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How did James Watson and Francis Crick determine the structure of DNA?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

James Watson and Francis Crick determined the structure of DNA in 1953 by constructing a molecular model that accounted for its known physical and chemical properties.12

Their breakthrough relied heavily on the following contributions:
* X-ray Diffraction: They utilized X-ray diffraction studies conducted by Maurice Wilkins and Rosalind Franklin, which provided essential evidence that DNA existed in a helical conformation.312
* Base Pairing: Building on the findings of biochemist Erwin Chargaff—who established that the amount of adenine (A) equals thymine (T) and guanine (G) equals cytosine (C)—Watson realized these bases must be linked in specific pairs.24

By creating a wire model, they demonstrated that DNA is a double helix consisting of two intertwined sugar-phosphate chains. The "rungs" of this structure are formed by the base pairs, which fit together like a lock and key. This model successfully explained how DNA could replicate itself and store genetic information, marking a cornerstone of modern genetics.124