Q.11 Watson and Crick base pairing is ?
1. Hydrogen bonding in a single DNA strand.
2. Non-covalent hydrogen bonding between two DNA strands.
3. Van der Waal’s forces between bases.
4. Photphodiester bonds between nitrogenous bases of opposite strands
Watson and Crick base pairing refers to the specific hydrogen bonding between complementary nitrogenous bases on opposite DNA strands in the double helix structure. The correct answer is option 2: Non-covalent hydrogen bonding between two DNA strands.
Option Analysis
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Option 1: Hydrogen bonding in a single DNA strand. Incorrect, as base pairing occurs strictly between complementary bases on antiparallel strands of the double helix, not within one strand.
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Option 2: Non-covalent hydrogen bonding between two DNA strands. Correct. Adenine (A) pairs with thymine (T) via 2 hydrogen bonds, and guanine (G) pairs with cytosine (C) via 3 hydrogen bonds, forming non-covalent interactions that hold the strands together.
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Option 3: Van der Waal’s forces between bases. Incorrect. While van der Waals forces contribute to base stacking along the helix axis for stability, the specific pairing specificity comes from hydrogen bonds, not these weaker forces.
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Option 4: Phosphodiester bonds between nitrogenous bases of opposite strands. Incorrect. Phosphodiester bonds form the sugar-phosphate backbone within each strand, linking nucleotides covalently; they do not connect bases across strands.
Introduction to Watson and Crick Base Pairing
Watson and Crick base pairing defines the complementary hydrogen bonding in the DNA double helix, where adenine (A) pairs with thymine (T) and guanine (G) with cytosine (C). This non-covalent interaction, proposed by James Watson and Francis Crick in 1953, ensures genetic fidelity during replication. For students preparing for exams like GATE Life Sciences, understanding this mechanism is key to mastering molecular biology.
How Watson-Crick Base Pairing Works
In the Watson and Crick model, two antiparallel DNA strands twist into a double helix, with bases projecting inward. Hydrogen bonds form specifically: A-T via two bonds and G-C via three, following Chargaff’s rules (A=T, G=C). These non-covalent bonds allow strands to separate easily for replication while maintaining specificity.
Why Hydrogen Bonding, Not Other Forces?
Hydrogen bonds provide precise complementarity due to donor-acceptor geometry between bases. Van der Waals forces aid stacking but lack specificity, and phosphodiester bonds build the backbone, not inter-strand links.
| Feature | Hydrogen Bonds (Watson-Crick) | Van der Waals Forces | Phosphodiester Bonds |
|---|---|---|---|
| Location | Between bases on opposite strands | Base stacking within helix | Sugar-phosphate backbone |
| Bond Type | Non-covalent, 2-3 per pair | Weak, non-specific | Covalent |
| Role in DNA | Pairing specificity | Structural stability | Strand integrity |
Importance in Molecular Biology
Watson and Crick base pairing underpins DNA replication, transcription, and repair, enabling the central dogma of biology. Mutations disrupting these bonds can lead to genetic disorders, highlighting their exam relevance in genetics and biotechnology.
