Final answer:
The APC/C regulates cell cycle transition by promoting the degradation of cyclins and inhibiting cyclin-Cdk complex activity, acting as a negative regulator to ensure controlled cell cycle progression.
Step-by-step explanation:
The anaphase-promoting complex/cyclosome (APC/C) is a key regulator of the cell cycle transition. Its main function is to promote the degradation of cyclins and inhibit the activity of cyclin-dependent kinase (Cdk) complexes. The APC/C targets cyclins for destruction by marking them with ubiquitin, which signals for their degradation by the proteasome. This process ensures that the Cdk/cyclin complexes, which are necessary for the progression of the cell cycle, are inactivated, thereby preventing premature progression to the next phase of the cell cycle. The degradation of cyclins leads to the inactivation of Cdk/cyclin complexes, ensuring the cell cycle does not proceed unchecked.
Therefore, the correct answer to the question is (a) APC/C promotes the degradation of cyclins and inhibits the activity of cyclin-cdk complexes, preventing cell cycle progression. By regulating the concentration of cyclins and the activation state of Cdk/cyclin complexes, the APC/C acts as a negative regulator by providing a checkpoint mechanism to ensure the cell cycle progresses in a controlled manner after previous phases have been successfully completed and ensuring that problematic conditions are resolved before moving forward.