Q73.Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A:
The density of RNA molecule is higher than that of DNA.
Reason R:
RNA has 2′OH group attached to the second carbon atom of the ribose ring.
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 correct explanation of A
(3) A is correct but R is not correct
(4) A is not correct but R is correct
The correct answer is option (2): Both A and R are correct but R is not the correct explanation of A.
Statement Analysis
Assertion A is true—RNA molecules have higher density (≈1.65-1.70 g/cm³ buoyant density in CsCl gradients) than DNA (≈1.50 g/cm³ for B-DNA), observable in ultracentrifugation where RNA bands below DNA. Reason R is correct—RNA’s ribose contains a 2′OH group at C2′ (absent in DNA’s deoxyribose), promoting A-form helix rigidity via H-bonding and 3′-endo sugar pucker. However, R doesn’t explain A: higher RNA density arises primarily from its compact A-helix (shorter, wider), increased hydration shell, and higher molecular weight per nucleotide (≈320 Da vs. DNA’s 303 Da), not directly from 2′OH alone.
Option Breakdown
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Option (1): Wrong—R true but doesn’t causally explain density difference (A-helix geometry dominates).
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Option (2): Correct—both facts hold, but 2′OH affects conformation/hydration indirectly, not density primarily.
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Option (3): Wrong—R accurately describes RNA’s structural hallmark.
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Option (4): Wrong—A confirmed by density gradient separation in molecular biology.
Introduction: Density of RNA Molecule Higher Than DNA Explained
NEET molecular biology tests nucleic acid properties via assertions like “density of RNA molecule is higher than that of DNA” versus 2′OH explanation. Both true but unrelated causally (option 2) distinguishes exam-level understanding. This analyzes buoyant density, structural causes, and prep strategy.
Why RNA Density > DNA
Buoyant density (CsCl ultracentrifugation):
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RNA: 1.65-1.70 g/cm³ (pellets below DNA).
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DNA: 1.40-1.50 g/cm³ (GC content dependent).
Primary causes: -
A-form helix: Compact (11 bp/turn, 2.3 nm rise) vs. B-DNA (10.4 bp/turn, 3.4 nm).
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MW/nucleotide: RNA ≈320 Da; ssDNA ≈304 Da.
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Hydration: 2′OH enables tighter water networks.
2′OH Role: True But Not Density Driver
R correct—ribose C2′ bears -OH (DNA: -H), enabling:
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A-form via 3′-endo pucker.
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RNA catalysis/H-bond networks.
Density link indirect: 2′OH → conformation → compactness → density.
NEET Winner: Option (2) Both Correct, R Not Explaining A
Trap: Assuming structural difference = causal. Distinguish correlation vs. mechanism.
Nucleic Acid Density Comparison
NEET Tip: Mnemonic—”RNA Denser: A-form + 2′OH → Compact.” Link to CsCl gradient separation PYQs.
