Double helix

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Double Helix of DNA
Watson and Crick deduced a structural model for DNA which is a double helix. The features of this Watson-Crick model are:
  1. Two helical polynucleotide chains coil around a common axis and run in opposite directions to each other.
  2. The outside of the double helix structure are the sugar-phosphate backbones whereas the inside are the bases (purine and pyrimidine).
  3. The bases are almost perpendicular to the common axis that being coiled around. They are separated from their adjacent ones by 3.4 A and there is a rotation of 36 degrees per base.
  4. The diameter of the double helix is 20 A.

The bases on one of the polynucleotide chains are held to the ones on the other polynucleotide chain on the same stack by specific hydrogen bonds. They are base pairs and the bases are complementary to each other so that purines form hydrogen bonds to pyrimadines. Cytosine (C) bonds only to Guanine (G) and Adenine (A) bonds only to Thymine (T). There are two hydrogen bonds between A and T and three hydrogen bonds between C and G. Bonding between base pairs are weak however they are protected within the DNA molecule and are therefore stable[1].


  1. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
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