Subchromosomal DNA synthesis in synchronous V-79 chinese hamster cells after X irradiation

A substantial fraction of replicon initiation events in Chinese hamster V-79 cells have been shown to be refractory to the effects of X irradiation immediately after exposure. This study examines the possibility that the initiation radiorefractive portion is the result of changes in replicon radiosensitivity as a function of position in S phase. The data obtained from DNA fiber autoradiograms and kinetic incorporation of radiolabeled thymidine from cells irradiated at various positions in S phase showed only slight changes in the proportion of replicons refractive to X irradiation immediately after exposure. These results indicate that initiation radiorefractive replicons may be an intrinsic property of V-79 cells and that cell-cycle-specific heterogeneity in radiation response cannot fully account for this phenomenon. The results also indicate that delayed inhibition of initiation events may play a larger role in the observed radiorefractive fraction than previously thought.