Question

You are studying a gene that encodes a particular protein; part of the amino acid sequence of that protein is shown below:

…-His-Val-Pro-Thr-Asp-Leu-Glu-…

You isolate a mutant version of this protein; the mutation abolishes the function of the protein. When you sequence the mutant protein,

you see the following amino acid sequence:

  

…-His-Val-Leu-Asp-Arg-Leu-Gly-…

Answer/do the following (refer to the Codon Chart below):

a. What was the most likely type of mutation (missense, nonsense, or frameshift) that occurred in the gene
encoding this protein that resulted in the mutated amino acid sequence above? (I am looking for the
simplest, most plausible explanation.)

b. In which codon did the mutation occur? What change actually happened?

c. Write out plausible RNA sequences that encode both the wild-type and mutant proteins

Answer 1

a. Answer- It is the frameshift type of point mutation where from the site of mutation, whole sequence of codon changes. As codon sequence changes, protein sequence also changed. In this example, after valine amino acid, whole sequence of protein changed. So that we can say that frameshift mutation occurred in the codon of proline.

b. Answer- in the codon of proline mutation occurred as from proline all further sequence of amino acid is changed.

Actually addition mutation has occurred at at the second nucleotide in proline codon. Proline is coded by 4 codons and each codon starts with CC and leucine is coded by 6 codons, out of 6 codons, 4 codons starts with CU as first two nucleotide. So that addition of U nucleotide between first and second nucleotide in proline codon CCU becomes CUC and as frame shift occurs, the last nucleotide of proline codon acts as first nucleotide for next codon. Next amino acid in normal polypeptide is threonine and it is coded by 4 codons and all starts with AC. So that G of proline acts as first nucleotide in threonine codon after addition mutation and new codon of proline becomes, CUC which codes for leucine.

c. Answer-

RNA sequence of wild type may be- • is site of addition mutation

....-CAU - GUA - C•CG - ACA - GAC - UUG - GAG-......

Sequence of wild type protein is-

....- His - Val - Pro - Thr - Asp - Leu - Glu -........

sequence of mutant RNA may be-

....-CAU - GUA - CUC - GAC - AGA - CUU - GGA - G-......

Sequence of mutant protein is-

....- His - Val - Leu - Asp - Arg - Leu - Gly -......