Answer:
(c) falls toward the end of M phase as a result of ubiquitylation and degradation.
Step-by-step explanation:
Cyclins are the regulatory proteins of the cell cycle. The levels of cyclins change during the cell cycle. The function of cyclins is to activate cyclin-dependent kinases. Mitotic cyclins undergo specific and precisely timed proteolytic breakdown which in turn regulates CDK activity throughout the cell cycle. A cell can continue the process of mitosis when cyclins A and B are the first activated and then degraded.
The mitotic cyclins contain a destruction box near their amino terminus which in turn targets them for degradation. The destruction box recognizing protein (DBRP) recognizes this sequence and initiates the process of cyclin degradation. DBRP facilitates the interaction of mitotic cyclin and ubiquitin. Cyclin and activated ubiquitin are covalently joined by the enzyme ubiquitin ligase followed by binding of several more ubiquitin molecules.
This process of binding of ubiquitin to the mitotic cyclins is called ubiquitylation and serves to signal a proteolytic enzyme complex, or proteasome, to degrade cyclin.