Final Answer:
For CDK4 to be active, Cyclin D must be elevated.Thus, the correct option is C.
Step-by-step explanation:
Cyclin-dependent kinase 4 (CDK4) is a crucial enzyme involved in the regulation of the cell cycle, specifically in the G1 phase. Its activity is dependent on the presence of specific cyclin proteins. In this case, Cyclin D forms a complex with CDK4 to activate it. The binding of Cyclin D to CDK4 allows the formation of an active complex, known as Cyclin D-CDK4, which phosphorylates key substrates, facilitating progression through the G1 phase of the cell cycle.
The specificity of cyclin-CDK interactions plays a pivotal role in the precise control of cell cycle events. In the context of CDK4 activation, Cyclin D is the partner that drives the activation of CDK4. This interaction is temporally regulated, ensuring that CDK4 becomes active at the appropriate stage of the cell cycle. The elevation of Cyclin D levels signals the cell that conditions are favorable for progression through the G1 phase, leading to the activation of CDK4 and subsequent cell cycle events.
Understanding the interplay between cyclins and CDKs is fundamental to comprehending the intricacies of cell cycle regulation. The precise coordination of these proteins ensures the orderly progression of the cell cycle, preventing uncontrolled cell division and contributing to the maintenance of cellular homeostasis. In summary, the elevation of Cyclin D is essential for the activation of CDK4, underscoring the specificity and regulation embedded in the intricate machinery governing the cell cycle.