Yvonne Bissett

Abstract: The control regulating commitment to the cell division cycle of eukaryotes seems to occur before the initiation of DNA replication1–4. In the budding yeast Saccharomyces cerevisiae, this control is called start and is the earliest gene-controlled event of the cell cycle3–5. A haploid cell which has completed start is committed to cell division and unable to undergo alternative developmental fates such as conjugation. Here, we describe an analogous start control in the fission yeast Schizosaccharomyces pombe. We have tested the ability of cdc mutants blocked at various stages of the cell cycle to undergo conjugation. Only mutants of cdc 2 and cdc 10 which block during the G1 period are able to conjugate. Other mutants which block during G1, S phase, G2 or mitosis are committed to cell division and cannot conjugate. The commitment control start is located in G1, and its completion requires the gene functions of cdc 2 and 10. Completion of start occurs at the beginning of the cell cycle in rapidly growing cells, but is delayed to later in the cell cycle at slow growth rates. The cdc 2 gene function also participates in another major cell cycle control which determines the timing of mitosis6–8. Therefore cdc 2 is a cell cycle control gene which acts at two separate points in the cell cycle: it is required in G1 for commitment to cell division and in G2 for the control of mitosis.