These cell-cycle checkpoints are the mechanisms that govern the order of the cell-cycle events. Mitosis is blocked when DNA duplication is incomplete or is damaged. The second checkpoint concerns the controls which operate during the
Cyclin-dependant kinase (CDK) regulates mitosis in fission yeast. A number of proteins are required for cell-cycle progression. Among these, Cdc2 protein kinase (CDK1) plays a central role. Loss of Cdc2 kinase activity arrests cells both before Start in G1 and in late G2 before the M phase, and dominant forms of Cdc2 advance entry into mitosis. Furthermore, Cdc2 is essential for viability.

Initiation of DNA synthesis and nuclear divisions are attributed to the activity of a single CDK catalytic subunit encoded by cdc2+. The equivalent gene in budding yeast is CDC28. People in the current literature are using different names for the same thing in a different host, and the same name--E2--for very different things.

After passing Start, Cdc2 becomes complexed with B-type cyclins Cdc13 and Cig2 and the Cdc2 complex is phosphorylated on tyrosine-15 by both the Wee1 and Mik1 tyrosine kinases. This tyrosine phosphorylation is maintained until the onset of mitosis. Dephosphorylation of this residue is carried out by the Cdc25 tyrosine phosphatase.