Ochre and amber are two types of nonsense mutations. Before the genetic code was worked out, Sydney Brenner, Anthony O. Stretton, and Samuel Kaplan applied different types of mutagens to bacteriophages in an attempt to determine the bases present in the codons responsible for amber and ochre mutations. They knew that ochre and amber mutants were suppressed by different types of mutations, demonstrating that each is a different termination codon. They obtained the following results
1) A single-base substitution could convert an ochre mutation into an amber mutation
2) Hydroxylamine induced both ochre and amber mutations from wild-type phages
3) 2-aminopurine caused ochre to mutate to amber
4) Hydroxylamine did not cause ochre to mutate to amber
These data do not allow the complete nucleotide sequence of the amber and ochre codons to be worked out, but they do provide information about the bases found in the nonsense mutations. What conclusion about the bases in the codons of amber and ochre mutations can be made from these observations?
1. The ochre codon contains an A or U, and the amber codon contains a G Both codons contain a G base
2. The ochre codon has a G, and the amber codon must have an A or a U
3. Both codons contain only A and U bases
4. The ochre codon can contain only an A, and the amber codon contains a G