FIGURE 19-18 Phosphorylation of the CDK subunit restrains mitotic CDK activity during S phase and G₂. Mitotic cyclins are synthesized during S phase and G₂ and bind to CDK1. However, the cyclin-CDK complex is not active because threonine 14 and tyrosine 15 of the CDK1 subunit are phosphorylated by the protein kinase Wee1. Once DNA replication has been completed, the protein phosphatase Cdc25 is activated and dephosphorylates CDK1. Active mitotic CDKs further stimulate Cdc25. At the same time, mitotic CDKs inhibit Wee1, the protein kinase that places the inhibitory phosphorylation on the CDK subunit. Ongoing DNA replication inhibits Cdc25 activity. How Cdc25 is initially activated upon completion of DNA replication to put these feedback loops in motion is not yet known. Cell size also affects this regulatory loop. Once cells reach the appropriate size, Cdr2 inhibits Wee1, allowing Cdc25 to activate mitotic CDKs.