Final answer:
The reason there are multiple D type cyclins is to provide precise control over the cell cycle at various checkpoints; cyclins fluctuate in concentration and regulate the cell cycle by activating cyclin-dependent kinases, which then phosphorylate specific proteins necessary for advancing through cell cycle phases.
Step-by-step explanation:
You may wonder why there are 3 different D type cyclins if their main role seems to involve activating Cdk4 and Cdk6, which then participate in the regulation of the cell cycle. The reason behind the existence of multiple cyclins is that they allow the cell to precisely control the cell cycle at various points. Different cyclins bind with Cdks at different points in the cell cycle to regulate specific checkpoints.
The concentrations of cyclin proteins fluctuate throughout the cell cycle and are responsible for the activation of Cdk/cyclin complexes at appropriate times. Once activated by binding with the appropriate cyclin, Cdk enzymes phosphorylate target proteins to advance the cell to the next phase. This is because cyclins show cyclic patterns of synthesis, whereas levels of Cdk proteins remain relatively stable through the cell cycle. This dynamic fluctuation in cyclin levels is one of the key mechanisms for cell cycle regulation.
Moreover, different cyclins have been coopted in evolution, allowing diversification in monitoring various cell cycle events and catalyzing phase-specific proteins. This specific concentration of fully activated cyclin/Cdk complexes is essential for the cell to proceed through the checkpoints.