Explain the 'Watson and Crick model' of DNA structure with a neat labeled diagram.

The Watson and Crick model (1953) describes the double-helical structure of DNA, a breakthrough in molecular biology. Their model was based on X-ray crystallography data (notably Rosalind Franklin’s Photo 51) and chemical evidence, confirming DNA’s role as the hereditary material.

Key Features of the Model

1. Double Helix Structure

   • DNA consists of two long polynucleotide chains coiled around a common axis, forming a right-handed double helix.
   • The two strands are antiparallel (run in opposite directions: 5′ → 3′ and 3′ → 5′).

2. Nucleotide Composition

   • Each strand is made of nucleotides, comprising:
     • A phosphate group
     • A deoxyribose sugar
     • A nitrogenous base (adenine A, thymine T, cytosine C, or guanine G).
   • The bases pair specifically via hydrogen bonds:
     • Adenine (A) pairs with Thymine (T) (2 hydrogen bonds).
     • Cytosine (C) pairs with Guanine (G) (3 hydrogen bonds).

3. Base Pairing and Helix Stability

   • The complementary base pairing ensures uniform width (~2 nm) of the helix.
   • The hydrogen bonds between bases hold the two strands together, while hydrophobic interactions between stacked bases stabilize the helix.

4. Sugar-Phosphate Backbone

   • The deoxyribose-phosphate backbone runs along the outside, providing structural support.
   • The bases project inward, forming the "rungs" of the helix.

5. Helix Parameters

   • Diameter: ~2 nm.
   • Distance between successive bases (rise per base pair): ~0.34 nm.
   • Complete turn of the helix: ~3.4 nm (10 base pairs per turn).

Biological Significance

• Replication: The complementary base pairing allows DNA to replicate semi-conservatively (each new DNA molecule contains one old and one new strand).
• Genetic Information Storage: The sequence of bases encodes genetic instructions for protein synthesis.
• Mutation and Variation: Changes in base sequences (mutations) lead to genetic diversity.

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