Mitotic delay in two biochemically different G1 cell populations in cultured roots of pea (Pisum sativum).

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 h...