456 Chapter 14 • Mutation, Repair, and Recombination Synonymous substitutions never alter the amino acid sequence of the polypeptide chain. The severity of the effect of missense and nonsense mutations on the polypeptide differs from case to case. For example, if a missense mutation replaces one amino acid with a chemically similar amino acid, referred to as a conserva-tive substitution, then the alteration is less likely to af-fect the protein’s structure and function severely. Alter-natively, chemically different amino acid substitutions, called nonconservative substitutions, are more likely to produce severe changes in protein structure and func-tion. Nonsense mutations will lead to the premature ter-mination of translation. Thus, they have a considerable effect on protein function. The closer a nonsense muta-tion is to the 3 9 end of the open reading frame, the more plausible it is that the resulting protein might possess some biological activity. However, many nonsense muta-tions produce completely inactive protein products. Like nonsense mutations, indel mutations (base-pair additions or deletions) have consequences on polypep-tide sequence that extend far beyond the site of the mu-
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.