DNA STRAND BREAKS, REPAIR, AND SURVIVAL IN X-IRRADIATED MAMMALIAN-CELLS

The yields of unrepairable single- and double-strand breaks in the DNA of X-irradiated Chinese hamster (V 79-379-A) cells were measured by low-speed neutral and alkaline sucrose density gradient sedimentation in order to investigate the relation between these lesions and reproductive death. After maximal single-strand rejoining, at all doses, the number of residual single-strand breaks was twice the number of residual double-strand breaks. Both double-strand and unrepairable single-strand breaks were proportional to the square of absorbed dose, in the range 10-50 krad. No rejoining of double-strand breaks was observed. In mammalian cells most double-strand breaks are not repairable, while all single-strand breaks are repaired except those that are sufficiently close on complementary strands to constitute double-strand breaks. Comparison with cell survival measurements at much lower doses suggests that loss of reproductive capacity corresponds to induction of approximately 1 double-strand break.