Structure of DNA

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•  Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction

Rosalind Franklin and Maurice Wilkins used a method of X-ray diffraction to investigate the structure of DNA

  • DNA was purified and then fibres were stretched in a thin glass tube (to make most of the strands parallel)
  • The DNA was targeted by a X-ray beam, which was diffracted when it contacted an atom
  • The scattering pattern of the X-ray was recorded on a film and used to elucidate details of molecular structure

Summary of the Process of X-Ray Crystallography

x-ray crystallography

From the scattering pattern produced by a DNA molecule, certain inferences could be made about its structure

  • Composition:  DNA is a double stranded molecule
  • Orientation:  Nitrogenous bases are closely packed together on the inside and phosphates form an outer backbone
  • Shape:  The DNA molecule twists at regular intervals (every 34 Angstrom) to form a helix (two strands = double helix)

Photo 51 – Evidence for the Structure of DNA via X-Ray Diffraction

      Evidence:                 Overview                 Helix                  Twists                  Bases                  Strands                  Orientation

Franklin’s data was shared by Wilkins with James Watson (without Franklin’s permission) who, with the help of Francise Crick, used the information to create a molecular model of the basic structure of DNA

  • In 1962, Watson, Crick and Wilkins (but not Franklin) were awarded the Nobel prize for their contributions to DNA structure identification

Link:  The Structure of DNA – Cooperation and Competition

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•  DNA structure suggested a mechanism for DNA replication

Franklin’s x-ray diffraction experiments demonstrated that the DNA helix is both tightly packed and regular in structure

  • Phosphates (and sugars) form an outer backbone and nitrogenous bases are packaged within the interior

Chargaff had also demonstrated that 
DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) 

  • This indicates that these nitrogenous bases are paired (purine + pyrimidine) within the double helix
  • In order for this pairing between purines and pyrimidines to occur, the two strands must run in antiparallel directions

When Watson & Crick were developing their DNA model, they discovered that an A–T bond was the same length as a G–C bond

  • Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three hydrogen bonds
  • If the bases were always paired this way, then this would describe the regular structure of the DNA helix (shown by Franklin)

Consequently, DNA structure suggests two mechanisms for DNA replication:

  • Replication occurs via complementary base pairing (adenine pairs with thymine, guanine pairs with cytosine)
  • Replication is bi-directional (proceeds in opposite directions on the two strands) due to the antiparallel nature of the strands

Complementary Base Pairing as a Mechanism for DNA Replication