Molecular model of DNA structure: A molecular model for DNA structure first proposed by J.D. Watson and F.H.C Crick (1953) has been universally accepted. They were awarded Nobel Prize for their discovery. The work of Crick and Watson has been supported by X-ray diffraction picture obtained by Wilkins and his associates.
According to this model, a DNA moleucle is composed of two chains that are spirally coiled and form a double helix, much like a spiral stairway. The distance between the two chains is uniform and it is maintained by base pairing. The back bone of the chain is made up of alternating sugar and phosphate molecules. At specific intervals, the chains are joined by ‘steps’.
Each step is made up of two nitrogen bases. Each nitrogen base is attached to the sugar molecule of the opposite chain. Between themselves, die nitrogen bases of a set are joined by hydrogen bonding. The pairing of the nitrogen bases is always specific. It is made up of a purine and a pyrimidine i.e., a double ring and a single ring.
Two purines would occupy too much space and cannot be accommodated between two chains; similarly two pyrimidines would form a short step and leave a gap between two chains. Hence the pairing is always between a purine and pyrimidine. Because of this pairing, in a DNA molecule, the total purine content is always equivalent to total pyrimidine content.
The purine, pyrimidine base pairing is further specific. It is always Adenine with Thymine and Guanine with Cytosine. Adenine and Thymine are joined by two hydrogen bonds through atoms attached to position 6′ and 1′. Cytosine and Guanine are joined by three hydrogen bonds at positions 6′ 1′ and 2′. The bonding of hydrogen is weak; but this facilitates easy separation of two strands during replication.
According to Crick and Watson, the helix has a diametre of 20 A0 and there is one complete turn of the helix at every 34 A0 and has a stack of 10 nucleotides at every turn.