Final answer:
Cyclins in biology regulate the cell cycle by forming complexes with cyclin-dependent kinases (Cdks) and advancing the cell through checkpoints. The levels of cyclin proteins fluctuate throughout the cell cycle and play a crucial role in determining when Cdk/cyclin complexes form.
Step-by-step explanation:
In biology, the concept of a circle relates to cyclins in their role of regulating the cell cycle. Cyclins, along with cyclin-dependent kinases (Cdks), are responsible for advancing the cell through different checkpoints during the cell cycle. The levels of cyclin proteins fluctuate throughout the cell cycle and determine when Cdk/cyclin complexes form, allowing the cell cycle to progress.