Final answer:
The retinoblastoma (Rb) protein regulates the cell cycle by controlling the transition from the G1 phase to the S phase, acting as a tumor suppressor to prevent uncontrolled cell growth.
Step-by-step explanation:
The normal activity of the retinoblastoma (Rb) protein in the cell is to regulate the cell cycle, particularly the transition from the G1 phase to the S phase. Unphosphorylated Rb binds to the transcription factor E2F, which controls the expression of genes required for this transition.
As a result, when Rb is bound to E2F, it prevents the cell from advancing to the S phase by blocking the production of necessary proteins. Cell growth leads to the phosphorylation of Rb, and once phosphorylated, Rb releases E2F. This allows E2F to activate the genes needed to progress the cell cycle into the S phase. Thus, Rb acts as a tumor suppressor by controlling cell division and preventing unregulated growth, which could lead to cancer.