Septin

Abstract: Mitotic cell division in eukaryotes is accomplished through a highly reproducible temporal sequence of events that is common to almost all higher organisms. An interval of time, Gl, separates the previous cell division from the initiation of DNA synthesis. Ohromosome replication is acco’mplished during the DNA synthetic period, S, which typically occupies about a third of the cell cycle. Another interval of time, G2, separates the completion of DNA synthesis from prophase, the beginning of mitosis, M. A dramatic sequence of changes in chromosome structure and of chromosome ,movement characterizes the brief mitotic period that results in the precise separation of sister chromatids to daughter nuclei. Mitosis is followed by cytokinesis, the partitioning of the cytoplasm into two daughter cells with separate plasma membranes. In some organisms the cycle is completed by cell wall separation. Each of these events occurs during the cell division cycle of the yeast, Saccharomyces cerevisiae (I) (Fig. 1) . However, two features which distinguish the cell cycle of S. cerevisiae from most other eukaryotes are particularly useful for an analysis of the gene functions that control the cell division cycle. First, the fact that both haploid and diploid cells undergo mitosis permits the isolation of recessive mutations in haploids and their analysis by complementation in diploids. Second, the daughter cell is recognizable at an early stage of the cell cycle as a bud on the surface of the parent cell. Since the ratio of bud size to parent cell size increases progressively during the cycle, this ratio pro-