In the year of 1953, Watson and Crick worked out the 3-D structure of DNA, beginning from X-ray diffraction photographs   taken through Wilkins and Franklin. They decreased which DNA is composed of two strands wound round each other to form a double helix with the bases on the inside and the sugar-phosphate back- bones on the outside. In the double helix that is represent in the figure, the two DNA standards are organized in an antiparallel   arrangement example for the  two  strands  run  in  opposite directions one is orientated 3'→5' and the other standard is orientated 5'→3'. The bases of the two strands produced hydrogen bonds to every other; the pairs with T and G pairs with C. This is called as complementary base pairing. Thus a large two-ringed purine is paired with a smaller single-ringed pyrimidine and the two bases fit neatly in the gap among the sugar-phosphate strands and handle the right spacing.  There would be insufficient space for two large purines to pair and too much space for two pyrimidines to pair that would be too far apart to bond.  The  A:T  and G:C base  pairing  also  maximizes  the  number  of effective  hydrogen  bonds  which  can  form  between  the  bases;  there  are  three hydrogen bonds between every G:C base pair and two hydrogen bonds between each A:T base pair. Thus A: T and G: C base pairs form the most stable conformation both from steric considerations and from the point of view of maximizing hydrogen bond formation.

      figure:   The DNA double helix.