Question

In a 125 amino acid sequence, if 25th amino acid is mutated to UAA, then
A. A polypeptide of 124 amino acids is formed.
B. A polypeptide of 25 amino acids is formed.
C. A polypeptide of 24 amino acids is formed.
D. Any of the above can be possible.

Answer 1

Hint: Mutation is the change in DNA sequence. It can arise due to faulty DNA duplication, autoimmune diseases or due to infectious diseases. It can also occur due to exposure to ionizing radiations.

Complete step by step answer:
To solve this question we should know the process of protein synthesis.
Protein synthesis is the process by which the cells make protein. It occurs in two steps.
Transcription- The DNA is converted to tRNA by RNA polymerase by the process of transcriptation.
Translation- It is the process of conversion of tRNA to amino acid chain. It takes place by the initiation of a polypeptide chain. The first codon to initiate is AUG i.e methionine. It is known as start codon. The Amino acid chain continues till it reads the stop codons- UAA, UGA, UAG. Once the stop codon is received, protein synthesis stops. The protein is translocated further to the target site.
In the above question, at the 25th position it is mutated to UAA. AS UAA is a stop codon, so protein synthesis is terminated. So the chain of 24 amino acids is formed.

Thus the correct answer is (C). A polypeptide of 24 amino acids is formed.

Note:
A stop codon terminates the translation of protein. UAA, UGA, UAG are called the nonsense codons as they do not code for any amino acid. Protein synthesis is necessary for growth, development and proper functioning of the body.