The Double Helix

The Double Helix

by

James D. Watson

Teachers and parents! Our Teacher Edition on The Double Helix makes teaching easy.

The Double Helix Summary

In his memoir The Double Helix, the influential but controversial molecular biologist James D. Watson recounts how he and Francis Crick discovered the structure of DNA at the University of Cambridge in 1953. This discovery was remarkable not only because it revolutionized genetics forever, but also because Watson and Crick were young and completely unknown when they made it: Watson was only 25 years old, and Crick, while much older, hadn’t even finished his PhD. In The Double Helix, Watson explains how a combination of input from other scientists, creative theorizing, and youthful ambition led him and Crick to the “key to the secret of life.”

Watson prefaces his book by emphasizing that he is merely presenting his own memory of events as he experienced them—and not the absolute truth. He also briefly reflects on the dynamism and intensity of his years at Cambridge. Then, he begins his first chapter by declaring, “I have never seen Francis Crick in a modest mood.” He remembers Crick’s boisterous personality and endless curiosity, which often led him to spend his days theorizing about other people’s data instead of doing his doctoral research on protein structure. In fact, while Crick’s colleagues at the Cavendish Laboratory were trying to understand the secrets of life and heredity by studying proteins, Crick belonged to the growing minority of biologists who thought that genes were really made of DNA. So did Watson.

The one man in England who took DNA seriously was Crick’s friend Maurice Wilkins, who tried to understand its structure through a crystallography technique called X-ray diffraction. However, Wilkins’s research advanced slowly because of his constant quarrels with his strong-willed assistant Rosalind Franklin. Watson saw Franklin as too independent, curt, and plainly dressed for a woman in science. In fact, he thought that Wilkins should have fired her at once. However, to truly understand Watson’s sexist remarks about Franklin, readers must know that he left a crucial truth out of the story: in reality, Franklin wasn’t Wilkins’s assistant at all, but rather his colleague and equal.

Watson first learned about Maurice Wilkins’s research at a conference in Naples in 1951. He had just finished his PhD in Indiana and moved to Copenhagen on a fellowship. Although Watson was supposed to be studying DNA under the biochemist Herman Kalckar, he couldn’t stand Kalckar’s methods, personality, or incomprehensible accent. Therefore, he started working with the phage researcher Ole Maaløe while he looked for another direction to take his research. When he went to Naples and saw Maurice Wilkins’s X-ray photo of DNA, he realized that he needed to go to England and study crystallography. He secured a place working with Max Perutz at the Cavendish Laboratory at Cambridge. While he waited to see if he could transfer his fellowship there, he moved in with Perutz’s collaborator John Kendrew and lived off his savings. That’s when he met Francis Crick.

Crick and Watson were fast friends: they started eating their meals together and spending their days talking about every scientific topic under the sun. Both were intensely interested in DNA. Crick told Watson about how Linus Pauling discovered the alpha helix protein structure using special molecular models, and the pair started wondering if they could use the same process to build a model for DNA. Crick and his colleague Bill Cochran developed a new theory of how to detect helix shapes through X-ray diffraction, and then Watson learned about Rosalind Franklin’s X-ray diffraction results at one of her talks. By combining Crick’s new theory with Franklin’s data, Crick and Watson theorized that DNA could be a three-strand helix with each strand’s sugar-phosphate backbone in the middle and its nitrogenous bases pointing outwards. They built a model of this hypothetical model, and then invited Maurice Wilkins, Rosalind Franklin, and their graduate students to visit Cambridge and take a look.

But Wilkins and Franklin’s visit was a catastrophe: they found a series of fatal flaws in Crick and Watson’s model. Worse still, Watson realized that he misquoted Franklin’s data to Crick, so their theory was based on the wrong numbers. Crick and Watson discarded their model, and the Cavendish Lab’s leader, Sir Lawrence Bragg, furiously ordered them to stop working on DNA. They agreed. Watson started doing X-ray diffraction experiments with tobacco mosaic virus, while Crick went back to his PhD research on protein structure.

But Crick and Watson continued to follow other scientists’ research on DNA. American researchers showed that viruses use DNA to infect bacteria, while the biochemist Erwin Chargaff discovered a pattern in DNA’s four nitrogenous bases: there tends to be exactly as much adenine as thymine and exactly as much guanine as cytosine. Crick and the theoretical chemist John Griffith determined that adenine and thymine could probably bond together, as could guanine and cytosine. Watson and Crick met Chargaff and explained their findings, but Chargaff dismissed the pair as inexperienced and overconfident. At a major conference in Paris a few weeks later, Watson also met the world-famous Linus Pauling, who was reportedly working on DNA.

That fall, Crick tried to convince Watson to drop his new research on bacterial mating and take another look at DNA. At first, Watson was hesitant. Linus Pauling’s son Peter came to Cambridge to pursue his PhD, and he initially reported that his father wasn’t working on DNA at all. But soon, this changed: Linus Pauling sent a letter to Peter, explaining that he had discovered the structure of DNA. Crick and Watson were astonished and disheartened all at once. But then, Pauling sent in a copy of his proposal: it looked just like the model that Crick and Watson showed Maurice Wilkins and Rosalind Franklin. It was dead wrong, and Crick and Watson could prove it. In fact, it was based on a basic chemistry error.

After Pauling’s blunder, Crick and Watson agreed that it was time to give DNA another shot. Pauling would soon realize and correct his mistake, so if Crick and Watson wanted to discover DNA’s structure before Pauling did, they had to act immediately.

Watson visited Maurice Wilkins to tell him about Pauling’s mistake, but got in an argument with Rosalind Franklin instead. After Watson and Franklin’s argument, Wilkins showed Watson one of Franklin’s secret X-ray diffraction images. Watson was stunned: Franklin’s diffraction picture clearly showed that DNA had a helical structure. As soon as he got back to Cambridge, Watson got permission to work on DNA with Crick. They immediately started putting together a new model, but for several days, they didn’t make much progress.

Then, one day, Watson remembered that the nitrogenous bases in DNA—adenine, thymine, guanine, and cytosine—are all capable of forming hydrogen bonds with themselves. This meant that DNA could look like a corkscrew-shaped ladder: it would have two strands, with the sugar-phosphate backbones on the outside and the nitrogenous bases of each nucleotide pointing inward. Each of the two strands would have an identical sequence of nitrogenous bases, and hydrogen bonds between these bases would hold the DNA molecule together.

Watson briefly thought he had solved the puzzle of DNA, but he soon realized that this structure wouldn’t work. Since all the nitrogenous bases were different sizes, a DNA molecule would be irregularly-shaped, full of random bulges and indentations. That wasn’t possible.

Two days later, however, Watson realized that, when adenine bonds to thymine, they form a combination with exactly the same shape as the combination of guanine bonded to cytosine. This meant that the DNA molecule could have an irregular sequence of nitrogenous bases and a perfectly regular shape. When Watson told Crick about his idea, Crick was immediately convinced. At lunch, he started telling everyone that he and Watson had discovered “the secret of life.” Watson and Crick built a new model of the double helix structure, measured it to make sure it was exactly right, and then showed it to their mentors and colleagues. Everyone who learned about their model was convinced—including Rosalind Franklin and even Linus Pauling. Crick and Watson published their results in the journal Nature, and their discovery went on to transform biology forever.