Some properties of a "G0" -model of the cell cycle. II. Natural constraints on the theoretical model in exponential growth conditions.

The two-phase (G and C phases) model first proposed by Burns & Tannock (1970) to describe the cell cycle kinetics has the major advantage of requiring only two parameters for a complete description of the kinetic behaviour of populations that are in a steady-state, or that grow exponentially (with no cell loss from the population). Steady-state populations were examined in paper I of this series. Exponential populations with no cell loss are investigated here. The model assumes two basic kinetic states--a 'C' phase which includes S, G2, M and perhaps part of G1, and a 'G' phase which cells enter after completing the C-phase and from which either are lost or return to C-phase randomly. The model assumes that transit time through C-phase is constant for all cells in the population. An original method is described which allows the determination of two independent parameters of the model from the experimental 'fraction of labelled mitoses' (FLM) curve; From those two parameters, the ratio of G-cells among the total number of cells (NG/N) has been calculated for each cell population studied, The range of the NG/N values thus obtained is fairly restricted, and the mean NG/N value for exponential growths is not statistically different from that found in steady-states considering in that case the only sub-population of cycling cells (i.e. the cells that will undergo a further mitosis).