Question

 What would happen, if in a gene encoding a polypeptide of 50 amino acids, 25th codon (UAU) is mutated to UAA?

A. A polypeptide of 49 amino acids will be formed

B. A polypeptide of 25 amino acids will be formed

C. A polypeptide of 24 amino acids will be formed

D. Two polypeptides of 24 and 25 amino acids will be formed

Answer 1

Hint: In a missense mutation, one base pair is changed in the codon that results in the termination of translation of protein in the early stages. A truncated protein is produced as a result of this mutation.

Complete answer:

A mutation is an heritable alteration in the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA.

A nonsense mutation is a type of point mutation in which a single base pair is substituted that leads to the formation of a stop codon that will terminate where previously there was a codon that could specify a particular amino acid. Due to the presence of a premature stop codon, a non-functional, truncated protein is produced. This type of mutation causes direct damage to the DNA that will induce mutations in the mRNA that is produced. This type of mutation introduces a stop codon in the upstream of the correct signal and stops the translation of the protein.

When a codon UAU is mutated to UAA then, this will result in early termination of the polypeptide chain because UAA doesn’t code for a protein; it is a stop codon that terminates the translation process. This will result in the formation of a polypeptide of 24 amino acids.

Hence, the correct option is (C) A polypeptide of 24 amino acids will be formed.

Note: A point-nonsense mutation can lead to the addition of a premature stop codon. It is also known as a truncating mutation. In this, the altered codon corresponds to a stop signal.