| Abstract: | Strains of Schizosaccharomyces pombe carrying the wee 1 mutation divide at a reduced cell size compared with the wild-type. In this paper, we investigate the mechanism which determines the time of division and cell size at division in wee 1 strains, using three experimental approaches. The evidence suggests that the wild-type control (a cell size control over entry into nuclear division) is absent in wee 1 strains. Instead, a mechanism operates which comprises a cell size control over the initiation of S phase plus a minimum incompressible period in G2 (“timer”) from S phase to nuclear division. The elements of this second control mechanism exist in wild-type cells, though the control is not normally expressed. In particular, the G2 interval in wild-type cells is normally longer than that in wee 1 cells, but can be reduced to this minimum value by delaying S phase. Thus there are two independent controls over entry into nuclear division, one of which operates in wild-type, and the other in wee 1 cells. |
