(a) The structures illustrated for the four DNA bases represent only the most stable tautomers. Draw one or more alternative tautomers for each of these heterocycles.

(b) In certain cases, the presence of a small amount of one of these less stable tautomers can lead to an error in DNA replication or mRNA synthesis due to faulty base pairing. One example is the imine tautomer of adenine, which pairs with cytosine instead of thymine. Draw a possible structure for this hydrogen-bonded base pair (see Figure 26-11). 

(c) Using Table 26-3, derive a possible nucleic acid sequence for an mRNA that would code for the five amino acids in met-enkephalin. If the mispairing described in (b) were at the first possible position in the synthesis of this mRNA sequence, what would be the consequence in the amino acid sequence of the peptide? 

Figure 26-11

Table 26-3

Transcribed Image Text:

H N-H. O CH3 0...H-N N.. H-N N-HN Sugar Sugar Sugar N-H ở H Sugar Adenine-thymine Guanine-cytosine