Life's Greatest Secret (11 page)

I have searched my memory and have failed to find any really extenuating circumstances for my failure to recognize the full significance of the discovery of transforming DNA.

The diffident response of the main members of the phage group – Delbrück, Luria and Hershey – had a rather different source, and all three of them later explained their behaviour in the same way: they were interested in genetics, not chemistry, and so they simply did not realise the potential implications. Typically robust, Delbrück said:

And even when people began to believe it might be DNA, that wasn’t really so fundamentally a new story, because it just meant that genetic specificity was carried by some goddamn other macromolecule, instead of proteins.

Luria recalled: ‘I don’t think we attached great importance to whether the gene was protein or nucleic acid. The important thing for us was that the gene had the characteristics that it had to have.’
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In 1994, Hershey explained that their focus was simply elsewhere – ‘as long as you’re thinking about inheritance, who gives a damn what the substance is – it’s irrelevant.’
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Ironically, Hershey is now best known for his attempt to resolve the issue of whether proteins or DNA are the basis of heredity, an experiment that students are now taught settled the question once and for all, even though it did not.

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Alfred (‘Al’) Hershey was a tall, skinny taciturn man with a toothbrush moustache and bad teeth. Although he was renowned for working long into the night, he was not solely focused on science – he often took afternoon naps and in the summer he would disappear for weeks on end, sailing his yacht on Lake Michigan. Like everyone else in the phage group, Hershey had followed the discussions around the chemical nature of Avery’s transforming principle. In May 1949, Hotchkiss sent Hershey an update on his progress in excluding any possible protein contamination from the DNA extracts of the transforming principle; after looking at the data, Hershey wrote to the younger man: ‘The experiments are very beautiful. … My own feeling is that you have cleared up most of the doubts.’
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But like Luria and Delbrück, Hershey’s initial interest in Avery’s experiments was unfocused – the members of the phage group could not see how chemistry could help them understand genetics.

Nevertheless, as phage researchers tried to understand how viruses reproduced, the question of chemistry became increasingly pressing. By 1949, electron microscope images had shown that a viral infection begins with the virus sitting on the outside of a cell; in ways that were unclear, the virus then took over the cell’s metabolic system and ‘lost its identity’ – no viruses could be detected inside the cell for a period, while the viral structures that were still on the outside of the bacterium lost their infective power. It was possible to burst viruses by subjecting them to a sudden change in the concentration of their surrounding medium; all that remained were ghost viruses – empty protein shells that would happily adhere to the outside of a bacterium but were not infectious. Researchers had begun to use radioactive tracers to explore this phenomenon – by growing phage and bacteria on radioactively labelled medium, radioactive phosphorus was taken up by nucleic acids, and radioactive sulphur could be used to mark proteins. It was therefore possible to track the fates of the two components of the phage virus, namely DNA and protein, by using radioactivity.

By late 1950, several phage researchers had begun to sketch out a hypothesis about the roles of DNA and protein in virus replication, explicitly acknowledging that Avery was right. John Northrop of Berkeley concluded one of his articles with an outline of this thinking:

The nucleic acid may be the essential, autocatalytic part of the molecule, as in the case of the transforming principle of the pneumococcus (Avery, MacLeod, and McCarty, 1944), and the protein portion may be necessary only to allow entrance to the host cell.

Roger Herriott of Johns Hopkins University wrote to Hershey:

I’ve been thinking – and perhaps you have, too – that the virus may act like a little hypodermic needle full of transforming principles; that the virus as such never enters the cell; that only the tail contacts the host and perhaps enzymatically cuts a small hole through the outer membrane and then the nucleic acid of the virus flows into the cell.

Thomas Anderson later recalled:

I remember in the summer of 1950 or 1951 hanging over the slide projector table with Hershey, and possibly Herriott, in Blackford Hall at the Cold Spring Harbor Laboratory, discussing the wildly comical possibility that only the viral DNA finds its way into the host cell, acting there like a transforming principle in altering the synthetic processes of the cell.

It was in this context that Al Hershey, together with his new technician, Martha Chase, decided to settle the matter. Hershey had recently moved to Cold Spring Harbor and had equipped his laboratory with the latest radioisotope technology.
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Chase, who was only 23 years old when she joined Hershey, had a round face and short hair; generally she was as reserved as her boss, but she was nonetheless prepared to complain loudly about her low pay.
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Their experiments, which were published in the
Journal of General Physiology
in the middle of 1952, have since taken on an iconic quality.
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They are reproduced in textbooks and are presented as a turning point, because they are now seen as showing that genes are made of DNA. The reality is rather different.

The Hershey and Chase paper describes several experiments in which they tried to identify the functions of protein and nucleic acid in bacteriophage reproduction. First, they confirmed and extended previous findings about the function and composition of ghost phage, which they showed were made of protein, were not infectious, could still attach to bacteria, and protected their DNA contents from enzyme attack. They next demonstrated that when the phage settled onto a bacterium, it injected DNA into the cell. All this supported Herriott’s hypodermic needle hypothesis, but there was no proof of what the DNA actually did, nor could they be certain that no protein entered the bacterial cell.

The final experiments are those most often taught to students today, but they are usually described inaccurately. They all involved the use of a Waring Blender, or Blendor as the Waring company trademark had it. This device was employed to agitate the viruses and their bacterial hosts, and the experiments that used it are now often known as the Blender experiments. This apparatus is often called a kitchen blender, which conjures up some kind of retro 1950s domestic device, all chrome and glass. Sadly this was not the case. Although the Waring company did make kitchen blenders, the apparatus used by Hershey and Chase was a highly specialised, unstylish bronze-coloured piece of laboratory equipment about 25 cm high that could run at speeds of up to 10,000 r.p.m. – much faster than anything you would have in your kitchen. It was not simply a centrifuge, it also produced what Hershey and Chase described as ‘violent agitation’, which they used to shake the protein-rich viral ghosts from the outside of the host cell. Using radioactive sulphur, they showed that they could remove up to 82 per cent of the phage protein from their preparations by separating out the ghost phage; a similar experiment with radioactive phosphorus showed that up to 85 per cent of the virus DNA was transferred into the bacterial cell.

Students are now generally taught that these experiments provided the evidence that DNA is the genetic material, but in fact they did no such thing, nor did Hershey and Chase claim that they did. The problem faced by Hershey and Chase was similar to that encountered by Avery and his colleagues, but in spades. Hotchkiss had reduced the protein component in his version of Avery’s experiment to effectively zero (at most 0.02 per cent), and still people did not accept his findings; in Hershey and Chase’s extracts around 20 per cent of the protein was still floating around. It was quite possible that some of this protein played a role in the reproduction of the virus. Furthermore, as Hershey and Chase put it, none of the experiments proved anything more than that DNA had ‘some function’ in viral reproduction. The paper concluded with Hershey’s typical terseness, beginning with the question of ‘adsorption’, or how the virus sticks to the outside of the bacteria:

The sulfur-containing protein of resting phage particles is confined to a protective coat that is responsible for the adsorption to bacteria, and functions as an instrument for the injection of the phage DNA into the cell. This protein probably has no function in the growth of intracellular phage. The DNA has some function. Further chemical inferences should not be drawn from the experiments presented.

Hershey remained troubled by his findings and later admitted, ‘I wasn’t too impressed by the results myself’.
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When he first presented his experiments in a small laboratory seminar at Cold Spring Harbor, he expressed his surprise that protein apparently had no function inside the infected cell. And when he made his first public presentation of the results, at the June 1953 Cold Spring Harbor meeting, speaking after the double helix structure of DNA had been described, Hershey was still sure that DNA could not be the sole carrier of hereditary specificity. He addressed this issue head-on by summarising the evidence from Avery, Boivin, Taylor and himself as follows:

1.     The amount of DNA in chromosomes is consistent in a species, not in a given kind of tissue in different species.

2.     DNA can transform bacteria.

3.     DNA plays some unidentified role in one kind of viral infection.

Hershey then told his audience that this evidence was not enough to support the conclusion that DNA was the hereditary material. He remained convinced that proteins must play a role:

None of these, nor all together, forms a sufficient basis for scientific judgement concerning the genetic function of DNA. The evidence for this statement is that biologists (all of whom, being human, have an opinion) are about equally divided pro and con. My own guess is that DNA will not prove to be a unique determiner of genetic specificity, but that contributions to the question will be made in the future only by persons willing to entertain the contrary view.

Hershey’s caution shows us the rigorous nature of his scientific thinking – strictly speaking, his interpretation was absolutely correct; he would go no further than the data allowed. It also shows the continued uncertainty about the possibility of contamination – this was an important problem in the Hershey and Chase experiment, although nobody pointed it out at the time.

Hershey later argued that the complex route from Avery’s 1944 discovery to the widespread acceptance that genes were made of DNA ‘shows that some redundancy of evidence was needed to be convincing and that diversity of experimental materials was often crucial to discovery’.
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Although that is undoubtedly true, it is also the case that while some people immediately embraced Avery’s discovery, others – including the phage group – were reluctant to recognise its significance. For a decade, scientists spent their time arguing over something that now seems blindingly obvious. There are many such moments in the history of science, and they can only be understood in terms of the evidence and attitudes of the time. In this case, the predominant problem was the power of the old ideas about the dominant role of protein, and the difficulty of imagining how in reality – not in theory – DNA could produce specificity.

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While the chemists and the microbiologists battled it out over the chemical nature of the gene, there were several bold attempts to look at gene function – how genes do what they do. In 1947, Kurt Stern, a 43-year-old German biochemist who had left Nazi Germany for the US, published a highly speculative article on what he called the gene code – one of the first uses of the word code since the publication of Schrödinger’s book three years earlier. In a prescient guess, Stern argued that the chemical basis of genes might take the form of helical coils – he assumed that genes were made of nucleoproteins, although he recognised that Avery could be right and DNA alone might be the genetic material.

Like Chargaff, Stern suggested that variations in the sequence of the bases in the DNA molecule could lie at the heart of gene specificity. According to Stern’s theory, genes were physical modulations, much like a groove on a vinyl record. The role of the nucleoprotein, Stern argued, was to fix the DNA molecule in a particular shape; removal of the protein would return the nucleic acid to its unmodulated form. To prove his point, Stern provided photographs of physical models that he had made, showing DNA and an associated protein spiralling around each other in a double helix, although he did not use the term. Despite the rich biochemical and structural data that were at the heart of Stern’s work, his model was too hypothetical to generate experiments, and his ingenious views had no influence.
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Although Stern used the term code, he did not embrace the idea that the code might abstractly represent protein structure; his vision was that of a template – genes were the physical form on which proteins were synthesised.

At the same time, André Boivin and Roger Vendrely came up with a hypothesis about the relation between the two kinds of nucleic acid found in a cell and the enzymes that were thought to be the product of genes, building on Torbjörn Caspersson and Jean Brachet’s investigations into the role of RNA in protein synthesis.
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Boivin and Vendrely’s idea was pithily expressed by the editor of the journal,
Experientia,
in an English-language summary:

the macromolecular desoxyribonucleic acids govern the building of macro-molecular ribonucleic acids, and, in turn the production of cytoplasmic enzymes.