Inhibitor dilution allows for size homeostasis.

(A) Influence diagram of the cell cycle model. Barbed edges indicate ‘activation’, blunt edges indicate ‘inhibition’, plus sign indicates positive feedback at Start. The synthesis of all proteins except Whi5 is assumed to be size-dependent. (B) Concentration of the size-independent inhibitor of Start, Whi5, and the size-dependent activator, Cln3, in G1-phase. Activation of SBF marks the onset of Start (dashed line). (C) Stable (solid) and unstable (dashed) steady states of active SBF with respect to cell volume. Arrow indicates Start transition. (D) Concentrations of cell cycle regulators (top and middle) and cell volume (bottom) over multiple generations. Dashed and dotted vertical lines mark Start and cell division, respectively. Model follows the daughter cell (bud) after each division. (E) Simulated amount of Whi5 over multiple cell cycles. The G1-phase and S/G2/M period are indicated. (F) Correlation between Whi5 concentration and cell size at birth. Cell size was varied by changing the specific growth rate (see Methods for details). (G) Correlation between G1 length and cell size at birth.