Worked Problems

1. A codon that specifies the amino acid Asp undergoes a singlebase substitution that yields a codon that specifies Ala. Refer to the genetic code in Figure 15.12 and give all possible DNA sequences for the original and the mutated codon. Is the mutation a transition or a transversion?

There are two possible RNA codons for Asp: GAU and GAC. The DNA sequences that encode these codons will be complementary to the RNA codons: CTA and CTG. There are four possible RNA codons for Ala: GCU, GCC, GCA, and GCG, which correspond to DNA sequences CGA, CGG, CGT, and CGC. If we organize the original and mutated sequences as shown in the following table, it is easy to see what type of mutations may have occurred:

Possible original sequence for Asp

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