The Double Helix

The Watson & Crick Model of DNA Structure (B-DNA)

by Prof. Siddharth Sanghvi Sir ( BSI SIR )

Introduction

In 1953, James Watson and Francis Crick proposed the double helix model for the structure of Deoxyribonucleic Acid (DNA). This discovery, published in the journal Nature, is considered one of the most significant biological breakthroughs of the 20th century. It provided the molecular explanation for how genetic information is stored and replicated.

Chronology of Contribution

The discovery was a culmination of work by several scientists. The "Race for the Double Helix" involved critical data from chemistry and physics.

1950

Erwin Chargaff: The Base Pairing Rules

Chargaff analyzed DNA from various species and discovered two critical rules (Chargaff's Rules):

  • The composition of DNA varies between species.
  • Equivalence Rule: In any double-stranded DNA, the number of Adenine units equals the number of Thymine units (A = T), and the number of Guanine units equals the number of Cytosine units (G = C). This hinted that bases were paired.
1951 - 1952

Rosalind Franklin & Maurice Wilkins: X-Ray Crystallography

Working at King's College London, Franklin and Wilkins used X-ray diffraction to study DNA fibers.

  • Photo 51: Rosalind Franklin produced an exceptionally clear X-ray diffraction image of B-DNA. The "X" pattern in the image was the signature of a helical structure.
  • Key Data: Her calculations revealed the DNA was a helix with a diameter of 2 nm and a repeating pattern every 3.4 nm.
  • Wilkins shared this data with Watson and Crick, confirming the helical nature.
1953

James Watson & Francis Crick: The Synthesis

At the Cavendish Laboratory in Cambridge, Watson and Crick synthesized the chemical rules of Chargaff and the physical dimensions provided by Franklin and Wilkins.

  • They built physical models using metal plates and rods.
  • They realized that A pairing with T and G pairing with C (purine + pyrimidine) resulted in a uniform width that fit Franklin's X-ray data perfectly.
  • They published "Molecular Structure of Nucleic Acids" in Nature on April 25, 1953.

Salient Features of the Watson-Crick Model

The model describes the B-form of DNA, which is the most common form found in living cells.

1. Double Helical Structure

DNA consists of two polynucleotide chains (strands) coiled around a common central axis to form a right-handed helix. This looks like a twisted ladder.

2. Antiparallel Polarity

The two strands run in opposite directions. One strand runs 5' → 3' and the other runs 3' → 5'. This orientation is crucial for replication and transcription.

3. Backbone vs. Steps

The backbone is made of alternating Sugar (Deoxyribose) and Phosphate groups found on the outside. The nitrogenous bases project inwards, perpendicular to the axis, forming the "steps" of the ladder.

4. Complementary Base Pairing

Bases pair specifically through Hydrogen Bonds:

  • Adenine (A) pairs with Thymine (T) via 2 H-bonds.
  • Guanine (G) pairs with Cytosine (C) via 3 H-bonds.

This pairing of a Purine (A, G) with a Pyrimidine (T, C) ensures a constant diameter.

Geometric Dimensions

Dimension Measurement Description
Diameter 20 Å (2.0 nm) Constant width due to Purine-Pyrimidine pairing.
Pitch (One Turn) 34 Å (3.4 nm) The length of one complete 360° turn of the helix.
Rise per Base Pair 3.4 Å (0.34 nm) The distance between two adjacent stacked bases.
Base Pairs per Turn 10 bp There are roughly 10 steps in one full turn.
Note on Grooves: The coiling of the two strands creates two unequal grooves along the surface of the helix: the Major Groove (wider) and the Minor Groove (narrower). These are critical binding sites for proteins that regulate gene expression.