Why do not all mutations in a nucleotide sequence lead to changes in polypeptide structure?

Mutations in a nucleotide sequence do not always lead to changes in polypeptide structure mainly due to the degeneracy of the genetic code. The genetic code is referred to as degenerate because multiple codons can code for the same amino acid. There are 64 possible codons but only 20 amino acids, meaning that several codons can specify the same amino acid.

For instance, if a mutation occurs in a DNA sequence resulting in a different codon that still codes for the same amino acid, the overall structure and function of the resulting polypeptide may remain unchanged. This provides a level of redundancy in the genetic code, which helps to buffer against the potential impacts of mutations.

Other options mention aspects such as the uniformity of the genetic code, harmful mutations, and the occurrence of mutations solely in exons, but these do not explain the phenomenon of why mutations may not influence polypeptide structure as successfully as the degeneracy of the code does.