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Modern Genetic Analysis Chapter 7. Gene Mutations
Solved Problem

2. | In Chapter 3, we learned that UAG and UAA codons are two of the chain-terminating nonsense triplets. On the basis of the specificity given in the test for aflatoxin B1 and ethyl methanesulfonate (EMS), describe whether each mutagen would be able to revert these codons to wild type. See answer

| Solution EMS induces primarily G·C A·T transitions. UAG codons could not be reverted to wild type, because only the UAG UAA change would be stimulated by EMS and that generates a nonsense (ochre) codon. UAA codons would not be acted on by EMS. Aflatoxin B1 induces primarily G·C T·A transversions. Only the third position of UAG codons would be acted on, resulting in a UAG:UAU change (on the mRNA level), which produces tyrosine. Therefore, if tyrosine were an acceptable amino acid at the corresponding site in the protein, aflatoxin B1 could revert UAG codons. Aflatoxin B1 would not revert UAA codons, because no G·C base pairs appear at the corresponding position in the DNA. See question

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