<human>:  How did Watson and Crick discover the structure of DNA?
<bot>: Watson and Crick made their discovery of the double-helix structure of DNA in 1953 after examining X-ray diffraction photographs taken by Rosalind Franklin. They used physical models to create a three-dimensional representation of the DNA molecule, suggesting that its structure was similar to a twisted ladder. This structure was explained through their hypothesis that DNA was composed of two interlocking strands of nucleotide bases held together by hydrogen bonds.