Further Data on DNA Strand Breakage by Various Radiation Qualities

Abstract

Thin films of dry DNA (∼ 30 per cent water content) from T7 bacteriophage were irradiated by charged particles with track segment LET_∞ values up to 255 keV/µm. Mean numbers of single- and double-strand breaks per molecule were determined by boundary sedimentation in an analytical ultracentrifuge. For both types of break, the number per unit dose increased only slightly up to about LET_∞ = 100 keV/µm, but more rapidly beyond that point; the ratio of the numbers of the two types of breaks did not change systematically throughout the whole range of LET_∞. The data have been analysed to deduce the separate contributions of the primary track cores and the delta-rays to total dose and to total single- or double-strand breakage. Breakage by the primary cores per unit dose from cores begins to increase markedly above a core LET₁₀₀ around 50 keV/µm. The increase could be explained by effective cooperation between two or more events of small size occurring together at high LET but which would be rather ineffective when occurring singly at low LET. On the basis of estimates of core diameters currently adopted in radiation chemistry, it appears that one intersection of the DNA molecule by a core with high LET₁₀₀ produces several single-strand breaks, implying long-range intramolecular transfer of energy. This conclusion could be avoided by postulating that core diameters are larger by an order of magnitude than the presently accepted values.