There are three different sorts of copy error atbase level, and the effect of a mutation depends on what type of base change it is.

• Point mutation A base in the DNA sequence changes to another base.

• Frame-shift mutation A mutation inserts a base pair into the DNA and alters the meaning of every base 'downstream' from the mutation. This will usually produce a completely nonsensical, functionless protein.

• Stop mutation A previously coding triplet mutates to a 'stop' codon; the resulting protein fragments will probably be functionless.

As well as errors at the DNA level, chromosomal mutations can also occur.

An animation illustrates the different types of base level mutations.

The image opposite is of a fruitfly (Drosophila ) which has suffered a dramatic mutation (called a macromutation) and gained an extra pair of wings.