What happens inside a cell to start cancer

Cancer starts when a single cell stops following the rules that keep it healthy. Normally, your cells divide in an orderly way: they grow, copy their DNA, divide into two cells, and then stop. They also repair damage to their DNA or die if the damage is too severe. Cancer cells break all three of these rules at once.

A cancer cell divides over and over without stopping. It ignores signals telling it to stop or die. And it accumulates damage to its DNA that a healthy cell would have fixed or died rather than pass on. This happens because of changes in specific genes—the instructions that tell a cell how to behave. When enough of these genes are damaged in the same cell, that cell becomes cancerous.

Key Takeaways

  • Cancer cells divide uncontrollably because mutations in their DNA disable the genes that normally tell cells when to stop dividing.
  • Cells have built-in repair systems that catch and fix DNA damage, but cancer cells either have broken repair systems or mutations that disable the genes controlling them.
  • A single mutation is not enough to cause cancer—usually four to seven key genes must be damaged in the same cell over time.
  • Some mutations are inherited, which is why certain families have higher cancer risk, but most cancer mutations happen randomly during a person's lifetime.
  • Carcinogens like tobacco smoke, UV radiation, and some viruses increase the chance that cells will accumulate the mutations needed for cancer to develop.

The genes that go wrong in cancer

Two main types of genes control cell division. Oncogenes are genes that tell a cell to grow and divide. Tumor suppressor genes are genes that tell a cell to stop dividing or to die if it is damaged. Cancer happens when oncogenes get stuck in the "on" position and tumor suppressor genes get turned off.

The most common tumor suppressor gene involved in cancer is called TP53. This gene produces a protein that acts like a quality control inspector: it checks DNA for damage and either fixes it or tells the cell to die. When TP53 is mutated, damaged cells survive and keep dividing instead. Another critical tumor suppressor is BRCA1, which repairs a specific type of DNA damage. Women with inherited BRCA1 mutations have much higher risk of breast and ovarian cancer because their cells cannot repair this damage as well.

Oncogenes work differently. A normal oncogene might tell a cell to divide when it receives a growth signal. A mutated oncogene can get stuck sending that signal all the time, even when the cell should stop. The RAS gene family is mutated in about one-third of all cancers. When RAS is mutated, it keeps telling the cell to divide regardless of what the rest of the body needs.

How DNA damage accumulates over time

Your cells face constant threats to their DNA. Radiation from the sun, chemicals in tobacco smoke, viruses, and even normal metabolism create damage. Most of the time, cells catch and fix this damage within hours. But sometimes a mutation happens in a gene that controls repair, and then damage starts to pile up.

Cancer usually requires multiple mutations in the same cell. A cell might get a mutation in an oncogene that makes it divide faster. Years later, a mutation in a tumor suppressor gene might disable the cell's ability to die when it should. Then a mutation in a DNA repair gene might prevent it from fixing new damage. Each mutation makes the cell more dangerous, and the more mutations it has, the more likely it is to become fully cancerous.

This is why cancer is largely a disease of aging. It takes time for enough mutations to accumulate in a single cell. A person who smokes for 30 years has had 30 years of exposure to carcinogens, giving cells many chances to pick up mutations. A person who gets a sunburn at age 20 might not develop skin cancer until age 50, because the cell needed decades to accumulate enough damage.

Why some cells become cancerous and others do not

Not every cell that gets damaged becomes cancerous. Your body has multiple defenses. The first is the DNA repair system itself—cells can fix most damage before it becomes a permanent mutation. The second is cell death: if damage is too severe to repair, the cell receives a signal to die rather than divide. The third is the immune system, which can recognize and kill cells that look abnormal.

Cancer cells evade all three defenses. They accumulate mutations that disable their repair systems. They disable the genes that trigger cell death. And they develop ways to hide from the immune system—they might produce proteins that tell immune cells to leave them alone, or they might change their appearance so immune cells do not recognize them as abnormal.

This is why inherited mutations matter. A person born with a BRCA1 mutation does not have cancer at birth, but every cell in their body starts with a disadvantage: it cannot repair certain DNA damage as well as normal cells can. This means fewer additional mutations are needed before a cell becomes cancerous. The person's risk is higher, but cancer is still not certain—it depends on whether cells accumulate the other necessary mutations during their lifetime.

What carcinogens do to cells

A carcinogen is anything that increases the chance that cells will develop cancer-causing mutations. Tobacco smoke contains over 70 known carcinogens. Ultraviolet radiation from the sun damages DNA in skin cells. Asbestos fibers can lodge in lung tissue and cause chronic inflammation, which damages DNA over time. Some viruses, like human papillomavirus (HPV) and hepatitis B, can integrate into a cell's DNA and disrupt genes that control division.

Carcinogens do not may provide cancer. A person who smokes has a much higher risk of lung cancer than a non-smoker, but not every smoker develops cancer. This is because cancer requires multiple specific mutations, and carcinogens increase the odds that cells will get them. The more exposure to carcinogens, and the longer the exposure, the more chances cells have to accumulate the mutations needed for cancer to develop.

Some carcinogens work by causing inflammation. Chronic inflammation—long-term irritation and immune response in a tissue—damages DNA in nearby cells. This is why people with inflammatory bowel disease have higher colorectal cancer risk, and why people with chronic hepatitis have higher liver cancer risk. The inflammation itself does not cause cancer, but it creates an environment where cells are more likely to pick up the mutations that do.

How cancer cells behave differently from normal cells

Once a cell becomes cancerous, it behaves in ways that normal cells do not. It divides much faster than the surrounding tissue needs. It ignores signals from neighboring cells telling it to stop. It can survive in conditions that would kill a normal cell—low oxygen, acidic environments, or nutrient-poor areas. It produces substances that encourage new blood vessels to grow toward it, bringing more oxygen and nutrients.

Cancer cells also change shape and lose the structures that normally hold them in place. A normal cell in the breast stays in the breast because it is anchored to other breast cells and to the tissue structure around them. A cancer cell can break free from these anchors and travel through the bloodstream or lymph system to other parts of the body. This process is called metastasis, and it is what makes cancer dangerous.

The mutations that cause these changes are not random—they are selected for. In a tumor, millions of cells are dividing. Most of them have the same mutations as their parent cells, but some pick up new mutations. Cells with mutations that help them survive or divide faster outcompete the others. Over time, the tumor becomes a population of increasingly aggressive cells, each one more likely to spread and resist treatment than the last.

Why different cancers develop differently

Not all cancers involve the same genes. Lung cancer often involves mutations in the KRAS gene and the TP53 gene. Breast cancer often involves mutations in BRCA1, BRCA2, or the HER2 gene. Colorectal cancer typically involves mutations in the APC gene early on, followed by mutations in KRAS and TP53. The specific mutations depend on which genes are most vulnerable in that tissue and which carcinogens the tissue is exposed to.

This is why cancer treatment is becoming more specific. A drug that works against one mutation might not work against another. Doctors now test tumors to find out which genes are mutated, and then choose treatment based on those mutations. A person with a lung cancer that has a specific EGFR mutation might respond well to a drug that targets that mutation, while someone with a lung cancer that has a different mutation might need a different drug.

Frequently Asked Questions

Can you inherit cancer, or do you inherit the risk of cancer?

You inherit the risk, not the cancer itself. If you inherit a mutated BRCA1 gene, you are born with that mutation in every cell in your body, but you do not have cancer. Cancer develops only if cells accumulate additional mutations over time. Some people with inherited mutations never develop cancer, while others do—it depends on whether their cells pick up the other necessary mutations during their lifetime.

If I have a mutation in one cell, will it spread to other cells?

No. A mutation in one cell affects only that cell and its descendants. If a cell with a mutation divides, both daughter cells will have the mutation. But cells in other parts of your body will not develop the same mutation unless they are exposed to the same carcinogen or unless they happen to develop the mutation independently, which is extremely rare.

Does stress cause cancer by damaging cells?

Stress does not directly damage DNA in a way that causes cancer. However, chronic stress can weaken the immune system, which normally catches and kills abnormal cells. Stress can also lead to behaviors that increase cancer risk, like smoking or drinking. The link between stress and cancer is indirect, not a direct cellular mechanism.

Why do some people who smoke never get cancer?

Cancer requires multiple specific mutations in the same cell, and this is partly random. Two people exposed to the same carcinogen for the same length of time will not necessarily develop the same mutations. Some people's cells may be better at repairing damage, or they may simply be unlucky enough to avoid the specific combination of mutations needed for cancer to develop in their lifetime.

Can a single mutation cause cancer?

No. A single mutation is not enough. Cancer typically requires mutations in at least four to seven key genes in the same cell. This is why cancer develops over time rather than appearing suddenly. The exception is childhood cancers, which sometimes involve fewer mutations, possibly because children's cells are dividing more rapidly and have less time to accumulate the multiple changes needed.