Who discovered DNA?
The double helix cracked the code of life — and repaired science’s most famous omission. DNA, Rosalind Franklin’s Photo 51, and the genome age.
Last updated: October 2026
The secret of secrets was a photograph — a blurry X of dark bands, taken in a London basement, that turned out to be the blueprint of every living thing. DNA is the code this series has chased since fire: the instruction set that writes forests, whales, and you.
Deoxyribonucleic acid, DNA, is the molecule that carries the instructions for building and running a living thing. Every cell of every plant, animal, fungus, and bacterium contains DNA, and every one of those organisms inherited its DNA from its parents. The instructions are written in a simple alphabet: four chemical letters, A, T, G, and C, arranged in long sequences that spell out genes, the units of heredity. To discover DNA was to discover the physical basis of inheritance, the answer to the oldest question in biology: how do living things pass their traits to their children? The discovery took nearly a century, and it was made by many hands.
The story begins, as so many in science do, with a substance nobody understood. In 1869, a Swiss physician named Friedrich Miescher, working in Tübingen in Germany, was studying white blood cells from pus on surgical bandages. From the nuclei of those cells he isolated a substance he had never seen before, rich in phosphorus, which he called nuclein. Miescher did not know what nuclein did, and he did not live to find out. But he had found DNA, and he had found it in the nucleus, the part of the cell where, as later work would show, heredity resides. The substance sat in the scientific literature for decades, regarded as a curiosity.
For a long time, scientists bet on the wrong molecule. Proteins, with their twenty amino acids and their complex shapes, seemed the obvious candidates for carrying heredity. DNA, with only four letters, seemed too simple to carry the complexity of life. The German biochemist Phoebus Levene, working in New York in the early twentieth century, studied DNA’s chemistry and concluded that its four letters repeated in a fixed pattern, a tetranucleotide, which made it look like a boring, repetitive molecule. Levene’s tetranucleotide hypothesis was wrong, but it was influential, and it delayed the recognition of DNA for a generation.
The first proof that DNA carries heredity came from bacteria. In 1928, the British bacteriologist Frederick Griffith showed that a harmless strain of pneumonia bacteria could be transformed into a deadly one by exposure to dead bacteria of the deadly strain. Something, a transforming principle, passed from the dead to the living. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty, working at the Rockefeller Institute in New York, identified the transforming principle as DNA. Their paper, one of the most important in biology, showed that pure DNA, with all protein removed, could change one strain of bacteria into another. The result was resisted, because it contradicted the belief that proteins carried heredity, and because Avery, a cautious man, understated his own conclusion.
The confirmation came in 1952, in one of the most elegant experiments in biology. Alfred Hershey and Martha Chase, at Cold Spring Harbor in New York, worked with bacteriophages, viruses that infect bacteria. A virus is mostly protein coat and DNA core, and Hershey and Chase wanted to know which part entered the bacterium and directed the making of new viruses. They labelled the protein with radioactive sulfur and the DNA with radioactive phosphorus, let the viruses infect bacteria, and then spun the mixture in a blender to strip the virus coats off the bacteria. The DNA went in; the protein stayed out. The Hershey-Chase experiment, the blender experiment, convinced most biologists that DNA, not protein, was the hereditary material.
In the same period, the Austrian-born biochemist Erwin Chargaff, working at Columbia University, found the rules that would unlock the structure. Analysing DNA from many species, Chargaff showed that the amount of adenine, A, always equals the amount of thymine, T, and the amount of guanine, G, always equals the amount of cytosine, C. The ratios of A to T and G to C were one to one, even though the ratio of A and T together to G and C together varied from species to species. Chargaff’s rules were a clue, and they pointed to a structure in which A pairs with T and G pairs with C. Nobody yet knew what that structure was.
The structure was found in 1953, in a race between two laboratories. At King’s College London, Maurice Wilkins and Rosalind Franklin were studying DNA by X-ray crystallography, a technique that bounces X-rays off a crystal and reads the pattern of the scattered rays to infer the arrangement of atoms. Franklin, a physical chemist with a reputation for meticulous work, produced the clearest X-ray image of DNA ever taken, known as Photo 51, in May 1952, with her doctoral student Raymond Gosling. The photograph showed a stark X shape, the signature of a helix, and it contained the information needed to work out the structure. Franklin was cautious, and she was working on the structure herself.
At Cambridge, James Watson, an American biologist, and Francis Crick, a British physicist, were trying to build a model of DNA. In early 1953, Wilkins showed Watson Photo 51, without Franklin’s knowledge or permission, and Crick also had access to a report of Franklin’s unpublished findings. Watson and Crick immediately saw what the photograph meant, and within weeks they had built a model: two strands, wound around each other in a double helix, held together by base pairs, A with T and G with C, in exactly the ratios Chargaff had found. The structure explained heredity: the two strands could separate, and each could serve as a template for a new strand, copying the information exactly. The model also explained mutation: a mistake in copying would change a letter, and the change would be inherited.
Watson and Crick published their model in the journal Nature on 25 April 1953, in a paper of just over a page, ending with one of the most famous 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. In the same issue, Wilkins and Franklin published their own papers, with the X-ray evidence. The double helix was not the work of two men, and the Nobel Prize that followed, in 1962, awarded to Watson, Crick, and Wilkins, did not include Franklin, who had died in 1958 at the age of thirty-seven, of ovarian cancer, and who could not receive a prize that is not awarded posthumously. The omission has become the most famous injustice in modern science, and Franklin’s contribution is now recognised as essential.
Knowing the structure was only the beginning. The next problem was the genetic code: how does a sequence of four letters spell out a protein made of twenty amino acids? The answer, worked out over the following decade, is that DNA is read in groups of three letters, codons, each of which specifies one amino acid or a stop signal. In 1958, Matthew Meselson and Franklin Stahl showed that DNA replicates semiconservatively, each new double helix keeping one old strand and one new one, exactly as the Watson-Crick model predicted. In 1961, Marshall Nirenberg and Heinrich Matthaei showed that the codon UUU specifies the amino acid phenylalanine, the first word of the code to be read, and the rest of the code was cracked over the following years, largely by Nirenberg, Har Gobind Khorana, and Robert Holley.
The code turned out to be universal, with small exceptions: the same codons mean the same amino acids in bacteria, in plants, and in people. That universality is one of the strongest pieces of evidence for the common descent of all life. It also made genetic engineering possible, because a gene from one organism can be read by another. The discovery of the code opened the way to recombinant DNA, to the production of human insulin in bacteria, to genetically modified crops, and, eventually, to the reading of whole genomes.
The next great project was to read the whole human genome, all three billion letters of it. The Human Genome Project, an international effort led by the United States, began in 1990 and published a working draft in 2001, with the completed sequence announced in 2003. The project found that humans have around twenty thousand protein-coding genes, far fewer than the hundred thousand many had expected, and that the differences between any two people are tiny, a fraction of one per cent of the genome. The genome also revealed that a large part of human DNA does not code for proteins, and that humans share the vast majority of their genes with other mammals, and a large fraction with far more distant relatives.
DNA has also transformed the world outside the laboratory. In 1984, Alec Jeffreys at the University of Leicester discovered DNA fingerprinting, the technique of identifying individuals by the unique patterns in their DNA, which is now used in criminal investigations, in paternity testing, and in the identification of disaster victims. Ancient DNA, recovered from bones and teeth, has rewritten the history of human migration, showing that modern humans interbred with Neanderthals and with Denisovans, and tracing the peopling of the world. In 2012, Jennifer Doudna and Emmanuelle Charpentier, building on the work of many others, turned the bacterial immune system CRISPR into a tool for editing DNA, a technology that has already produced new therapies for genetic diseases and new crops, and that raises profound ethical questions.
The discovery of DNA did not belong to one person, and it did not happen in one year. Miescher found the substance. Avery proved it carried heredity. Chargaff found its rules. Franklin photographed its shape. Watson and Crick built the model. Hundreds of others cracked the code, read the genome, and turned the knowledge into medicine. The molecule that Miescher scraped from a bandage in 1869 turned out to be the instruction book for life, and the reading of it is the great scientific adventure of the modern age, still under way.
This entry draws on Watson and Crick’s 1953 paper in Nature, Franklin and Gosling’s and Wilkins’s companion papers, Avery, MacLeod, and McCarty’s 1944 paper, the Encyclopaedia Britannica, the Nobel Prize organisation’s records, and the standard histories of molecular biology, including Horace Judson’s The Eighth Day of Creation. The account of Photo 51 and of Franklin’s role follows the evidence set out in those sources.