The 1953 discovery at Cambridge that the DNA molecule is a double helix — two strands twined together whose paired chemical letters spell out and copy the instructions for life. James Watson and Francis Crick built the model, but their leap rested crucially on the X-ray crystallography of Rosalind Franklin, whose key image was shown to Watson without her knowledge. The breakthrough revealed how heredity works at the molecular level and helped found the science of molecular biology.
The shape of the molecule was the answer to the question. In 1953, two researchers at Cambridge worked out that DNA, the chemical increasingly recognized as the carrier of heredity, is a double helix: two strands wound around each other like a twisted ladder, whose rungs pair up in a fixed way. That simple pairing meant each strand was a template for the other, so the molecule could copy itself. As Watson and Crick wrote, the pairing immediately suggested how the genetic material might be copied. It was the day heredity became readable chemistry. The leap was real, and so was its cost: it depended on the X-ray photographs of Rosalind Franklin, one of which was shown to James Watson without her knowing.