Abstract
It is argued in this lecture that in most eukaryotic cells onset of mitosis is coupled to attainment of a critical cell mass and to completion of the previous S-phase. In fission yeast these controls operate through a regulatory gene network that activates the p34cdc2 protein kinase at mitosis. This is brought about by dephosphorylation of a tyrosine residue located in the ATP binding site of the kinase. The p34cdc2protein kinase is also important for regulating the onset of mitosis in vertebrate cells suggesting that there is a universal control regulating mitosis in all eukaryotic cells.