The G1 Cell Cycle Interval in Yeasts

Abstract

Like most other eukaryotic cells, yeast cells regulate the cell division process during the G1 interval of the cell cycle (3,9). There have been two countervailing views of the nature of G1. One view holds that the G1 interval exists mainly to accommodate specific events after nuclear division which are required for the subsequent S phase. These specific events would necessarily he part of the DNA-division sequence, a concept proposed by Mitchison (6) to encompass periodic events in the replication and segregation of nuclear DNA. An alternative view of G1, which we and others support (1,10), is that most of G1 is without functional significance for the DNA-division sequence and exists mainly to satisfy needs for ongoing processes of mass accumulation, here termed “growth”. For yeast cells growth to what can be measured as a critical cell size is required for performance of an event of the DNA-division sequence prior to S phase (3,9). Clearly for these cells a G1 will be present when sufficient mass has not accrued during the previous cell cycle. Here we summarize experiments using the budding yeast Saccharomyces cereivisiae, and report new results using the fission yeast Schizosaccharomyces pombe, which show that it is solely this dependency of the DNA-division sequence upon growth that accounts for a G1 interval. For both yeasts, increasing cell size at division shortens G1.