DEFICIENT REPAIR AND DEGRADATION OF DNA IN X-IRRADIATED L5178Y S-S CELLS - CELL-CYCLE AND TEMPERATURE-DEPENDENCE

The rejoining of DNA strand breaks induced by X rays in the radiosensitive S/S variant of the L5178Y murine leukemic lymphoblast has been studied by alkaline-EDTA-sucrose sedimentation using swinging-bucket and zonal rotors. After irradiation, incubation resulted in an increase in DNA size, but the DNA structures were not restored in all cells, even when the X-ray dose was only 50 rad. Subsequently, 10 to 20 hr after irradiation, heavily degraded DNA began to appear. When cells were irradiated at different parts of the cycle, the extent of DNA degradation varied in a fashion similar to survival: Least DNA degradation was found after irradiation at the most radio-resistant stage (G1 + 8 hr), and most DNA degradation occurred after irradiation at the radiosensitive stage (G1). Changes in cell survival caused by postirradiation hypothermia were also reflected in the extent of DNA degradation. Populations of G2 cells, which show marked increases in survival after postirradiation hypothermic exposure, exhibited a lower level of DNA degradation, whereas populations of G1 + 8 hr cells, whose survival is affected little by postirradiation hypothermia, showed limited changes in DNA degradation. The onset of degradation was delayed by hypothermia in all cases.