The decline of MPF (Maturation Promoting Factor) activity at the end of mitosis is due to:

• A. the degradation of cyclin
• B. the synthesis of new cyclin
• C. the phosphorylation of CDK
• D. the activation of DNA polymerase

Answer: (A)

Verified step by step guidance

1. Understand the role of MPF (Maturation Promoting Factor): MPF is a complex of cyclin and cyclin-dependent kinase (CDK) that regulates the progression of the cell cycle, particularly the transition from G2 phase to mitosis.
2. Recognize that MPF activity is dependent on the presence of cyclin: Cyclin binds to CDK to activate MPF. Without cyclin, MPF cannot function.
3. Learn about cyclin degradation: At the end of mitosis, cyclin is targeted for degradation by the proteasome. This process is mediated by ubiquitination, where cyclin is tagged for destruction. The degradation of cyclin leads to the inactivation of MPF.
4. Eliminate incorrect options: The synthesis of new cyclin occurs after mitosis to prepare for the next cell cycle, but it does not cause the decline of MPF activity. Phosphorylation of CDK and activation of DNA polymerase are unrelated to the decline of MPF activity.
5. Conclude that the decline of MPF activity at the end of mitosis is due to the degradation of cyclin, as this directly leads to the inactivation of the MPF complex.