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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. Part II. 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. Even with low doses of neutrons, there is a visible increase in the production of a smaller subset of DNA fragments which arise only after very high dose X-irradiation. In both cell lines, dsbs induced by neutrons are rejoined more slowly than those induced by X-rays. At long repair times (4 and 17 h) there are no significant differences in the fractions of unrejoined dsbs between neutrons and X-rays. We propose that neutron-induced dsbs have a higher probability of becoming lethal because they are more likely to be misrepaired during the slow stage of rejoining.