X-ray inhibition of DNA synthesis at discrete times during S phase in synchronous human diploid fibroblasts

Synchronous human diploid fibroblasts were exposed to 1000 rad of X rays at various times in S phase, and thymidine incorporation, an indicator of DNA synthesis, was measured throughout the cell cycle. When cells were irradiated just before the time when thymidine incorporation peaked in control cells, the peak was strongly suppressed. Irradiation after appearance of an incorporation peak in control cells resulted in some depression of thymidine incorporation but had much less effect than irradiation before the peak. These results are in agreement with the interpretation that, in human diploid fibroblasts, most replicons initiate during relatively restricted times in S phase, leading to the multiple thymidine incorporation peaks observed in these cells.