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The geniuses and the gene

Why the discovery of the DNA model some fifty years ago by Watson and Crick still awakens so much enthusiasm

Rogério Meneghini

Some fifty years ago the magazineNature published, on the 25th of April 1953, an article by James Watson and Francis Crick in which the 3-dimensional structure of the salt of the acid, deoxyribonucleic acid (DNA) was presented. There were some two pages in which the first sentence began:We wish to suggest a structure for the salt of deoxyribo nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest .” And was wrapped up with:It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for genetic material .

This final sentence has since been considered as being one of the most falsely modest ever in scientific literature. Much has been written about the story of this discovery in books and articles. They include such material as the “best-seller” in 1968 by Watson,The Double Helix , to more recent books, among them two by Brazilian authors,História da Biologia Molecular [History of Molecular Biology] by de Rudolf Hausmann, who is currently at the University of Freiburg, in Germany, andWatson e Crick, a História da Descoberta do DNA , [Watson and Crick, the Story of the Discovery of DNA] by Ricardo Ferreira (Federal University of Pernambuco).

Probably the most famous areThe Double Helix andThe Eighth Day of Creation by Horace F. Judson (Simon and Schuster, New York, 1979). The first is a version by Watson himself of the story of the discovery where, perhaps for wishing to turn the book into a “best-seller”, or through his very personality, produced a manuscript that highlighted the most novelistic episodes of what had occurred. He relates how he managed to obtain, in a rather dark manner, information about crystallography data of his rivals (Rosalind Franklin, chemist and crystallographer at King?s College in London), scoffs upon by another rival (Erwin Chargaff, biochemist at Colombia University in New York), who had caustically criticized the ignorance of Crick and Watson with respect to the chemical structure of the bases of DNA, Adenine (A) Thymine (T), Guanine (G) e Cytosine (C).

At the end of the 40s, Chargaff had discovered that in all of the samples of DNA analyzed the amount of A had equaled that of T, while the amount of G had equaled that of C and had attempted to demoralize Linus Pauling (a chemistry icon of the 20th century, at that time a professor at the California Institute of Technology, who published in 1952 a suggestion for the structure of DNA, where three helixes had intertwined and where the phosphate appeared protonated and without charge; in physiological pH conditions, close to 7.0, the phosphate should have a negative charge, as students of a basic course in chemistry learn). Watson was twenty-three years old when he revealed the double helix formation.

Since that moment his caustic personality has been inclined to attribute to him the role of the villain during the discovery. Entertaining, but not trustworthy in historical terms, the bookThe Double Helix was written at a time in which Watson had already given a scientific career and had turned himself into an administrator of science (most certainly the attributes demonstrated in his book qualify him for this position). At the end of the 80s, Watson led the start of the project on the sequencing of the human genome. Without a doubt, anyone who wants to form his or her own opinion on the discovery (and many may well have been invented, as readers can note in other books) must fall back on the bookThe Eighth Day of Creation by Judson. It is a majestic and extensive work (650 pages) of scientific journalism.

Judson took seven years interviewing the personalities involved in the unfolding of the happenings, frequently contrasting statements and analyzing documents, scientific papers and laboratory notebooks. He attained the climax of his professional activity when, as well as relating and interpreting the various facets of the collected information, he even explains, with his own terms and understanding, the technical part that surrounded the discovery. Why one more article? I believe thatPesquisa FAPESP cannot refrain from remembering this happening in the same manner, as have so many other scientific magazines.

As to me personally accepting the invitation to write something on this question, it was because I developed a lot of enthusiasm in 1963 when I learned know about this discovery and I have followed a lot of what has been written afterwards about it. After all, since 1965, I had worked in one of the few laboratories that dealt with molecular biology in Brazil in that era, that of Professor Francisco J. S. Lara. Why has this discovery awakened such effusive enthusiasm and enchantment over the years that followed? Watson stated in his bookThe Double Helix that it was the greatest discovery in biology since the theory of evolution by Darwin.

On the other hand, Crick, on the day of the conception of the structure, went into a pub on the campus of the University of Cambridge, shouting: “We have discovered the secret of life!” Apart from these testimonies of the authors, the fact is that many other scientists came to partake of these affirmations throughout the years. What led to the uniqueness of this discovery? It was probably an amalgam of factors, with a very small probability of it happening again, that mixed science with adventure and art. In the first place the structure has beautiful symmetry, perceptible even to the layman. In the second place, there is chemical logic in the structural arrangement that immediately allows a biological interpretation for the molecule of DNA.

Although the term molecular biology had been coined in 1935 by Warren Weaver, of the Rockefeller Foundation, in order to describe how biological phenomena might be fundamentally understood by the knowledge of the structures of the molecules and of the interactions and alterations of them, only in 1953 was it to be perceived in a dramatic manner in this correlation structure-function, with the discovery of the double helix.

This discovery is, nevertheless, a marker between the past and the future, with respect to molecular biology, today better known as structural molecular biology.In the third instance it was a fight of two young Davids against a powerful Goliath. Actually, Linus Pauling together with Robert Corey had already discovered the helicoidal structure of proteins (a -helix), and had information that the structure of DNA could also be helicoidal. Pauling was an eminent personality in physics-chemistry, then more than fifty years of age, and was going to win the Nobel Prize in 1954 for his work on the structure of proteins. Watson and Crick knew of Pauling?s intention and obviously were fearful about being overtaken in the race.

The figure of the structure of DNA is so ubiquitous that there is no way of demonstrating it here. In summary, the structure possesses two helixes that intertwine, forming a double helix. Imagining an axis that runs up the center of the double helix, the framework of each helix is formed by the sequence of a sugar molecule (deoxyribose) linked to a phosphate, this unit links itself to another identical innumerable times, in a manner parallel to the axis of the double helix. The result would be sugar-phosphate-sugar-phosphate and so on. A simple helix has polarity, that is to say, ranging over it from one side is different when ranging over it from another.

A fundamental piece of data is that the two helixes of DNA intertwine with opposing polarities (called anti-parallel helixes). The double helix twists itself clockwise. Attempt to twist into a spiral a role of paper around a pencil. It is possible to carry it out in two manners, clockwise or anti-clockwise. The two helixes thus obtained are asymmetrical and different, one being the mirror image of the other. It is the same thing as a hand reflected in a mirror. The right hand is the image of the left and vice-versa. The bases A, T, C, and G correspond to the third component of DNA. They have a planar structure and are also linked to the sugar molecule.

However, their planes are situated orthogonally to the axis of the helix. They have a specific pairing on the same plane: A on one helix counterbalances the T on another or G on one helix to the C of another. For this reason the helixes are said to be complementary. It can be noted, because of this specific pairing, in a certain DNA that the quantity of G bases is equal to C and the quantity of A is equal to that of T, as Chargaff had discovered. The chemical bonding that links the bases is called hydrogen bonding. These forces are important elements in the stabilization of the double helix.

Between A and T there are two hydrogen bonds, while between G and C there are three. If we look at these pairs of bases from above, we will see that they have dimensions and forms that are almost identical, in such a way as to allow that the diameter of the double helix remains constant along its axis. If we look at the profile of the double helix, we will see that the pairs of bases are stacked up, in an orthogonal fashion to the axis, and with a partial covering because to the turning of the helix.

It would be like taking dominos (representing the pairs of bases) and stacking them up, placing one on top of the other at a displaced angle of 36 degrees each time, one to the other, from bottom to top. If we were to pile up ten dominos we would complete 360 degrees, that is, one revolution of the double helix corresponds to ten pairs of bases. The distance between the pairs of bases is 3.4 Angstroms (one Angstrom is equivalent to 1×10-9 meters), in such a way that the step of the double helix (the distance along the axis corresponding to a complete revolution) is 34 Angstroms.

Most certainly the fundamental elements of this structure provide a complete explanation on the two most important properties of the gene: the codifying of the proteins, given by the sequence of bases, and the duplicating of the gene itself: the complementary strands A and B separate and are copied, A giving a new B strand, and B a new A strand. Two new double helixes AB and BA, identical to the mother helix AB, will be formed. The meeting of Watson with Crick is what one might call a fortuitous complement, molding the discovery that they were to make.

Both had read the book written by the famous physicist Erwin SchrödingerWhat Is life?, published in 1944. Many biologists, physicists and chemists became magnetized by the speculations made by Schrödinger with respect to the chemical nature of the gene, up until that point unknown. Schrödinger had called the genetic material solid aperiodic. Aperiodic because it could not be repetitive like a crystal of sodium chloride, otherwise, how could it code so many distinct characteristics of an organism? Solid, because the gene could not have the properties of a common organic substance, that is, to suffer chemical changes at a level relatively high to environmental temperature, incompatible with the stability of the gene.

Schrödinger had argued this stability as the example of the members of the Habsburg dynasty of Austria, whose portraits ? which went back two centuries – frequently showed malformation of the lip. Without a doubt the stability of the genetic material is today explained by the process of the repairing of the DNA and certainly Schrödinger had used the term solid as a metaphor. Furthermore, genetic material must have properties that would allow for its reproduction. In 1946, Oswald Avery and his collaborators demonstrated that DNA constituted part of genetic material. Therefore, it is not surprising that various scientists had become interested in DNA at the start of the 50s: virologists, physicists, chemists and structural biologists.

Nor is it surprising that Watson, finding himself at a congress in Naples with Maurice Wilkins, had become excited about the interest of the scientists of King?s College, to which group Wilkins pertained, and of the Cavendish Laboratory in Cambridge, in structural studies on DNA. He managed to leave Copenhagen, where he had been carrying out his post-doctorate, and with the help of Salvador Luria, a renowned geneticist at that time at the University of Illinois and supervisor for Watson?s doctorate degree, he eventually shifted to Cambridge, and the Cavendish Laboratory.

There, in October of 1951, he met Crick who – thirty-five-year at that time – had been working on the structure of hemoglobin as the material of his doctorate thesis (in his youth he had worked during the Second World War as a physicist on radar, hence the lateness of his studies.) Thus the great adventure began. Both of them had the same interests. They were young and unknown. They knew very little about the chemical structure of DNA. Some x-ray diffraction results from King?s college, obtained through Maurice Wilkins and worked out by Rosalind Franklin, suggested a helicoidal structure.

They spoke with great enthusiasm and were bright. They managed to win over Max Perutz ? who would later work out the 3-dimensional structure of hemoglobin ?, who for his part convinced Sir Lawrence Bragg (the head of the Cavendish Laboratory) to allow them to work with DNA. Crick insisted that they should not worry themselves about the details but about the structure in itself, basing their work on x-ray diffraction data and using a mixture of intuition and deduction, and making use only of the models of atoms introduced by Pauling in order to define protein structures.

Looking at it from another angle, what else could they have done? They didn?t know how to prepare samples of DNA, they had never worked an x-ray machine in order to obtain photos of x-ray diffraction, and had only partial chemical knowledge on genetic material. Crick held an advantage, which was knowing how to interpret data from x-ray diffraction.

The knowledge of the genetics of viruses and bacteria from Watson, brought to the laboratory by Luria, helped little on this scenario. How was it possible then that in a year and a half they would be publishing their work in the magazineNature with the definition of the double helix structure? There are various relevant episodes: in November of 1951, Watson appeared at a seminar given by Rosalind Franklin at King?s College, who talked about her x-ray diffraction photographs on fibers of DNA.

Various important data was presented: the crystalline unit cell (the unit cell that repeats and which provides the diffraction standard pattern) indicated a large helix containing two, three or four chains, various water molecules (around eight) and in which the phosphates were in the interface of the helix and of an aqueous solvent (that is to say, on the external side of the helix, contrary to what the structure proposed by Pauling had demonstrated). Various reports indicate that Watson managed to understand only a little of what Rosalind had been discussing, and worse still, took note of nothing.

When questioned by Crick, he provided, from memory, wrong data: mainly erring as to the numbers of the high content of water per crystalline unit. The first model was constructed based on these erroneous premises, and shortly afterwards discredited by various colleagues at Cavendish and King?s College. Crick later argued that this abortive attempt was not only Watson?s fault for having provided erroneous data, but his own as well because he didn?t know enough chemistry to perceive that the charges on the phosphate would imply a high water content, not taken into account in the model.

Through the interference of John Randall (King?s College) with Bragg, Watson and Crick had to stop their work on DNA, leaving this to other people at King?s College. Crick returned to his thesis on hemoglobin and Watson dealt with growing crystals of protein for Randall. What else could they do, without work and in a transitory situation, to move on from where they were? Nevertheless, fifteen months later Watson and Crick would publish the correct structure of DNA. The turning point for their comeback came from the publication of a possible DNA structure by Pauling, also chemically inconsistent.

However, the simple publication worked up enthusiasm at Cavendish. Unhappy for having lost the race to Pauling concerning the discovery of the alpha-helix and beta-leaf folded structures of proteins, they could not lose out yet again with that of DNA. Bragg reactivated Watson and Crick. During this interval both Watson and Crick had concerned themselves in establishing more solid theoretical bases for their pretensions. Crick, together with William Cochran and Vladimir Vand, published a theoretical article on the interpretation of x-ray diffractions in helicoidal structures.

For his part, Watson attempted to better understand the structures of the bases making up DNA. The great leap forward came about when the report from the King?s College team to the Medical Research Council (that gave financial support to the crystallography group) passed through the hands of Perutz, who gave it to Crick. The recent data from Rosalind Franklin about measurements on the x-ray diffraction of DNA had been meticulously described.

It revealed data picked up from the watchful eye of an observer such as Crick that had gone unnoticed by Rosalind Franklin. Years later, André Lwoff (Pasteur Institute) and Erwin Chargaff had separately published articles questioning if Perutz had been ethical in making the report available to King?s College and Crick. Horace Judson, in the bookThe Eighth Day of Creation , relates that he examined in detail the laboratory notebooks of R. Franklin. According to him, Franklin had not given importance to her data. She had discovered that under more humid conditions the DNA shifted from form A to B, which clearly demonstrated its helicoidal nature. Nevertheless, she turned her back on structure B and concerned herself with structure A, questioning if this corresponded to a helicoidal structure.

Structure B, as well as being more revealing in terms of definition by way of x-ray diffraction, should be closer to the physiological structure in an aqueous environment. Crick clearly perceived the geometrical parameters of the crystal unit cell using Franklin?s data (after all, she was developing her doctorate thesis in which the structure of proteins unequivocally dispensed with these parameters) that permitted him to conclude that there were two helixes, running in an anti-parallel manner, and in which the bases were unquestionably in the inside of the double helix.

Nevertheless, what was missing was to understand how the bases of one and the other chain interacted, maintaining a more rigid structure. Crick had not accepted hydrogen bonding, so popular at that moment due to the discoveries of Pauling of their importance in the structure of proteins. Based on data in books, texts, Crick admitted that the bases had an enolic and not “keto” structure (a carbon chain with an “OH” group linked to a carbon to which another carbon is joined by a carbon by a double bond, could be in tautomeric equilibrium with a structure in which this same carbon is linked by a double bond to an oxygen atom).

At this point a further personality appears, Jerry Donohue, coming from the Pauling group, and who understood chemistry better than any investigator at Cavendish, and who at that moment in time was working at that institution. He immediately perceived that Crick was being directed by an erroneous argument, since the stable structures of the bases must be in the “keto” form, which would allow the formation of hydrogen bonding.

The structure of DNA then practically turned itself into a mounted structure in the heads of Crick and Watson. Nevertheless, the final piece of the puzzle appears to have been held by Watson. Not managing to wait for the building of atomic models by the Cavendish workshop, that would be used to construct the DNA structure compatible with all of this information, he began working with paper models that he himself built. It is almost unbelievable that Watson should have had the final word.

Not having contributed neither before nor after to any major scientific revelations and for having been attached to non-elucidating conjectures during the episode of the discovery (such as, for example, that bridges between ions of magnesium and phosphate stabilize the structure of DNA), had a revelation that would bring him to the final point of the discovery. With his home-made models, he could perceive that the pairing of guanine and cytosine and that of thymine and adenine had comparable geometrical configurations and that two hydrogen bonds in these pairs would be, according to the teaching of Donohue, responsible for the stability of the double helix.

In this manner the diameter of the double helix would remain constant along its axis. No other type of pairing would permit this. As well, and of equal importance, these two pairings gave justice to the data given by Chargaff, that, in truth, were taken more seriously by Watson than by other personalities involved. It is curious that both Watson and Crick knew of Chargaff?s rule and had inferred that it was important in complement replication. However, as Crick himself noted: “The paradox was that, when we had all of the pieces of the structural puzzle ready, we had not used Chargaff?s rule. We were pushed to it”. The structure of DNA using the models from the Cavendish workshop was finalized a few days later.

Nevertheless, on the day following the vision by Watson, the 28th of February 1953, he and Crick knew, though only in their heads, all of the structure: it had emerged from the shadow of billions of years, absolute and simple, and was seen and understood for the first time, according to the report by Judson. The fascination of the discovery is perceived by some other facts: Watson and Crick, though young and without any academic position, managed through their own brilliance to be the center of attention of a community of scientists of grandiose proportions.

A number of them received the Nobel Prize after the discovery of the double helix. Directly involved with the saga of the double helix were Lawrence Bragg (Physics Nobel Prize in 1915), Linus Pauling (Chemistry Nobel Prize in 1954), Alexander Todd (Chemistry Nobel Prize in 1957), Maurice Wilkins (Medicine or Physiology Nobel Prize in 1962), Max Perutz and John Kendrew (Chemistry Nobel Prize in 1962). As “messengers” there were André Lwoff and Jacques Monod (Medicine or Physiology Nobel Prize in 1965) and Max Delbruck, Alfred Hershey and Salvador Luria (Medicine or Physiology Nobel Prize in 1969).

Crick and Watson were to receive the Medicine or Physiology Nobel Prize in 1962, nine years after the announcement of the double helix structure. How complex is the structure of DNA? Here is the observation made by Perutz, who accompanied all of the episodes of the discovery: “A protein is a thousand times more difficult than DNA. DNA was comparatively simple and could be elucidated by the method of trial and error. There was little information beyond a photograph of its x-ray diffraction pattern; what Crick and Watson had was really three limited measurements: the width, the height between piled up parallel bases and the height of one complete revolution of the helix. From this data they knew that the same pattern periodically recurred along the axis of the fiber.

Obeying these three parameters, they managed to resolve the structure with construction models. It is not possible to resolve the structure of a protein using this method, since there are no patterns of repetition. In order to determine such structures one needs to determine various thousands of parameters starting from x-ray photographs”. An assertion by Jacques Monod, another theorist in molecular biology would appear to place Crick within his due context: “Francis (Crick) in fact studied more than we did. Nobody discovered or created molecular biology.

Nevertheless, a man intellectually dominated in this area because he knew more than the rest of us and understood more than we did: Francis Crick!” What would happen if the names of Watson and Crick were to be erased from the history of science, as an intellectual exercise? Gunther Stent, the famous molecular biologist, argued the following: “If Watson and Crick had not discovered the structure of DNA, instead of it being revealed in all of its glamour, it would have been presented like a slow drip, in a manner that its impact would be considerably less”. With this argument Stent gave the sentence that a scientific discovery can be more of a work of art than is generally admitted.

Rogério Meneghini is a retired professor at the Department of Biochemistry of the Chemical Institute of USP and the coordinator of SMolBNet – the Structural Molecular Biology Network