Life's Greatest Secret (10 page)

Some scientists were more enthusiastic about the importance of nucleic acids. In an article that appeared in 1948 in a new genetics journal entitled
Heredity,
Joshua Lederberg, who had been inspired by Avery’s paper to study bacterial genetics and was still only 22 years old, wrote:

The total absence of all other components is not readily established, although none can be detected with available methods. The criticism of Mirsky and Pollister (1946) should be noted, however. The further chemical characterisation of the transforming principle is one of the most urgent problems of present-day biology, since it behaves like a gene which can be transferred by way of the medium from one cell to another.

In 1948, four years after Avery’s first paper appeared, Boivin was arguing that all genes are made of DNA, Chargaff had hypothesised that nucleic acid specificity might involve differences in the sequence of bases, and Lederberg was urging his colleagues that characterising the transforming principle was a central task of biology. As Astbury had put it in 1945, this was indeed a ‘heroic age’.

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Oswald Avery played no part in any of this. He left Rockefeller at the end of 1948 and moved to Nashville to live with his brother, Roy. He received no further official recognition of his part in the discovery of the genetic role of DNA; although he was nominated for a Nobel Prize, the committee apparently ‘found it desirable to wait until more became known about the mechanism involved’.
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This view may have been reinforced by the fact that the Swedish scientific community was not closely following the debate in the US and the UK about the nature of the hereditary material.
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The brief obituary that appeared in the
New York Times
when Avery died in 1955 did not even mention DNA.
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As science journalists began to report on the growing wave of excitement within the scientific community about DNA, history was rewritten almost as soon as it happened. In January 1949, the
New York Times
informed the world that a Rockefeller Institute researcher had discovered that ‘genes consist in part of a substance called desoxyribonucleic acid’. The researcher’s name was Dr Alfred Mirsky.
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Research into the transforming principle was continued by Rollin Hotchkiss at Rockefeller and by Harriett Taylor, who had moved to Paris, where she married the geneticist Boris Ephrussi. Taylor extended the number of bacterial characteristics that could be transformed, thereby showing the generality of transformation and its similarities to genetic factors in higher organisms, while Hotchkiss responded to Mirsky’s carping criticism about the protein content of the extracts by carrying out very precise experiments.
²⁶

The first public presentation of Hotchkiss’s new data took place in Paris at the end of June 1948. André Lwoff, a bacteriologist at the Institut Pasteur, had organised a small meeting with the rather pompous title ‘Biological units endowed with genetic continuity’ – these ‘units’ were bacteria and viruses. Hotchkiss described results from a range of techniques that were intended to eliminate proteins from the transforming principle. After treatment, at most 0.2 per cent of the extract was protein; this was within the margin of error of a result of 0.0 per cent, so it was quite possible that there was no protein at all in his samples.
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It was impossible to be more precise. Despite this clarity, when Lwoff summed up the week’s discussions, he insisted that nucleic acids ‘could and should be normally combined with another constituent, most likely a protein.’
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The old ways of thinking died hard.

Although Hotchkiss’s paper was initially published only in French, news of his findings soon began circulating in the US. Another participant at the Paris meeting was the US virus biologist Max Delbrück, and in the spring of 1949 Al Hershey, a member of Delbrück’s informal phage group, presented Hotchkiss’s data as part of the DNA transformation story at a round-table discussion on nucleic acids organised by the Society of American Microbiologists.
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By the end of the 1940s, it was widely known that the levels of protein in the transforming principle were effectively zero.

Boivin, who by now had moved to Strasbourg, began his talk at the Paris meeting by stating that although DNA specificity had been shown only in bacteria, the conclusion was overwhelming: ‘these facts lead us to accept – until formal proof to the contrary – that this specificity is an example of a general phenomenon, which everywhere plays a major role in the biochemistry of heredity.’
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In his conclusion, Boivin reported that in a wide range of organisms – including many animals – the nuclei of different cells in the same organism had the same amount of DNA; similarly, members of a given species all had the same amounts of DNA, whereas eggs and sperm had only half the amount found in normal cells. This was a decisive discovery. It had been known since Sutton’s observations in 1902 that when most sexual species reproduced, the creation of the egg and sperm cells involved the halving of the usual double or diploid complement of chromosomes, so that one set of chromosomes went into each egg or sperm, to form what is called a haploid cell. When the egg and sperm fused to form the new organism, these two haploid components formed a new diploid set of chromosomes. Boivin’s observation that the amount of DNA corresponded to the chromosome complement at different phases was what would be expected of a gene – nothing like this had been found for any protein. In bacteria, plants and animals, Boivin argued, ‘each gene can, in the final analysis, be considered as a macromolecule of DNA.’
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This was one of Boivin’s last lectures. The cancer he had been suffering from returned and he died in July 1949. In the meantime, doubts began to be raised about his reports on transformation in
E. coli
– researchers in the US were unable to replicate his findings, and his original strains were lost.
³²
Despite – or perhaps because of – Boivin’s bold statements and prophetic vision, an air of disbelief accumulated around his discoveries. This took years to dissipate – his findings were eventually confirmed in the 1970s, and his vision of the nature of heredity and the future of biology were also shown to be true.
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By the end of the 1940s, support for the hypothesis that DNA had a fundamental role in heredity had grown much stronger. In the summer of 1950, the cell biologist Daniel Mazia gave a lecture at Woods Hole Marine Biology Laboratory that summed up many people’s thinking. Mazia could not be absolutely certain that genes were made of DNA, but it certainly looked that way: ‘The “physical basis of heredity” is something in the chromosome which may or may not be DNA, but which follows DNA for all practical purposes’, he said.
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Following Boivin, Mazia outlined four criteria that had to be met by what he called the vehicle of heredity, whatever its chemical composition might be. There had to be the same amount in every diploid cell of a given species, that amount should double just before ordinary cell division, and it should be halved during the creation of haploid sex cells. It should be stable, it should be capable of specificity and it should be able to be transferred from one cell to another and act like a gene. Proteins failed at the first hurdle – there was no evidence that the levels of protein in cell nuclei were the same in all tissues of an organism. All that proteins had going for them was that they were known to be complex. Both Chargaff and Gulland had suggested that nucleic acids could vary by the sequence of the bases, perhaps providing a source of complexity. While still not excluding the possibility that proteins were involved, Mazia concluded: ‘DNA is the most likely candidate so far for the role of the material basis of heredity.’
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A couple of months earlier, in April 1950, Mazia had chaired a session at a special conference on the biochemistry of nucleic acids, held at the US Atomic Energy Commission’s Oak Ridge Laboratory in Tennessee. Among the speakers was Arthur Pollister, who with Mirsky had criticised Avery’s conclusions two years earlier. Pollister was changing his tune; he enthusiastically reported the data from Boivin’s laboratory that showed that the amount of DNA was constant in all diploid cells of a given species, before discussing the idea of a ‘DNA-gene’ and raising the possibility that the chemical structure of the gene was within reach. Nevertheless, Pollister was not completely convinced: the complexity of gene function led him to suggest that ‘important genic components other than DNA remain to be discovered.’
³⁶

Another speaker at the Oak Ridge meeting was Erwin Chargaff, who was acquiring some of the most telling evidence in favour of Avery’s conception of the role of DNA. Chargaff had been an early supporter of Avery’s hypothesis, and in 1950 he presented data on the precise base composition of nucleic acids, using paper chromatography to identify the bases by weight. He found that the proportion of the different bases was constant in all tissues of any species, but differed wildly between species. As Chargaff pointed out, these data disproved the tetranucleotide hypothesis, to the extent that anyone still thought it was true.
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DNA was clearly not boring.

The 1951 Cold Spring Harbor symposium was on the topic ‘Genes and mutations’, and one of the speakers was Harriett Ephrussi-Taylor, who took the opportunity to survey the seven years that had passed since Avery, MacLeod and McCarty had published their landmark paper. She was downbeat:

Considering the interest which was aroused by the publication of the results of the chemical and biochemical study of the capsular transforming agent of pneumococcus, it is surprising that so few scientists are at present working in this field.

As she admitted, transformation was difficult to study – for example, transformation in Boivin’s
E. coli
system ‘occurred only with some irregularity’ – and many researchers were not familiar with the pneumococcal system in which transformation had first been described. She glumly concluded that the study of transformation remained isolated: ‘as yet,’ she said, ‘no bridge can be seen leading over into classical genetics’.

Ephrussi-Taylor’s lament was related to what now appears to be an odd feature of genetics research in the second half of the 1940s – many biologists, including geneticists, simply did not ‘get’ Avery’s result. Not only did they not immediately accept that genes were made of DNA, they did not even attempt to test the hypothesis in the systems they studied. For example, Max Delbrück first heard of Avery’s breakthrough in May 1943, when his Vanderbilt colleague Roy Avery showed him the letter from New York that announced the discovery. Delbrück later recalled his ‘total shock and surprise’ at the contents of the letter, ‘which I read there standing in his office in the spring sunshine’.
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But despite his ‘shock’, Delbrück did nothing. He did not start studying the role of DNA in viruses, nor did any of his colleagues, even though they were all intimately aware of the results that were coming out of Avery’s laboratory. Delbrück later explained that the suggestion that genes were made of DNA left them nonplussed. As he put it ‘you really did not know what to do with it’.
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With the easy wisdom of hindsight, this lack of interest in what led to the most remarkable biological discovery of the twentieth century looks remarkably short-sighted. And at one level, it was. The phage group did not react in the way that Lederberg, Boivin and others did. Their diffident attitude was one component of the failure of Avery’s discovery to immediately transform biology.
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Avery’s findings now look so obvious, and yet many scientists at the time responded to them with hostility or bemusement.

One of the scientists who did not immediately embrace Avery’s findings was the young Gunther Stent, who worked with Delbrück. In 1972, Stent sought to explain the lack of widespread recognition of the importance of the Avery group’s discovery by suggesting that the result was ‘premature’.
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This term does not explain anything; in fact it obscures the historical reality of how Avery’s work was received, and it does not explain why some scientists accepted the finding but others rejected it. There were two valid criticisms of Avery’s suggestion that DNA was the hereditary material in the transforming principle, each of which gradually became weaker. First, there was the issue of potential protein contaminants, which led the Avery group to employ increasingly precise techniques, the results of which all indicated that protein contamination was not the cause of transformation. Second, there was the conundrum of how exactly specificity might be represented in what were supposed to be boring molecules – if DNA was essentially composed of four bases, a way needed to be discovered that enabled it to bring about the almost infinitely different effects produced by genes. The leading chemists of DNA such as Gulland were happy to imagine that specificity could reside in DNA through the sequence of bases, or their proportions, but this had yet to be demonstrated. Nevertheless, as time wore on, there were fewer reasons not to accept Avery’s findings.

Some scientists had strong personal reasons to reject the DNA hypothesis. Mirsky’s career was based on the study of nucleoproteins and he was clearly not going to give up his world view without a fight. Through his articles, his lectures and his interventions at conferences, Mirsky sowed doubt among the undecided. Similarly, Wendell Stanley turned a blind eye to the work of the Avery group, even though he too had been familiar with it before publication. In 1936, Stanley had crystallised the tobacco mosaic virus and announced that it was a protein; this was finally shown to be wrong in 1956 – the hereditary material in this virus is in fact RNA, and small amounts of RNA in his protein extract accounted for his results. In 1946, Stanley won the Nobel Prize in Physiology or Medicine for his mistaken claim; he later said that he ‘was not impressed’ by Avery’s discovery – otherwise he would have immediately tested tobacco mosaic virus RNA for specificity. In 1970, he concluded, somewhat shamefacedly: