Final answer:
The p53 protein is a tumor-suppressor protein that operates at the G1 restriction point to halt the cell cycle when DNA damage is detected. It plays a key role in DNA repair and can trigger apoptosis to prevent cancer development. The protein also increases p21 production to reinforce cell cycle inhibition.
Step-by-step explanation:
The tumor-suppressor protein that normally functions to maintain a cycling cell at the G1 restriction point is known as the p53 protein. p53 is a critically important protein that acts as a checkpoint in the cell cycle, specifically at the G1 phase, to prevent cells with damaged DNA from continuing to divide. When p53 detects DNA damage, it halts the cell cycle and activates repair mechanisms. If repair is not possible, p53 can initiate apoptosis, which is the process of programmed cell death, to eliminate the potential for cancerous growth. Additionally, p53's activation can lead to the production of another protein called p21, which reinforces the stoppage of the cell cycle by inhibiting the action of Cdk/cyclin complexes that are necessary for the cell to proceed to the S phase. This controlling mechanism is essential for maintaining the integrity of the genome and preventing uncontrolled cell division, which is characteristic of cancerous cells.