DNA-REPLICATION IN X-IRRADIATED CHINESE-HAMSTER CELLS MADE PERMEABLE TO DEOXYRIBONUCLEOSIDE TRIPHOSPHATES

Chinese hamster ovary cells that were X-irradiated (500-2000 rad) and permeabilized to deoxyribonucleoside triphosphates by exposure to 1% Tween 80 in 0.25 M sucrose incorporated [3H]dTMP to the same extent as did unirradiated control cells. If the cells were returned to growth conditions ('holding' experiments) for 30 to 120 min after irradiation, but before permeabilization, subsequent [3H]dTMP incorporation was depressed by about 20%. By 3 hr postirradiation, such cells had fully recovered. This 20% depression of DNA synthesis is too much to be accounted for by a failure of irradiated cells to enter S-phase. Furthermore, autoradiography showed that the same fraction of the cells was actively synthesizing DNA whether or not they had been irradiated and/or held. The data support the conclusions that low doses of X rays inhibit initiation of DNA synthesis in individual replicons.