A codon that specifies the amino acid Asp undergoes a single-base substitution that yields a codon that specifies Ala. Give all possible DNA sequences for the original and the mutated codon. Is the mutation a transition or a transversion?

Solution Strategy

What information is required in your answer to the problem?

A list of all possible original codons and mutated codons that would cause a change from Asp to Ala.

What information is provided to solve the problem?

For help with this problem, review:

Types of Mutations in Section 13.1 and the genetic code in Figure 11.5.

Solution Steps

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 the mutated sequences as shown in the following table, the types of mutations that may have occurred can be easily seen:

If the mutation is confined to a single-base substitution, then the only mutations possible are that CTA mutated to CGA or that CTG mutated to CGG. In both, there is a T → G transversion in the middle nucleotide of the codon.