Final answer:
Tumor-suppressor genes code for proteins that inhibit cell division and function as the 'brakes' of the cell to prevent uncontrolled proliferation, playing a crucial role in preventing cancer.
Step-by-step explanation:
Tumor-suppressor genes are segments of DNA that code for proteins which inhibit cell division. Unlike oncogenes which promote cell division and help move the cell cycle forward, tumor suppressor genes act as negative regulators. These genes can be thought of as the cellular brakes that prevent uncontrolled cell proliferation, which is crucial in preventing cancer. For example, proteins such as retinoblastoma protein (RB1), p53, and p21, when functioning correctly, can halt the cell cycle until specific conditions are met, thereby putting a 'roadblock' to cell-cycle progression. If these genes are mutated, the cell can divide without control, similar to a car that's lost its brakes, leading potentially to the development of tumors.