Final answer:
Cyclins are present inside the cell during the cell cycle, and their levels change in a predictable pattern, closely tied to the cell cycle's checkpoints. Cyclin levels rise and fall, ensuring that the cell progresses through the cycle correctly, and are degraded as the cell passes each checkpoint.
Step-by-step explanation:
Cyclins are proteins whose levels fluctuate throughout the cell cycle. These fluctuations are predictable and vital for the regulation of the cell cycle checkpoints. The cycle of cyclin synthesis and degradation is directly correlated with the progression through various stages of the cell cycle, including three major checkpoints found near the end of G1 phase, at the G2/M transition, and during metaphase. As the cell transitions from one phase to another, the cyclins associated with the previous phase are degraded by cytoplasmic enzymes. This is crucial because the concentration of active cyclin/Cdk complexes needs to be regulated for the cell to proceed through the checkpoints. Without these precise concentrations, the cell cycle could either stall or proceed unchecked, which can lead to issues such as cancer.