摘要：

Temperature-sensitive mutations in one gene (cdc1) of Saccharomyces cerevisiae confer a defect in bud emergence. Asynchronous cultures of cells defective in cdc1 collect uniformly as unbudded cells (or cells with very tiny buds) following a shift from the permissive to the restrictive temperature. Studies with synchronous cultures demonstrate that the thermolabile product of cdc1 completes its function (the execution point) for bud emergence at the time of bud emergence (0.2 fractions of a cell cycle). When this function is not completed at the restrictive temperature. cells complete DNA replication but do not undergo nuclear division. Temperature-sensitive mutations in four genes (cdc3, 10, 11, and 12) result in a defect in cytokinesis. At the restrictive temperature these mutant strains develop multiple elongated buds that do not separate from the parent cell. An assay for cytokinesis in yeast was developed and the mutants were shown not to have completed this process. The mutants undergo several rounds of DNA replication and nuclear division at the restrictive temperature and hence become multinucleate. The execution points for these gene products were determined in synchrony experiments to be at approx. 0.08 (cdc11), 0.27 (cdc3) and 0.58 (cdc10) fractions of a cell cycle. Cytokinesis takes place at 0.9 fractions of a cell cycle. I conclude from these observations that bud emergence is not a necessary prerequisite for the completion of DNA replication but is apparently necessary for nuclear division. Cytokinesis and cell separation are not necessary prerequisites for bud emergence, DNA replication, or nuclear division.

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