Continued DNA synthesis after a mitotic block in the double mutant cut1 cdc11 of the fission yeast Schizosaccharomyces pombe.

DNA synthesis is normally dependent on a cell having previously gone through mitosis. Hirano et al. (1986), however, found that DNA synthesis continued at the restrictive temperature in the double mutant cut1 cdc11 of Schizosaccharomyces pombe even though mitosis was blocked in some of the cells. We have confirmed this result with bulk DNA assays of asynchronous cultures. Synchronous cultures of a diploid double mutant at the restrictive temperature showed two peaks of incorporation with an interval between them that was approximately the same as the doubling time in cell length. Flow cytometry showed that the cells had increased their DNA content from 4C (the diploid value) to about 16C after 7h. The cytological appearance at this time was mixed, with uninucleate, binucleate and dead cells, but fluorescence measurements on single cells indicated that about half the population had single nuclei with about the 16C value and had therefore gone through two rounds of DNA synthesis without mitosis.