EXPONENTIAL-GROWTH, RANDOM TRANSITIONS AND PROGRESS THROUGH THE G(1) 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 phen omenon discussed before the present expanding era of cell cycle resear ch. 'Random transition' and exponential slopes of alpha- and beta-curv es were conceived in the 1970s and early 1980s to explain cell cycle p rogression. An exponential behaviour of the beta-curve was claimed as being necessary and sufficient for a 'random transition' in the cell c ycle. In our present work, similar slopes of those curves were shown t o materialize when the increase in mass of single cells was set as exp onential in a structured cell cycle model where DNA replication and in crease in cell mass were postulated to be two loosely coupled subcycle s of the cell cycle, without introducing any 'random transition'. Find ings published in the 1980s demonstrating the effect of serum depletio n of 3T3 Balb-c cells were simulated and the shallower slope of the al pha- and beta-curves found experimentally could be attributed to the r educed rate of exponential growth in cell mass, rather than to a reduc ed 'transition probability'.