by Luisa Bonolis
Nobel Prize in Physiology/Medicine 1962 together with James Watson and Maurice Wilkins
"for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".
From physics to biology
Francis Harry Compton Crick was born in 1916 in Northampton, where he spent his early years. After World War I, the family moved to London where he attended the Mill Hill School, showing a strong interest in physics, chemistry and mathematics. In 1934, he entered University College in London to study physics and after graduating three years later with a B.S, he conducted research on the viscosity of water at high temperatures. His work was interrupted in 1939 by the outbreak of World War II. Crick joined the British Admiralty Research Laboratory and spent the war years devising detonators for magnetic and acoustic mines. After the war he remained at the Admiralty for two years, and it was during this time that he read Erwin Schrödinger's influential book, What is Life? The Physical Aspects of the Living Cell, which was published in 1944. In asking, “How can the events of space and time which take place within ... the living organisms be accounted for by physics and chemistry?” Schrödinger proposed that through quantum physics it was possible to understand biological growth and the nature of life itself. At the time, Crick was already 30, considering a career in particle physics, but like many young scientists of his generation, he was so stimulated by the idea that fundamental biological problems could be thought about using the concepts of physics and chemistry that he began to think of making a completely fresh start as a biologist. In that same period, Maurice Wilkins, who would be involved with Crick in the race for the DNA structure, was fascinated as well by the novelty of these new concepts involving crossover methodologies of previously unrelated disciplines and decided to turn away from nuclear physics to pursue biophysics.
Crick succeeded in obtaining a Medical Research Council studentship, and in 1947 he began working on experimental cytology at Strangeways Research Laboratory at Cambridge. There he learned biology, organic chemistry, and the techniques of X-ray diffraction, a method used to determine the three-dimensional structure of molecules that at the time was in full bloom.
Crick was fascinated by the borderline between the living and the non-living, as typified by proteins, viruses, and bacteria, so he hoped that by studying the structure of elemental reproducing molecules, he might find evidence for a starting point for life. Initially he desired to work with Bernal, who, in 1934 had been the first to obtain clear images of X-ray diffraction patterns of the protein crystal pepsin, becoming the founder of protein crystallography. His research group was working on the structure of viruses at Birkbeck College in London. But Bernal was abroad when he visited the College, and his secretary discouraged Crick saying: “Do you realise that people from all over the world want to come to work with the Professor?” He then had the opportunity of entering the Strangeways Laboratory, but by early 1949, as his research on the viscosity of cytoplasm approached completion, he was faced with a decision about future employment. Crick remembered how, on his first visit to Cambridge, it had been suggested to him to visit Max Perutz at the Cavendish Laboratory. Perutz had been working at Cavendish since 1936 and had begun his career working with John Bernal. When Rutherford died in 1937, Lawrence Bragg was appointed as his successor and the Cavendish became a world-leading centre for X-ray crystallography. While still a young student, Bragg, together with his father Henry, had pioneered the application of X-ray diffraction, discovered in 1912 by Max von Laue, to the analysis of crystals. In 1915, when he was only 25 years old, Lawrence Bragg shared with his father the Nobel Prize in Physics for their discoveries related to X-Ray diffraction. The Braggs led the field of X-ray crystallography for several decades, encouraging a new generation of researchers, including John Desmond Bernal and William Astbury, both pioneers of molecular biology. In his activity as scientific organiser after the war, Lawrence Bragg promoted the establishment of specific laboratories for research into the molecular structure of biological systems. He organised the research effort, found support for the project of protein analysis, and assembled a team to tackle these new problems. Perutz was named director of the Unit for Molecular Biology established in 1947 by the Medical Research Council in the Cavendish Laboratory that attracted more and more researchers who felt that the new field of molecular biology had great promise. Among Bragg's students were Dorothy Crowfoot Hodgkin, Rosalind Franklin, who would play a major part in the discovery of the structure of DNA, Aaron Klug, who later developed crystallographic electron complexes and was awarded the Nobel Prize for Chemistry in 1982; and of course Max Perutz.
Perutz was struck by Crick and thought that he, as a physicist, might be very useful for the project of using methods of X-ray analysis to determine the structure of proteins and allied molecules. Thus, in June 1949, Francis Crick began to work under Perutz on investigations into the molecular structure of proteins at Cavendish Laboratory. When Perutz started his X-ray work on crystalline proteins in the Cavendish Laboratory in 1937, Bernal and Dorothy Hodgkin had just demonstrated that protein crystals could be made to yield sharp X-ray diffraction patterns. Both Hodgkin and Perutz would be awarded the Nobel Prize in Chemistry during the 1960s for solving the structure of important biochemical molecules. In their initial work in the 1910s, the Braggs had applied the technique of X-ray analysis to relatively simple crystals, such as sodium chloride, which are composed of only a few types of atoms. Bernal, however, was interested in the far more complex structure of proteins and he hoped that diffraction studies would eventually enable him to elucidate the function of a given protein. Stimulated by Bernal's visionary faith in the power of X-ray diffraction, in the late 1930s Perutz began investigating haemoglobin, the carrier of oxygen from the respiratory organs to the rest of the body. But this was only the beginning of a long and ambitious research path. Gradually, it turned out that each unit repeat volume in a crystal of haemoglobin has about 12000 atoms. In 1937, X-ray analysis had only solved structures containing less than about 100 atoms. Perutz resumed his studies after the war, at the time when the great British tradition of X-ray crystallography initiated by Henry and Lawrence Bragg was flourishing and financial support allowed the establishment of specific laboratories for the research into the molecular structure of biological systems. It took Perutz more than 20 years of hard work to solve the structure of haemoglobin.
Learning the difficult art of crystallography
When Crick joined Perutz's unit in 1949, its members included John Kendrew, who was working on the structure of myoglobin, a small protein acting as a temporary storehouse for the oxygen brought by the haemoglobin in the blood, and Hugh Huxley, who had turned from haemoglobin to myosin and actin, the motility proteins of the muscle fibre. All three were keen on solving protein structures using the technique of X-ray crystallography.
A polypeptide chain of perhaps hundreds of links can be arranged in space in an almost infinite number of ways. Chemical methods give only the order of the links; equally important is their arrangement in space, the way in which particular side chains form cross-links to bind the whole structure together into a nearly spherical object (as most proteins are known to be). How is it possible to discover the three-dimensional arrangement of a molecule as complicated as a protein? The key to the problem is that luckily many proteins can be persuaded to crystallise, and often their crystals are as regular and as nearly perfect in shape as the crystals of simpler compounds. But protein molecules contain at minimum thousands of atoms, and have enormously convoluted and irregular formations that are extremely difficult to elucidate. In the 1930s, when Bernal, Hodgkin and Perutz performed the earliest crystallographic studies of proteins at Cambridge’s Cavendish Laboratory, they soon realised that the intricacies of the three-dimensional structure of proteins were too complex for analysis by conventional methods. Each type of crystal has its own characteristic arrangement of atoms and so will produce its own specific X-ray pattern, the features of which can be unambiguously, if tediously, predicted by calculation if the structure of the crystal is known. X-ray analysis involves the reverse calculation: Given the X-ray pattern, what is the crystal structure that must have produced it? Each spot on a diffraction pattern provides information only about the intensity of the X-ray waves that have been deflected off the atoms in the crystal into the path of that spot. Ideally, researchers also needed to know the phase of the wave, i.e. which particular point in the undulating cycle of movement from one wave peak to the next these X-ray waves are at when they hit the film and form each spot in the diffraction pattern.
In the case of simple molecules containing only tens of atoms the diffraction pattern and wave intensity information was sufficient to identify a structure. Although it was a painstaking task, researchers in the early years of crystallography could almost always rely on the technique of Fourier synthesis, together with a healthy dose of trial-and-error and informed intuition, to hit upon the right structure of these compounds. However, larger molecules with thousands of atoms posed a much greater problem. In his work on haemoglobin, Perutz, too, faced fundamental problems in the interpretation of his X-ray pictures. The number of reflections and interactions that occur within crystals of this size is so complex that to translate X-ray patterns into molecular structures requires more than just knowledge about the intensity of waves, that could be directly measured on the diffraction pictures. Without knowledge of the phases researchers did not have any chance of deducing the structure. Researchers called the case of this missing information `the phase problem', and at the time solving this was the ultimate goal for anyone interested in trying to determine protein structures from X-ray crystallography. During the 1930s, Lindo Patterson had devised a variation of the Fourier synthesis, developing a method that partially avoided the phase problem. However, for several reasons many workers felt that it was not satisfactory as a direct means of solving protein structures, because it was not guaranteed to be successful with more complex compounds.
Several months after his arrival at the Cavendish, Crick had not yet succeeded in finding a suitable protein of his own to work out its structure, as both Bragg and Perutz expected. However, he had read Perutz's papers on haemoglobin, coming to the conclusion that most of the assumptions made in those papers were not substantiated by the facts. He had thoroughly studied the methods of interpretation of X-ray data being used by Bragg, Perutz and Kendrew in their attempts to find the structure of haemoglobin and other proteins and in July 1951, he was asked to give a seminar on prospects and methods of X-ray crystallography related to these attempts. The seminar turned into a critique of the unit's work on haemoglobin in which Crick tried to demonstrate that they were wasting their time using all those methods, including Patterson's synthesis and Bragg's optical method called the Fly's eye. Crick thought that all but the so-called “isomorphous replacement” method, which had yet to be attempted, were wrong.
The method had been proposed by Bernal more than a decade earlier: heavy atoms - preferably a metal – incorporated into crystals by chemical substitution without altering the structure of the substance to be studied, would act as strong scatterers of X rays. Taking X-ray diffraction pictures of crystals identical in shape and molecular structure, but differing in that in one case a heavy atom is present, while in another case it is absent (or replaced by a different heavy atom), the positions of the diffracted beams do not alter, and only the intensities vary. By comparing the original and modified patterns, it would now be possible to determine the location of specific atoms and hence gain important information about the structure of the crystal. The method had been used successfully to solve small organic structures. However, nobody thought that it would be possible to apply it to such huge molecules as proteins, because the X-ray scattering contribution by a metal atom was thought to be too weak to observe, and moreover, inserting sufficiently heavy atoms would distort the protein structure.
Crick had calculated that this approach had some prospects of success and had clearly stated his views during the seminar, criticising interpretations based on the standard techniques used by the team. Bragg came to regard him as a nuisance, while Perutz in later years regarded the event with admiration, even if at the time it certainly cannot have been a pleasant experience for him having the new research student telling his supervisor that his work was wrong. But Crick was quite right, and actually, images of complex molecules proved impossible to interpret until 1953, when Perutz actually succeeded in the difficult task of applying the strategy of isomorphous replacement to such a complex problem as a protein. He succeeded in crystallising haemoglobin including heavy mercury atoms, making it possible, in principle at least, to solve the complex X-ray pattern of a protein crystal and to produce a model of the structure of the molecule. Bragg was so thrilled that he went around telling everyone that Perutz had “discovered a goldmine.” This was a turning point in his work, and a great breakthrough in protein crystallography that opened the way to solving molecules as complex as proteins. In 1960, the publication of the 6-angstrom structure of haemoglobin by Perutz and his collaborators along with Kendrew's findings on the myoglobin molecule at 2-angstrom truly heralded a new era in biology. For being the first to successfully identify the structures of complex proteins, John Kendrew and Max Perutz were honoured with alacrity by the award of the 1962 Nobel Prize in Chemistry.
Ten years earlier, all this was still a challenge for future work, and Crick was still finding his way at the Cavendish. He lacked a definite problem for his dissertation but Braggs now asked him to work on the determination of the kind of diffraction pattern that a helical molecule should produce. British researchers thought that alpha keratin (a very common protein, the stuff of hair, fingernails, and animal horn) and other proteins might have a molecular structure like a spiral. In October 1950, Bragg, Kendrew, and Perutz had published a paper enumerating potential protein helices, proposing in particular a specific model for the alpha keratin. Soon after, between February and March 1951, the US chemist Linus Pauling had published a series of articles in the Proceedings of the National Academy of Sciences, some in collaboration with H. R. Branson and especially with Robert Corey, a skilled X-ray crystallographer, in which he deduced the two main structural features of proteins: the alpha-helix and beta-sheet, now known to form the backbones of tens of thousands of proteins. Actually, Pauling had already succeeded in building a helical mental model of the alpha helix already in 1948. He had conceived it while ill in bed, using a sheet of paper on which he had drawn a polypeptide chain that he folded to construct a helical model. But something was wrong: it was not possible to correlate the model with the X-ray diffraction pattern of alpha-keratin. The discrepancy could not be removed by minor adjustments and thus he refrained to publish his findings at the time. He did it only after three years, following the British scientists' article, because they were proposing potential helical structures for alpha keratin that were all unacceptable in Pauling's view. As he later clarified: “I knew that if they could come up with all of the wrong helices, they would soon come up with the one right one, so I felt the need to publish it.” Bragg was completely upset. He and his colleagues had missed this important achievement lacking the necessary chemical insight that provided the clue to the actual structure of the alpha helix. But the confirmation of the structure came from Max Perutz, who realised that Pauling and Corey's alpha helix was actually like a spiral staircase in which the amino acid residues formed the steps and the height of each step was 1.5 angstrom. It was a thrilling experience when he immediately observed in keratin the characteristic X-ray reflection of this regular repeating pattern, confirming the existence of this fundamental structure.
Helices had thus become a hot topic at Cavendish. In fall 1951, in conjunction with the crystallographers Vladimir Vand and William Cochran, Crick succeeded in formulating a theory predicting several major features of the diffraction pattern yielded by helical, long-chain molecules, packed tightly together and lined up in the direction of the fibre. Crick was also able for the first time to interpret the early diffraction pattern obtained by Astbury in terms of Pauling's helices.
By the end of 1951, Crick was thus beginning to give original and significant contributions with the derivation of the kind of diffraction pattern produced by a helical molecule and its application to protein fibre structures. He had also become a convert to the crystallographer's use of model building to help solve the structure of fibrous proteins and synthetic polypeptides, considering this approach as a form of experimental science. It was the great lesson that Pauling, by his example of the alpha helix, had taught Crick.
DNA and transmission of hereditary information
At that time, in the fall of 1951, James Watson arrived at the Molecular Biology Unit from the United States. He was only 23 years old, and already on a postdoctoral fellowship, whereas Crick was 35 and had yet to complete his doctorate. Watson knew no X-ray crystallography, whereas Crick was fully immersed in it. But the sight of an X-ray diffraction pattern of DNA shown by Maurice Wilkins at a meeting in Naples had fired Watson's enthusiasm to pursue its molecular structure, having become strongly convinced of its importance.
Crick well knew Wilkins, who worked since 1946 as assistant director of the Biophysics Research Unit at King's College in London. They had first met in 1947. Both formerly physicists, inspired by Schrödinger's book, they shared an eager desire to explore the molecular basis to the life of the cell. In May 1950, Wilkins had got samples of the finest quality DNA available, provided by the biochemist Rudolf Signer. At that time it was known that the nucleic acids occur in two forms: DNA and ribonucleic acid (RNA). In 1929, the biochemist Phoebus Levene, the discoverer of ribose and deoxyribose sugars, had identified the components that make up a DNA molecule: four nitrogen-containing bases (adenine, thymine, guanine, and cytosine), a 5-carbon sugar (deoxyribose), and phosphate. He showed that the components of DNA were linked in the order phosphate-sugar-base, a unit that he called a nucleotide. He suggested that the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, the latter forming the backbone of the molecule. In 1937, William Astbury at Leeds started pioneering X-ray diffraction studies of DNA fibres and found evidence of considerable regularity. In 1947, he had deduced that nucleotides must be stacked along the fibre axis 'like a pile of pennies' each 3.4-angstrom thick.
Working with his graduate student Raymond Gosling, Wilkins had succeeded in obtaining X-ray diffraction photographs containing dozens of well-defined spots on a clear background. This clearly showed for the first time that DNA was truly crystalline, and that one “could be very hopeful that its structure could be derived from X-ray patterns...” In his Nobel lecture Wilkins immediately remarked that, on the other hand, they knew that, “genes had to be complicated and therefore DNA had to be complicated.”
However, when Wilkins set up to work with DNA, there were already many different indications of simplicity and regularity in its structure. In the electron microscope, DNA was seen as a uniform unbranched thread with a diameter of about 20 angstrom. But nobody had the slightest idea of what the molecule might look like. Working closely with Gosling, Wilkins obtained detailed X-ray diffraction data of unprecedented quality of what we now know as the A-form of DNA, presenting 100 diffraction spots. As soon as good diffraction patterns were obtained, the mathematician Alex Stokes at King's worked out the theory of diffraction by a helix. Inspired by Pauling's publication on the alpha helix, Wilkins searched in his experimental data for evidence that DNA was also helical.
Wilkins presented his photographs in Naples, Italy, in May 1951, at a Conference on large molecules attended by James Watson. Watson was a younger member of the “phage group,” led in the USA by Max Delbrück and Salvador Luria. This group, mostly geneticists, studied bacterial viruses, bacteriophages. In October 1950, Watson had moved to Copenhagen, where he had been sent to learn nucleic acid chemistry. Analysis showed that chromosomes contained both nucleic acid and proteins. Proteins were well known as versatile molecules that played many exciting roles, but virtually nothing was known about the functional roles of the nucleic acids, so they were seen as materials with a purely structural function while most scientists thought that genes were proteins. In the mid- 1940s, Oswald Avery, Colin MacLeod, and Maclyn McCarthy had presented evidence that DNA might be the hereditary material, but that conclusion was not widely accepted. Watson was already convinced that DNA was the genetic material, and he was so inspired by Wilkins' slides showing clear evidence that DNA had a simple, regularly repeating structure, that he converted to a structural approach and decided to move to Cambridge to learn X-ray diffraction techniques under Lawrence Bragg.
A central, unsolved problem was how genetic information is transmitted from an organism to its offspring. There was little awareness in the community at large that the problem could be attacked at the molecular level. Watson and Crick immediately discovered their common interests and began to discuss these exciting topics. During his investigations on the molecular structure of proteins, Crick had also been very struck by the work of the Cambridge biochemist Frederick Sanger, also working at the Medical Research Council since 1951. Sanger was revealing the sequences of amino acids in the polypeptide chains of the protein insulin, a work for which he would be awarded the Nobel Prize in Chemistry 1958. Amino acids are the building blocks of proteins and Crick believed that such chains, following no obvious pattern, had to be laid down on templates, and some form of these templates were then stored and transmitted to the next generation. He thought that there would be a different template for each kind of protein, which, in hereditary transmission, would determine the one-dimensional sequence of amino acids in the polypeptide chain of that specific protein. These one-dimensional chains then fold to form the three-dimensional molecules in globular proteins such as haemoglobin and the numerous enzymes. The problem was how the hereditary information could be expressed. DNA was clearly implicated in the hereditary process, but how? Crick had already developed an interest in the genetic code for the amino acid sequence of protein molecules, but it was the arrival of the young American biologist James Watson that really focused his attention on DNA.
Learning from a complete failure
Crick was deeply impressed by Pauling's achievement with the alpha helix. The powerful method of combining the process of creative and conjectural model building with attention to correct empirical data on the directions, degrees of freedom, and lengths of the bonds between atoms in the molecule, might lead to the creation of molecular structures, narrowing down the different possibilities coming from raw data. Crick thought that Watson and he might well be able to repeat with DNA Pauling's triumph with the alpha helix. Within a few days after Watson's arrival, they knew what to do: imitate Linus Pauling and beat him at his own game.
While Wilkins and his group were beginning to work at DNA on a broader front, Rosalind Franklin, a physical chemist who had studied the structure of carbons using X-ray diffraction methods, had arrived at King's College, becoming Wilkins's colleague. During the first year of work, Franklin made important improvements in the preparation of the DNA fibres and assembled a new X-ray tube with Gosling, which provided sharper diffraction patterns. Franklin also made a crucial advance showing that, depending on the water content of the fibre specimen, two forms of the DNA molecule actually existed, which she later labelled A and B, also defining the conditions for the transition between them. The two forms of DNA give radically different diffraction patterns, corresponding to slight structural differences mainly related to the arrangement of their ribose units, as it was later clarified. It became clear that all previous workers had been working mostly with a mixture of the two forms, thus explaining for the first time the difficulties of earlier attempts to decipher such mixtures in terms of a single phase. In Franklin's draft notes prepared for a colloquium on her work she gave in November 1951 at King's College, it is clearly stated that both DNA forms are likely to be a helical bundle of two or three chains. She realised, too, that any correct model must have the phosphate groups on the outside, accessible to water. The group of chains would be linked together by hydrogen bonds between the bases that would be in the centre of the molecule.
Watson attended Franklin's seminar of November 1951, after which Crick cross-examined him about what he had gleaned from the talk. But Watson was in trouble. He had not taken any notes and he did not know enough of the crystallographic jargon; some of the reported data later proved to be faulty. Still, they decided they had enough elements to start in earnest their collaboration and test their ideas on the possible structure of DNA. Inspired by Pauling's technique, they began to build a three-dimensional stick-and-ball model, also based on what Watson remembered about the structural information from Franklin's seminar. After a week, in late November 1951, Wilkins, Franklin and other colleagues were invited to Cambridge to see a tentative three-chain model of the DNA molecule, with the phosphates on the inside and with the bases pointing outwards, which Watson and Crick had just built. Franklin immediately expressed complete disagreement, saying that their model did not fit the diffraction data or rules of chemistry and insisted that the sugar-phosphate backbones must be on the outside, not the inside. After this failure, Lawrence Bragg, the head of the Cavendish Laboratory, firmly vetoed any further work on DNA. Crick returned to his protein studies for his thesis and Watson turned to the other form of nucleic acid -- RNA -- in the tobacco mosaic virus.
Towards the three-dimensional structure of DNA molecule
The tetranucleotide hypothesis stated that DNA was a simple molecule composed of a boring repetition of each of its four constituent “bases” attached to a sugar-phosphate backbone. In 1947, Erwin Chargaff had discovered an important regularity related to the bases: although their sequence along the polynucleotide chains was complex and the base composition of different DNA's varied considerably from species to species, for particular pairs of nucleotides - adenine and thymine, guanine and cytosine - the two nucleotides are always present in equal proportions. A real breakthrough in Crick and Watson's enterprise occurred in 1952, when Chargaff visited Cambridge and inspired Crick with a description of his experiments, that Crick had not been aware of. Shortly before meeting Chargaff, Crick had asked the young Cambridge mathematician, John Griffith, to calculate what attractive forces there would be between the bases and he had found out that adenine attracted thymine, and guanine attracted cytosine. Crick was quite satisfied, because this meant that this preferential attraction between the two couples would give rise to complementary replication. The idea of a complementary scheme, which had been circulating in the circle of theoretical geneticists intrigued by gene duplication, was now taking a more concrete sense. Thus, in June 1952, Crick had become aware of a fundamental piece of the jigsaw: base pairing could be the cause of the Chargaff rules -- the one-to-one ratio-- and, on the basis of Griffith's calculations, of the possibility that a structural relationship between pairs of bases might explain them.
At that time, the American microbiologists Alfred Hershey and Martha Chase had completed their decisive set of experiments showing that when a phage infects its bacterial host, only its DNA enters the cell. The phage protein remained outside devoid of any further function in the reproductive process. It provided evidence that the DNA component of the phage, not the protein, carries the genetic information. During their work, completed in early 1952, they established that DNA directs the biosynthesis of enzymes and thus controls the biochemical processes of the cell. From these results it could be inferred that nucleic acids had important biological roles that were connected with protein synthesis. Hershey continued to work along this line and in 1969 he would share the Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria “for their discoveries concerning the replication mechanism and the genetic structure of viruses.”
According to Watson's memoir, The Double Helix, learning about the Hershey-Chase experiment in the spring of 1952 drove them to intensify their efforts to work out the structure of DNA. However, Pauling, too, was becoming eager to solve the three-dimensional structure of DNA, as he had already done with other biological molecules. In the summer of 1952, as soon as he learned about the Hershey-Chase experiments, suggesting that hereditary continuity is carried by the nucleic acid and not the protein, he immediately switched his attention to DNA, feeling that it should be possible to decipher the structure of this substance by building models along similar lines to those of his protein work. During the last week of January 1953, Crick and Watson saw a draft of his already submitted paper through Pauling's son, Peter, who was at the time working with them as a graduate student in the Cavendish Laboratory. The paper, written with Corey, described the DNA molecule as a triple-stranded helix, very reminiscent of the three-stranded model Watson and Crick had abandoned fourteen months before. The three strands were twisted around each other in rope-like fashion, with the phosphoric acid groups triangularly arranged in the centre and with the various bases pointed outward. The analysis was unfortunately based on rather poor photographs published many years before by Astbury and on equally poor photographs made in Pauling and Corey’s laboratory.
Being familiar with every subtlety of the subject, Watson immediately spotted a chemical nonsense in the three-chain structure as proposed by Pauling: the hydrogen bonds that held Pauling's triple helix together at the core were not ionised, as they should have been, since DNA is an acid. Virologists and organic chemists in Cambridge reassured Watson that DNA is indeed an acid, so of course the phosphate groups must be ionised. Moreover, the structure ignored Chargaff's rules and in general it gave no clue to its own reduplication. Crick and Watson felt that thanks to Pauling's failure, they still had a chance to interpret the structure first. Crick immediately explained to Perutz and Kendrew that “no further time must be lost on this side of the Atlantic.” When his mistake would become known, Pauling would not stop until he had captured the right structure. They now felt in open competition with the world-famous chemist Linus Pauling. As soon as the paper would be spread around the world, then it would be only a matter of days before the error would be discovered. They had anywhere up to six weeks before Pauling was in full-time pursuit of DNA.
The next year Pauling would be awarded the Nobel Prize in Chemistry 1954 “for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances.” In 1962, for his opposition to weapons of mass destruction, he was awarded the Nobel Peace Prize, becoming the only person ever to receive two undivided Nobel Prizes.
The double helix
In the meantime, at King's College in London Wilkins had continued his experiments working on sepia sperm and getting much clearer patterns than the previous year. They very clearly offered strong evidence for a helical structure for DNA. Franklin, too, had continued to work on her step-by-step detailed analysis of her diffraction data. At the end of January 1953 Watson visited Kings' College and Wilkins showed him what became famous as the picture B51, an X-ray picture of the B form of DNA, that Franklin had obtained in May 1952. In his book The Double Helix Watson himself recalled that it was a memorable moment: “The instant I saw the picture my mouth fell open and my pulse began to race.” The pattern was so much simpler than that of the A-form. The black cross of reflections that dominated the picture could arise only from a helical structure, also giving several of the vital helical parameters. The characteristic feature of the B-form is the X-shaped pattern of streaks arranged in a set of layer lines, from which it could be directly deduced that DNA in this form is a helix with an axial repeat of 34 angstrom and an axial spacing between nucleotides of 3.4 angstrom. Franklin, however, had chosen to concentrate on the difficult analysis of the more complicated A-form, actually offering the possibility of an objective -- even if more difficult -- crystallographic analysis, because of the greater wealth and precision of the diffraction data available. If correctly interpreted, the A pattern would yield more precise information about the DNA molecule. She was actually able to determine the dimensions and symmetry of the unit cell and embarked on the calculation of the Patterson maps in an attempt at solving the structure. Watson did not engender any excitement in Wilkins and that same evening, as he rode back to Cambridge, he sketched what he could remember of the B form's diffraction pattern. By the time he was back he had decided that the range of possible densities in the molecule did not, after all, rule two chains out.
The next day Watson reported to Perutz and Bragg what he had learned during his visit in London and said that he was going to ask a Cavendish machinist to make models of the purines and pyrimidines. Bragg made no objection and encouraged him to get on with the job of building models.
Unleashed from Bragg's earlier ban, Watson began to build tentative models as soon as he received the first group of atoms. Every evening, after he got back to his rooms, Watson tried to puzzle out the mystery of the bases: “My aim was somehow to arrange the centrally located bases in such a way that the backbones on the outside were completely regular - that is, giving the sugar-phosphate groups of each nucleotide identical three-dimensional configurations. But each time I tried to come up with a solution I ran into the obstacle that the four bases each had a quite different shape. Moreover, there were many reasons to believe that the sequences of the bases of a given polynucleotide chain were very irregular. Thus, unless some very special trick existed, randomly twisting two polynucleotide chains around one another should result in a mess... There was also the vexing problem of how the intertwined chains might be held together by hydrogen bonds between the bases.” In the meantime they learned from Wilkins that he, too, was going to work on model building, so they asked him whether he would mind if they started to play about with DNA models. Wilkins' answer was no, so they went ahead without any embarassment.
A most lucky coincidence opened the way to the final stage of the race. In the second week of February, Perutz, a member of the Medical Research Council Committee, and Crick's PhD supervisor, showed him a progress report from King's College that included Franklin's report to that Committee of December 1952. Wilkins himself had mentioned the existence of such report, which was not meant to be confidential, but should help in establishing contact between the different groups of people working for the Council in the same field. It confirmed much of what they already knew, but a highly significant piece of information that Crick derived from the report was related to the fact that Franklin had identified the A-form as having a face-centred monoclinic cell with the space symmetry group C2. Up to that moment everyone had assumed that the component chains lay parallel, all pointing in the same direction. A lucky coincidence led Crick to grasp such crucial piece of information: C2 was the same space group as that of oxyhaemoglobin, the material he was supposed to be studying. This meant that the motif units occur in pairs (related by the two-fold axes) with one pointing upwards and the other downwards, a crucial factor whose significance did not escape Crick's attention. As the A- and B-forms can be interconverted simply by changing the level of hydration, Crick argued that they must be very similar in structure. The dimensions of the unit cell proved that the axis of symmetry between paired elements of the structures was perpendicular to the long axis of the cell, suggesting two chains running in opposite directions, definitely excluding a three-chain structure. If the A-form had an anti-parallel double-stranded structure, the B-form must be the same and the chains had to be twisted at twice the rate, so that the repeat distance was a complete 360° turn for each helix, while the repeat distance for a parallel double-chain structure is 180° turn for each strand. Discovery of the characteristic symmetry of the molecule settled immediately the spatial arrangement of the backbones as they spiralled up the outside around the core. Watson was now convinced about Franklin's claim made more than 1 year earlier that the sugar-phosphate chains must be on the outside of the molecule. But how were the bases to be fitted inside the two helices? And how were Chargaff's ratios to be accounted for in the model?
During the last week of February, Jerry Donohue, an excellent structural chemist, sharing an office with Watson and Crick, set them on the right track about the correct position of hydrogen atoms, essential for cross-bonding. This allowed Watson to fit in adenine-thymine as a pair, and also guanine-cytosine as a pair. All this information had a key role in Watson's final insight: the adenine-thymine bond was exactly as long as the cytosine-guanine bond: an adenine-thymine pair held together by two hydrogen bonds was identical in shape to a guanine-cytosine pair held together by at least two hydrogen bonds. The hydrogen bonds formed naturally, making the two types of base pairs identical in shape. Chargaff's ratios now automatically arose as a consequence of Watson's base-pairing scheme, standing out as a consequence of a double-helical structure for DNA. Always pairing adenine with thymine and guanine with cytosine meant that the base sequences of the two intertwined chains were complementary to each other. Given the base sequence of one chain, that of its partner was automatically determined. The structure of DNA was solved. Crick was so excited that he told everyone that they had found the secret of life.
When the metal plates of the bases were ready, Watson made a model in which for the first time all the DNA components were present. He arranged the atoms in positions that satisfied both the X-ray data and the laws of stereochemistry. Because of the helical symmetry, the locations of the atoms in one nucleotide would automatically generate the other positions. The resulting helix was right-handed with the two chains running in opposite directions.
Their three-dimensional metal skeleton of atoms assembling a double helix was finished in early March. Bragg was delighted and immediately caught on to the complementary relation between the two chains and saw how an equivalence of adenine with thymine and guanine with cytosine was a logical consequence of the regular repeating shape of the sugar-phosphate backbone. Wilkins was the first person outside of Cambridge to see it. According to the model, the double helix of the DNA molecule consists of two antiparallel strands of deoxyribose phosphate (alternating units of sugar and phosphate) on the outside, joined by complementary pairs of bases within the helix. As stated by Chargaff, adenine is paired with thymine, guanine with cytosine, and the bases to one another by hydrogen bonds. When Franklin saw the model, she had enough elements to understand that it was a reasonable model, because in the meantime she had arrived very near to the solution, as can be inferred from her notebooks and from two papers sent to press before Franklin knew of the Watson-Crick model. After a whole afternoon of difficult conversation about how much the King’s work had helped Watson and Crick, they agreed that Wilkins and his colleagues would publish their data jointly with Watson and Crick’s announcement in three papers with continuous pagination in Nature. The three papers appeared on 25 April 1953, grouped together under the title “Molecular Structure of Nucleic Acids”. In their paper Crick and Watson acknowledged that they had been ”stimulated by ... the unpublished results and ideas” of Wilkins and Franklin. Gosling and Franklin added a short note describing the helical evidence in the B photographs, and Wilkins co-authored his paper with Stokes and H. R. Wilson. They made a preliminary description of the experimental evidence for the polynucleotide chain configuration being helical and existing in this form when in the natural state. During the following 10 years Wilkins led a team that performed a range of meticulous experiments to establish the helical model more rigorously.
After Watson and Crick had seen the two King's College papers showing how strongly X-ray evidence supported their structure, they wrote a second article published on May 30 containing the first clear statement on DNA carrying the genetic code: “... it therefore seems likely that the precise sequence of bases is the code which carries the genetic information”. It also offered a mechanism of replication: “If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself.” The complementarity of the two strands in the structure provided a mechanism for inheritance, in that each single strand could act as a template for assembling its complement - leading to two identical duplex molecules. The information is in the sequence of the bases.
In 1962, Crick, Watson and Wilkins were jointly awarded the Nobel Prize in Physiology or Medicine for “discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.”
Unfortunately, by that time Rosalind Franklin had died. After moving to Bernal's laboratory at Birkbeck College, Rosalind Franklin did very fruitful work on the tobacco mosaic virus and also began work on the poliovirus. In the summer of 1956, she became ill with cancer and died less than two years later at the age of 37.
The discovery of the DNA three-dimensional structure gradually emerged as an event with great heuristic consequences and the steady accumulation of new evidence for the double helix made it apparent that this was a transforming milestone in the evolution of biological science. During the following years Crick turned his attention to deciphering the genetic code and had an important role in establishing the detailed nature of the mechanism of transfer of information, written in the sequence of the bases in DNA, to specify the amino acid sequence and, hence, the structure and function of the proteins which carry out their cellular roles. The elucidation of the genetic code launched the new subjects of molecular genetics and, combined with biochemistry, the molecular biology of the gene. The following decades saw what has been called the genetic revolution in biotechnology, after the development of many powerful methods tools for handling and manipulating DNA that led to great advances in understanding the regulation of gene expression and eventually to the human genome project and to the comparative genomes of other organisms. The image of the DNA double helix itself has turned into one of the icons of the 20th century.
Judson H. F. (1979) The Eighth Day of Creation: Makers of the Revolution in Biology (1979). Touchstone Books
Klug A. (2004) The Discovery of the DNA Double Helix. Journal of Molecular Biology 335: 3-26
Klug A. (1968) Rosalind Franklin and the Discovery of the Structure of DNA. Nature 219: 808-810, 843-844
Lydon J. E. (2003) The DNA double helix -- the untold story. Liquid Crystals Today 12 (2): 1-9
Olby R. (1970) Francis Crick, DNA, and the Central Dogma. Daedalus 99 (4): 938-987
Olby R. (1974) The Path to the Double Helix. The discovery of DNA. Macmillan 1974
Olby R. (2009) Francis Crick. Hunter of Life's Secrets. Cold Spring Harbor Laboratory Press
Stent G. S. (2006) Francis Crick, 8 June 1916 •28 July 2004. Proceedings of the American Philosophical Society 150(3): 467-474
Wasson, T. (ed.) (1987) Crick, Francis. In Nobel Prize Winners, H. W. Wilson Company, New York, pp. 230-232
Specify width: px
Specify width: px