Effects of Ionizing Radiations on DNA Replication in Cultured Mammalian Cells

Abstract

presence of cold thymidine reflected a depression of net DNA synthesis. Radiosensitive steps in DNA synthesis were examined by the use of alkaline sucrose gradient centrifugation. The rate of replication along the DNA strands was inhibited to a lesser extent than that of over-all DNA synthesis. The labeling patterns of DNA exposed to ['H]thymidine for 20 min indicated that ionizing radiation preferentially interfered with the formation of small-size 'H-labeled DNA pieces. These results suggest that the initiation of DNA replication is more radiosensitive than the elongation of DNA strands whose replication has already been initiated.