THE EFFECT OF I-125 DECAY AT DIFFERENT STAGES OF S-PHASE ON SURVIVAL,EXPRESSION OF MICRONUCLEI AND CHROMOSOME-ABERRATIONS IN CHO CELLS

Chinese hamster ovary (CHO) cells were synchronized in M phase by mito tic selection, and then re-synchronized with aphidicolin at the G(1)/S phase border. The cells were labelled in early-S phase by 10 min expo sure to I-125-iododeoxyuridine and then cultured (chased) in non-radio active medium for 0.5, 3 or 5 h, followed by harvesting and freezing t o accumulate the desired number of I-125 decays. Cell damage was asses sed by evaluating colony formation, micronucleus formation and chromos ome aberrations. These biological estimators of damage showed that the cytocidal effect of I-125 decay increased with the duration of the po st-labelling chase period: the highest level of damage was found in ce lls from the 5 h chase period and the lowest in the cells from the 0.5 h chase period. Survival curves for the three chase periods displayed low-dose hyper-radiosensitivity for 0 to 20 I-125 decays cell(-1). Th e results indicate that the repair of DNA double-strand breaks (DSBs) may depend on the maturation stage of chromatin and an explanation of this finding is proposed which invokes the homologous recombination mo del for DSB repair.