Watson and Crick did not discover DNA. Friedrich Miescher first isolated it in 1869, long before either man was born. What they worked out was its shape: two strands twisted into a double helix, held together by paired bases. They did it largely without running experiments of their own, building models from other people’s data, including Rosalind Franklin’s X-ray photographs.1National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953 So the story is as much about whose evidence made the answer possible as about the answer itself.
What Scientists Knew About DNA Before 1953
DNA is built from four chemical units called bases, usually shortened to A, T, C and G. From the mid-1910s to the early 1950s, most scientists believed DNA was just those same four bases repeating in a fixed pattern.2PubMed Central. The “scientific catastrophe” in nucleic acids research that boosted molecular biology. That made it look far too uniform and simple to carry the information that makes each living thing different.3PubMed Central. The “scientific catastrophe” in nucleic acids research that boosted molecular biology. Many assumed proteins, with their 20 building blocks, must be the real carriers of heredity.
In 1944, Oswald Avery’s team showed that a substance from one strain of bacteria could permanently change another strain.4Genome.gov. 1944: DNA is “Transforming Principle” They destroyed proteins, fats and RNA in the extract, and it still worked.5Genome.gov. 1944: DNA is “Transforming Principle” An enzyme that breaks down DNA wiped out its power.6Wikipedia. Avery–MacLeod–McCarty experiment So DNA, not protein, carried the lasting, inherited change.7Genome.gov. 1944: DNA is “Transforming Principle”
The result wasn’t accepted overnight. Skeptics worried that tiny traces of protein left in the extract might be doing the real work, which is why the DNA-destroying enzyme was such an important check. And Avery’s finding was about bacteria, so it took further experiments by other researchers before many scientists accepted that DNA was the genetic material in general.
Chemistry then supplied a clue no one could yet explain. In the late 1940s, Erwin Chargaff developed a careful way to measure each base in DNA samples.8Journal of Biological Chemistry. Chargaff’s Rules: the Work of Erwin Chargaff His results showed the amount of A matched T and the amount of C matched G, a pattern known as Chargaff’s rules.9PubMed Central. The Discovery of the Double Helix, 1951-1953 DNA was not a simple repeat after all, and those matching amounts were a puzzle waiting for a structure to explain them.
Once DNA was known to be genetic, its shape became urgent. Knowing a molecule carries genes doesn’t tell you how it stores information or copies itself. Biologists expected a molecule’s shape to reveal how it works, as it had with proteins, so by around 1950 DNA’s structure was one of biology’s biggest open questions. Several labs in Britain and the US raced to answer it.
What the Double Helix Looks Like
Watson and Crick proposed two chains coiled around the same axis, forming a double helix.10Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid Both chains twist in the same right-handed direction, like an ordinary screw.11Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid The two chains run in opposite directions, though, one up and one down.12Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid Their backbones of sugar and phosphate sit on the outside, with the bases tucked on the inside.13Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid
Picture the molecule as a twisted ladder. Each rung is one base from each strand, joined by weak chemical links.14Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid For a rung to fit, one base in every pair must be a larger type and the other a smaller type.15Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid A and G are the larger bases, built from two rings, while C and T are smaller, built from a single ring. Two large bases would make a rung too wide, and two small ones would leave a gap too narrow to link up. A large-small pair keeps the rung the same width all along, which lets the backbones run as smooth, evenly spaced rails.
Only two pairs work: A with T and G with C.16Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid So the amount of A must equal T and C must equal G, exactly what Chargaff had measured.17PubMed Central. The Discovery of the Double Helix, 1951-1953
Other details matched the measurements too. The bases lie flat, stacked at right angles to the length of the molecule.18Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid X-ray evidence showed about ten stacked bases in each full turn of the helix.19King’s College London. Francis Crick and James Watson’s Discovery of DNA
How Watson and Crick Worked Out the Structure
At Cambridge’s Cavendish Laboratory, Watson and Crick tried to build a model of DNA from what was already known and what other labs’ research suggested.20National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953 They worked with cardboard cut-outs and metal pieces scaled to real atom sizes. A model had to obey known chemistry and match the X-ray measurements, so wrong ones could be ruled out quickly.
That’s how their first attempt in November 1951 failed. It was a three-strand model with the phosphates inside, and it was wrong.21National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953 The negatively charged phosphates would repel each other with nothing to hold them together, and the model didn’t fit the X-ray data from King’s College, which experts there recognized when they saw it. Then in early 1953, Linus Pauling proposed a similar three-strand model, which was also wrong, spurring them to try again before he corrected it.22National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953
Franklin’s photograph gave the decisive picture. Scientists beam X-rays through DNA fibres and record the pattern the fibres scatter, then infer the shape. The photograph doesn’t show the molecule directly. Researchers work backward from the spots to the shape that could have produced them.
Photograph 51, taken in May 1952 by Franklin’s lab, showed a clear X-shaped “cross” pattern.23National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953 That cross is the signature a helix makes when X-rays pass through it from the side.24King’s College London. Francis Crick and James Watson’s Discovery of DNA The spacing of the spots revealed about ten bases stacked per turn.25King’s College London. Francis Crick and James Watson’s Discovery of DNA Measurements like these limited the possible models, since any proposed structure had to reproduce them.
Watson and Crick saw this evidence without Franklin’s knowledge. Wilkins showed him the photo,26National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953 and a research report containing Franklin’s data reached Watson and Crick through Max Perutz.27National Institutes of Health (NIH) / National Library of Medicine. The DNA Riddle: King’s College, London, 1951-1953
One more piece fell into place through a chemist’s correction. Watson first drew the bases in shapes copied from textbooks.28freaktakes.com. Watson’s Double Helix, one small clue, and the importance of knowing what you don’t know American crystallographer Jerry Donohue told him those shapes were probably wrong and gave the likely correct forms.29Acta Crystallographica Section D. Getting the chemistry right: protonation, tautomers and the importance of H atoms in biological chemistry Molecules can shift a single hydrogen atom between two positions, which changes how they bond, and textbooks of the time often showed the less likely position without good reason. With the wrong version, the bases simply would not pair cleanly.
On February 28, 1953, Watson redrew the bases in cardboard and saw that an A-T pair had nearly the same shape as a C-G pair.30PubMed Central. The Discovery of the Double Helix, 1951-1953 Both pairs fit neatly between two backbones, matching Chargaff’s rules and the X-ray evidence at once.31PubMed Central. The Discovery of the Double Helix, 1951-1953
Rosalind Franklin, Maurice Wilkins and the Question of Credit
Maurice Wilkins and his student Raymond Gosling began X-ray studies of DNA at King’s College London around 1950.32Science History Institute. Francis Crick, Rosalind Franklin, James Watson, and Maurice Wilkins Wilkins led that work before Franklin arrived and started the methods that later produced Photograph 51.33Wikipedia. Maurice Wilkins Franklin and Gosling took Photograph 51 as the best of her “wet,” or B-form, images of DNA.34King’s College London. Francis Crick and James Watson’s Discovery of DNA
Her contribution went beyond that one picture. She had worked out that DNA exists in two forms, a drier one and a wetter, more hydrated one, and that they give different X-ray patterns. Separating them made the clearer wet-form pictures possible, and the sharper pattern gave cleaner evidence for a helix. Her careful measurements also supported the idea that the phosphate backbones sit on the outside.
On April 25, 1953, Nature printed three papers side by side: by Watson and Crick, by Wilkins and colleagues, and by Franklin and Gosling.35PubMed Central. James D. Watson (1928–2025): Influencer of science and society In 1962, Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine for discovering DNA’s structure.36NobelPrize.org. Nobel Prize in Physiology or Medicine 1962 Franklin had died of cancer in 1958, and the prize is not given after death.
A Nobel Prize can be shared by at most three people, so had she lived, the committee could not simply have added her as a fourth. Whether Franklin would have been included had she lived remains debated.
What was unfair, exactly? Her data were seen and used without her knowledge or permission. Wilkins showed Photograph 51 to Watson, and a report prepared for a research committee held her measurements and passed to Watson and Crick through Perutz, a committee member. Franklin had agreed to neither, which is the core of the fairness complaint.
Some things are often exaggerated. Her work appeared in Nature alongside Watson and Crick’s, so she was not erased at the time. The bigger issue was later accounts, including Watson’s memoir, that minimized her. Saying she “did all the work” oversimplifies, because the result combined several people’s contributions.
Some historians argue “discovery” should include making the structure visible, not just the final insight.37Philosophy of Science. Reassessing Discovery: Rosalind Franklin, Scientific Visualization, and the Structure of DNA On that view, Franklin’s photographs count as part of the discovery itself.38Philosophy of Science. Reassessing Discovery: Rosalind Franklin, Scientific Visualization, and the Structure of DNA Wilkins’s role in leading and coordinating the King’s work is also often underplayed.39Wikipedia. Maurice Wilkins
How the Double Helix Explains Copying Genes
Watson and Crick noted that the base pairing had obvious meaning for how genes are copied.40PubMed Central. The Discovery of the Double Helix, 1951-1953 Once you know one strand’s sequence, the other’s is automatically fixed.41PubMed Central. The Discovery of the Double Helix, 1951-1953 So when the two strands separate, each can guide the building of a new matching strand.42PubMed Central. The Discovery of the Double Helix, 1951-1953 Each new DNA molecule ends up with one old strand and one new one.
Take a strand reading A-T-G-C. Its partner must read T-A-C-G. Unzip the two strands down the middle of the ladder, and free bases line up against each exposed strand by the pairing rule. The result is two identical ladders, each half old and half new. Genetic information can therefore pass to new cells with very few mistakes.43Pearson. What did the Watson–Crick model suggest about the replication44Pearson. What did the Watson–Crick model suggest about the replication
Their first paper was careful about all this, saying only that the pairing they proposed immediately suggested a possible copying mechanism. That understated line pointed to a larger idea, which they spelled out in a follow-up paper on genetic implications. It was a prediction rather than a proof, which is why later experiments were needed.
The structure also fed into the question of how cells build proteins. Crick later proposed small adaptor molecules that carry each amino acid to the genetic template. This idea grew into the basic flow of information from DNA to RNA to protein.
How the Model Was Proven
In experiments published in 1958, Matthew Meselson and Franklin Stahl grew bacteria on food containing heavy nitrogen, making their DNA heavier than normal.45Embryo Project Encyclopedia. The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl They then switched the bacteria to ordinary light nitrogen and weighed the DNA after each round of copying.46Embryo Project Encyclopedia. The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl
Heavy nitrogen has extra weight, so DNA built with it is slightly denser. The researchers spun DNA samples in a salt solution at very high speed, and the DNA settled into bands at a position matching its density. The position of a band told them how much heavy and light material each sample held.
After one round, all the DNA was exactly halfway between heavy and light.47The River Divides into Thousands of Branches. Chapter 8: Meselson and Stahl provide empirical support for semi-conservative DNA replication Only one band appeared, midway between the two positions, so every molecule was a hybrid with one heavy strand and one light strand. That immediately ruled out the idea that the original heavy molecule stayed intact while an all-light copy was made beside it.
After a second round, half was halfway and half was fully light.48The River Divides into Thousands of Branches. Chapter 8: Meselson and Stahl provide empirical support for semi-conservative DNA replication If DNA copied as a whole new molecule, you’d see separate heavy and light bands rather than a halfway band. If strands were chopped and mixed, the bands wouldn’t split cleanly in round two. Only the half-old, half-new model fit both rounds: each copy kept one old strand and added one new strand, just as Watson and Crick predicted.49Embryo Project Encyclopedia. The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl
What the Discovery Led To
Knowing DNA’s structure opened the way to genetic engineering, which moves genes between organisms.50International Journal of Frontiers in Medicine. The Impact of the Discovery of DNA Double Helix Had Scientists can cut DNA at chosen places, take a gene from one organism and join it into another’s DNA. This works because DNA has the same basic structure in all living things, so a gene from one species can be read by another’s cells. Making human insulin in bacteria is the classic example.
DNA sequencing then let scientists read genes in detail and trace the roots of many diseases.51ASBMB Today. Discoveries made possible by DNA Those efforts led to the Human Genome Project, which produced the first reference sequence of the human genome.52ASBMB Today. Discoveries made possible by DNA Sequencing went from slow, labour-intensive methods to fast modern machines,53Preprints.org. From the Double Helix to Precision Genomics: A Comprehensive Review of DNA and Its Transformative Role in Biomedical Sciences and tools like CRISPR now let scientists edit DNA with high precision.54Preprints.org. From the Double Helix to Precision Genomics: A Comprehensive Review of DNA and Its Transformative Role in Biomedical Sciences
The 1953 insight is used directly, not just remembered as history. Reading DNA works by building a matching copy and noting each letter added, and gene editing tools find their target by matching bases to it.
You meet the results in everyday life. Ancestry and paternity tests read your DNA’s letter sequence. Some cancer treatments are chosen by reading a tumour’s genes. Insulin for diabetes is made by bacteria given a human gene.