Question

If a specific mRNA sequence produces a 100 amino acid long polypeptide, but it was seen that the ${75}^{\mathrm{th}}$ codon (UAU- which codes for amino acid tyrosine) got mutated to UAG. What would be the consequence of the protein formed?

• A. A polypeptide of 100 amino acids will be formed
• B. A polypeptide of 75 amino acids and a polypeptide of 23 amino acids will be formed
• C. A polypeptide of only 74 amino acids will be formed
• D. A polypeptide of only 75 amino acids will be formed

Answer 1

The correct option is C A polypeptide of only 74 amino acids will be formed

The mechanism of protein synthesis is studied in three stages- initiation, elongation and termination. Three sets of nucleotide bases in the mRNA code for a particular amino acid which is referred to as a codon. There are 61 possible combinations of bases possible with a set of three nucleotides that encode amino acids and 3 combinations that signal for halt/termination of the polypeptide synthesis. These do not encode any amino acids and are called stop codons. UAG, UAA and UGA codons in the reading frame of mRNA signals for the translation termination.

If a 100 amino acid long polypeptide making mRNA gets mutated at the ${75}^{\mathrm{th}}$ codon position and the protein-encoding codon gets mutated to one of the stop codons then the process of translation/protein synthesis will get terminated in that particular mutated codon. Therefore, a polypeptide of 74 amino acids will be formed and then the process of translation will be terminated due to the nonsense mutation (when a normal codon changes to a stop codon due to mutation) of the open reading frame in mRNA.