Gamma-ray induced double-strand breaks in DNA resulting from randomly-inflicted single-strand breaks: temporal local denaturation, a new radiation phenomenon?

The induction of single- and double-strand breaks in DNA by gamma-rays has been measured. The maximum number of nucleotide pairs (a) between two independently induced single-strand breaks in opposite strands of the DNA which cannot prevent the occurrence of a double-strand break was found to amount to about 16. This value did not differ significantly for the four types of bacteriophage DNA investigated (T4, T7 and PM2 DNA, and replicative form DNA of phage phiX174) and was the same in 10(-2) M phosphate buffer containing 0, 0.5 or 1 M NaCl. In 10(-3) M phosphate buffer a was 34 nucleotide pairs. Evidence is presented that the relatively large value of a has to be ascribed at least partly to a temporal local denaturation accompanying the induction of a single-strand scission. A contribution of base damage that labilizes the DNA-helix, between two single-strand breaks to the high value of a can not be excluded.