Exponential growth, random transitions and progress through the G1 phase: computer simulation of experimental data.

At a time of increasing knowledge of gene and molecular regulation of cell cycle progression, a re-evaluation is presented concerning a phenomenon discussed before the present expanding era of cell cycle research. 'Random transition' and exponential slopes of alpha- and beta-curves were conceived in the 1970s and early 1980s to explain cell cycle progression. An exponential behaviour of the beta-curve was claimed as being necessary and sufficient for a 'random transition' in the cell cycle. In our present work, similar slopes of those curves were shown to materialize when the increase in mass of single cells was set as exponential in a structured cell cycle model where DNA replication and increase in cell mass were postulated to be two loosely coupled subcycles of the cell cycle, without introducing any 'random transition'. Findings published in the 1980s demonstrating the effect of serum depletion of 3T3 Balb-c cells were simulated and the shallower slope of the alpha- and beta-curves found experimentally could be attributed to the reduced rate of exponential growth in cell mass, rather than to a reduced 'transition probability'.