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The background of the model of the double helix
Watson and Crick determined the structure of DNA in 1953.
They based their model on the results achieved by researchers such as Erwin Chargaff, who studied base equivalence; and Rosalind Franklin and Maurice Wilkins, who worked on the diffraction of X-rays on DNA fibres. They all produced data about DNA, such as the following examples:
DNA is a long rigid molecule.
There is base equivalence, meaning that the contents in purine bases are the same as for pyrimidine bases, or (and this is the same thing) the proportion of adenine is equal to that of thymine, and that of cytosine is equal to that of guanine.
There are structures in the molecule which repeat every 0.34 and 3.4 nm.
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3. The Watson and Crick model
DNA is made up of two chains of nucleotides, twisted around an imaginary axis, forming a 2nm-diameter double helix.
2 nm in diameter
Two chains of nucleotides
Twisting towards the right
The chains are dextrorotatory, which means they twist clockwise.
The nitrogen bases are on the inside of the double helix and the sugar-phosphate backbone is on the outside.
Nitrogen bases
Sugar-phosphate backbone

4. The Watson and Crick model
Each complete turn of the double helix is made up of ten pairs of nucleotides, which means that the turn is 3.4 nm long.
Complete turn every ten pairs of nucleotides. The length of turn is 3.4 nm
The distance between each pair of nucleotides is 0.34 nm.
0.34 nm separation between bases
Complementariness between chains
There is complementariness between the chains. The adenine unites with the thymine and the guanine with the cytosine.
The nitrogenous bases are opposite each other, making hydrogen bonds between them.
Links between hydrogen bonds between complementary bases
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5. Rosalind Franklin’s discovery
Rosalind Franklin ( ), was born in England. She studied biophysics and devoted most of her research to the application of X-ray diffraction to the study of molecular structures.
In 1953 she produced a publication containing what is known as photograph 51, an image of X-ray diffraction, providing experimental support which led Watson and Crick to establish the hypothesis of the “double helix”, for which they received the Nobel Prize for Physiology and Medicine in 1963, alongside the scientist Maurice Wilkins.
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