Question

In a polypeptide chain of 125 amino acids, if the ${25}^{th}$ amino acid is mutated to UAA, then

• A. A polypeptide of 124 amino acid is formed.
• B. A polypeptide of 25 amino acid is formed.
• C. A polypeptide of 24 amino acid is formed.
• D. Any of the above can be possible.

Answer 1

The correct option is C A polypeptide of 24 amino acid is formed.

Protein synthesis is the process of formation of proteins from mRNA. This process is also referred to as translation and is carried out by ribosomes. Ribosomes read the information present in mRNA as codons. Codons are the three letter amino acid code that constitute the genetic code. There are 64 codons present in the genetic code. Of this, 61 codons code for amino acid whereas three codons donot code for any amino acid. The translation unit which is translated into a polypeptide consists of a start codon, intermediate codons that code for peptide and stop codon which donot code for any amino acid. Therefore, if the polypeptide synthesis initiates with a start codon during initiation and is elongated. If the codon coding for 25th amino acid is mutated to UAA which is a termination/stop codon, the polypeptide synthesis will terminate here. As no amino acid is incorporated against a stop codon. Therefore, a resulting polypeptide of 24 amino acids will be formed. This will lead to the formation of a truncated protein since the polypeptide chain consists of 125 amino acids originally.

Therefore, the correct answer is option C.