[Effect of irradiation at different stages of the mitotic cycle on the formation of sister chromatid exchanges in a Chinese hamster cell culture].

Cultured Chinese hamster cells (line 237) grown in a medium containing 5-bromo-deoxyuridine were irradiated with 1 Gy of 60Co gamma-rays and fixed at different times there after 1-18 hours. Sister chromatid exchanges (SCE) were analysed in harlequine-stained cells. It was established that SCE may occur involving only part hitherto only recorded in plant cells. Radiation produced 18 fold increase of the frequency of SsCE, whereas the increase of usual SCE was less than two fold. The shape of stage-effect curves for induces SCE and SsCE was the same; the maximal yield was observed in the middle of the S stage, and no exchanges were produced in the G2 stage. It is concluded that SsCE is nothing other than minute intercalary SCE produced by the single-hit mechanism.