Brent Cornell

    
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

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

Franklin’s data was shared by Wilkins with James Watson (without Franklin’s permission) who, with the help of Francis 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

    
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