DNA structure

• The structure of DNA holds the key to understanding its function
• Linus Pauling - Chemist as California Institute of Tech, discovered the structures of alpha helix protein, held by H bonds
• 1950-1952, Maurice Wilkins, Rosalind Franklin, etc., at King’s College (London)

  • A and B DNA
  • Franklin used X-ray diffraction to reveal the 3-dimensional structure of DNA
• Erwin Chargaff (1950)

  • Identified DNA as genetic material
  • From his paper - base pair purines were equal to the # of pyrimidines
  • The amount of A is proportional to T and C is proportional to G. But the % of C+G doesn’t necessarily equal the % of A+T
• James Watson

  • phD in early 20s from Indiana university
  • Cavendish labs - Cambridge England
  • Studied DNA
  • Worked with francis crick
• Watson and Crick discovered the structure of DNA through observation, clues, and analysis
• W&C (1953) proposed that DNA is a right-handed double helix in which the 2 strands are antiparallel and the bases are stacked on one another. The 2 strands are connected by A-T and G-C base pairing, and there are 10 base pairs per helix turn
• Double helix

  • Distance 34 A
  • Distance between pairs: 3.4A
  • 10 base pairs between 2 turns
  • 20A width
  • Strands are antiparallel
  • 5’->3’ and 3’->5’
  • Major and minor groove
• The a-t and g-c base pairing provides complementarity of the 2 strands and chemical stability to the helix
• A-t base pairs form 2 H bonds, G-C pairs form 3 H bonds
• The arrangement of sugars and bases along the axis provides another stabilizing factor
• Under different conditions of isolation, difference conformations of DNA are observed
• The W&C DNA model of B-DNA is seen under aqueous, low-salt conditions and is believed to be the biologically significant conformation
• A-DNA is more compact than B-DNA and is prevalent under high-salt or dehydration conditions. Doubtful to occur in vivo
• C-DNA, D-DNA, E-DNA, and P-DNA are also right-handed forms of DNA that are less compact than B-DNA
• Z-DNA forms a left-handed double helix.
• DNA might have to assume a structure other than the B form for some of its genetic functions