End group characterization in DNA of thymocytes after low doses of ionizing radiation.

Investigations into the configuration of the radiation induced strand breaks in the low dose range are presented. DNA sections containing the radiation induced strand breaks were separated from the undamaged sections in order to increase the concentration of lesions. The configuration of 3'terminals in damaged DNA from gamma-irradiated thymocytes was analysed studying the priming ability for the DNA polymerase I. The experiments show that soon after irradiation with a dose of 10 krad DNA strand breaks carry 3'OH end groups as well as damaged 3'terminals not susceptible to Dna polymerase I. The fraction of damaged terminals increases with dose. We conclude that also after low doses an exonucleolytic action is required in cells for removing damaged 3'terminals before repair DNA synthesis can take place.