A modular systems biology analysis of cell cycle entrance into S-phase

A modular systems biology approach to the study of the cell cycle of the budding yeast Saccharomyces cerevisiae is presented. Literature on the structure of yeast population and its relevance to the study of yeast cell cycle is reviewed. A model for the control of yeast cell cycle, with emphasis on a threshold mechanism controlling entrance into S-phase is presented. The simple model has been used as a framework to derive a molecular blow-up of the major upstream events controlling the G1 to S transition that involves two sequential thresholds cooperating in carbon source modulation of the critical cell size required to enter S-phase, a hallmark response of the cell cycle to changing growth conditions. The model is discussed as an aid to filter and give structure to post-genomic data. The iterative application of this approach allows to obtain more refined models capturing the major regulatory features and the molecular details of the circuits connecting cell growth to cell cycle.