Radiation-induced mitotic delay of exponentially growing populations (immediate decline of mitotic index followed by recovery of mitotic index) was studied in cultured mouse leukemic (L5178Y) cells by using five different methods of localizing the site of the radiation-induced block of cell progress in their life cycle. This was compared with the site of the blocks caused by actinomycin D and puromycin and with the time of switching-off of RNA synthesis. It was found that the x-ray block induced by 200 rads occurred in the middle of the G2 stage; the puromycin-induced block (10 μg/ml) also occurred in the middle of the G2 stage; however, the actinomycin D-induced block (0.5 μg/ml) was localized somewhere at the late S or in the early G2 stage. The switching-off time of RNA synthesis was about 10 minutes before the end of the G2 stage. In the recovery phase of mitotic delay, actinomycin D (0.5 μg/ml), ouabain (10-4 M), and dinitrophenol (10-4 M) failed to interfere with the reappearance of mitotic cells, but puromycin (10μg/ml) significantly interfered with recovery from the mitotic block. From these results, it is concluded that radiation-induced mitotic delay is caused by a complete but transient blockage of cell progress in the middle of the G2 stage, and it is speculated that radiation-induced mitotic delay involves inhibition of protein synthesis.

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