• Learn about cancer
  • What is cancer?
    • A brief history of cancer
    • What are we made of?
    • How do cells become cancerous?
    • How does cancer make you ill?
    • How many different types of cancer are there?
  • What causes cancer?
  • Detecting cancer
  • Cancer treatment
• Cancers at a glance
• Research highlights
• About cancer research
• Understanding 'risk'
• Our history and achievements

How do cells become cancerous?

The cells of our bodies grow and multiply in a process known as cell division. It must be extremely tightly controlled if all the cells in your body are to grow in the right place, and for all our organs and tissues to function properly. If cells divide too quickly the consequences can be disastrous.

When a cell divides, it first makes an exact copy of its DNA via a process called DNA replication, before splitting in half to form two 'daughter' cells that are genetically identical. Cell division involves hundreds of proteins (and therefore genes).

Some proteins tell the cell when or when not to divide. Others are responsible for making sure the DNA is copied accurately. Yet more are involved in physically pulling the duplicated chromosomes apart as the cell splits in two.

Cancer is essentially a disease of cell division. Uncontrolled cell division can have many causes, and can happen in any type of cell in the body, but it usually results from defects or damage in one or more of the genes involved in cell division.

If these genes become damaged (mutated) in some way, for example by exposure to cigarette smoke or ultraviolet radiation, the cell can start to divide uncontrollably. These defective cells can multiply to form a lump of abnormal tissue called a tumour.

Different cancer-causing genes

There are four main types of gene involved in cell division. Most tumours have faulty copies of more than one of these genes:

Oncogenes

Oncogenes are genes that, under normal circumstances, play a role in telling cells to start dividing. We can think of them as being a bit like the accelerator pedal in a car. When oncogenes are activated, they speed up a cell's growth rate. When one of them becomes damaged, causing cancer, it is like the accelerator becoming stuck down - the cell, and all its daughter cells, are permanently instructed to divide.

Tumour suppressor genes

These genes make proteins whose normal function is the opposite of oncogenes. They tell the cell NOT to divide, and must be switched off by other proteins before a cell can grow. They are like a car's handbrake - it is supposed to be on when the car is at rest. One of the most important tumour suppressor genes is called p53. This gene was co-discovered in 1979 by Cancer Research UK scientist Professor Sir David Lane.

Suicide genes

Apoptosis, or cell suicide, is a highly complex and hugely important process. Cells are usually able to commit suicide whenever something goes wrong, to prevent damage to their neighbours. There are many different genes (and therefore proteins) involved. If these 'suicide genes' become damaged, then a faulty cell can keep dividing and become cancerous.

DNA-repair genes

The DNA in every cell in your body is under constant assault from a variety of directions. But cells contain many different proteins whose job is to repair damaged DNA. Thanks to these, scientists think that the vast majority of DNA damage is repaired immediately, with no ill effects.

But if the DNA damage occurs to a gene that makes a DNA repair protein, a cell's ability to repair itself will be reduced, and this can allow errors to accumulate in other genes over time. This can cause cancer.

Cancer Research UK is at the forefront of the search for genes that cause cancer when they're damaged. See the 'Our current research' section for more details of the charity's work in this area. And you can find out more about the charity's past achievements in the 'Our History and achievements' section.