What are proteins that regulate progression through the cell cycle called?

Proteins that regulate progression through the cell cycle are known as cyclins. Cyclins are a family of proteins that undergo specific fluctuations in their levels throughout the cell cycle. Their primary role is to activate cyclin-dependent kinases (CDKs), which are essential for various cell cycle transitions, allowing the cell to move through different phases such as the G1, S, G2, and M phases.

When the concentration of a particular cyclin rises, it binds to its corresponding CDK, leading to the phosphorylation of target proteins that drive the cell cycle forward. For instance, cyclin D is involved in the transition from the G1 phase to the S phase, while cyclin B is crucial for the transition into mitosis. This regulatory mechanism is vital for ensuring that the cell proceeds through the cycle at the right time and in a regulated manner, preventing errors that could lead to issues like cancer.

The other choices provided do not fulfill this specific role. Chromatids are the replicated forms of chromosomes during cell division, hormones are signaling molecules that regulate physiological processes, and enzymes are not primarily involved in cell cycle regulation but serve as biological catalysts for biochemical reactions.