Question

The gene encoding protein Bog in E. coli has been changed by a point mutation in its open reading frame. Originally the gene encoded a 108 amino acid long functioning protein whose amino acid composition favored nonpolar and negatively charged amino acids. After the mutation it encodes a 110 amino acid long non-functioning protein that is now rich in polar uncharged amino acids and positively charged amino acids. Which mutation scenario best explains this observation.

A) A frame-shift caused by a single nucleotide deletion early in the open reading frame, with a nonsense codon not encountered in the new reading frame until shortly after the original stop codon.

B) A nucleotide substitution resulting in a nonsense codon early in the open reading frame.

C) A nucleotide substitution in the stop codon that extends the open reading frame of the mRNA by an additional 2 amino acids.

D) A double nucleotide addition just upstream of the stop codon in the original reading frame that extends the length of the ORF by 2 codons.

E) A nucleotide addition followed by a nucleotide deletion in the second and third codons of the open reading frame encoding this protein.

Answer 1

Ans. Given, the mutated gene encodes 110 AA protein compared to 108 AA in normal gene. Since, each amino acid is encoded by a triplet codon, an addition of two more triplet codons in the gene during mutation would be required to increase peptide length by 2 AA.

So, any mutation occurring should increase the number of coding triplet codons to get two more AA.

#1. Incorrect. Deletion of one nucleotide and downstream shift of stop codon by 1 nucleotides results no change in the number of amino acids in the mutated protein.

#2. Incorrect. Appearance of stop codon in mid of ORF causes premature termination of translation of mutated mRNA. Thus, peptide chain length reduces.

#3. Incorrect. The ORF extended by 2 amino acids due to mutation. Thus, the resultant mutated protein has 2 additional amino acids in it.

However, addition of 2 AA at the end of peptide chain (Note: Stop codon extended by 2 codons) causes least effect to the overall folding of protein. Because, the sequence of remaining 108 AA is common to both normal and mutated proteins.

#4. Correct. The chain length increases by 2AA.

However, additional of 2 nucleotides (Note: 1 codon has 3 nucleotides) in ORF changes the creating the new sequence of triplet codons, i.e. shifts the ORF by 1 nucleotides. So, there would be least similarity in AA sequence of mutated protein and normal protein. Due to change in AA sequence, the folding pattern changes leading to a denatured protein.

#5. Incorrect. Deletion of nucleotides makes protein shorted than 108AA.