Q.34 DNA and RNA are acidic in nature due to the presence of
(A) pentose sugar
(B) nitrogenous bases
(C) phosphate groups
(D) large number of hydrogen bonds
DNA and RNA are acidic in nature due to the presence of phosphate groups. This makes option (C) the correct answer.
Option Analysis
Pentose sugar, such as deoxyribose in DNA or ribose in RNA, provides the structural backbone but lacks acidic functional groups like hydroxyls that ionize significantly at physiological pH. Nitrogenous bases (adenine, guanine, cytosine, thymine/uracil) are primarily involved in base pairing and genetic coding; they remain mostly neutral without contributing protons. Phosphate groups form phosphodiester linkages in the backbone, ionizing to release H+ ions and create negative charges, akin to phosphoric acid. A large number of hydrogen bonds stabilizes the double helix in DNA but does not impart acidity.
Introduction to Nucleic Acid Acidity
DNA and RNA acidic nature phosphate groups define their key chemical property in molecular biology. These nucleic acids carry genetic information, with phosphate groups in the sugar-phosphate backbone ionizing at cellular pH to confer negativity. For CSIR NET aspirants, understanding this explains extraction techniques and stability.
Molecular Structure Breakdown
Nucleotides consist of pentose sugar, nitrogenous base, and phosphate. Phosphodiester bonds link 3′ and 5′ carbons, where phosphate (PO4) acts as a weak acid, dissociating as H3PO4 → H+ + H2PO4-. This ionization protects DNA from hydrolysis by repelling nucleophiles.
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Deoxyribose (DNA) vs. ribose (RNA): Sugars neutral.
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Bases: Purines/pyrimidines for H-bonding.
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Phosphate: Sole acidic contributor.
Why Not Other Components?
Pentose sugar anchors but imparts no acidity. Nitrogenous bases enable specificity, not pH effects. Hydrogen bonds maintain structure, irrelevant to acidity. Phosphate groups alone match phosphoric acid behavior.
Exam Relevance for CSIR NET
This question tests biomolecule chemistry. Phosphate negativity aids supercoiling, packaging, and counterion binding like histones. Master for units on nucleic acids and biochemistry.
