Contribution of Various Types of Damage to Inactivation of a Biologically-active Double-stranded Circular DNA by Gamma-radiation

SummaryThe double-stranded circular DNA of the bacteriophage PM2 has been irradiated in oxygenated solution by 60Co gamma rays. The following quantities have been determined as a function of dose: The average number of single-strand breaks and that of double-strand breaks per molecule, the biological activity of the irradiated sample as such, the biological activity after mild denaturation (to denature molecules containing a break) and the biological activity after denaturation and membrane filtration (i.e. the biological activity of DNA without single- and double-strand breaks.)From the collected data it can be deduced that 4·5 ± 0·5 per cent of the inactivation is a consequence of double-strand breaks, 8·5 ± 4·2 per cent of single-strand breaks and 87·0 ± 4·2 per cent of nucleotide damage. Only about 2 per cent of the single-strand breaks is lethal, whereas the efficiency of inactivation of nucleotide damage is about 30 per cent of the nucleotide damage which is lethal if present in single-stranded DNA.Preliminary experiments with circular double-stranded RF-DNA of bacteriophage ϕX174 gave similar results.