The Double helix model of DNA was proposed by James Watson and Francis Crick in 1953...
The structure of double helical DNA
The Double helix model of DNA was proposed by James Watson and Francis Crick in 1953 based on the X-ray diffraction data produced by Maurice Wilkins and the Rosalind Franklin and Erwin Chargaff's rules of base pairing.
Chargaff's rule
The amount of adenine is always equal to the amount of thymine and the amount of guanine is always equal to the amount of cytosine, i.e., [A] = [T], [G] = [C].
Adenine is joined to thymine with 2 hydrogen bonds and guanine is joined two cytosine by 3 hydrogen bonds.
The ratio of adenine and guanine to that of thymine and cytosine is always equal to one.
[𝐴+𝐺] / [𝑇+𝐶] = 1
Salient features of DNA
DNA is made up of two polynucleotide chains, where the backbone is made up of sugar and phosphate groups and the nitrogenous bases project towards the centre.
The two Chains have anti-parallel polarity. It means, if one chain has the polarity 5'→3', the other has 3'→5'.
The bases in two strands are paired through hydrogen bond forming base pairs (bp). Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine with three hydrogen bonds. As a result, always a purine comes opposite to a pyrimidine.
The two chains are coiled in a right-handed fashion. The pitch of the Helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm.
The diameter of the strand is always constant due to pairing of purine and pyrimidine. This in addition to hydrogen bonds confers stability of the helical structure.