Question

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.

Answer 1

Answer: (A)

A polypeptide of 24 amino acids will be formed.

The codon UAA is a stop codon and causes termination of protein synthesis. When this stop codon will occupy the ribosomal A site, it causes hydrolysis of the terminal peptidyltRNA bond to release the polypeptide and the last tRNA and finally dissociation of the 70S ribosome into its 30S and 50S subunits. The whole process results in premature termination of the polypeptide at 25th codon. Since 24 codons are translated properly, the polypeptide will have 24 amino acids, not 49 or 25 amino acids. "AUG" is the most common start codon. The start codon specifies the first codon of mRNA transcript that is translated by a ribosome. It codes for methionine in eukaryotes and a modified Met in prokaryotes. Translation initiation requires the presence of initiation codon and promoter sequences (Pribnow box, TATA box etc.) which are not present in the middle of any gene. Hence, after premature termination at 25th codon, the gene can not be translated at its 26th codon; two polypeptides of 24 and 25 amino acids can not be formed. Thus, the correct answer is A.