Fig 2. Schematic representations of stochastic cell cycle checkpoint models.

(A) The DDT model random variable is depicted as a ball stochastically traversing a staircase. The ball can move both up and down the staircase (diffusion) but the probability of taking a step up is greater than taking a step down (drift; thick arrow vs. thin arrow). Checkpoint passage occurs immediately upon the ball reaching the top of the staircase (threshold), emulating a bistable switch. (B) The EMPF model is depicted as a staircase connected to a basin. Once the ball reaches the top of the staircase it falls into the basin, from which time to escape is exponentially distributed. In contrast to the DDT model, here the ball can only move up the staircase (single arrow). The transition-probability model of Smith and Martin [15] corresponds to the special case in which the time to traverse each step is constant (delayed exponential). The EMG [38, 39] relaxes this assumption, so that the total time to traverse the staircase is Gaussian distributed. See S1 File for a detailed mathematical comparison of the two models.