Watson and Crick base pairing

 In 1953 James Watson and Francis Crick discovered the structure for DNA .

They had initially hypothesised this arrangement from multiple sources of data including a DNA x-ray of diffracted fibres created by Rosalind_franklin and Maurice Wilkins.

Watson and Crick believed that the photograph was indicative of a structure containing two separate strands, coiled together in a helical conformation.

Furthering their understanding from this other contemporary findings, they proposed an arrangement of DNA with the following properties:

1. A common axis with two polynucleotide chains wound around eachother in a right handed screw sense, forming a regular helix. Chains are antiparallel and consequently have opposite polarity.
2. The presence of pyrimidine and purine bases which are localised to inside of the helix due to hydrophilic interactions and the presence of a sugar phosphate backbone found on the outside of the structure.
   
3. Bases are separated from each other by 3.4 Å and are almost perpendicular to the helical sugar phosphate backbone. Every 3.4 Å the helix repeats equating to 10 bases per turn. There is a 36° rotation between a base and the one immediately beneath it.
4. The helix diameter is 20 Å.

Further, Watson and Crick also proposed that purines must be paired with pyrimidines. Base pairing of the same size and shape (purine-purine or pyrimidine-pyrimidine) did not seem to occur. This observation was supported by Erwin Chargaff's analysis of ratios of bases across multiple species; G-C ratio was similar in comparison to a highly variable G-A ratio. Watson and Crick discovered that guanine and cytosine paired separately to adenine and thymine to form similarly shaped complexes. This was the basis for defining purine-pyrimidine Watson- Crick base pairing:

A - T

C - G