Final answer:
Cyclins and cyclin-dependent kinases (Cdks) are protein complexes that control progression through the early stages of the cell cycle.
Step-by-step explanation:
In the early stages of the cell cycle, progression from one phase to the next is controlled by characteristic protein complexes called cyclins and cyclin-dependent kinases (Cdks). These proteins work together to regulate the cell cycle by activating or inactivating target molecules through phosphorylation. Cyclins activate Cdks, which then trigger the cell to advance to the next stage of the cycle. The levels of cyclin proteins fluctuate throughout the cell cycle and are triggered by both external and internal signals.