Unravelling the Double Helix

by Gareth Williams

  Muller did not answer that one. His Nobel lecture stuck to the facts of what million-volt X-rays did to chromosomes and genes. One day, he speculated, ‘individual genes could be changed to order’ but there was ‘no evidence that anything of the sort had been done artificially’. He also skated over absent colleagues and friends, mentioning Thomas Hunt Morgan, who had died just over a year earlier, only briefly and Nikolai Vavilov not at all.

  Finally, to Moscow in July 1945, and the first delegation of foreign scientists to visit since the war began, to celebrate the 220th anniversary of the Soviet Academy of Sciences. Nikolai Vavilov was one of many who were conspicuously absent. Visitors who asked after him were shown the Roll of Academicians, from which his name had been erased, and told not to enquire further.

  In November 1945, the Soviets allowed some information to filter out. As Nature reported: ‘News has recently been received of the death in the Soviet Union of Nikolai Ivanovich Vavilov. The circumstances are not precisely known, but the time was after December 1941 and the place probably Saratov.’ Nature’s brief obituary focused on Vavilov’s colourful life and science, with warm recollections of his larger-than-life personality and unfailing optimism: ‘Wherever he went, he took sunshine and courage.’

  When the details of Vavilov’s ordeal emerged some years later, it was clear that those qualities had been severely tested. After his arrest in the Ukraine in August 1940, Prisoner 7002 was taken to Lubyanka prison (Figure 17.1). One of Vavilov’s favourite sayings was ‘Life is short: hurry’, but his tormentors took their time. He stood trial – all ten minutes of it – a year later, after 400 sessions of interrogation, some lasting thirteen hours, to make him confess to being an English spy sent to destroy Soviet agriculture. Vavilov and two other colleagues were sentenced to death; both the others were shot, but Vavilov’s plea to be allowed to live was granted and his punishment commuted to twenty years in prison.

  Figure 17.1 Prisoner No. 7002: Nikolai Vavilov. Lubyanka Prison, Moscow, August 1940.

  The prison they chose was surely no coincidence: in Saratov, the city on the Volga where he had been a young professor. There, he raised spirits by organising his fellow prisoners to talk about the things they loved – science, agriculture, trees. The lecture series ran to over 100 hours; to avoid the wrath of their captors, the talks were delivered in whispers. Vavilov served just over a year of his sentence. In late January 1943, he was taken into the prison hospital with a fever. The doctors merely documented severe malnutrition, consistent with the prison diet. Vavilov died on 23 January and was buried in an unmarked grave in the prison grounds, without informing his next of kin.

  Trofim Lysenko made no public comment about Vavilov’s death. Neither did Professor J.D. Bernal FRS, even when challenged on a live radio debate about the disturbing state of science in the Soviet Union.

  Vavilov’s obituary in Nature carried a footnote – ‘When Leningrad came to be besieged, the residue of his collection [of seeds] was eaten by the famished people’ – which was both poignant and inaccurate. Leningrad was blockaded by the Nazis for twenty-eight months from September 1941 until January 1944, at a cost of one and a half million lives. Most of the victims died of starvation.

  Throughout the siege, the seed-bank of the Academy of Agricultural Sciences was guarded by Vavilov’s colleagues. Out on the streets, people were trying to survive on whatever they could get hold of: rats, tree-bark and the glue from book-bindings. Inside the Institute, there was also famine, but in the midst of plenty. Individual seed samples in the bank were small, but even a single species – for example, the 30,000 strains of wheat – could have kept the guardians and their loved ones alive for months.

  But like Vavilov himself, these were people of principle. When the siege ended, twenty-eight of the guardians had died of starvation but all the samples in the bank were untouched.

  18

  TIPPING POINTS

  The spring of 1947 should have found Bill Astbury content with his lot: not yet fifty and at the peak of his powers, Fellow and Croonian Lecturer of the Royal Society, professor and head of his new department. Instead, this normally ebullient man was reduced to ‘the picture of depression’ when he joined his staff in the tea-room one afternoon in March. The stuffing had been ripped out of him by the arrival of the news that he had been dreading. All he could say was, ‘Randall has been given the MRC grant.’

  Astbury reacted as though he had been bereaved, and in a sense he had. His own approach to the MRC – at their invitation – had foundered. Then he had rubbed salt into his own wounds by agreeing to review Randall’s grandiose bid to the same organisation. By comparison with the empire that Randall was going to build on the Strand in London, Astbury’s own Department of Biomolecular Structure now looked as amateurish as its name – a terraced house that had been fine for bringing up a Victorian family, but was so ill-suited to science that renovation was going to take eighteen months.

  Many successes still lay ahead for Bill Astbury, but the crushing disappointment of watching Randall’s ‘biophysics’ triumph over his own, original ‘molecular biology’ left its mark on him. The Astbury that Kathleen Lonsdale had known at the Royal Institution – ‘so full of enthusiasm that it was impossible not to rejoice with him’ – allowed cynicism and bitterness to sink their claws into him. This was the moment that marked the beginning of the decline in his career.

  The months that followed were also a testing time for some of the thinking that shaped the history of DNA. A major catalytic event was the Twelfth Cold Spring Harbor Symposium on ‘Nucleic acids and nucleoproteins’, held from 11 to 20 June. This brought together 145 scientists from different backgrounds and succeeded admirably, both as a showcase for new ideas and as a gladiatorial arena in which to challenge prejudices and sloppy thinking. To set the scene, it might be helpful to survey the landscape as it appeared in the run-up to the conference.

  The story so far (mid-1947)

  Genetics, chemistry, physics and bacteriology are still separate disciplines, each proud of its individuality and reluctant to venture outside its personal silo. Visionaries like John Randall and Bill Astbury intend to bridge the divide between biology and physics, but biophysics/molecular biology is still in transit from the drawing-board to the laboratory bench.

  At present, humans have 48 chromosomes, but because the smaller fiddly ones are hard to pin down, this estimate may be wrong. There is universal acceptance that the genes are arranged linearly along the chromosomes. The molecular weight of a typical gene (whatever it might consist of) has now crept up to around 48 million, while its size could lie anywhere between 10 and 300 cubic Å.

  It is also agreed that genes consist of nucleoprotein, the massive complexes of DNA and proteins that make up the chromosomes. The business end of the gene could, in theory, be either protein or DNA. However, proteins are still the only game in town for almost everyone who matters, be they geneticists (who are not really interested in what genes are made of), biologists or biochemists. More nebulous ideas about genes are still floating around. Schrödinger’s enigmatic ‘aperiodic crystals’ have caught the attention of a few physicists, none of whom has any real clue what this means. Some of those initially seduced by the poetry of the notion have now decided that Schrödinger was a false prophet. Maurice Wilkins, for example, has written him off for ‘rehashing’ the ideas of Niels Bohr, who argued during the 1930s that genes – no matter how wondrous they might appear – must consist of atoms that obey the laws of quantum physics.

  DNA remains a dull substance and a biochemical dead end. Virtually all experts, notably Mirsky (biochemist) and Astbury (molecular biologist), are still locked in the stranglehold of the tetranucleotide; all evidence against Levene’s hypothesis has either been ignored or manipulated to make it supportive. It is true that the molecular weight of DNA (now 2 million or more) is vastly bigger than that of four nucleotides, but embarrassment is easily avoided by postulating that DNA consists of many tetranucleotid
e units, all identical, joined together. Rollin Hotchkiss has recently proved that this cannot be true, by showing that DNA contains unequal amounts of the four bases – but his evidence is buried in a low-profile French journal and doomed to remain invisible for another few years.

  The structure of DNA is so uninspiring that it could have been designed to kill curiosity. The best bet appears to be Astbury’s ‘pile of pennies’, with flat tiles each consisting of deoxyribose stuck to one of the bases, tied together into an immensely tall stack by a tenuous thread of phosphate groups running down its side. As dictated by the tetranucleotide hypothesis, Astbury believes that the nucleotides are arranged in groups of four – one of each, one after another – repeated ad nauseam along the molecule.

  It follows that DNA cannot do anything exciting. Most biochemists assume that it is a scaffold, propping up the really important nuclear substances (proteins, of course), the histones and protamines. The only faint glimmer of interest lies in Astbury’s suggestion that DNA helps proteins to be assembled in the nucleus: their constituent amino acids snuggle up to the DNA strand and slot into what he believes are docking berths, spaced out at the magical interval of 3.34 Å.

  This naturally rules out any possibility that DNA carries the ‘coding script’ of heredity envisaged by Schrödinger. A well-worn circular argument leads back to the inescapable conclusion that only proteins are clever enough to serve as what Astbury has called ‘the long scroll on which the instructions for life are written’. Recently, Oswald Avery has thrown in a wild card by claiming that DNA, not protein, mediates the strange bacterial phenomenon of transformation. This change is so fundamental that some geneticists have called it a mutation, but other geneticists (including a Nobel laureate) are deeply sceptical about this ‘revolutionary’ finding. Anyway, those who believe that DNA and genes are the same thing in bacteria cannot be heard over the strident attacks led by Alfred Mirsky.

  Conclusion: whatever it does, DNA has nothing to do with genes.

  Summer of discontent

  Alfred Mirsky (Figure 18.1) was in confident mood at the 1947 Cold Spring Harbor Conference. Now forty-seven years old, he had secured his position as Phoebus Levene’s natural heir in nucleic acid research. Paradoxically, Avery had helped to consolidate Mirsky’s authority, by giving him plentiful opportunities to slap down the notion that genes were made of DNA. And now that Avery had disappeared to Nashville, Mirsky could broadcast without fear of challenge.

  Mirsky had come to talk about a new research seam which he and his colleague Hans Ris had begun to chisel out: isolated chromosomes. These were not the abnormally gigantic structures which Miss Melland had painstakingly dissected out of the salivary glands of midges, but were harvested from the nuclei of ordinary cells in the blood of fish (‘live carp may be had in the fish stalls of many cities’), turtles and toads, and in the thymus of calves. Nuclei were isolated as usual, then smashed up in a kitchen blender and centrifuged. This yielded a jumble of threadlike objects, like microscopic vermicelli in a watery minestrone. To the untutored eye, the threads might have looked like scraps of nuclear debris, but Mirsky’s expert gaze revealed their true nature: ‘microscopic analysis makes it certain that we are dealing with isolated chromosomes’. Biochemical analysis showed that they contained DNA with a novel protein ‘entirely different’ from histone – an exciting breakthrough, as this could be the elusive hereditary protein of which genes must be made. After his talk, nobody questioned Mirsky’s assertion that the microscopic threads really were isolated chromosomes.

  Figure 18.1 Alfred Mirsky, left, and Gulland Masson, at the 1947 Cold Spring Harbor Symposium.

  However, Mirsky soon faced other challenges. The first was from André Boivin, a well-respected biochemist-bacteriologist from the medical faculty in Strasbourg. The title of Boivin’s talk was clumsy – ‘Direct mutation of colon bacilli by an inducing principle of desoxyribose nature’ – but his findings were ominously clear. Boivin had repeated Avery’s experiment in E. coli, a rod-shaped bacterium which is happiest in human faeces. He managed to transform the E. coli equivalent of R into S with an extract of dead S, and had shown that the active material (which formed pure white fibres) was DNA. To confirm its identity, he had removed all protein and proved that the transforming material was resistant to RNase and protein-digesting enzymes, but was instantly broken down by DNase. Boivin went further than Avery, by demonstrating that only DNA from E. coli could transform these bacteria; DNA from other species was completely inactive. This strongly suggested that DNA varied between different species, and that each species might have its own unique variant of DNA.

  Here, according to Boivin, was new evidence to support Avery’s view that bacterial genes consisted of DNA, not protein. He added boldly that ‘in all likelihood’ this applied ‘in higher organisms as well’. In conclusion, he explained that Avery had inspired him and that he ‘took pleasure in acknowledging the priority of the American authors in the field’.

  During question time, Mirsky rode into battle on his hobby horse. He could have been quoting from the 1946 paper in which he had tried to bury Avery’s findings. It was ‘going beyond the experimental facts to assert that the specific agent in transforming bacterial types is a desoxyribonucleic acid’. The agent responsible was obviously a protein that Boivin – who had repeated all of Avery’s mistakes – had failed to detect.

  Avery had never risen to Mirsky’s challenge, but the Frenchman now fought back with relish. It could not be ‘an absolute certainty’ that genes were made of DNA, but it was ‘extremely likely’. Like Avery, Boivin had isolated DNA ‘as pure as it is possible to obtain’; no protein had slipped through. He threw the gauntlet back to Mirsky: ‘It seems to us that the burden of proof now rests upon those who would postulate the existence of an active protein lodged in an inactive nucleic acid.’ For once, Mirsky had nothing to say.

  Another challenge soon followed from Erwin Chargaff, Assistant Professor of Biochemistry at Columbia University in New York (Figure 18.2). Chargaff was forty-two years old, five years younger than Mirsky, and his résumé combined elements from those of Phoebus Levene and Herman Muller. Born in Czernowitz in the Austro-Hungarian Empire;* studied chemistry in Vienna after the First World War; visiting fellow (1925-30) at Yale, which he intensely disliked; returned to Europe in 1930, to be chased successively away from posts in Berlin and Paris during the anti-Jewish purges.

  Chargaff was cultured and erudite, with a sense of humour that was often spiked with craftily poisoned barbs. Like Mirsky, he was opinionated, fearless in discussion and intolerant of fools (these were mostly different from the fools identified by Mirsky). And he was another unashamed convert to the gospel of Avery, which promised such excitement that he had dropped his current research and switched his entire group on to studying DNA. Now, three years later, he had preliminary results and disconcertingly bold plans.

  Taking great pains, Chargaff had extracted ‘very highly polymerised’ DNA from unusual sources, including yeast and the tuberculosis bacterium. Unlike the ‘sorry fragments’ of degraded DNA yielded by routine methods, his DNA appeared to be undamaged; its physical properties in solution suggested that it had a molecular weight in the millions, and was a long, very thin molecule, perhaps 400 times longer than its width.

  Figure 18.2 Erwin Chargaff.

  For now, that was all, but Chargaff was clearly a man to watch – and in Mirsky’s case, with foreboding. Chargaff found Avery ‘very convincing’ and was setting out to prove that DNA was the genetic material. He believed that DNA could carry genetic specificity, for example if there were ‘differences in the proportions or the sequences of the nucleotides forming the nucleic acid chain’. As a first step, he intended to measure the amounts of the four bases in DNA from a variety of sources, looking for divergences from the constant 25 per cent of each that the tetranucleotide hypothesis predicted. He did not say how he intended to do that, and Alfred Mirsky did not bother to ask.

  Mirsky
should also have been unsettled by what was later described as ‘one of the most significant papers of the Symposium’, notable for the ‘warm personality’ of the presenter as well as ‘the excellence of his work’. The speaker was Masson Gulland, a Scot and a gentleman, and Britain’s leading biochemical researcher in nucleic acids.

  Gulland (Figure 18.1) was born in Edinburgh in 1898, the same year as Bill Astbury. Unlike Astbury, he was the product of a Miescher-style academic dynasty. His father was the Professor of Medicine at Edinburgh, and even as a young man, Masson’s ultimate ambition was to occupy the Chair of Biochemistry in the same university. In the meantime, he contented himself with Oxford (a First in Chemistry) and then the Lister Institute in London, where he became fascinated by the nucleic acids. One of his early successes was to work out which atoms linked the sugar deoxyribose to the purine bases in DNA.

  In 1936, he moved in the right direction (north) by being appointed Professor of Biochemistry at University College in Nottingham. Gulland spent eleven successful and enjoyable years there, building up an energetic department that attracted funding from local industry (the Boots Pure Drug Company) and a steady stream of high-quality PhD students. However, Nottingham (roughly one-third of the way from London to Edinburgh) was never more than a stepping-stone towards the Chair in his favourite city.

  Gulland was famous for ‘the scholarliness and incisiveness of his thinking’, which led him to ask awkward questions about venerable but dubious concepts that nobody else dared to challenge. In 1943, he became a leader of the underground movement to overthrow the tyranny of the tetranucleotide, observing that ‘had the true molecular size of DNA been recognised earlier, it is doubtful whether the conception would have gained such firm hold’.