Cancer occurs when the rate of cell multiplication is faster than the rate of cell death. This causes the growth of a tumour.
Cancer is caused by environmental damage to DNA from
- physical factors such as UV light and asbestos
- chemical carcinogens such as those in the tar in cigarette smoke
- viruses may trigger cancer by altering the DNA
Chemicals called radicals are produced by the cell metabolism and can damage DNA. Fresh fruit and vegetables contain antioxidants to destroy radicals.
The cause may also be genetic. About 5% of cancers are due to an inherited gene.
The progression through the cell cycle (G1, S, G2, M) is controlled by:
- Oncogenes which stimulate the cycle. Mutations can result in the cycle being continually active and lead to excessive cell division and tumour formation
- Tumour suppressor genes which stop the cycle. Mutations mean there is no brake on the cycle and control is lost.
If tumours are not removed, cancer cells can spread to other parts of the body through the blood and lymphatic systems. This is called metastasis.