DL 4

Mistakes can happen as DNA is copied. The base sequence is changed and mistakes occur in the manufacture of proteins.

This type of change is called a mutation. Mutations may arise spontaneously (by chance) or result from damage to a strand of DNA. UV radiation, nuclear radiation and certain chemicals such as nicotine and asbestos can cause mutations.

If the mutation occurs in the eggs or sperm, then there is a chance that it will be passed on to the next generation.

Types of mutations

The cells ‘read’ the genetic code a little bit like you read words on a page. A change to the order of the letters may have no effect on your understanding or it may mean that the text can no longer be understood.

• Silent mutations are changes in the genetic code that do not affect the individual. They occur when a single base on the DNA strand is changed and this does not affect the protein that is made using the information on that gene.

• Mis-sense mutations are changes that don’t stop the gene from making a protein. However, the protein produced is a different one. The protein may not function correctly and cause disease. A missense mutation is responsible for sickle-cell anaemia.

• Nonsense mutations are more a significant type of mutation. The mutation causes the cells to stop reading the information on the gene before its end. The protein created is incomplete and cannot function at all. Cystic fibrosis, Duchenne muscular dystrophy and Beta thalassemia are diseases resulting from nonsense mutations (Figure 1.3.10).

• Frameshift mutations are caused by the insertion or deletion of a single base. The insertion or deletion causes all the information following it to become jumbled so that it cannot be ‘read’ to make a protein. Frameshift mutations frequently result in severe genetic diseases such as Tay-Sachs disease (a genetic disorder that results in the destruction of nerve cells in the brain and spinal cord.)

Mutations add to the diversity of organisms. Natural selection works on this diversity by selecting the individuals most suited to an environment. Farmers rely on this diversity to improve crops and to increase productivity.