Q.74 Nonsense suppressor mutation is found in _________________.
(A) rRNA
(B) tRNA
(C) start codon of mRNA
(D) stop codon of mRNA
The correct answer is (B) tRNA.
Nonsense suppressor mutations primarily occur in tRNA genes, enabling these molecules to recognize and translate premature stop codons during protein synthesis. This mechanism allows readthrough of nonsense mutations, restoring partial protein function. Such suppressors are crucial in genetic studies and have applications in biotechnology and therapeutics.
Option Analysis
Nonsense mutations introduce premature termination codons (UAG, UAA, UGA) in mRNA, halting translation and producing truncated proteins.
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(A) rRNA: Rare rRNA mutations in specific ribosomal regions can influence termination efficiency or ribosomal ambiguity, but they do not directly bind stop codons like classic nonsense suppressors. These are not the primary or standard site for such mutations.
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(B) tRNA: Mutations alter the tRNA anticodon to base-pair with a nonsense codon (e.g., amber suppressor tRNA recognizes UAG), inserting an amino acid and suppressing termination. This is the most common and well-documented mechanism in bacteria, yeast, and eukaryotes.
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(C) Start codon of mRNA: The start codon (AUG) initiates translation; mutations here typically prevent initiation rather than suppress downstream nonsense codons. No suppressor role exists for start codon alterations in this context.
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(D) Stop codon of mRNA: Stop codons signal termination via release factors, not suppression. Mutations in natural stop codons would disrupt normal termination but do not create suppressors; suppression acts on mutated premature stops elsewhere.
Nonsense suppressor mutation in tRNA plays a key role in molecular biology, allowing cells to overcome premature stop codons during translation. This phenomenon is essential for understanding genetic suppression in exams like CSIR NET Life Sciences.
What is Nonsense Suppressor Mutation?
A nonsense suppressor mutation alters a tRNA anticodon, enabling it to pair with termination codons like UAG (amber), UAA (ochre), or UGA (opal). Instead of halting translation, the mutant tRNA inserts an amino acid (e.g., tyrosine or glutamine), promoting readthrough and producing full-length or near-full-length proteins. This intergenic suppression restores function in organisms carrying nonsense mutations in essential genes.
Mechanism of Suppressor tRNA
During translation, a premature nonsense codon enters the ribosomal A-site. Normally, release factors (RF1/RF2 in bacteria) bind and terminate synthesis. Suppressor tRNA competes effectively due to its altered anticodon, outcompeting release factors and inserting its cognate amino acid. Efficiency depends on codon context, tRNA concentration, and cellular factors; cells maintain low suppressor levels to avoid disrupting normal termination.
Why tRNA, Not Other Options?
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rRNA suppressors: Exist rarely (e.g., in 16S/23S rRNA loops increasing mistranslation), but primary nonsense suppression is tRNA-based.
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mRNA elements: Start or stop codons in mRNA do not mutate to suppress; they are templates decoded by tRNAs and factors.
Suppressor tRNAs are classified by codon: Su1 (tyrosine, amber), Su2 (glutamine, amber), etc.
Applications in Research & Therapy
Engineered suppressor tRNAs enable unnatural amino acid incorporation in synthetic biology and readthrough of disease-causing nonsense mutations (e.g., cystic fibrosis). Aminoglycosides enhance suppression, but tRNA-based gene therapy shows promise.
CSIR NET Exam Tips
For MCQs like “Nonsense suppressor mutation is found in _____,” focus on tRNA as the standard answer. Practice distinguishing from ribosomal or frameshift suppressors. Key concepts: anticodon mutation, amber/ochre/opal, intergenic suppression.
