Question

What would be the result if a gene encoding a polypeptide of 50 amino acids undergoes nonsense mutation at the 25th codon, i.e., UAC into UAA?

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

Answer 1

Answer: (D)

Every amino acid is coded by a triplet of 3 adjacent nitrogen bases. This triplet is called a codon. To code for 50 amino acids, 51 codons are needed i.e., 50 codons + 1 stop codon.
However, the sequence of DNA has changed here due to the occurrence of a nonsense mutation. This mutation has resulted in a stop codon (UAA) at the 25th codon in the gene. This will stop the normal elongation of the amino acid chain to form the final protein. Thus, a polypeptide having only 24 amino acids will be formed.