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.

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