At the start of the 20th century, biologists knew that hereditary information was stored "somewhere" in cells but did not know where or how. In 1944 Oswald Avery showed DNA was the carrier molecule; but the structure of DNA was still behind a curtain. Without the structure, function could not be fully understood.

In 1952 at King's College London, Rosalind Franklin took X-ray diffraction images of DNA crystals — most famously "Photograph 51". Her images clearly showed DNA had a helical structure. At Cambridge, James Watson and Francis Crick gained access to Franklin's data (in ways she had not authorised) through Maurice Wilkins, and built a model: two long backbones twisting around each other in a helix, with four bases (A, T, G, C) pairing in the middle — A always with T, G always with C.

The importance of this pairing was enormous. When the two halves of the helix separate, each can act as a template for a new helix using complementary bases — meaning DNA can copy itself. On 25 April 1953 Watson and Crick announced the structure in an 800-word note in Nature, ending with one of the most famously understated sentences in science: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

The decades that followed exploded with techniques built on this structure: Sanger sequencing in 1977, PCR in 1983, the Human Genome Project in 1990–2003, CRISPR gene editing from 2012. All were possible because DNA's structure had been solved. Franklin died of ovarian cancer in 1958, aged only 37 — probably linked to her X-ray work. Nobel rules don't allow posthumous awards, so the 1962 Prize in Medicine went to Watson, Crick, and Wilkins; Franklin has since come to be remembered as one of the foundational figures of modern biology.