No abstract

The yields of immediate and residual breaks in DNA of X- and peak pion irradiated Chinese hamster cells were measured by hydroxylapatite chromatography in order to investigate the r.b.e. of immediate break production and the relation between residual breaks and survival in the same low dose range of up to 10 Gy. A linear dose response for immediate break induction was observed with an r.b.e. of 0.37 for peak pions, whereas the formation of residual breaks showed a linear-quadratic dose dependence for both types of radiation. The dose-effect curves of residual break can directly be correlated with the corresponding survival curves for both radiation types used indicating that residual breaks are lethal events (most probably unrepaired double strand breaks) formed in this low dose range by single- and multi-hit interactions. Despite their lower efficiency in formation of immediate breaks peak pions produce more residual breaks per dose than X-rays.

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.