Association between the division delay target and DNA late in the cell cycle

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/ml 125IUdR 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) before 125I decays were accumulated at 4 degrees 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 of 125I-labeled cells had entered a sensitive phase of the cell cycle where 125I 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 S/G2 boundary), the labeled DNA comes into sufficiently close contact with a non-DNA structure to facilitate damage to this structure by overlap irradiation from 125I decays in the DNA.