Cyclins are a group of proteins that play a crucial role in regulating the cell cycle, which is the process that cells go through to grow and divide. They help control the timing of cell division by activating specific enzymes called cyclin-dependent kinases (CDKs). When cyclins bind to CDKs, they trigger various phases of the cell cycle, ensuring that the cell progresses through its stages at the right time. Different types of cyclins are produced at specific points in the cell cycle, and their levels fluctuate throughout the process. For example, cyclin D is important for the transition from the G1 phase to the S phase, while cyclin B is essential for the transition from the G2 phase to mitosis. This precise regulation is vital for normal cell function and growth.