Also the total concentration of purines (A +G) always equal to the total concentration of pyrimidine (T +C). However, the (T+ A)/ (G+C) ratio was found to vary widely in DNAs of different species.
When X rays are focused through isolated macromolecules or crystals of purified molecules, the X ray are deflected by the atom of the molecules in specific patterns called diffraction patterns.
It provides the information about the organization of the components of the molecules.
Watson and Crick had X ray crystallographic data on DNA structure from the studies of Wilkins and Franklin and their coworkers.
These data indicated that DNA was a highly ordered, multiple stranded structure with repeating sub structures spaced every 3.4 Ao (1 Angstrom = 10-10 m )
X-ray diffraction patterns of DNA – Rosalind Franklin and Maurice Wilkins
The central cross shaped pattern as indicative of a helical structure. The heavy dark patterns (top and bottom) indicate that the bases are stacked perpendicular to the axis of the molecule.
Watson and Crick proposed that DNA exists as a double helix in which two polynucleotide chains are coiled above one another in a spiral.
Each polynucleotide chain consists of a sequence of nucleotide linked together by Phosphodiester bonds.
The two polynucleotide strands are held together in their helical configurations by hydrogen bonding.
The base pairing is specific
That is, adenine is always paired with thymine and guanine is always paired with cytosine
Thus, all base-pairs consists of one purine and one pyrimidine.
Once the sequence of bases in one strand of DNA double helix is known, it is possible to know the other strand sequence of base because of specific base pairing.
In their most structural configuration, adenine and thymine form two hydrogen bonds, where as guanine and cytosine form three hydrogen bonds.
The two strands of a DNA are complementary (not identical) to each other. It is this property, that makes DNA uniquely suited to store and transmitting the genetic information.
The base-pairs in DNA are stacked 34Ao apart with 10 base-pairs per turn (3600) of the double helix
The sugar – phosphate backbones of the two complementary strands are antiparallel, that is they have opposite chemical polority.