberrations were different, suggesting two separate mechanisms for their formation: SCE activity takes place when DNA damage occurs before the DNA replication, and aberration activity when the DNA damage occurs chiefly after the DNA replication.