Due to the degenerate nature of the genetic code, not all such mutations…

The idea

There are 64 possible DNA triplets but only 20 amino acids, so most amino acids are coded for by more than one triplet: the code is degenerate. The alternative triplets for an amino acid usually differ in the third base, so a substitution at that third position often still specifies the same amino acid. When it does, the amino acid sequence of the polypeptide is unchanged, the tertiary structure is unchanged and the protein works normally. This is called a silent mutation, and it is why many mutations have no effect on the phenotype at all. Substitutions in the first or second base are far more likely to change the amino acid.

What earns the marks

• More than one triplet or codon coding for the same amino acid.
• The substituted triplet still coding for the same amino acid.
• No change to the amino acid sequence of the polypeptide, so no change to its tertiary structure or function.
• Stating that a substitution in the third base of a triplet is the most likely to be silent.
• Distinguishing degenerate from universal, where a question about expressing a gene in another species requires universal.

Common mistakes and how to put them right

• Answering why bacteria can make a human protein with the code is degenerate, which mark schemes explicitly reject.

  Put it right: The required point is that the code is universal.

• Claiming degeneracy means one triplet can code for several amino acids, which reverses the definition.

• Assuming every substitution is silent.

• Thinking a silent mutation means no mutation happened, when the DNA base sequence has genuinely changed.

• Using codon for the DNA triplet when the question refers to DNA.

Exam tips

• Learn the three properties of the code as a set, each with a typical question, so you pick the right one under pressure.
• If asked why a mutation had no effect on the protein, name degeneracy and say the new triplet codes for the same amino acid.
• Count triplets and amino acids in your answer: 64 against 20 is a quick, creditable justification.