Fig. 1. The effects of altering SIN and MEN activity. Panel A shows wild-type S. pombe cells stained with DAPI (to reveal the DNA) and Calcofluor (to reveal the position of the division septum). Panel B shows cells in which SIN signaling is defective. Note that cells become elongated and multinucleated, as nuclear division continues in the absence of cytokinesis. Panel C shows the effects of increased SIN activity. Note that the cells undergo multiple rounds of septum formation without cell cleavage. The red cross indicates that loss-of-function mutations produce the indicated phenotype. The bold black type `OP' indicates that increased expression of a gene produces the indicated phenotype. Scale bar, 10 µm. Panel D shows an arrested S. cerevisiae cdc15 mutant. The cells are stained with DAPI (blue) and antibody to tubulin (red). Note that the chromosomes are separated, and that the spindle is highly elongated. This image is provided by Rosella Visintin and Angelika Amon (MIT, Cambridge, MA). Panel E shows the mutant dbf2-2. The phosphoprotein phosphatase has been tagged with GFP. Note that in the arrested cells, Cdc14p remains in the nucleolus. This image is provided by Elmar Schiebel (Patterson Institute for Cancer Research, Christie Hospital, Manchester, UK).