Regulation of DNA Replication during the Yeast Cell Cycle

Abstract

One central event that must occur exactly once per cell division cycle is the replication of the genomic DNA. One defining observation of the eukaryotic cell cycle is that the nuclear DNA replicates during a short period (the S phase) and that during that period each part of every chromosome is entirely replicated. Although there are many origins of DNA replication per chromosome, and it is known that not all of them are required to act in every cycle, nevertheless reinitiation in the same cycle at any one of them is never observed. For these reasons, it has long been assumed that the initiation of DNA replication is under very stringent control. When Hartwell et al. (1970) began systematic isolation of cell-division-cycle (cdc) mutants more than 20 years ago, it was expected that mutants specifically defective in DNA replication, especially those involved in initiation of DNA replication, would be prominent among the mutants that exhibit cdc phenotypes. Growing yeast cells show a morphology characteristic for the position they have reached in the cell cycle: The bud emerges just about the time that S phase begins and has reached its full size at the time that mitosis begins. Thus conditional-lethal mutants defective in DNA elongation were expected to arrest, under nonpermissive conditions, with a large bud. Indeed, temperature-sensitive mutations in genes specifying DNA polymerases (cdc2,