Watson and Crick base pairing
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:
- 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.
- 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.
- 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.
- 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
- ↑ Berg et al., 2007: 117