Watson and Crick were wary of Linus Pauling, a renowned chemist known for building complex molecular models, fearing he would soon decipher DNA. They suspected DNA was a helix. After Watson heard Franklin report on her work, he and Crick built a three-chain helical model with bases inside, which Franklin disproved due to Watson's misremembered data. This embarrassing failure led the Cavendish leadership to forbid them from working on DNA, pushing their quest underground.

In the early 20th century, while physicists and chemists were unlocking atomic secrets, the mystery of life, particularly inheritance, remained. Scientists knew traits were passed down, but not how. They were convinced a biological molecule with special qualities was responsible for storing and transmitting this information, explaining both life's stability and mutability for evolution.

In 1951, two unknown scientists, 23-year-old American James Watson and Englishman Francis Crick, embarked on solving this fundamental secret. They met at the Cavendish Laboratory, Cambridge, connecting instantly over their shared passion for science and the belief that finding the gene's structure was paramount, despite their different backgrounds.

By the 1920s, genes were located in cell nuclei and associated with chromosomes, known to be made of proteins and DNA. The debate was whether genes were made of DNA or protein. Protein seemed more likely due to its diversity, while DNA, with its simple repeated units, was considered uninteresting. However, Oswald Avery's work demonstrating DNA's ability to carry genetic information began to sway some, including Watson and Crick.

Watson and Crick believed solving DNA's molecular structure would reveal how genetic information is stored. X-ray crystallography, a powerful technique for determining atomic positions, was crucial. While Cavendish was known for this, its director avoided DNA research to prevent competition with Maurice Wilkins' group at King's College. Wilkins, a physicist, was also interested in the gene problem, but lacked urgency. His strained relationship with the talented crystallographer Rosalind Franklin, who believed she should lead the DNA research, further complicated their work. Franklin's Photo 51 would later become critically important.

In January 1953, news arrived that Pauling was preparing a DNA paper. Watson obtained a copy and was relieved to find Pauling’s proposed triple helix was similar to their discarded model. Watson then visited Franklin, who was uninterested in his news. Afterwards, Maurice Wilkins showed Watson Franklin's Photo 51, a crucial X-ray diffraction image that immediately revealed DNA was a helix, and Watson suspected it was a double helix. Around the same time, Crick saw a report on Franklin's work, which included an observation on DNA's symmetry, leading him to realize the two backbones had to run in opposite directions and be on the outside with bases inside.

Watson began building models again, initially trying to pair like with like (adenine with adenine, thymine with thymine). However, a colleague pointed out that such pairings were chemically incorrect. Crick reminded him of Erwin Chargaff's rules, which showed that the amount of adenine (A) always equaled thymine (T), and guanine (G) always equaled cytosine (C) in DNA. With this in mind, Watson used cardboard cutouts on a Saturday morning, discovering that A paired with T and G paired with C. This explained the structure's symmetry and, most importantly, how genetic information is stored and replicated through complementary base pairing.

The completed model, unveiled on February 28, 1953, fit both X-ray diffraction data and Chargaff's rules. This structure immediately revealed how DNA works: information is stored by the sequence of bases, and mutations occur when this sequence changes. The discovery was celebrated for its elegance and profound implications, reported in Nature, and later earned Watson, Crick, and Wilkins a Nobel Prize. It opened up a new world for biologists, allowing them to decode life's mysteries, explaining both stability and mutability in evolution.