In this mutation, stop codon - UAG is replaced by another stop codon- UAA. So, this is silent mutation. In silent mutations, there is no change in primary structure of protein. OTHER QUESTIONS FREQUENTLY ASKED ON THIS TOPIC: Silent Mutation:The codon containing the changed base codes for the same amino acid.For example, if the serine (Ser) codon UCA is given a different third base, U, to become UCU, it still codes for Ser. This is termed a "silent" mutation. Missense Mutation:The codon containing the changed base codes for different amino acid. For example, if the Ser codon UCA is given a different first base, C, to become CCA, it will code for a different amino acid (in this case, proline ). This is called a "missense" mutation. Nonsense Mutation:The codon containing the changed base become a termination codon. For example, if the Ser codon UCA is given a different second base, A, to become UAA, the new codon causes termination of translation at that point and the production of a shoened (truncated) protein.