06. Genetic mutation

What happens if one letter in a sentence is changed?

Suppose, for example, 'The cat sat on the mat' becomes 'the cat sat on the rat'? One letter has been changed, but the meaning of the sentence is completely different. The same happens with DNA (which contains a 4-letter rather than a 26-letter alphabet) - changes to the genetic code can change the protein produced by a gene.

A mutation is a change in the DNA of an organism. The change may or may not result in a change to the physical characteristics (or phenotype) of the organism.

Mutations that occur in a gamete (sperm or egg) can be passed onto the next generation, whereas mutations that occur in body (or 'somatic') cells may be harmless or may result in diseases such as cancer.

Mutations can result in DNA being copied incorrectly. Mutagens can also cause mutations. Examples of mutagens include:

• X-rays

• ionizing radiation

• chemicals (for examples, the tar from cigarettes).

A point mutation is a change in one base pair in the genetic code, it can be the result of:

• the change of one base to a different base (often caused by the DNA being copied incorrectly) - this is called a substitution mutation.

• the insertion of an additional base into the DNA

• the deletion of a base from the DNA.

Activity: research and select one form of genetic mutation that affects humans. Produce a single sheet A3 poster that explains:

• • What the affliction is,

  • How it is caused by genes,

  • How common it is,

  • Any treatment there may be.

• Figure 1 A substitution mutation. What is happening here and what is the effect?

Because bases work in triplets (threes) to determine which amino acid is needed in a protein, an addition or deletion can have a profound effect. The deletion of insertion of a base results in what is called a frameshift.

• Figure 2 Base deletion resulting in a 'frameshift'. The base 'A' is removed, resulting in all bases before it to the left moving forward one place. Because bases work in threes, the first two amino acids in the sequence change - this would result in a different protein to the original.