Final answer:
The correct answer to the question is a. cyclins and Cdks. These proteins have fluctuating expression levels throughout the cell cycle, with cyclins showing variable concentrations that directly affect the formation and activity of Cdk/cyclin complexes, which are essential for cell cycle regulation.
Step-by-step explanation:
The expression levels of different proteins fluctuate throughout the cell cycle. Specifically, the expression levels of cyclins and Cdks vary as the cell progresses through different stages. Cyclins are regulatory proteins whose concentrations change predictably throughout the cell cycle. These changes are crucial for the cell cycle to proceed, as cyclins regulate the cycle only when they are bound to cyclin-dependent kinases (Cdks). While the levels of Cdks remain relatively stable, it is the cyclin levels that determine when Cdk/cyclin complexes form to regulate cell cycle checkpoints.
To be fully active, the Cdk/cyclin complexes also need to be phosphorylated at specific locations, which is a process executed by Cdks. Phosphorylation activates proteins by changing their shape, and it is these phosphorylated proteins that drive the cell to the next phase. The cyclic fluctuations of cyclin concentrations are not based on specific events but on the timing of the cell cycle itself, playing a pivotal role in enabling or inhibiting the transition through various checkpoints.