Mapping the mitotic clock by phase perturbation

In synchronized V79 cells perturbed by serum, heat shock, or ionizing radiation at half-hour intervals through a modal 8.5-hour cell cycle, phase-response curves show a characteristic biphasic pattern of advances and delays in subsequent cell divisions. These observations, together with previous observations of quantizement of generation times in this an other cell lines have led us to consider a model incorporating, in the simplest case, a two-component oscillator with two threshold crossings required per cell cycle. By assuming that oscillator variables respond in a simple way to the experimental perturbations, for example, by first order destruction due to heat shock, a map of the qualitative features of the oscillator can be obtained by matching simulated with experimental phase response curves. Random fluctuations in oscillator variables about a fixed trajectory lead to subthreshold oscillations and result in a distribution of generation times which is roughly a negative exponential, but quantized within this exponential envelope. The extent of the random fluctuations can be determined from comparison with data on desynchronization of a cell population after mitotic selection. The same parameters which correctly simulate phase response and the desynchronization data also give good agreement with generation time distribution data.