Final answer:
The expression levels of proteins fluctuate throughout the cell cycle, controlled by cyclin proteins. The levels of the four cyclin proteins increase and then are degraded as the cell progresses through the various checkpoints of the cell cycle.
Step-by-step explanation:
The expression levels of different proteins fluctuate throughout the cell cycle. Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints. The levels of the four cyclin proteins fluctuate throughout the cell cycle in a predictable pattern, increasing in concentration triggered by both external and internal signals, and then being degraded after the cell moves to the next stage of the cell cycle. This process is shown in Figure 10.11 and 13.11.