Cell cycle dynamics of an individual fission yeast cell. Simulation results of three different cell cycle models. Inactive MPF is depicted by dark green curves and light green denotes activated MPF. (A) Stochastic variant of the cell cycle model presented in . Simulation parameters are similar to the parameters that have been used to produce Figure 3a in , i.e. k3 = 180 min-1 and k4 = 0.9 min-1. (B) Model includes downward causation by dynamic adjustment of the MPF dissociation rate due to an increase of the cell volume. Parameters are equal to those presented for model 1 in the additional file 2. (C) Multi-level model comprising of downward and upward causation. MPF dissociation depends on the cell volume and at the same time, transitions from one cell cycle phase to another depends on the intracellular amount of active and inactive MPF. The entire model as well as initial solution and parameters can be found in additional file 2, model 1.