The DNA double helix was discovered in 1953 (with further details in 1954) based on X-ray diffraction images of DNA (most notably photo 51, taken by Raymond Gosling and Rosalind Franklin) as well as base-pairing chemical and biochemical information. Prior to this, X-ray data being gathered in the 1950s indicated that DNA formed some sort of helix, but it had not yet been discovered what the exact structure of that helix was. There were therefore several proposed structures that were later overturned by the data supporting a DNA duplex. The most famous of these early models was by Linus Pauling and Robert Corey in 1953 in which they proposed a triple helix with the phosphate backbone on the inside, and the nucleotide bases pointing outwards. A broadly similar, but detailed structure was also proposed by Bruce Fraser that same year. However, Watson and Crick soon identified several problems with these models:
* Negatively charged phosphates near the axis repel each other, leaving the question of how the three-chain structure stays together.
* In a triple-helix model (specifically Pauling and Corey's model), some of the van der Waals distances appear to be too small.
The initial double helix model discovered, now termed B-form DNA is by far the most common conformation in cells. Two additional rarer helical conformations that also naturally occur were identified in the 1970s: A-form DNA, and Z-form DNA.