Mitotic Delay and the Regulating Events of Plant Cell Proliferation: DNA Replication by a G1/S Population

Earlier studies with inhibitors suggested that impaired synthesis of requisite proteins for cycle progression was the cause of mitotic delay in irradiated G1 cells. Cells synchronized at the G1/S boundary were not delayed in initiating DNA synthesis, but they were subject to mitotic delay. The present experiments were designed to determine precisely where in the cycle the cells were delayed and whether disrupted protein synthesis was involved. Synchronized cells were exposed to puromycin or irradiated prior to or during DNA synthesis. Neither puromycin exposure nor irradiation prevented the cells at G1/S from initiating DNA synthesis. With populations irradiated at G1/S, DNA replication occurred uninterrupted, followed by a major prolongation of the G2 period. Exposure to puromycin or irradiation during the DNA synthetic stage, however, delayed the transit of S, demonstrating a separation of the events controlling initiation and completion of DNA replication. It was concluded that (a) the radiosensitivity...