DNA's double helix was revealed in 1953

On 25 April 1953, James Watson and Francis Crick published a roughly 900-word paper in Nature describing the structure of DNA as a double helix: two strands twisting around each other like a spiral ladder, with rungs of paired bases that fit together precisely, so that A always pairs with T, and C always pairs with G.

That pairing rule didn’t come from nowhere. Biochemist Erwin Chargaff had already shown that in any organism’s DNA the amount of adenine equals the amount of thymine, and guanine equals cytosine. Chargaff’s rules were the numerical clue that bases come in matched couples, exactly what a complementary two-strand ladder requires.

The model also leaned heavily on X-ray diffraction images of DNA made at King’s College London by Rosalind Franklin and Maurice Wilkins. The sharpest of these, Photo 51, revealed the telltale helical cross. Watson was shown it by Wilkins without Franklin’s knowledge or consent, and it proved decisive. Franklin died of ovarian cancer in 1958, four years before the 1962 Nobel Prize went to Watson, Crick, and Wilkins; the prize is not awarded posthumously, so her role long went underacknowledged.

“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

With that famously understated line, the paper hinted at how DNA replicates: unzip the strands, and each templates a perfect new copy. In 1958, Matthew Meselson and Franklin Stahl confirmed it, labeling DNA with heavy nitrogen and watching the copies come out half-old, half-new, proving replication is semi-conservative. It remains one of the defining discoveries of 20th-century science.