Final answer:
pRb, along with p53 and p21, acts as a tumor suppressor by inhibiting cell cycle progression, especially in the presence of DNA damage, to prevent uncontrolled cell proliferation and tumor progression.
Step-by-step explanation:
The proteins pRb (Retinoblastoma protein), p53, and p21 play critical roles in cell cycle regulation and tumor suppression. pRb inhibits cell cycle progression, particularly at the G1 checkpoint, by holding the cell in a non-dividing state. This action helps to promote cellular differentiation and prevent uncontrolled cell proliferation, which is a hallmark of tumor progression. The question asks which of the listed processes pRb directly influences to suppress tumor growth. The correct answer is the inhibition of cell division, ultimately functioning as a tumor suppressor. This regulation is especially important when a cell experiences DNA damage or stress, as it provides time for repair, or in cases where repair is not possible, can lead to apoptosis to prevent the propagation of damaged DNA. p53 plays a synergistic role with pRb in this process, by halting the cell cycle, activating DNA repair enzymes and, if necessary, inducing apoptosis. Proteins such as p21 enforce the cell cycle halt by inhibiting the Cdk/cyclin complexes critical for cell cycle progression. Therefore, in the context of tumor progression, the primary function of tumor suppressor genes like pRb, p53, and p21 is to stop certain cells from dividing, particularly those with potentially oncogenic mutations.