5-Bromodeoxycytidine in the study of sister chromatid exchanges in human lymphocytes

When [3H]dC was added with a high dose (4 x 10(-1) mM) of dT to human blood lymphocyte cultures, much heavier labeling of interphase nuclei and metaphase chromosomes was observed compared with that in cultures treated with [3H]dC alone. This observation indicates that in the presence of excess dT, exogenous dC is included into cytosine bases of DNA, releasing the cells from the thymidine block. BrdC 5 x 10(-2) mM added with a high dose of dT (4 x 10(-1) to 1.0 mM) to the cultues did not relieve the thymidine block as determined from the percentage of metaphases of the first to third divisions. It is concluded that BrdC, in contrast to dC, is not utilized as a cytosine DNA precursor even in the presence of high concentrations of dT. The frequency of SCEs per cell was the same when studied with the aid of BrdC and BrdU used under similar conditions. The distribution of SCEs among chromosomes was also identical for both analogues: The number of SCEs was significantly higher than expected in chromosomes of group B and lower than expected in chromosomes of groups E, F, and G.