Transition probability and the origin of variation in the cell cycle

Abstract

IT is well established that the cell cycle times of cells cultured in vitro are highly variable although the origin of this variability has remained unclear^1,2. A number of theories have been advanced to explain the variation in cell cycle times although none of them give entirely satisfactory fit to the experimental data². Any proposal as to the origin of this variability must take account of the observation that most of the variability occurs in the G₁ phase of the cell cycle and that it is the length and variability of this phase which is the major determinant of the length and variability of cell cycle times within a cell population¹. A new model of the cell cycle has been proposed independently by two groups that suggests that variability in the G₁ phase of the cell cycle arises as a result of a probabilistic transition occurring within G₁ (refs 3, 4). Using time-lapse cine-photography I have examined the possible role of a number of other factors which might determine this variability and have come to the conclusion that the observed variation in cell cycle times in vitro is qualitatively and quantitatively in accord with variation predicted by the transition probability model of the cell cycle⁴.