123 Skin Cancer: A Non-Inherited Mutation

Key Idea: Skin cancer is caused by mutations in somatic cells. These mutations are not inherited because skin cells are not involved in the production of gametes.

` Mutations can occur due to mutagens in the environment, e.g. cigarette smoke or exposure to ultraviolet light. The effect of these mutagens tends to build up over a life time so that the chance of mutation increases with age. This can be seen in the occurrence of cancers, which are more commonly seen in adults than in young people. ` Skin cancer is commonly caused by exposure to ultraviolet light. It is an example of a non-heritable mutation, as skin cells are not involved in the production of reproductive cells (gametes). ` It has been estimated that there might be 1012 mutations in the cells of a tumour, occurring both before and after the formation of the tumour.

Melanoma

UV Light Thymine dimer

DNA of tumour suppressor gene

` After exposure to UV light (a potent mutagen), adjacent thymine bases in DNA become cross-linked to form a 'thymine dimer'. This disrupts the normal base pairing and affects gene function. In some cases, mutations trigger the onset of cancer by disrupting the genes regulating the cell cycle. ` A large number of repair mechanisms constantly repair mutations so many genes may need to be affected by mutations before cancer occurs. Tests have shown that the use of sunscreen significantly reduces the production of thymine dimers. However tests also show that in unprotected skin 50% of dimers were removed after 24 hours and 75% were removed after 72 hours. A single incidence of sunburn generally won't lead to cancer because the damage is repaired but repeated cases increase the risk.

1. Discuss the difference between a non-inherited disease, such as skin cancer, and an inherited disease, such as cystic fibrosis:

2. Explain why skin cancer is less likely to be seen in young people than adults:

3. Explain why a large number of mutations (generally) need to occur before cells become cancerous: