(*A*) The *wee1*^{−} cell cycle. Simulated time courses for the major components. Parameters given in Table . Rum1 (total) = *R* + *G1R* + *G2R* + *PG2R*, Cdc13 (total) = *G2K + G2R + PG2 + PG2R*, Cig2 (total) = *G1K + G1R*. The cycle time is 140 min, identical to the mass doubling time. (*B*) Phase portrait of the Start transition. As explained in the , we select from Fig. only those steps involved in the synthesis, degradation, and interactions of Cdc13 and Rum1, thereby reducing the complete system to just two differential equations (**A6**, **A7**). The properties of this two-dimensional subsystem are conveniently portrayed as balance curves (solid lines) in the phase plane (). The dashed curve shows how the activity of Cdc13/Cdc2 is quenched as Rum1 accumulates. In pre-Start, the balance curves intersect in two stable steady states (the • labeled **G**_{1} and **G**_{2}) and an intermediate unstable steady state (○). In post-Start, the balance curves intersect only in the **G**_{2} steady state. As the cell grows, the Rum1 balance curves moves down, causing the **G**_{1} state to disappear (by a saddle-node bifurcation) and the system to proceed along the dotted trajectory to the **G**_{2} state. (*C*) Unbalanced growth and division in *wee1*^{−}* rum1*Δ mutants. Parameter values as in Table , except *k*_{3} = 0 (therefore, *R* = *G1R* = *G2R = PG2R* = 0). Both size controls, at G_{1}/S and G_{2}/M, are inoperative. There is no stable steady state (checkpoint) at which the cycle can pause to query cell size. Instead the control system executes autonomous (limit cycle) oscillations with a division time (85 min) shorter than the mass doubling time (140 min). Hence cells get smaller each cycle.

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