Studies of DNA Structure – Quick Review

Key Researchers

  • Linus Pauling: proposed helical structures in proteins; attempted analogous DNA model
  • Maurice Wilkins & Rosalind Franklin (King’s College): produced DNA XX-ray diffraction images
  • James Watson & Francis Crick (Cavendish Laboratory): built physical models integrating available data

Experimental Techniques

  • XX-ray crystallography: passed XX-rays through DNA fibers → diffraction patterns revealing helical symmetry
  • Model building: spatial assembly of known chemical components to satisfy experimental constraints

Crucial Evidence

  • Franklin’s diffraction photograph: showed DNA has two repeating helices
  • Chargaff’s base-composition data (1951): constant pairing ratios across species
    • [A]≈[T][A] \approx [T]
    • [G]≈[C][G] \approx [C]
    • [A]+[G]=[C]+[T][A]+[G] = [C]+[T] (purines = pyrimidines)

Chargaff’s Ratios (examples)

  • Human DNA: AT=1.00\frac{A}{T}=1.00, GC=1.00\frac{G}{C}=1.00
  • E. coli: AT=1.09\frac{A}{T}=1.09, GC=0.99\frac{G}{C}=0.99
    (Consistent 1:1 trend indicates fixed base pairing)

Watson–Crick Model (1953)

  • Two antiparallel nucleotide chains → double helix
  • Complementary base pairing via hydrogen bonds:
    • AA–TT (2 H-bonds)
    • GG–CC (3 H-bonds)
  • Specific pairing implies a templated copying mechanism for genetic replication

Core Conclusion

  • DNA’s informational fidelity arises from complementary base pairing within a stable double-helical structure, providing a direct molecular basis for heredity.