1. Which one of the following situations could be termed as intergenic suppression of a mutation in a coding sequence?
(1) A nonsense mutation in the codon leading to lack of interaction with cognate tRNA anticodon
(2) The amber mutation in the codon recognized by the altered anticodon of a tRNA
(3) A read-through of a stop codon by the ribosomes
(4) A frameshift mutation skipping an otherwise stop codon

Intergenic suppression occurs when a mutation in one gene restores the function lost due to a mutation in a different gene, specifically here involving a coding sequence mutation like a nonsense codon. This MCQ tests understanding of suppressor mechanisms in translation, common in CSIR NET Life Sciences genetics topics. The correct answer is (2) The amber mutation in the codon recognized by the altered anticodon of a tRNA.​

Option Analysis

Option 1: A nonsense mutation in the codon leading to lack of interaction with cognate tRNA anticodon
This describes the primary nonsense mutation itself (e.g., changing a sense codon to UAG amber), causing premature termination due to failed tRNA binding, not suppression. No second mutation in another gene is involved, ruling out intergenic suppression.​

Option 2: The amber mutation in the codon recognized by the altered anticodon of a tRNA (Correct)
An amber mutation (UAG nonsense codon) in the coding sequence is suppressed intergenically by a mutation in a tRNA gene, altering its anticodon (e.g., to CUA) to recognize UAG and insert an amino acid, allowing translation to continue. This classic informational suppressor exemplifies intergenic suppression.​

Option 3: A read-through of a stop codon by the ribosomes
Ribosomal read-through of natural stop codons (UAA, UAG, UGA) can occur spontaneously or via drugs but lacks a second mutation in another gene, so it is not intergenic suppression. It does not specifically address suppression of a coding sequence mutation via another gene product.​

Option 4: A frameshift mutation skipping an otherwise stop codon
Frameshift mutations that restore reading frame or skip stops are typically intragenic (second mutation in the same gene), not intergenic, as seen in Crick’s experiments. No involvement of a separate gene like tRNA qualifies it as intergenic suppression.​

Key Concepts for CSIR NET

• Intergenic vs. Intragenic: Intergenic involves different genes (e.g., tRNA mutation suppressing protein-coding mutation); intragenic is within the same gene.​

• Nonsense Suppressors: Amber (UAG), ochre (UAA), opal (UGA) suppressors arise from tRNA anticodon mutations, restoring protein function.​
  Practice such MCQs to excel in Unit 8 Genetics for CSIR NET Life Sciences.​