Regulation of Cell Cycle Practice Test

In a cell-free extract, cyclin binds CDK to form an active complex that phosphorylates multiple substrates. A phosphatase removes these phosphates, opposing CDK signaling. When cyclin concentration is held constant, adding a phosphatase inhibitor causes substrate phosphorylation to remain high for longer, even though CDK activity is unchanged. In intact cells, sustained phosphorylation of mitotic substrates correlates with delayed exit from mitosis. Which change would most likely shorten mitotic duration in cells treated with the phosphatase inhibitor?

A signaling module at the G2/M transition uses positive feedback: active CDK–cyclin activates a phosphatase that removes the CDK inhibitory phosphate, and simultaneously inhibits the kinase that adds the inhibitory phosphate. This creates a rapid switch from low to high CDK activity. A mutation prevents CDK from activating the phosphatase, but cyclin binding and the inhibitory-kinase inhibition remain normal. Which effect would most likely be observed in the mutant cells?