Q.42 Give below are two statements : One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : Prokatyotes have three RNA polymerases. Prokaryotic RNA pol II synthesizes ribosomal RNAs.
Reasons R : In Eukaryotes RNA Polymerase II synthesizes mRNA and is sensitive to low concentration of a-
amanitin.
In the light of the above statements. choose the most appropriate answer from the options given below :
1. Both A and R are correct and R is the correct explanation of A
2. Both a and R are correct but R is NOT the con•ect explanation of A.
3. A is correct but R is not correct.
4. A is not correct but R is correct.
Correct Answer: 4. A is not correct but R is correct
Prokaryotes have a single RNA polymerase, not three, and it transcribes all RNAs including rRNA, while eukaryotic RNA Pol II handles mRNA and is inhibited by low α-amanitin concentrations.
Assertion (A) Analysis
Assertion A claims prokaryotes have three RNA polymerases, with prokaryotic RNA Pol II synthesizing rRNA. This is incorrect: prokaryotes (e.g., E. coli) use one core RNA polymerase (α2ββ’ω) with sigma factors for promoter specificity, transcribing mRNA, tRNA, and rRNA. Eukaryotes have three (Pol I: rRNA; Pol II: mRNA; Pol III: tRNA/5S rRNA). No “RNA Pol II” exists in prokaryotes.
Reason (R) Analysis
Reason R states eukaryotic RNA Polymerase II synthesizes mRNA and is sensitive to low α-amanitin. Correct: Pol II transcribes pre-mRNA (hnRNA) for protein-coding genes, and low α-amanitin (from mushrooms) inhibits it specifically, unlike Pol I (resistant) or Pol III (sensitive at higher doses).
Option Breakdown
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Option 1: Both correct, R explains A—False. A wrong; R unrelated to prokaryotes.
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Option 2: Both correct, R not explanation—False. A incorrect.
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Option 3: A correct, R incorrect—False. A wrong, R right.
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Option 4: A incorrect, R correct—True. Matches facts.
Prokaryotic vs Eukaryotic RNA Polymerase: Assertion Reason Explained
Prokaryotic eukaryotic RNA polymerase differences define transcription regulation, with prokaryotes using one versatile enzyme vs. eukaryotes’ specialized Pol I/II/III. Crucial for molecular biology exams, this covers misconcepts like prokaryotic “RNA Pol II.”
Prokaryotic RNA Polymerase
Prokaryotes rely on a single RNA polymerase for all transcription:
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Core: α2ββ’ω subunits.
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Holoenzyme: +σ factor for promoters.
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Products: mRNA, tRNA, rRNA (16S/23S/5S).
No separate polymerases; sigma variants handle specificity. Couples with translation sans nucleus.
Eukaryotic RNA Polymerases
Eukaryotes split tasks across three nuclear polymerases:
| Polymerase | Location | Products | α-Amanitin Sensitivity |
|---|---|---|---|
| Pol I | Nucleolus | Most rRNA (28S/18S/5.8S) | Resistant |
| Pol II | Nucleoplasm | pre-mRNA, snRNA, miRNA | Low concentration sensitive |
| Pol III | Nucleoplasm | tRNA, 5S rRNA, small RNAs | High concentration sensitive |
Pol II needs general factors (TFII) for promoters; highly regulated via enhancers.
α-Amanitin Inhibition
This bicyclic peptide blocks Pol II’s bridge helix at low doses (nM), halting mRNA synthesis—lethal in poisoning. Diagnostic for Pol II activity in assays. Pol I/III resist longer.
Exam Tips: Assertion-Reason
Such questions test recall: Prokaryotes ≠ 3 polymerases (common error). R is eukaryotic fact, irrelevant to A. Choose based on individual correctness + link. Aligns with GATE/NEET syllabi on transcription machinery.
