Increase in macromolecular amounts during the cell cycle of Tetrahymena: a contribution to cell cycle control

Increases in RNA, protein and cell size were determined cytophotometrically during the cell division cycle of Tetrahymena. For these parameters different patterns were found. RNA accumulates slowly during G1 period and faster during macronuclear S. This agrees with the changing uridine incorporation rate which is at least partly related to the varying macronuclear DNA amount. Increases in protein content and cell size occur mainly during G1 and G2. This pattern was confirmed by determining the RNA : protein ratio in individual cells. It is minimal at the end of the G1 period. These findings and evidence from the literature suggest that initiation of DNA replication is under negative control by the relative RNA content of the cell.