A strain of E. coli carries a mutation that completely inactivates the enzyme encoded in the gene. Several revertants, mutants of the mutant with partly or fully restored activity, were selected, and the amino acid sequence of the enzyme was determined. The only differences found were at position 10 in the polypeptide chain. Revertant 1 had Thr.
Revertant 2 had Glu.
Revertant 3 had Met.
Revertant 4 had Arg.
Assume that the initial mutation itself, as well as each revertant, resulted from a single nucleotide substitution.
a. What amino acid is present at position 10 in the mutant protein?
b. What codon in the mRNA would encode this amino acid?

The correct answer is - a) Lys b) AAG

Explanation:

As mention in question that whatever mutation took place was single nucleotide substitution which changes the complete amino acid here what one can do is he can analyze all 64 codons which code for different amino acids to find out.

Revertant 3 which had methionine in the 64 codons one codon codes for methionine which is AUG so we will search for most similar and found that-

Threonine = ACG

Glutamate = GAG

Arginine = AGG

Methionine = AUG

Lysine = AAG

In the mutant protein most possible amino acid present could be lysine as lysine coded by AAG in E.coli and if we do point mutation in the codon of lysine we can get all other four codon for given revertants :

AAG= lysine if second nucleotide converted to C so it will become ACG and will code for threonine as in Revertant 1.

AAG= lysine if first nucleotide converted to G - become GAG and will code for Glutamate as in Revertant 2.

AAG =lysine if second nucleotide converted to U - AUG and will code for methionine as in Revertant 3.

AAG= lysine if second nucleotide converted to G  - AGG and will code for arginine as in Revertant 4.

So, lysine codon AAG is the only position in the genetic code that can, in one step, mutate to either Thr, Glu, Met, or Arg.

Thus, the correct answer is - a) Lys b) AAG

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