The first transition point of the mutant cdc2.33 in the fission yeast Schizosaccharomyces pombe

We show that the first of the two transition points of cdc2.33, a mutant of Schizosaccharomyces pombe, exists in exponential phase cells. Using flow cytometry and a double-block experiment, we have measured the position of this transition point both in the single mutant and in the double mutant cdc2.33 wee1.6. In the single mutant, this point is in early G1. In the double mutant, however, this point is only delayed slightly, if at all, despite much larger delays in the S period and in the transition point of cdc10, another 'start' mutant. There is therefore a significant dissociation in the timing of what are thought to be two start events, and the first one appears not to be subject to a size control and to be associated with the completion of mitosis rather than the G1/S boundary.