RELATIVE CONTRIBUTIONS OF LEVELS OF INITIAL DAMAGE AND REPAIR OF DOUBLE-STRAND BREAKS TO THE IONIZING RADIATION-SENSITIVE PHENOTYPE OF THE CHINESE-HAMSTER CELL MUTANT, XR-V15B .2. NEUTRONS

We have compared DNA double-strand break (dsb) induction and rejoining , using field-inversion gel electrophoresis, with survival in mutant ( XR-V15B) and in wild-type parental (V79B) hamster cell lines after low dose neutron and X-irradiation. We found that neutrons do not appear to induce more dsbs than X-rays and deduce that increased sensitivity to neutrons is therefore not due to a higher initial yield of dsbs. Ev en with low doses of neutrons, there is a visible increase in the prod uction of a smaller subset of DNA fragments which arise only after ver y high dose X-irradiation. In both cell lines, dsbs induced by neutron s are rejoined more slowly than those induced by X-rays. At long repai r times (4 and 17 h) there are no significant differences in the fract ions of unrejoined dsbs between neutrons and X-rays. We propose that n eutron-induced dsbs have a higher probability of becoming lethal becau se they are more likely to be misrepaired during the slow stage of rej oining.