DNA STRUCTURE

In 1951, the then 23-year old biologist James Watson traveled from the United States to work with Francis Crick, an English physicist at the University of Cambridge. Crick was already using the process of X-ray crystallography to study the structure of protein molecules. Together, Watson and Crick used X-ray crystallography data, produced by Rosalind Franklin and Maurice Wilkins at King's College in London, to decipher DNA's structure.

• This is what they already knew from the work of many scientists, about the DNA molecule:
  
  
• DNA is made up of subunits which scientists called nucleotides.
  
  
• Each nucleotide is made up of a sugar, a phosphate and a base.
  
  
• There are 4 different bases in a DNA molecule:

adenine (a purine)
cytosine (a pyrimidine)
guanine (a purine)
Thymine (a pyrimidine)

• The number of purine bases equals the number of pyrimidine bases
  
• The number of adenine bases equals the number of thymine bases
  
• The number of guanine bases equals the number of cytosine bases
  
• The basic structure of the DNA molecule is helical, with the bases being stacked on top of each other
  

Working with nucleotide models made of wire, Watson and Crick attempted to put together the puzzle of DNA structure in such a way that their model would account for the variety of facts that they knew described the molecule. Once satisfied with their model, they published their hypothesis, entitled "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" in the British journal Nature (April 25, 1953. volume 171:737-738.) It is interesting to note that this paper has been cited over 800 times since its first appearance!

Here are their words:

"...This (DNA) structure has two helical chains each coiled round the same axis...Both chains follow right handed helices...the two chains run in opposite directions. ..The bases are on the inside of the helix and the phosphates on the outside..."

"The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases... The (bases) are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side...One of the pair must be a purine and the other a pyrimidine for bonding to occur. ...Only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine)."

"...in other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined."

and

"...It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

And with these words, the way was made clear for tremendous strides in our understanding of the structure of DNA and, as a result our ability to work with and manipulate the information-rich DNA molecule.