How does cancer start?

This page tells you about how cancer starts. There is information about

Cell changes and cancer

All cancers begin in cells. Our bodies are made up of more than a hundred million million (100,000,000,000,000) cells. Cancer starts with changes in one cell or a small group of cells. 

Usually, we have just the right number of each type of cell. This is because cells produce signals to control how much and how often the cells divide. If any of these signals are faulty or missing, cells might start to grow and multiply too much and form a lump called a tumour.

A primary tumour is where the cancer starts. Some types of cancer start from blood cells. These are called blood cancers, such as leukaemia. They don't form solid tumours. Instead, the cancer cells build up in the blood and sometimes the bone marrow Open a glossary item.

For a cancer to start, certain changes take place within the genes Open a glossary item of a cell or a group of cells.

Genes and cell division

Different types of cells in the body do different jobs. But they are similar.

All cells have a control centre called a nucleus. Inside the nucleus are chromosomes made up of thousands of genes. Genes contain long strings of DNA (deoxyribonucleic acid), which are coded messages that tell the cell how to behave. 

Each gene is an instruction that tells the cell to make something. This could be a protein or a different type of molecule called RNA (ribonucleic acid). Together, proteins and RNA control the cell. They decide:

  • what sort of cell it will be
  • what it does
  • when it divides
  • when it dies

Gene changes within cells (mutations)

Genes Open a glossary item make sure that cells grow and make copies (reproduce) in an orderly and controlled way. They keep the body healthy. 

Sometimes a change happens in the genes when a cell divides. This is a mutation. It means that a gene has been damaged or lost or copied too many times.

Mutations can happen by chance when a cell is dividing. Some mutations mean that the cell no longer understands its instructions. It can start to grow out of control. There have to be about 6 different mutations before a normal cell turns into a cancer cell.

Mutations in particular genes may mean that:

  • a cell starts making too many proteins that trigger a cell to divide
  • a cell stops making proteins that normally tell a cell to stop dividing
  • abnormal proteins may be produced that work differently to normal

The video below explains what cancer is and how it starts. It is 2 minutes 42 seconds long. 

What is cancer and how does it start? | Cancer Research UK (2021)


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View a transcript for the video about what is cancer and how does it start.

It can take many years for a damaged cell to divide and grow and form a tumour big enough to cause symptoms or show up on a scan. 

How mutations happen

Mutations can happen by chance when a cell is dividing. They can also be caused by the processes of life inside the cell. Or by things coming from outside the body, such as the chemicals in tobacco smoke. And some people can inherit faults in particular genes that make them more likely to develop a cancer.

Some genes get damaged every day and cells are very good at repairing them. But over time, the damage may build up. And once cells start growing too fast, they are more likely to pick up further mutations and less likely to be able to repair the damaged genes.

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