The Cell Cycle

SYNOPSIS. I review recent advances in our knowledge of the eucaryotic cell cycle: the set of processes by which cells grow and divide. Genetic approaches to the cell cycle of somatic cells identified a pathway of events where the initiation of each event was dependent on the successful completion of the preceding event, as well as a single key gene, cdc2, that is required both at the beginning and at the end of the cell cycle. The alternative approach of studying the cell cycle biochemically in early embryos provided evidence for a cytoplasmic oscillator which alternated between mitosis-inducing and interphase-inducing states and identified the mitosis-inducing component as maturation promoting factor (MPF). These two very different views of the cell cycle initially seemed irreconcilable. However, a link between the somatic and embryonic cell cycles was provided by the recent discovery that the cdc2 protein is one of the components of MPF. In the embryonic cell cycle the activation of MPF and induction of mitosis is triggered by the accumulation of a protein named cyclin which becomes a component of MPF. Somehow, MPF induces the proteolytic degradation of cyclin, which inturn allows MPF to be inactivated and allows the cell cycle to pass from mitosis into interphase. The more complex cell cycle of somatic cells is probably derived from the embryonic cyclin-based oscillator by imposing a system of checks and balances on the accumulation and destruction of cyclin. I also present some thoughts on the relationships between science and society, and comment on the way in which scientists describe their work to the lay world.