| Abstract: | The controls acting over the timing of DNA replication (S) during the cell cycle have been investigated in the fission yeast Schizosaccharomyces pombe. The cell size at which DNA replication takes place has been determined in a number of experimental situations such as growth of nitrogen-starved cells, spore germination and synchronous culture of wee mutant and wild-type strains. It is shown that in wee mutant strains and in wild type grown under conditions in which the cells are small, DNA replication takes place in cells of the same size. This suggests that there is a minimum cell size beneath which the cell cannot initiate DNA replication and it is this control which determines the timing of S during the cell cycle of the wee mutant. Fast growing wild-type cells are too large for this size control to be expressed. In these cells the timing of S may be controlled by the completion of the previous nuclear division coupled with a requirement for a minimum period in G1. Thus in S. pombe there are two different controls over the timing of S, either of which can be operative depending upon the size of the cell at cell division. It is suggested that these two controls may form a useful conceptual framework for considering the timing control over S in mammalian cells. |
