Cell proliferation involves both control of progress through the current cell cycle and coordination of successive cell cycles. We have focused our attention on the events that trigger traversal of the G1/S boundary of the cell cycle. A protein kinase activity was found in preparations of the DNA-replicative complex from the budding yeast Saccharomyces cerevisiae. The activity phosphorylated only a few of the proteins present in the replicative fraction, and it displayed a marked preference for a 48-kDa polypeptide. Most importantly, the protein kinase activity was heat-sensitive in replicative fractions from cdc7 cells, a mutant that arrests at the G1/S boundary at restrictive temperature. The results suggest that phosphorylation of components of the replication machinery may play a role in control of initiation of DNA replication during the cell cycle. We have also begun an analysis of cellular aging in yeast, as a means of addressing the problem of coordination of successive cell cycles. Yeast cells have a finite life span defined by reproductive capacity. With age, the generation time of yeast cells lengthened. The cell cycle of the daughter cell was under the control of the mother. This control was transient, and the daughter cell began dividing at the rate characteristic of its own age within three divisions of its birth. This suggests that the senescent phenotype, as manifested by lengthened generation time, is a dominant feature in yeast cells, and that it is determined by a diffusible cytoplasmic molecule(s) that undergoes turnover in young cells. In a search for this putative senescence factor(s), we are cloning genes that differentially expressed during the yeast life span. Several such genes have been isolated and partially characterized. Our goals are to determine whether the expression of one or more of these genes is casually associated with cell longevity. We propose the Cell Spiral model to describe the relationship between the cell cycle and cellular aging.