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How do normal cells become cancer cells?

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Precancerous growth in a human colon — cancer View through an endoscope of a polyp, a benign precancerous growth projecting from the inner lining of the colon.

Normal cells become cancer cells through a complex, multi-step process involving the accumulation of genetic and epigenetic alterations that rewire the cell's control circuits for growth, differentiation, and death.12

The Role of Mutation

Cancer arises when mutations disrupt the genes responsible for regulating the cell cycle. This typically involves two main classes of genes:3
* Proto-oncogenes: These normally encourage cell growth. When mutated into oncogenes, they become overactive, acting like a stuck accelerator that forces the cell to divide continuously.143
* Tumor suppressor genes: These act as the "brakes" on cell division. Mutations that inactivate these genes remove necessary growth restraints.14

Accumulation of Damage

While cells possess DNA repair mechanisms to fix errors, these systems can become faulty due to inherited or acquired damage. When repair fails, mutations accumulate rapidly. Furthermore, cancer cells often bypass apoptosis (programmed cell death) and senescence, allowing them to achieve a form of immortality.156

This transformation is a gradual process often described as "initiation" followed by "promotion," requiring multiple genetic insults over time to fully deregulate the cell. Once transformed, these cells may acquire further mutations that facilitate invasion and metastasis, enabling them to spread to other parts of the body.217