The precise cell cycle time of association between labeled DNA (the radiation source) and the non-DNA cell structure whose damage is responsible for radiation-induced division delay was measured. Mitotic cells were selected from a monolayer of Chinese hamster ovary cells for 80 min (nine shakes) to establish the rate of cell progression into mitosis. The cell monolayers were then exposed to 0.1295 MBq/ml125 IUdR for 10 min to label the cells in S phase. After pulse labeling, mitotic cell selection was continued for various times (between 0 and 120 min) before125 I decays were accumulated at 4°C. After 2 h in the cold, the cells were rewarmed and the selection of mitotic cells was continued. (Cooling had a small, transient affect on subsequent cell progression.) As the time between labeling and cooling was increased, the fraction of cells selected in mitosis decreased, indicating that an increasing proportion of125 I-labeled cells had entered a sensitive phase of the cell cycle where125 I decays are particularly effective in producing radiation-induced division delay. It is hypothesized that during this sensitive period (from -25 to +90 min of the${\rm S/G}_{2}$ boundary), the labeled DNA comes into sufficiently close contact with a non-DNA structure to facilitate damage to this structure by overlap irradiation from125 I decays in the DNA.

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