Genetic control of the cell division cycle in yeast. III. Seven genes controlling nuclear division

Temperature-sensitive mutations in seven genes (cdc 2, 6, 7, 9, 13, 14, and 15) of Saccharomyces cerevisiae confer a defect in nuclear division. Following a shift from the permissive to the restrictive temperature, asynchronous populations of these mutant cells accumulate uniformly in nuclear division. Depending upon the particular gene defect, mutants appear to terminate development at an early stage of nuclear division (cdc 2), at a medial stage (cdc 6, 7, 9, 13), or at a late stage (cdc 14, 15). Experiments employing temperature shifts, time-lapse photomicroscopy, and synchronous cultures permitted a determination of the time in the cell cycle at which each of the thermolabile gene products completes its function at the permissive temperature (the execution point) for the ensuing nuclear division. These studies reveal a program of gene activity required for nuclear division that commences at the beginning of the cell cycle. Subsequent to the block in nuclear division the mutant strains do not undergo cell separation or bud initiation. Apparently, nuclear division is normally a prerequisite for the latter two processes.