DNA synthesis in relation to cell division in Tetrahymena pyriformis

The effect of different nutritional conditions on the durations of the G1, S, G2 and D periods of exponentially replicating Tetrahymena pyriformis strain HSM were determined with autoradiographic techniques and observation of individual cells. The data indicate that, with decreased availability of nutrients, DNA synthesis is initiated proportionately later in the cell division cycle and that the proportion of the cell cycle spent in DNA synthesis increases. The effects of temperature on the relationship of the phases of the cell cycle were examined. Between 29° and 20°C, the generation time and the time spent in the G2 period about double while the time spent in the S phase remains about the same. This phenomenon results in a reduction in the proportion of the generation time spent in the S phase and a compensatory increase in the G1 phase proportion. These results are discussed as they relate to nutritional factors involved in initiation and maintenance of DNA synthesis.