Radiation-Induced Cycle Delay in Synchronized Chinese Hamster Cells: Comparison between DNA Synthesis and Division

Chinese hamster ovary fibroblasts (MI > 90%) were synchronized by mitotic selection and x-irradiated at two-hour intervals during the cell cycle with 150, 300, or 600 rad. Cell progression was monitored by pulse labeling with [³ H] TdR at one-hour intervals, followed by autoradiography and liquid scintillation counting. Irradiated mitotic and G₁ cells were delayed in initiating DNA synthesis (0.4-2.7 hr), but the rate of entry into and out of S, the duration of S, and the rate of DNA synthesis remained unchanged. Cells irradiated during S incorporated [³ H] TdR at a reduced rate and were delayed in entering G₂. The fraction of cells capable of entering S was not reduced by any time-dose combination. The delay in entering and leaving the S phase which resulted from a dose of 150 or 600 rad was about 40% of that from 300 rad. The division delay, however, increased linearly with dose and exponentially with cell age. The ratio of division delay to S delay for G₁ cells irradiated with 150 and 300 rad was approximately one. However, after irradiation at all other time-dose combinations, the ratio was considerably greater than one, indicating the existence of a G₂ block responsible for division delay. A possible relationship between radiation-induced S delay and a defect in translation is discussed.