Induction and repair of DNA double strand breaks

The induction of DNA double strand breaks (DSB) by different types of ionising radiations was measured in diploid yeast cells using pulsed field electrophoresis. Apart from 80 kV-X-rays and alpha particles a large range of accelerated heavy ions were studied obtained either from GSI, Darmstadt, or HMI, Berlin. Results are reported as induction cross sections, i.e. the probability of a single particle to cause a DSB. Cross sections increase for LET values with ion charge approaching a transient plateau. With heavier ions they increase with ion energy (i.e. with decreasing LET) yielding 'hook' structures. In this regime the action of each ion has to be represented by a separate curve and there is no unique dependence on any physical parameter. The relative biological effectiveness (RBE) is clearly above unity in the LET region between 100 and about 400 keV/μm but falls for higher LET-values. Rejoining of DSB was also measured in a number of selected experiments. For X-rays there seems to be a two-phasic disappearance of DSB with time, with heavy charged particles only the slower component persists. (orig.)