RNA SYNTHESIS IN THE DIFFERENT PHASES OF CELL CYCLE OF CHICK EMBRYO CELLS

RNA synthesis was studied at different phases of the cell cycle of chick embryo fibroblasts, which were synchronized by medium replacement in the confluent phase. The synthesis of DNA started at 4 hr and continued for 8 hr. RNA synthesis increased with time after medium change. The ratio of total amount of radioactivity in nuclear RNA prepared at 0, 2 and 8 hr was 1.0:1.03:5.05. The distribution of radioactive RNA in the sedimentation pattern was similar, showing remarkable incorporation in 45S region of ribosomal precursor RNA. The base composition of newly synthesized RNA, however, varied at different time intervals after medium replacement. Even within the G1 phase, the molar percentage of G and C was quite different. Treatment with actinomycin D at a concentration of 0.02 μg/ml for 1 hr specifically inhibited ribosomal RNA synthesis. At 2 hr after medium change, ribosomal and AU-rich RNA including larger than 28S were synthesized in about equal amounts.