The mitotic delay of two biochemically different G1 cell populations of cultured 48-hour stationary phase (S.P.) and synchronized pea root meristems was measured after exposure to 300 R of gamma rays. The indices of delay were the transition from G1 to S and the time required for the population to divide (G1 → S → G2 → M). The criteria for biochemical differences were the time required to enter the S period and the sensitivity of DNA synthesis initiation to actinomycin D, puromycin, and cycloheximide. The population of S.P. meristems was delayed in G1 for approximately 5 hours but subsequently traversed S and G2 at a control rate; puromycin treatment prevented all cells from advancing to S for at least 12 hours and none divided for 24 hours. Actinomycin D treatment failed to stop about 10% of the G1 cells from moving to S, and a few of these eventually divided. Synchronized cells at the G1/S boundary progressed to S unimpaired but were delayed somewhere in either S or G2 and they appeared in mitosis 6-7 hours later than the controls. When irradiated and treated with an inhibitor, these cells were not prevented from advancing to S. It was concluded that defective protein synthesis was the cause of delay in all irradiated G1 cells and that the time and cycle period where it occurred varied and was dependent on whether the requisite proteins for functional completion of a given period were already formed at the moment of exposure.

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