Frequency of sister-chromatid exchanges depending on the amount of 5-bromodeoxyuridine incorporated into parental DNA.

Chinese hamster D-6 cells were grown for two cell cycles. The effect of 5-bromodeoxyuridine (BrdU) on the frequencies of sister-chromatid exchanges (SCEs) in these cells was investigated by the BrdU-labeling method. A low concentration (5 microM) of BrdU was inoculated in the first cell cycle for SCE counting. When excess concentrations (100-1000 microM) of BrdU were added subsequently in the second cell cycle, a 1-2-fold increase of SCE frequencies was observed. When excess thymidine (dT) (100-1000 microM) was supplied instead of BrdU, the incidence of SCE also increased. When cells were exposed to high concentrations (50-200 microM) of BrdU in the first cell cycle, a 1-4-fold increase in SCE frequencies was observed. This incidence of SCE was largely dependent on the concentration of BrdU and dT used in the second cell cycle. These results suggest that efficient SCE induction by BrdU is related to the BrdU residue incorporated into parental DNA strands.