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G

The normal base sequence shown here consists of nine bases that code for three amino acids: GCA codes for arginine, TTC for leucine and CAG for valine. Look at the first example of a mutation where there has been a substitution. The first base in the original sequence, guanine, has been replaced by adenine. The first three bases now read АСА, not GCA. This codes for a different amino acid, cysteine. This mutation will therefore result in cysteine replacing arginine in the protein that is produced. The second example of a substitution is slightly different. Although the third base in the original sequence, adenine, has been replaced by cytosine, the altered sequence, GCC, also codes for arginine. To summarize, a mutation involving the substitution of a single base may affect a single amino acid in the protein for which it codes. It may have no effect at all.