The Mysterious World of the Human Genome

by Frank Ryan

  Back in early 1950, Wilkins had complained of a poor-quality X-ray apparatus that was not designed for the scrutiny of exquisitely fine fibers. At his suggestion, the department had purchased a new and better-quality X-ray tube to be set up in the basement, but it had lain there for a year or more unused while Wilkins was distracted by the multiple tasks that fell to a busy deputy director of the unit. On her arrival, Franklin, not unnaturally, believed that she was there to take over the DNA work as her personal project. However, the returning Wilkins expected that Franklin had been brought in as his collaborator, to take up the research from where he had already developed it. He would subsequently admit that he was unqualified to take the X-ray diffraction work further and needed exactly such a dedicated and qualified collaborator. “That's why we hired Rosalind Franklin.”

  Unfortunately, Franklin and Wilkins now disagreed as to her role. Even so, rancor was neither necessary nor inevitable between the two scientists, personally or scientifically. These difficulties, provoked by Randall's vagueness, might have been readily overcome with goodwill on both sides, but Franklin, in the opinion of both her biographers, was not inclined to cooperate.

  Much has been written about prejudicial attitudes to women in science at this time. In particular, an American journalist, and personal friend of Franklin's, Anne Sayre, would write a biography of her in which she suggested that King's College was particularly unfriendly to female scientists, with Franklin struggling to assert her presence in a domain that was almost exclusively male. But when another American journalist, Horace Freeland Judson, looked into this claim, he discovered that of the 31 staff working at King's at this time, eight were female, including some working in a senior position in Franklin's unit. A second biography of Franklin, by Brenda Maddox, confirmed that women were, on the whole, well treated at King's College. Crick made the same point in his biography—and Crick had come to know Franklin well in the years following the DNA discovery. Even in Sayre's more trivial complaint—that the main dining room was exclusively forbidden to women, who were thus precluded from lunchtime conversation—is misleading. There were two dining rooms. One was limited to men, but this, in the main, was used by Anglican trainees. The main dining room, used by the departmental staff, including Randall himself, was open to all.

  The frosty relationship between Wilkins and Franklin was not the result of anti-female prejudice—it even seems unlikely to be the result of Randall's peculiar wording in the letter—but it appears to be more directly related to a personality clash between the two scientists. Of the two, only Wilkins ever seems to have made any attempt at compromising. He asked other colleagues what he should do, but Alexander (Alec) Stokes, his closest colleague, was even meeker than he was. In Brenda Maddox's opinion, the two should have got on well; Wilkins was gentle in manner and, despite his lack of self-confidence, was attractive to women. He was mathematically fluent and immersed in the very problems that concerned Franklin. But “confrontation,” in Maddox's words, “was Franklin's tactic, whenever cornered.” In an earlier confrontation with her professor R. G. W. Norrish, when working on a postgraduate research project at Cambridge, she would confide, “When I stood up to him…we had a first-class row…he has made me despise him so completely I shall be quite impervious to anything he may say in the future. He gave me an immense feeling of superiority in his presence.”

  Sayre, who championed her friend, would admit that Franklin's ogreish depiction of her professor was unkind and inaccurate. Professor Norrish was awarded the Nobel Prize in Chemistry in 1967.

  Sayre had a correspondence with Norrish in which she described Franklin as “highly intelligent…and eager to make her way in scientific research,” but also “stubborn, difficult to supervise” and, perhaps most tellingly, “not easy to collaborate with.” In Maddox's opinion, “If Rosalind had wished, she could have twisted Wilkins around her little finger.” The fact is she had no wish to collaborate with him. This left Wilkins isolated locally so instead he turned to Crick and Watson at Cambridge. It also meant that Franklin was equally isolated. To the commonsensical Crick, this may have been a crucial factor when it came to working out the molecular structure of DNA. “Our advantage was that we had evolved…fruitful methods of collaboration, something that was quite missing in the London group.”

  In that same year of Franklin's appointment, just before Wilkins headed for America, he asked his colleague, Alec Stokes—another Cambridge-educated physicist—if he could work out what kind of diffraction pattern a helical molecule of DNA would project onto an X-ray plate. It took Stokes just twenty-four hours to do the mathematics, largely figuring it out while traveling home on the commuter train to Welwyn Garden City. A helical model fit very closely with the picture Gosling and Wilkins had obtained in their diffraction pictures of DNA. It would appear that if anybody first confirmed that DNA had a helical structure, the credits must surely include Wilkins, Gosling, and Stokes—the latter would subsequently lament that, in retrospect, he might have merited 1/5000th of a Nobel Prize.

  In November 1951, Wilkins told Watson and Crick that he now had convincing evidence that DNA had a helical structure. Watson had only recently heard Franklin say something similar in a talk about her research during a King's College research meeting. This inspired Watson and Crick to attempt their first tentative three-dimensional model for DNA.

  But where to begin?

  Taking their cue from Linus Pauling, Watson and Crick decided that they would attempt to construct a three-dimensional physical model of the atoms and molecules that made up DNA with their covalent and hydrogen bond linkages to one another. On the face of it, the structure was made up of a very limited number of different molecules. There were the four nucleotides—guanine, adenine, cytosine, and thymine—but they also knew that the structure contained a sugar molecule—deoxyribose—and a phosphate molecule. The phosphate was likely to be playing a structural role, perhaps holding the thread together, much as phosphate is a key structural component of our bony human spine. In the colloquium at King's, attended by Watson, such was his lackadaisical absence of focus that he completely missed the importance of Franklin's statement that the phosphate-sugar “spines” were on the outside, with the coding nucleotides, the GACT, on the inside. As usual, he had eschewed making notes. All that seemed to intrigue Watson was the fact that the King's people were uninterested in the model-building approach developed, with such aplomb, by Pauling.

  In 1952, Franklin appears to have undergone a drastic change of heart in her own thoughts on the structure of DNA. She had in her possession a brilliantly clear X-ray picture of DNA, taken by Gosling, that clearly showed a helical structure to the molecule. She called this her “wet form” and also her “B form.” But she had even clearer pictures of a different structure of the same molecule in its “dry form,” or “A form,” that did not appear to suggest a helix. The contrast between the two forms caused Franklin to dither as to whether the DNA molecule was helical. There is a suggestion that she may have asked the opinion of an experienced French colleague, who advised her to place her bets on whichever form gave the clearest pictures. She must have been altogether aware of the advice her ignored colleague, Wilkins, would have given. Unfortunately, she ended up putting the B form into a drawer, meanwhile focusing most of her research over that year into the A form.

  Early that same year, Watson and Crick made a first attempt at building a triple-stranded helical model of DNA, with a central phosphate-sugar spine. When Wilkins brought Franklin and Gosling up to Cambridge to view the model, they broke out into laughter. The model was absolute rubbish. It did not fit at all with the X-ray diffraction predictions. Thanks to Watson's lackadaisical focus and his failure to take notes at Franklin's colloquium, he had made the cardinal error of putting the phosphate-sugar spine at the dead center of their helix and not on the outside, as Franklin and Gosling had clearly deduced.

  Sayre, who rightly defended Franklin from the egregious caricature depicted by Watso
n's book, loses track of the contribution of Wilkins and Gosling. It is true that Franklin and Gosling had produced some of the clearest pictures yet of the B form of DNA, pictures of such clarity that they did come astonishingly close to the truth of its molecular structure. But then, confused for a year by the two seemingly different patterns of the A and B forms, Franklin veered away from her own earlier conclusions, and for a year she took the view that DNA wasn't helical at all. Sayre appears to refute this, but Gosling would subsequently confirm Wilkins's account of how, on Friday, July 18, 1952, Franklin goaded Wilkins with an invitation to a wake. The invitation card announced, with regret, the death of the DNA helix (crystalline) following a protracted illness. “It was hoped that Dr. M. H. F. Wilkins would speak in memory of the deceased.” At the time, Wilkins assumed it was typical of Gosling's sense of humor. But many years later he would discover that it was Franklin who had written the card, and it confirmed her refutation of any helical structure of DNA in that confused year.

  In the middle of the same year, 1952, Crick struck up a conversation with a local young Welsh mathematician called John Griffith, whom he met one evening after a talk at the Cavendish by theoretical astronomer Thomas Gold. Gold had captured Crick's imagination with the notion of “the perfect cosmological principle.” Wondering if there might be some equivalent “perfect biological principle,” Crick pressed Griffith, who was interested in how genes replicated, about the work of an American chemist, Erwin Chargaff, who had discovered that the nucleotides in DNA formed flat linkages with one another. It was curiously reminiscent of Pauling's discovery of how the amino acids that made up the primary chains of proteins, known as “peptide bonds,” also joined up to form flat two-dimensional planes. In Crick's mind it invoked a vague notion that this might be something to do with DNA replicating itself. He asked Griffith if he could work out which of the four nucleotides would pair off with which. Griffith confirmed that the likely pairing was C with G and A with T…

  But even then the penny did not drop.

  Erwin Chargaff was yet another Austrian scientist who fled Europe in the years leading up to World War II and headed to the US, where he became professor of biochemistry at Columbia University. His interest was the study of nucleic acids. We might recall that much of the disbelief around Avery's discovery centered on the fact that geneticists had been misled by the erroneous notion of Levene's “tetranucleotide hypothesis,” which proposed that DNA comprised repeats of the same cluster of four nucleotides. Such a simple formula would be incapable of storing the vast memory demanded for the molecule of heredity, so it was wrongly assumed that DNA could not be the answer to the gene.

  Chargaff didn't give a damn what the geneticists thought of Avery: he was deeply impressed by his findings. And if Avery was right, and DNA was the molecule of heredity, the DNA sequences of a horse, say, would be different from that of a cat, or a mouse, or a human being. In Chargaff's words, “There should exist chemically demonstrable differences between [their] deoxyribonucleic acids.” These differences should be demonstrable in the proportions of the four different nucleotides. It might appear that a four-letter code would be limited in delimiting the wide variety of genes that occurred in nature, but if we were to regard the nucleotides as letters in a four-letter alphabet, and the genes as words, the potential for different arrangements of just four “letters” in words a thousand or more letters long could easily explain the complexity needed for the makeup of genes.

  Technology was limited in the late 1940s and early 50s, but Chargaff modified a technique, known as paper chromatography, to read off the different proportions of the four nucleotides in any given sample of DNA.

  After four years of laborious experiment, analyzing the DNA of yeast, bacteria, oxen, sheep, pigs, and humans, Chargaff had his answer: the four nucleotides that made up the word of the gene were not present in the equal proportions one might expect from Levene's hypothesis. For example, human DNA, extracted from a gland in the chest called the thymus, yielded 28 percent adenine, 19 percent guanine, 28 percent thymine, and 16 percent cytosine. The tetranucleotide hypothesis could now be ditched. But Chargaff took it further. He demonstrated that the proportions of the nucleotides varied between species—meanwhile, the proportion of nucleotides was always the same for members of the same species and indeed was the same from organ to organ and tissue to tissue within the same species. He also noticed something else: by inference, the sum of the molecules of adenine and thymine was very similar to the sum of the molecules of cytosine and guanine.

  This was a groundbreaking discovery.

  In May 1952, by remarkable happenstance, Chargaff arrived in person at Cambridge University where Kendrew introduced him, over lunch, to Watson and Crick. Chargaff was offended by how little they knew about his work. In his opinion, they appeared to know next to nothing about the actual chemistry of nucleotides. In Chargaff's subsequent description to Judson: “I explained our observations…[that] adenine is complementary to thymine, guanine to cytosine.” But as far as Chargaff could see, all that preoccupied Watson and Crick was the race to construct a DNA helix to rival Pauling's alpha helix for proteins. Watson would subsequently recall Chargaff's open scorn for the “two men who knew so little—and aspired to so much.”

  Chargaff was largely right in his assessment of Watson and Crick's ignorance of the biochemistry at the time. Crick knew nothing about Chargaff. No more did he understand that the pairing of the nucleotides involved not the covalent chemical bonding found in stable molecules but the weaker hydrogen bonding. What then was he to make of Chargaff explaining the importance of the one-to-one ratios of cytosine and guanine, and adenine and thymine, in the molecule of DNA?

  Crick then had a brainwave: What if this signified a natural chemical attraction between these nucleotides?

  Might this not play a vital role when an existing strand of DNA copied itself to a daughter strand? Every C would attract a G, and every A would attract a T in the daughter sequence—and the whole thing would revert to the maternal sequence when the daughter strand replicated in its turn. He took it a stage further. What if DNA comprised two threads, complementing one another in exactly this way? If and when the two threads broke apart and copied themselves, they would create an identical pair of threads, a new identical chain.

  It seemed too incredible to be true that the great and profound mystery of heredity might be explained on the basis of these simple chemical couplets, with their specific attractions to one another.

  Then Crick and Watson made a mistake, not in scientific terms, but in human terms. They sat back and thought about all that they knew but did nothing about constructing a new model. It almost cost them everything. In December 1952, Peter Pauling, son of Linus, then working as a graduate student at the Cavendish, informed Watson that he had just received a letter from his father to say that he had worked out the structure of DNA. The following month, Peter showed everybody an advance copy of the paper, which was scheduled to be published in February 1953 in the Proceedings of the National Academy of Sciences. Watson and Crick would later confess with what sinking hearts they read the paper, which proposed a triple helix with the phosphate-sugar spine at the center. For a brief interval they were dumbfounded, wondering if their own model, dismissed by Wilkins and Franklin, might have been correct after all. But then they realized that all of the scorn heaped on them by the X-ray crystallographers also applied to Pauling's model. This time it was the great chemist's turn to blunder.

  The race was now on again to get it right. Where the Cambridge duo had agreed to stay away from DNA, Watson was now convinced that if they did so, Pauling would beat them all to the prize.

  A few days after reading Pauling's paper, Watson took it down to King's College London, where, according to his biography, he talked about it first with Franklin—who, according to Watson, flew into a rage. In Watson's opinion, her rage was provoked by his criticism of her rejection of helical structures. But it would also appear that Watson
deliberately provoked Franklin into that response. Discovering that…“Rosy was not about to play games with me, I decided to risk a full explosion. Without further hesitation I implied that she was incompetent in interpreting X-ray pictures.”

  It was hardly surprising that Franklin flew into a rage.

  Much has also been made of the fact that, without consulting Franklin, Wilkins showed Watson the photographic copy of the particularly clear X-ray picture of the wet form of DNA obtained the previous May—a picture that confirmed without a shadow of a doubt that the molecule of DNA was a helical structure. In fact, Watson, Crick, and Wilkins were already long convinced of the helical structure of DNA. Wilkins makes clear in his belated biography, published in 2003, just a year before his death, the X-ray photograph that Watson crowed about was not stolen from Franklin but was passed to him by Gosling, who had taken the photograph in the first place and who would have assumed, now that Franklin was leaving, that she could have no objection to his doing so. Gosling still needed to complete his PhD thesis without the departing Franklin's supervision and so had every reason to show his own work to the deputy director of the unit who would now be tutoring him. Gosling himself would confirm that, “Maurice had a perfect right to that information.” Gosling was clearly fed up with the rancor provoked by Franklin's refusal to collaborate with Wilkins, bemoaning a time when, “There was so much going on at King's before Rosalind came.”