The timing of initiation of DNA synthesis in Paramecium tetraurelia is established during the preceding cell cycle as cells become committed to cell division.

The timing of initiation of DNA synthesis (IDS) in Paramecium is established before cell division at a point located at about 0.75 in the preceding cell cycle. This point occurs about 90 min prior to fission and coincides with the point at which cells become committed to cell division. The location of the point at which the timing of IDS is set was deduced from a series of nutrient-shift experiments. Changes in nutrient level lead to changes in the duration of the subsequent G1 interval when they occur more than 90 min prior to fission. Perturbation of the cell cycle so that the timing of commitment to cell division is altered, results in a parallel shift in the point at which the timing of IDS is established.