KINETICS OF CHROMOSOME REJOINING IN NORMAL HUMAN FIBROBLASTS AFTER EXPOSURE TO LOW-LET AND HIGH-LET RADIATIONS

To determine whether chromosome breaks produced by alpha particles are processed differently from those produced by X rays, the premature chromosome condensation technique was used to follow chromosome rejoining after irradiation. Doses of 90 and 200 keV/mu m alpha particles (2.7 Gy) and 250 kVp X rays (6 Gy) were chosen to produce approximately the same number of initial chromosome breaks (about 30 excess fragments per cell). Frequencies of excess fragments were assessed at eight times to 24 h after irradiation with the final yields being about 2, 4 and 8 excess fragments per cell for 250 kVp X rays and 200 and 90 keV/mu m alpha particles, respectively. For each radiation the time for the initial measured fragment frequency per cell to be halved (i.e. to about 15) was the same (about 100 min). The results were fitted to three models of kinetics of the rejoining, and the initial and residual number of excess chromosome fragments as well as the rate of rejoining were determined. Even with eight times, discrimination between the models of the kinetics was not possible, such that a single-cornponent first-order reaction could not be rejected for either X-ray- or alpha-particle-induced breaks. Although rejoining proceeds at similar rates, the probability of ''correct'' rejoining is apparently reduced for alpha-particle-irradiated cells.