The DNA Molecule

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The DNA Molecule

In the autumn of 1951, James Watson (left) and Francis Crick (right) started work on unravelling the structure of DNA. It was known at the time that DNA was present in the nucleus of every living cell, and that it had something to do with heridity, but without a knowledge of its structure little more could be understood about how it actually worked. They approached the problem with the same methodology that had been pioneered by Linus Pauling, who after years of exhaustive study had earlier discovered that many proteins exhibited a helical structure. Their task was to devise a structure which would account for all the chemical and X-ray evidence, and at the same time be consistent with all the structural features of the units involved - such as the size and shape, bond angles and lengths, configurations and conformations. X-ray diffraction photographs of DNA fibres taken by Rosalind Franklin and Maurice Wilkins showed a distinctive X-shape, which was characteristic of a helix structure, but strong arcs on the meridian indicated a repeating structure 3.4 Å apart. And from the chemical evidence, it was known that part of the structure was comprised of 4 heterocyclic bases, adenine (A), guanine (G), cytosine (C) and thymine (T), somehow linked together with sugar units and phosphates. One of the biggest puzzles was that although the proportion of these bases varied from one DNA to another, it was always found that the number of A = T, and G = C.

Adenine

Guanine

Cytosine

Thymine

The 4 bases which make up DNA

(Click on each image to get its 3D molfile).

Using molecular models, Watson and Crick devised a structure in which all of the building blocks fitted together without crowding ...

... middle of paper ...

...tracted from organic remains (blood, saliva, etc) left at crime scenes to identify the criminal. It can also be used to determine parentage, the gender of animals and birds (where it is difficult to do so by just looking at them!), and to prove whether traditional medicines contain extracts from endangered species. This process is called DNA fingerprinting.

Bibliography:

The Path to the Double Helix: The Discovery of DNA, Robert C. Olby (Dover Pubns; ISBN: 0486681173, 1994)

Organic Chemistry, Morrison and Boyd (Allyn and Bacon, Boston, 1983).

Biochemistry, L. Stryer (Freeman, San Francisco, 1975).

Interactive DNA structure (Imperial College, requires Chime)

Interactive DNA Structure from the University of Massachusetts (requires Chime).

The Curtis model of H-bonding in the T-A and C-G base pairs (Imperial College London, requires Chime).

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