Substitution of bases

- A nucleotide is replaced by another nucleotide that has a different base - Can form stop codon - So production of the polypeptide would be stopped prematurely - Final protein different, not perform its normal function - Forms codon for a different amino acid, structure of polypeptide different, different shape + not function properly - Forms different codon but for the same amino acid, genetic code is degenerate, has no effect