The question of who first identified DNA does not have a single, simple answer, because the molecule’s identification occurred in stages over nearly a century. This process involved multiple scientists, each making a distinct and foundational discovery, from the initial isolation of the substance to the final determination of its functional three-dimensional shape. The answer depends on whether one means the first time the chemical substance was isolated, its molecular components described, or its information-carrying structure revealed. Deoxyribonucleic acid, or DNA, is now universally recognized as the fundamental blueprint of life, carrying the genetic instructions for all known organisms.
Isolating the Genetic Material
The first physical identification of the substance we now call DNA occurred in 1869 with the work of Swiss physician Friedrich Miescher. Working in Germany, he focused his research on the chemical composition of the cell nucleus. Miescher chose to study white blood cells because they have large nuclei and were readily available.
Miescher developed a process to separate the cell nuclei from the cytoplasm, treating the remaining material with an alkaline solution followed by acidification. This procedure resulted in a precipitate rich in phosphorus and nitrogen but lacking the sulfur that characterized proteins. He named this unique, acidic, phosphorus-containing material “nuclein” because he had isolated it from the nucleus. This moment marks the first isolation and chemical identification of the substance itself, though its biological purpose was completely unknown.
Defining the Chemical Components
The next major steps in identifying DNA involved breaking down Miescher’s nuclein to understand its constituent parts. This chemical analysis was primarily driven by Russian-American biochemist Phoebus Levene in the early 20th century. Levene’s work characterized the distinct molecular components that make up the DNA molecule.
Levene correctly identified the three primary parts of the building block, which he termed a nucleotide: a phosphate group, a five-carbon sugar, and one of four nitrogenous bases. He identified the sugar as deoxyribose in 1929 and characterized the four bases as adenine (A), guanine (G), cytosine (C), and thymine (T). Despite accurately identifying these components, Levene made a misleading error in his proposed structure. His “tetranucleotide hypothesis” suggested that the four bases existed in equal amounts and were arranged in a simple, repeating loop. This supposed simplicity led many scientists to dismiss DNA as the genetic material, believing proteins must carry the hereditary information.
The flawed tetranucleotide hypothesis was later disproven by the work of Austrian biochemist Erwin Chargaff in the late 1940s. Chargaff used advanced techniques to analyze the base composition of DNA from various organisms. He found that the proportions of the four bases varied significantly between different species, directly contradicting Levene’s claim.
Chargaff’s second finding became known as Chargaff’s Rules. He observed that within any single species, the amount of adenine (A) always closely equaled the amount of thymine (T), and the amount of guanine (G) always closely equaled the amount of cytosine (C). This consistent 1:1 ratio—A=T and G=C—strongly suggested a pairing mechanism between the bases. This chemical understanding was foundational data necessary for solving the final structure of the molecule.
Determining the Three-Dimensional Structure
The final step in the identification of DNA was the determination of its three-dimensional structure, which provided the functional meaning of the molecule. This process relied on the work of multiple researchers in the early 1950s. At King’s College London, Rosalind Franklin and Maurice Wilkins used X-ray diffraction to study the structure of DNA fibers.
Franklin produced an exceptionally clear X-ray photograph of the “B” form of DNA, which she and her student Raymond Gosling labeled Photo 51. The cross-shape and dark reflections in this image were evidence of a helical structure, and the pattern provided precise measurements for the helix dimensions. Although Franklin was close to solving the structure herself, Maurice Wilkins showed Photo 51 to James Watson, who was working with Francis Crick at Cambridge University.
Watson and Crick were focused on building physical models, and the data from Photo 51 provided the missing physical constraints they needed. They combined this X-ray evidence with Chargaff’s rules to construct their model of the double helix. Their model, published in 1953, described DNA as two intertwined strands wrapped around a common axis, like a twisted ladder.
The structure featured a sugar-phosphate backbone on the outside and the nitrogenous bases paired in the interior, held together by hydrogen bonds. The model also incorporated the anti-parallel nature of the strands, meaning one strand runs in the opposite direction of the other. The pairing of A with T and G with C explained the consistency of Chargaff’s Rules. This double helix structure immediately suggested a mechanism for genetic replication, establishing DNA as the carrier of hereditary information.