Repair of radiation-induced DNA strand breaks does not occur preferentially in transcriptionally active DNA

Certain DNA base lesions induced by ionizing radiation or oxidative stress are repaired faster from the transcribed strand of active genes compared to the genome overall, In this study, it was investigated whether radiation-i nduced DNA strand breaks are preferentially repaired in active genes compar ed to the genome as a whole in CHO cells. The alkaline unwinding technique coupled to slot-blot hybridization with specific DNA probes was used to stu dy the induction and repair of DNA strand breaks in defined DNA sequences. Results using this technique showed a linear dose response for the formatio n of radiation-induced DNA strand breaks in the dihydrofolate reductase (DH PR) gene. Furthermore, the half-life of radiation-induced strand breaks was less than 5 min in the DHFR gene, in the ribosomal genes, and in the genom e as a whole. These results suggest that the repair of DNA strand breaks is fast and uniform in the genome of mammalian cells, (C) 1999 by Radiation R esearch Society.