Final answer:
The third regulatory checkpoint is at the metaphase stage; essential proteins include cyclins and Cdks (positive regulators), and Rb, p53, and p21 (negative regulators), which ensure proper cell cycle progression and prevent uncontrolled cell division.
Step-by-step explanation:
The third regulatory point of cell division is during the metaphase stage of the cell cycle. Proteins crucial to this process include cyclin-dependent kinases (Cdks) and cyclins, which are known as positive regulators. These proteins fluctuate in concentration throughout the cell cycle, enabling the cell to progress through various checkpoints.
The negative regulatory molecules, including retinoblastoma protein (Rb), p53, and p21, play a key role at the G1 checkpoint, which ensures the cell is ready for DNA replication. These tumor suppressor proteins respond to damaged DNA by potentially halting the cell cycle and initiating repair processes or apoptosis if necessary. Poor functioning or damaged copies of these regulatory proteins can lead to uncontrolled cell replication and cancer.