Final answer:
The biomolecules that regulate the cell cycle are primarily regulatory proteins such as cyclins and Cdks. They control the cycle by promoting or inhibiting progression through checkpoints, ensuring that cells divide correctly.
Step-by-step explanation:
The biomolecules involved in regulating the cell cycle include a variety of regulatory proteins such as cyclins and cyclin-dependent kinases (Cdks). These proteins can act as either positive regulators, which promote the progression of the cell through the various phases, or as negative regulators, like retinoblastoma protein (Rb), p53, and p21, which can halt the cell cycle to prevent errors. The specific Cdk inhibitors are crucial as they ensure that the cell's checkpoints are not passed through until certain conditions within the cell are met.