Survival of Excess Thymidine Synchronized Cell Populations in Vitro after X-irradiation in Various Phases of the Cell Cycle

Summary

Cells from a human kidney-cell line were synchronized in vitro by a double excess thymidine treatment. Exposure to radiation at various times after thymidine synchronization revealed a high radioresistance of cells at the end of S-phase and the beginning of G₂-phase. A blocking of DNA-synthesis at the beginning of S-phase by a third thymidine exposure inhibited progression to a more radioresistant phase; blocking of DNA-synthesis at the end of S-phase maintained the cells in the radioresistant phase. A third thymidine exposure at the beginning of G₂ had no effect on changes in radiosensitivity.