| Abstract: | The time of median cell division in V79 Chinese hamster cells following high serum pulses was determined for two synchronous cell generations following mitotic selection. Differences in cell cycle time for each pair of pulse and control cultures were computed and plotted as a function of time of serum pulse. This phase response curve for hamster cells with an 8.5 h cell cycle shows a characteristic biphasic pattern. Beginning 0.5 h after mitotic selection, pulses with serum produce delays in the midpoint of the subsequent mitotic waves. Delay is maximum at 1.5 h. Delays give way abruptly to advances at 2.5 h and the amount of advance then decreases as pulses are given between 3 and 5 h into the cycle. At 5 h decreasing advances become delays, with increasing delays due to serum pulses occurring between 5 and 6 h. Delays again give way abruptly to advances at 6 h and again the amount of advance decreases through the late portion of the cycle. Pulses very late in the cycle appear to generate phase delays. This biphasic response to serum is interpreted as an expression of an underlying time-keeping oscillator whose period is nominally of 4 h duration. |
