Association between the Division Delay Target and DNA Late in the Cell Cycle Available to Purchase

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.