Radiosensitivity of mammalian cells. IV. Effects of x-irradiation on the DNA synthetic period in synchronized cells.

The effect of X-irradiation on the timing of DNA synthesis in the Chinese hamster ovary cells has been investigated. Mitotically synchronized cells irradiated in mitosis or early G(1) exhibited a fixed, dose-independent (150-2000 rad) delay of 1.6 hr in entry into S, while the duration of S was unaffected. Cells irradiated during late G(1) or the first 0.8 hr of S were not affected either in time of initiation or duration of S. However, when cells 0.8 hr or more into S were irradiated, completion but not initiation of DNA synthesis was delayed, indicating a very precise separation of X-ray effects upon initiation and replication. There was no indication of a re-ordering of cells following irradiation and recovery, since cells in G(2) at the time of irradiation always divided before cells irradiated in S. The results suggest that two separate functions required for initiation and continued replication of DNA may be differentially sensitive to X-irradiation.