Fig. 7.

Robustness of the Drosophila model to stochastic noise. A, Simulation of circadian oscillations. Time courses are displayed for the numbers of molecules of all forms of PER (top, gray time course) and of dCLOCK (bottom, black time course). Parameter values in the standard set were scaled to convert units from a nanomolar concentration to number of molecules. Randomness in the timing of single-molecule synthesis and degradation events was incorporated as discussed in the text. The time step was kept small (5 × 10⁻⁶ hr) so that during no time step did the probability of any given reaction occurring exceed 2%. B, Expanded view displaying oscillations of the numbers of molecules of dCLOCK (gray time course) and of the PER-dCLOCK complex (black time course).