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.

The sucrose gradient and native DNA S20,W, an examination of measurement problems

Sedimentation coefficients of T7, T2H AND T4 DNA were determined with isokinetic sucrose gradients in both 0.1 M and 1 M NaCl. The s values were completely equivalent to those measured by analytical ultracentrifuge and no reduction of s20,w was observed due to the presence of sucrose (anomalous sedimentation). s20,w values are calculated on the basis of both partial specific volume (v) and apparent specific volume (0). Using the latter value s20,w molecular weight relations are derived for 0.1 M and 1 M NaCl solvents. The glucosylation of T2H and T4 DNA appears to influence s20,w in a manner disproportionate to the molecular weight added by glucose.