Final answer:
The role of specific signalling molecules like cyclins and cyclin-dependent kinases (Cdks) in driving the cell cycle was discovered through pivotal experiments such as the fusion of cells at different stages and the isolation of Maturation Promoting Factor from Xenopus eggs.
Step-by-step explanation:
We learned that the cell cycle is driven by specific signalling molecules present in the cytoplasm through key experiments. One crucial discovery was the isolation of a cytoplasmic factor from Xenopus eggs, known as the Maturation Promoting Factor (MPF), which showed evidence of chemical regulation of the cell cycle. This factor was instrumental in demonstrating that specific molecules could control progression through the cell cycle. Another significant experiment fused cells in different stages (S and G₁ phase), revealing that specific chemical signals in the cytoplasm, such as cyclins and cyclin-dependent kinases (Cdks), trigger DNA synthesis. These experiments conclusively showed that internal molecular signals orchestrate cell transitions through various checkpoints. For example, growth factors can prompt a cell to divide by interacting with its plasma membrane, while cyclins and Cdks regulate the cell's progress at checkpoints in G₁, G₂/M transition, and metaphase. The delicate balance of positive regulators like cyclins and Cdks, and negative regulators, like tumour suppressor proteins, ensure that the cell cycle proceeds accurately. Disruptions to these regulatory mechanisms can lead to cancer due to uncontrolled cell division.