Final answer:
The concentration of mitotic cyclin changes throughout the cell cycle, with a direct correlation between cyclin accumulation and the three major cell cycle checkpoints. Cyclin levels rise gradually after cytokinesis, peak at the next mitosis, and then fall. Cyclin reaches a threshold concentration in G₂ phase, triggering entry into mitosis.
Step-by-step explanation:
The concentration of mitotic cyclin (M cyclin) changes throughout the cell cycle. There is a direct correlation between cyclin accumulation and the three major cell cycle checkpoints. Cyclin levels rise gradually after cytokinesis, peak at the next mitosis, and then fall. The levels of cyclin gradually increase in G₁ phase, binding to more cyclin-dependent kinase (cdk) subunits. Cyclin reaches a threshold concentration in G₂ phase, triggering entry into mitosis. Following each checkpoint, cyclin levels sharply decline as it is degraded by cytoplasmic enzymes.