One discovery, many steps
“The discovery of DNA” is really several discoveries spread across almost a century: finding the substance, showing that it carries heredity, and working out its shape. The 1953 model of the double helix is the best-known moment, but it depended on earlier work by people who are less often named. It is also a story where credit has been argued over, and where careful historians still disagree on some points.
Miescher and nuclein
In 1869 the Swiss physician Friedrich Miescher, working in Tübingen in Germany, was studying the chemistry of white blood cells. He obtained them from pus on discarded surgical bandages and found that the nuclei contained a substance unlike the proteins he expected. It was rich in phosphorus and did not break down in the same way. He called it “nuclein.” Later chemists, notably Albrecht Kossel and Phoebus Levene, identified its building blocks: sugar, phosphate and four types of base.
For decades nuclein was thought to be a minor structural material. Levene proposed that the four bases repeated in a simple, fixed pattern, which made it seem too uniform to carry the complex instructions of heredity. Most biologists therefore assumed that proteins, with their greater variety, were the genetic material. That view later proved wrong, but it was reasonable given what was known.
Evidence that DNA carries heredity
In 1928 Frederick Griffith, a British bacteriologist, noticed that harmless pneumonia bacteria could become dangerous after being mixed with dead dangerous bacteria. Something from the dead cells had transformed the living ones. He did not know what the substance was.
In 1944 Oswald Avery, Colin MacLeod and Maclyn McCarty, at what is now Rockefeller University, purified the transforming material and reported that it was DNA. Their result was important, but it was not accepted at once; some scientists suspected a small amount of leftover protein was responsible. In 1952 Alfred Hershey and Martha Chase used viruses that infect bacteria, tracking radioactive labels, and their results were widely read as strong support for DNA as the genetic material. Around 1950 Erwin Chargaff also reported that, in DNA from many species, the amount of one base matched the amount of its partner (adenine with thymine, guanine with cytosine). That pattern turned out to be a vital clue.
The race to find the structure
By the early 1950s several groups wanted to know DNA’s shape. At King’s College London, Maurice Wilkins and Raymond Gosling had begun producing X-ray diffraction images of DNA fibers, and Rosalind Franklin, an experienced crystallographer who joined the lab in 1951, took over much of the experimental work. X-ray machines had been used in medicine for decades, but diffraction work of this kind uses the rays to infer molecular structure from the patterns they scatter. Franklin found that DNA could exist in two forms, a drier and a wetter one. In May 1952 she and Gosling produced the sharp image of the wetter form now known as Photo 51, which showed a distinctive X-shaped pattern consistent with a helix.
At the Cavendish Laboratory in Cambridge, James Watson and Francis Crick were building physical models. Their first attempt in 1951 had errors and was discouraged. Linus Pauling in California also proposed a three-stranded model that turned out to be wrong.
The 1953 model
Watson and Crick’s breakthrough came in early 1953. They had seen Photo 51, which Wilkins showed to Watson, apparently without Franklin’s knowledge or permission. They also had access to a report of Franklin’s measurements prepared for a Medical Research Council committee, which reached Crick through Max Perutz. Using these, along with Chargaff’s base ratios, they built a model of two strands wound around each other in opposite directions, with the bases paired in the middle: adenine with thymine and guanine with cytosine.
The model made the biology clear almost at once: pairing explained how the molecule could be copied, with each strand serving as a template for the other. Their short paper appeared in the journal Nature on April 25, 1953, along with separate papers from Wilkins’s group and from Franklin and Gosling that supported the structure with data.
The credit debate
In 1962 Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine. Franklin had died in 1958, aged 37, of cancer, and the prize is not awarded after death, so the question of whether she would have been included remains hypothetical.
What is clear: Franklin’s experimental work was central, and the way her data reached Watson and Crick was not an ordinary scientific exchange. Her contribution was under-acknowledged for years. Watson’s 1968 memoir, The Double Helix, portrayed her unflatteringly and was criticized for it. What is less clear is how to weigh each person’s role. Some historians stress that Watson and Crick’s insight was the model-building and the base pairing, and that Franklin had been close to the answer herself; others argue that she was treated unfairly. Reasonable accounts disagree, so it is best to treat any single-sentence verdict with caution.
What came next
Once the structure was known, the question became how the sequence of bases specifies proteins. Over the 1960s researchers worked out the genetic code, and later decades brought DNA sequencing, which now produces so much information that analyzing it relies on big data methods, and tools such as biochips that read many genes at once. The discovery also set the stage for genetic engineering and, much later, gene editing. Understanding the double helix did not end the story; it opened a field.