Final answer:
The material does not specify an exact number of Cdk proteins at the G1 checkpoint in animal cells but indicates that Cdk proteins are involved in cell cycle regulation by forming complexes with cyclins, which fluctuate in concentration.
Step-by-step explanation:
The number of Cdk proteins that operate at the G1 checkpoint in animal cells is not explicitly stated in the material provided. However, we know from the provided information that Cdks, paired with specific cyclins, regulate the cell cycle at different checkpoints, including the G1 phase. It is also mentioned that p53 and p21 are crucial in regulating the G1 checkpoint, with p21 inhibiting the activity of the Cdk/cyclin complexes.
The levels of Cdk proteins are relatively stable throughout the cell cycle, which means that while the different cyclins fluctuate and get degraded as the cell moves on to the next stage, the Cdks remain constant and form complexes with newly synthesized cyclins as needed. The exact number of Cdks at the G1 checkpoint may vary depending on the type of cell and the circumstances under which cell division is taking place, such as the presence of DNA damage or stress signals.