81. 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
Why Are DNA and RNA Acidic in Nature?
Correct Answer
(C) Phosphate groups
Introduction
DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are the two major nucleic acids responsible for the storage, transmission, and expression of genetic information in all living organisms. Every nucleotide of DNA and RNA consists of three fundamental components: a pentose sugar, a nitrogenous base, and a phosphate group. These nucleotides are linked together through 3′–5′ phosphodiester bonds, forming the characteristic sugar-phosphate backbone of nucleic acids. While sugars provide the structural framework and nitrogenous bases encode genetic information, the phosphate groups are responsible for the unique chemical property that makes nucleic acids acidic.
The acidic nature of DNA and RNA originates from the phosphoric acid residues present in every nucleotide. At physiological pH, these phosphate groups lose hydrogen ions (H⁺), becoming negatively charged phosphate ions. As a result, every nucleotide contributes a negative charge to the nucleic acid backbone, making DNA and RNA polyanionic molecules. This negative charge influences DNA stability, protein binding, electrophoretic mobility, chromatin organization, and numerous molecular biology techniques.
Understanding the Concept Behind the Question
Each nucleotide contains:
- One pentose sugar
- One nitrogenous base
- One phosphate group
Among these components, only the phosphate group ionizes under physiological conditions.
The phosphate groups release hydrogen ions according to the reaction:
–PO₄H ⇌ –PO₄⁻ + H⁺
Because every nucleotide contains a phosphate residue, DNA and RNA possess multiple negatively charged phosphate groups along their backbone.
Therefore, nucleic acids behave as strongly acidic molecules.
Analysis of Option (A)
Pentose Sugar
This statement is incorrect.
DNA contains deoxyribose, whereas RNA contains ribose.
Although these sugars contribute to the structural framework of nucleic acids, they are electrically neutral under physiological conditions.
They do not release hydrogen ions responsible for acidity.
Therefore,
Option (A) is incorrect.
Analysis of Option (B)
Nitrogenous Bases
This statement is incorrect.
The nitrogenous bases include:
- Adenine
- Guanine
- Cytosine
- Thymine
- Uracil
These bases contain amino and nitrogen-containing ring structures.
While they participate in hydrogen bonding and base pairing, they are not responsible for the acidic character of DNA and RNA.
Therefore,
Option (B) is incorrect.
Analysis of Option (C)
Phosphate Groups
This statement is correct.
Each nucleotide contains one phosphate group.
These phosphate groups form phosphodiester bonds between adjacent nucleotides.
At physiological pH, phosphate groups are negatively charged because they dissociate hydrogen ions.
Consequently:
- DNA carries a large negative charge.
- RNA also carries a large negative charge.
- Both molecules migrate toward the positive electrode during electrophoresis.
Therefore,
Option (C) is correct.
Analysis of Option (D)
Large Number of Hydrogen Bonds
This statement is incorrect.
Hydrogen bonds stabilize:
- DNA double helices
- RNA secondary structures
However, hydrogen bonding has no role in determining whether DNA or RNA is acidic.
The acidity depends exclusively on ionization of phosphate groups.
Therefore,
Option (D) is incorrect.
Structure of the DNA and RNA Backbone
The backbone of nucleic acids consists of alternating:
- Pentose sugar
- Phosphate group
Neighboring nucleotides are connected through:
3′ → 5′ phosphodiester bonds
Each phosphodiester linkage retains a negatively charged phosphate group.
This repeating negative charge gives nucleic acids their characteristic acidic nature.
Biological Importance of Phosphate Groups
The negatively charged phosphate backbone performs numerous biological functions. It increases the solubility of DNA and RNA in aqueous environments, prevents spontaneous passage through lipid membranes, facilitates interactions with positively charged proteins such as histones, and enables separation of nucleic acids by gel electrophoresis. The phosphate backbone also stabilizes the double helix and contributes to the overall structural integrity of chromosomes and RNA molecules.
High-Yield Points
- DNA and RNA are polyanions.
- Acidity is due to phosphate groups.
- Phosphate groups form 3′–5′ phosphodiester bonds.
- DNA contains deoxyribose sugar.
- RNA contains ribose sugar.
- Nitrogenous bases are A, G, C, T (DNA), and U (RNA).
- DNA migrates toward the positive electrode during electrophoresis because of its negatively charged phosphate backbone.
- Histones bind DNA through electrostatic interactions with phosphate groups.
Frequently Asked Questions
Why is DNA negatively charged?
DNA contains one phosphate group in every nucleotide. These phosphate groups lose hydrogen ions at physiological pH, leaving the backbone negatively charged.
Do nitrogenous bases contribute to DNA acidity?
No. Nitrogenous bases encode genetic information but are not responsible for the acidic nature of nucleic acids.
Why do DNA molecules move in agarose gel electrophoresis?
Because the negatively charged phosphate backbone causes DNA to migrate toward the positive electrode when an electric field is applied.
Key Takeaways
DNA and RNA are acidic molecules because every nucleotide contains a phosphate group that ionizes under physiological conditions, producing negatively charged phosphate ions. These phosphate groups form the phosphodiester backbone, giving nucleic acids their characteristic negative charge and acidic properties. In contrast, pentose sugars provide structural support, nitrogenous bases store genetic information, and hydrogen bonds stabilize nucleic acid structures but do not contribute to acidity. Therefore, the correct answer is Option (C).
Final Answer
Correct Option: (C) Phosphate groups
Explanation
DNA and RNA are acidic because their nucleotides contain phosphate groups, which become negatively charged after releasing hydrogen ions under physiological conditions. These phosphate groups are linked together by 3′–5′ phosphodiester bonds, forming the sugar-phosphate backbone of nucleic acids. The accumulation of negatively charged phosphate residues makes DNA and RNA polyanionic molecules, responsible for their acidic nature, water solubility, and electrophoretic behavior. Pentose sugars, nitrogenous bases, and hydrogen bonds do not determine the acidic character of nucleic acids. Therefore, Option (C) Phosphate groups is the correct answer.
