Fission yeast cells have been stained with a dye (DAPl ) that reveals DNA and hence shows the cell 
nucleus. These particular mutant cells lack two genes (mikl and wee/ ) that encode the inhibi- 
tory cdc2 tyrosine kinase. They are therefore entering mitosis prematurely, before the completion 
of DNA replication, and are dividing themselves to death. 
Research of David H. Beach. 
28 
