Question

What would happen if in a gene encoding a polypeptide of 50 amino acids, 25${}^{th}$ 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.

Answer 1

Answer: (A)

A polypeptide of 24 amino acids will be formed.

The process of translation requires transfer of genetic information from a polymer of nucleotides to a polymer of amino acids. The three letter nucleotide code of DNA and RNA is termed as codon. These are termed so since they code for amino acids by the ribosomes during the translation process. There are 64 codons in total in the genetic code of which 61 codons code for amino acids and 3 codons do not code for any amino acid and called as stop codons since protein synthesis terminated when stop codons are read. The codon is a triplet. UAA, UAG and UGA are the three stop codons. A gene is encoding a polypeptide of 50 amino acids implies that this gene has 51 codons where the 51st codon is a stop codon. Therefore, a protein of 50 amino acids is encoded. If 25th codon i.e., UAU coding for tyrosine is point mutated to UAA which is a stop codon, the protein formation process will stop here. Therefore, option A is correct