(HT only) Mutations occur continuously. Most do not alter the protein, or…

The idea

Mutations are random changes in the base sequence of DNA that happen continuously, including during DNA replication before cell division. Because the genetic code is degenerate and many amino acids have more than one codon, a base substitution often still codes for the same amino acid, so the primary structure and therefore the folded protein are unchanged. Even when a different amino acid is inserted, it may sit in a region that is not critical for the protein's shape or activity, so the appearance or function is only slightly altered or unaffected. For example, a substitution in a gene for an enzyme may change an amino acid far from the active site, leaving the substrate binding site unchanged and the enzyme still functional. Only a minority of mutations produce a protein with a markedly different shape or function.

What earns the marks

• Mutations are random changes in the base sequence of DNA and occur continuously, including during DNA replication.
• The genetic code is degenerate, so a base substitution may still code for the same amino acid and leave the protein unchanged.
• A mutation may change one amino acid in a part of the protein that is not essential for its shape or function, so the appearance or function is only slightly altered.
• Most mutations do not alter the protein, or alter it only slightly, so the organism's characteristics are usually unaffected.
• The effect of a mutation depends on where in the gene it occurs and whether the changed amino acid affects the protein's folding or active site.

Common mistakes and how to put them right

• Thinking that every mutation changes the protein.

  Put it right by stating that the code is degenerate and many substitutions still code for the same amino acid.

• Believing that mutations are always harmful.

  Put it right by explaining that most have no effect on the protein or its function, and some can be beneficial.

• Assuming that a change in one amino acid always changes the protein's shape.

  Put it right by noting that the altered amino acid may be in a region that does not affect folding or function.

Exam tips

• Use the phrase 'random change in the base sequence of DNA' when defining a mutation, and link it to DNA replication.
• Explain the role of the degenerate genetic code when a substitution does not change the amino acid sequence.
• When describing effects, refer to the protein's shape, active site or function rather than saying the mutation 'does nothing'.