455 14.1 Point mutations The molecular consequences of point mutations on gene structure and expression What are the functional consequences of these differ-ent types of point mutations? First, consider what hap-pens when a mutation arises in a polypeptide-coding part of a gene. For single-base substitutions, there are several possible outcomes, but all are direct conse-quences of two aspects of the genetic code: degeneracy of the code and the existence of translation termination codons (Figure 14-3). • Synonymous mutations . The mutation changes one codon for an amino acid into another codon for that same amino acid. Synonymous mutations are also referred to as silent mutations. • Missense mutations . The codon for one amino acid is changed into a codon for another amino acid. Missense mutations are sometimes called non-synonymous mutations. • Nonsense mutations . The codon for one amino acid is changed into a translation termination (stop) codon. Point Mutations at the Molecular Level
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