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When cultured Chinese hamster ovary (CHO) cells are exposed to acute doses of ionizing radiation at different times during the cell division cycle, there is a characteristic cell-cycle response for radiation-induced cell killing and induced resistance to 6-thio-guanine (6TG). For cell killing the sensitive periods of the cell cycle are the G1, G2, M, and early S periods, as others have reported. For mutation induction the sensitive stage is the G1 period with the maximum sensitivity near the boundary between the G1 and the S period. Cells appear to be very refractile to induction of 6TG resistance in other periods of the cell cycle. These results suggest that chromosomal rearrangements of the X chromosome are most likely to occur in the G1 period before the gene for hypoxanthine-guanine-phosphoribosyl-transferase replicates, most likely due to genetic recombination. Clones resistant to 6TG after exposure to x rays are most likely induced by a different mutagenic pathway than ones stimulated by ultraviolet (uv) or ethylnitrosourea treatments, since the mutation induction patterns in the cell cycle are quite different.

Ionizing Radiation-Induced 6-Thioguanine-Resistant Clones in Synchronous CHO Cells