Targets for Radiation-induced Cell Death: Target Replication During the Cell Cycle Evaluated in Cells Exposed to X-rays or 125I Decays

Chinese hamster ovary cells were labelled with 125I-iododeoxyuridine (1·15 × 103 Bq/ml) for 12 h, then synchronized by mitotic selection, plated for cell cycle traverse, and harvested during successive stages of the cell cycle for freezing and accumulation of 125I decays. Cell viability was evaluated by the colony-forming assay. Cells subjected to 125I decays during the G1 phase exhibited exponential survival curves with an N = 1 and a D0 = 38–41 decays/cell. A continuous increase in 125I resistance was observed as cells progressed through the S phase and cells in late-S/G2 yielded shouldered survival curves with a N = 2 and a D0 = 78–84 decays/cell. After mitosis, the radiation resistance of cells returned to G1 values. These findings suggest that the primary target for radiation-induced cell death is duplicated during S phase, with G1 cells containing one target and G2 cells two targets. Dual targets, although located within a single cell, act as independent entities as if already distributed between tw...