DNA Replication Kinetics in X-Irradiated Chinese Hamster Ovary Cells

Abstract

The kinetics of semiconservative DNA replication have been studied in both asynchronous and synchronized Chinese hamster ovary cells (CHO) irradiated with x-ray doses up to 3000 rad. Amounts of DNA replicated were determined by isopycnic gradient centrifugation of DNA from cells which were incubated after irradiation in medium containing 5-bromodeoxyuridine (50 ..mu..g/ml) and 5-fluorodeoxyuridine (0.1 ..mu..g/ml). The results confirm that cells irradiated in early G/sub 1/ phase experience a delay in their entry into S phase. This G/sub 1/ block is dose independent in the range from 300 to 3000 rad and is 0.5 to 0.7 hr in length. Cells at the G/sub 1//S boundary are insensitive to x-ray induced perturbations of bulk DNA synthetic rates when exposed to doses less than 1000 rad. At doses in excess of 1000 rad, these cells are inhibited from replicating their DNA for a time, but ultimately replicate near-control levels of their DNA. Cells irradiated in S phase again show no effects of x-ray doses below 1000 rad on their ability to replicate bulk DNA. After a 3000-rad exposure, however, the rate of DNA replication in these S-phase cells is markedly reduced compared to that of controls. Irradiation of asynchronous cells with dosesmore » from 150 to 3000 rad does reduce the rate of semiconservative DNA replication in these cultures in a dose-dependent manner. These results confirm that x-ray doses greater than 1000 rad reduce the rate of DNA synthesis in irradiated S-phase cells, thus prolonging the length of S phase. The combined data from asynchronous or synchronized cultures, irradiated with x-ray doses less than 1000 rad, indicate that at least a portion of the reduction in DNA replication rates in irradiated asynchronous CHO cultures is due to the x-ray induced G/sub 1/ block, which reduces the overall number of cells in S phase after irradiation.« less