A G1 rate model accounts for cell-cycle kinetics attributed to 'transition probability'.

Smith and his colleagues have proposed that the duration of the cell cycle is determined by a random transition, analogous to the random decay of a radioactive nucleus, by which a cell passes from an 'A state' within the G1 phase to a 'B phase' that includes the rest of the cycle. The experimental support for this transition probability hypothesis is the tendency of a cumulative plot of differences of cycle times of sibling cells (the beta curve) to be exponential ad parallel to the exponential tail of a cumulative plot of the cycle times themselves (the alpha curve). However, a close examination of four of the most extensive sets of experimental data now suggests that the two beta curves with the steepest slopes may not, in fact, be exponential. These and all the other characteristics of the experimental curves are best matched by computer simulations using a cell-cycle model that will be termed here a G1 rate model. This model is consistent with differences in cell metabolism, rather than a transition at an inherently unpredictable time, being the physiological basis for differences in cycle times within a cell population.