2.2 The Double Helix

The Watson-Crick Model (1953)

James Watson and Francis Crick proposed the double helix structure of DNA, using X-ray diffraction data from Rosalind Franklin and Maurice Wilkins, along with Chargaff's base pairing rules. This discovery revolutionized biology.

Key Features

Structural Parameters (B-DNA)

  • Diameter: 2.0 nm (20 Å)
  • Rise per bp: 0.34 nm (3.4 Å)
  • Bases per turn: 10.5
  • Pitch: 3.4 nm (34 Å)
  • Twist: 36° per bp
  • Major groove: 22 Å wide
  • Minor groove: 12 Å wide
  • Handedness: Right-handed

Antiparallel Strands

5'---ATCGATCG---3'

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3'---TAGCTAGC---5'

The two strands run in opposite directions (5'→3' and 3'→5'). This is essential for replication.

Base Pairing

A-T Base Pair

  • • 2 hydrogen bonds
  • • Weaker than G-C
  • • AT-rich regions melt first
  • • Common at origins of replication

G-C Base Pair

  • • 3 hydrogen bonds
  • • Stronger than A-T
  • • Higher melting temperature
  • • Stabilizes DNA structure

Major and Minor Grooves

The double helix has two grooves of different widths, created by the geometry of base stacking:

Major Groove

Wider (~22 Å). More accessible. Most protein-DNA interactions occur here. Unique pattern of H-bond donors/acceptors allows sequence-specific recognition.

Minor Groove

Narrower (~12 Å). Less sequence information. Some small molecules and minor groove binding proteins interact here.

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