Final answer:
Cyclins are the series of proteins that regulate progression at the three main checkpoints of the cell cycle. They work in conjunction with cyclin-dependent kinases (Cdks) to advance the cell through these checkpoints. Cyclins and Cdks function as positive regulators, allowing the cell cycle to proceed to the next stage.
Step-by-step explanation:
The series of proteins that regulate progression at the three main checkpoints of the cell cycle are called cyclins. Cyclins, along with cyclin-dependent kinases (Cdks), are responsible for advancing the cell through these checkpoints. The levels of cyclin proteins fluctuate throughout the cell cycle, and their concentrations determine when Cdk/cyclin complexes form and regulate different checkpoints.
For example, cyclin B controls the G₂/M transition checkpoint, ensuring the separation of chromatids during mitosis. Cyclins and Cdks function as positive regulators, allowing the cell cycle to advance to the next stage, while negative regulator molecules can halt the cycle until specific requirements are met.