The observations in solution state, however, predicted the existence of secondary structure in DNA. Taking these facts into consideration Watson and Crick in 1953 proposed the secondary structure in the form of the famous double helix model –
(a) It was known through base analyses that there is as much adenine as thymine and as much guanine as cytosine (A/T and G/C = 1). Therefore, the sum of purines is equal to the sum of pyrimidincs (A+G = C+T). It is known as Chargaffs rule. Also, experimental results suggested the polydcoxyribonuclcotidc chains were held together by hydrogen bond.
(b) X-ray diffraction studies (Wilkins, 1952), which suggested a helicoidal configuration. According to this model, DNA has a double stranded structure where two polydeoxyribonucleotide chains twisted around one another in a double helix. Both the helices are held together by means of hydrogen bonds existing between the nitrogen bases. The diameter of the DNA molecule is 20A0 (2nm). The length of the DNA in one complete turn is 34A° (3.4nm), which incorporates 10 base pairs. Therefore the distance between two adjacent base pairs is 3.4 A0.
Both the strands have sugar phosphate backbone and are antiparallel to one another. The antiparallel nature is given by orientation of the deoxyribose sugar which is opposite in both the strands.
Therefore the carbon atom of the sugar molecule, which is exposed at one end of a strand (5′ end), faces the 3r^ carbon atom of the sugar in the opposite strand (3′ end). The strands are also complementary to each other. This nature is based on the purine-pyrimidine links i.e. if a strand is having a purine base (adenine or guanine) the opposite must be its pyrimidine counterpart (thymine or cytosine) e.g. A = T and G = C. in DNA the nucleosides are joined by means of phosphodicstcr bonds.