Question

What would happen, if the vertebrate mitochondrial DNA undergoing the process of translation and forming a polypeptide, a sudden mutation changes the ${25}^{\mathrm{th}}$ codon of the mRNA into UGA?

• A. The ribosome will read over the mutated codon and continue the synthesis of the polypeptide
• B. The ribosome will stop at the mutated codon and synthesise a polypeptide of 25 amino acids
• C. The ribosome will stop at the mutated codon and synthesise a polypeptide of 24 amino acids
• D. Polypeptide synthesis will not occur at all

Answer 1

The correct option is A The ribosome will read over the mutated codon and continue the synthesis of the polypeptide

The genetic code decides how and what protein will be synthesised after translation. There are 64 possible combinations of three bases, among which 61 specify amino acids and 3 combinations are stop codons. A three-base sequence in the mRNA that signals to halt the process of protein synthesis is called the stop codon. The sequences are UAG, UAA and UGA. Among the 64 possible combinations, 61 codons encode amino acids as they are read by the tRNA molecule and the three stop codons are recognised by the release factors.

But in the case of vertebrate mitochondrial DNA, the codon UGA codes for the amino acid tryptophan rather than being a stop codon. Ribosomes will continue moving ahead reading the scratch of mRNA and the process of translation will continue even after the mutation even after changing the codon of the reading frame to UGA.