Strand Breaks and 5′ End-groups in DNA of Irradiated Thymocytes

SummaryRat thymocytes suspended in Hank's solution were irradiated with 10 MeV electrons up to 6 Mrads in air at 0°C. Single- and double-strand breaks in the DNA were determined in the analytical centrifuge. 5′ end-groups were characterized by specific phosphorylation with polynucleotide kinase before and after phosphatase treatment.The single-strand breaks are formed with an efficiency value of 109 ± 6 eV per break and the double-strand breaks with 1940 ± 110 eV. 14 per cent of the 5′ terminals carry a hydroxyl group and 20 per cent a phosphate group. Polynucleotide kinase does not react with the other 66 per cent 5′ end-groups.For comparison, dry calf-thymus DNA was irradiated in vacuo. Under these conditions, 22 per cent of the 5′ terminals carry a phosphate end group, whereas no 5′OH groups could be found.