Q.67 Intergenic suppression involves mutation in
(A) rRNA (B) mRNA (C) tRNA (D) cDNA

Intergenic suppression occurs when a mutation in one gene restores the function lost by a mutation in a different gene, most commonly through changes in tRNA that allow read-through of premature stop codons. The correct answer to the multiple-choice question is (C) tRNA.

What is Intergenic Suppression?

Intergenic suppression involves a second mutation in a different gene that counteracts the effect of the first mutation, restoring wild-type function. Unlike intragenic suppression, which occurs within the same gene, intergenic types act across genes via interacting products like tRNAs.
This mechanism is key in molecular genetics for studying protein synthesis and codon recognition, often seen in nonsense or frameshift mutations.

Correct Answer: (C) tRNA

Mutations in tRNA genes create suppressor tRNAs with altered anticodons that recognize premature stop codons (UAG, UAA, UGA) as sense codons, inserting an amino acid and allowing translation to continue. These “nonsense suppressor tRNAs” exemplify intergenic suppression by acting on mutated protein-coding genes elsewhere in the genome.
For example, a su3 tRNA mutation in E. coli changes its anticodon to pair with amber (UAG) stops, suppressing defects in unrelated genes.

Why Not the Other Options?

Option (A) rRNA

rRNA forms the ribosome’s structural core and aids peptidyl transfer but lacks the anticodon needed to decode specific codons directly. Mutations in rRNA can affect global translation accuracy (e.g., streptomycin resistance) but do not provide codon-specific suppression like intergenic suppressors.

Option (B) mRNA

mRNA carries the coding sequence from DNA to ribosomes and is the target of suppression, not the suppressor itself. A mutation in mRNA would be transient and cis-acting, not heritable or trans-acting across genes as required for intergenic suppression.

Option (D) cDNA

cDNA is synthetic DNA reverse-transcribed from mRNA, used in cloning and expression studies, not a natural cellular component involved in translation or suppression. It plays no role in live-cell genetic suppression mechanisms.

Key Takeaways for Exams

• Focus on tRNA for intergenic (extragenic) nonsense suppression in MCQs.

• Distinguish from intragenic (same gene) or informational suppression via tRNA anticodon changes.
  This concept appears in genetics curricula for biotech and molecular biology students.