2003 AIPMT Molecular Basis of InheritanceMutations: Nonsense Mutation Medium Question 171 of 200 in Molecular Basis of Inheritance

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 24 amino acids will be formed
B Two polypeptides of 24 and 25 amino acids will be formed
C A polypeptide of 49 amino acids will be formed
D A polypeptide of 25 amino acids will be formed

The answer is A.

Solution

UAU codes for the amino acid tyrosine, while UAA is a stop (termination) codon. Changing a sense codon into a stop codon is a nonsense mutation.
During translation, the ribosome reads codons 1 to 24 and adds 24 amino acids.
When it reaches codon 25, now UAA, no tRNA pairs with it. A release factor binds and translation stops, and the polypeptide is released.
The remaining codons (26 to 50) are not translated, and no second polypeptide is made, because translation cannot restart without a new start codon.
So a polypeptide of 24 amino acids will be formed.

Concept Premature stop codon

Molecular Basis of Inheritance in past papers

200 questions from this chapter have appeared across 27 exam years.