Induction and repair of DNA strand breaks were measured in X-irradiated CHO, V79 and EAT cells using either the alkaline unwinding or the neutral filter elution techniques. After irradiation on ice, the cells were incubated at 37 degrees C for various times to allow for repair. The repair curves obtained were quite similar and the fast initial decline was always characterized by a half-time of 6-8 min and the final slow phase by a half-time of about 200 min independently of the technique used. The curves were found to differ only in the relative fraction of damage repaired during the fast or slow phase. From the similarity of the half-times it is concluded that the fast phase recorded by alkaline unwinding reflects the repair of single-strand breaks but also the repair of fast rejoining double-strand breaks. Comparing the known ratio of induced single- and double-strand breaks with the measured ratio of fast and slowly rejoining strand breaks as derived from the kinetics, it can be further concluded that the slow phase measured by alkaline unwinding covers the repair of both slowly rejoining double strand breaks as well as slowly rejoining single strand lesions.
