The Hershey-Chase Blender Experiments

Martha Chase and Alfred Hershey, 1953.

Martha Chase and Alfred Hershey, 1953.

“When asked what his idea of happiness would be, [Hershey] replied, ‘to have an experiment that works, and do it over and over again.'”

Jonathan Hodgkin, 2001

In 1944 the Avery-MacLeod-McCarty experiments demonstrated that DNA, rather than proteins, is the carrier of genetic information.  Though the work appeared to be well-supported, and was endorsed by other researchers, the trio met with resistance from much of the scientific community.  For nearly a decade, the Avery group was forced to repel attacks on the validity of their experiments, defending both their findings and their reputations.

Finally, in 1952, Alfred Hershey, a Carnegie Institution researcher working at Cold Spring Harbor Laboratory, set out to conclusively settle the issue.  Like many of his contemporaries, Hershey believed that proteins, with their complicated structures, were more likely to be the carriers of genetic information than was the simple DNA molecule.  Hershey, however, was about to make a discovery that would turn his own notions on end.

In order to show that proteins carry genetic information, Hershey and his lab technician, Martha Chase, decided to track the transfer of proteins and DNA between a virus and its host.  For their experiment, they chose to use the T2 bacteriophage as the vehicle for delivering genetic material.  Like all bacterial viruses, the T2 is comprised of only a protein-based outer wall and a DNA core, its simple structure making it the perfect research candidate.  The phage reproduces by injecting its genetic material into a bacterium, leaving its protein shell attached to the host.  Then, through a microscopic takeover, the virus seizes control of the bacterium’s reproductive mechanisms and uses them to duplicate itself, destroying the host in the process.

Though it was known that the protein shell remained outside the bacterium, researchers thought it possible that certain proteins were transferred from the virus to the bacterium upon attachment. If genetic material was in fact carried by proteins, this would explain how a phage is able to reproduce within a bacterium without the entirety of the protein shell penetrating the bacterium’s membrane.  In order to prove that proteins are the carriers of genetic information, Hershey and Chase needed to demonstrate that at least a portion of the phage’s protein mass was transferred to the interior of the bacterium.

In their first experiment, Hershey and Chase tagged the T2 phage DNA with Phosphorous-32, a radioactive form of the element.  Because phosphorous can be found in large quantities in DNA, but in only trace amounts in protein, the researchers could track the location of DNA and protein according to the radiation concentrations.  They then allowed the tagged phages to begin infecting samples of E. coli.  After introducing to the phage culture to the bacterial sample, they used a Waring blender to violently disturb the infected bacteria, causing the protein shells to detach from their hosts.  Then, using a centrifuge, they separated the bacterium from the phages and protein.

The Hershey-Chase Blender Experiment.  Diagram by Eric Arnold.

The Hershey-Chase Blender Experiment. Diagram by Eric Arnold.

Once the separation was complete, they measured the radiation concentrations in the E. coli cells and the protein shells.  The phosphorous tracer appeared in large quantities only in the bacterial sample, demonstrating that DNA was transferred from the bacteriophage to the host organism.  Further, despite the protein shells being detached while reproduction of the phage should have been taking place, the virus was still copied in each of the host cells. This, in turn, suggested that the proteins shell itself was not necessary to the replication process following the initial insertion of genetic material.

Shocked by their findings, Hershey and Chase decided to perform the test once again, this time using a different tracer molecule.  They chose sulfur for the second test, because it appears in the amino acids that make up proteins, but is not present in DNA.  This allowed them to track the same process as in the first experiment, but in reverse.  After tagging the proteins, infecting the E. coli cells, and separating the shells from the host, the researchers tested for the presence of sulfur.  In accordance with their previous results, the sulfur could only be found in the protein shells and not in the bacteria. And again, the phage’s genetic material was replicated despite the protein shell being disconnected from the bacteria via the blending process.

Sufficiently impressed by the significance of his findings, Hershey returned to the phosphorous-tagged batch to engage in some follow-up research.  Upon examining the offspring of the phages, the researchers found that the young bacteriophages also possessed phosphorous-tagged DNA, but their protein lacked any trace of radioactivity.   The implications of their first experiments were reinforced.

At first, the pair was inclined to believe that the experimental or data-collection procedures were flawed.  They rechecked the experiment design, the equipment, and the bacterial cultures.  It was all in vain, though.  Hershey was a notoriously cautious researcher and his experiments were always well-planned and precisely executed. The results were no mistake and the import of their work was clear: Hershey and Chase had elucidated direct, irrefutable evidence that DNA, not protein, is the source of genetic material.

Alfred Hershey, 1960.

Alfred Hershey, 1960.

Later that year, the pair reported their findings in a short paper in The Journal of General Physiology titled “Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage.”  This publication catalyzed a storm of activity in the scientific community, with researchers all over the world clamoring for details on the experiments.  Alfred Hershey’s lectures on the subject were attended by the greatest scientific minds in Europe and North America; Pauling was one of hundreds to hear him speak.  In the years following the discovery, DNA became a major focus for researchers all over the world, resulting in Pauling’s own attempts to deduce its structure and the eventual success of Watson and Crick.  Even today, our genetic research traces its roots from the work of Alfred Hershey and Martha Chase.

For more on the story of the quest for DNA, see our documentary history website on the subject.  For more information about Linus Pauling, visit the Linus Pauling Online portal.

The Pauling-Corey Structure of DNA

Today, the structure of DNA series is continued with the model proposed by Linus Pauling and Robert Corey in 1953. As a result of insufficient data and an overloaded research schedule, Pauling’s structure turned out to be incorrect. However, it is interesting to see the ways in which one of the world’s leading scientists went wrong with his approach to the structure of this hugely-important molecule.

Linus Pauling played around with nucleic acids as early as 1933 when he hypothesized a structure for guanine, a base ring. In the summer of 1951, he again became interested in DNA when he heard that Maurice Wilkins at King’s College had developed a few good photographs of nucleic acids. Unfortunately for Pauling, Wilkins was unwilling to share his research. In November of that same year, a structure of nucleic acids was proposed and then published by Edward Ronwin. Pauling could tell almost immediately that Ronwin’s structure wasn’t correct, but it did contain a few good ideas that got him thinking about other possible structures. Pauling hypothesized that DNA was likely helical in shape, with the large base groups facing out and the phosphate groups stacked in the core. At this juncture, however, Pauling was again distracted by other research and let the project drop.

Until 1953 nucleic acids weren’t considered to be very important. At the time, proteins, rather than DNA, were considered by most scientists to be the carriers of genetic material. Partly because of this, Pauling’s attention was focused on proteins, not DNA. In May of 1952, Pauling was scheduled to attend a special meeting of the Royal Society where he would address questions pertaining to his protein structures. This trip would also give him an opportunity to discuss DNA with Rosalind Franklin, who was Maurice Wilkins’ assistant. She had recently developed an especially clear photograph of DNA which likely would have saved Pauling from making some key mistakes when determining the structure of DNA.

As a result of his very-public anti-war and anti-nuclear activities, Pauling’s initial request for a passport was denied, though he was granted a limited passport only ten weeks later. However, when Pauling arrived in England, he did not visit King’s College. He was preoccupied with his protein research and he assumed that Wilkins still wouldn’t be willing to share his data.

Soon after his visit to England, Pauling was granted a full passport and traveled to France. Here he was informed, through an experiment performed by Alfred Hershey and Martha Chase, that DNA was in fact the genetic master molecule. Upon learning this, Pauling decided that he would solve the structure of DNA. However, when he returned to California, he continued to work primarily with proteins. It wasn’t until November 25, 1952 that Linus Pauling would make a serious attempt at the structure of DNA.

Unfortunately, when Pauling did decide to put in some time with DNA, he still had insufficient data to correctly deduce its structure. Using only a few blurry x-ray patterns done by William Astbury in the 1930s and a photograph published by Astbury in 1947, Pauling decided that DNA was indeed a three-chain helix with the bases facing outward and the phosphates in the core.

Astbury's 1947 photographs of DNA.

Astbury's 1947 photographs of DNA.

However, it was immediately clear that making room for so many phosphates in the center of the molecule would be quite a task. Pauling spent a great deal of time manipulating his model, and eventually produced a satisfactory representation. He then asked Robert Corey, his chief assistant at Caltech, to perform detailed calculations on the proposed atomic positions. Corey’s calculations proved that, despite Pauling’s efforts, there still wasn’t enough room for all of the atoms. Pauling, refusing to consider the possibility that his structure was incorrect, resorted to further manipulation. (In fact, Pauling refused to concede even after a colleague pointed out that there was no room for sodium ions in the core of his model, a feature that is essential in the creation of sodium salts of DNA.) Convinced that the finer details would later fall into place, Pauling and Corey spent the last week of the year writing up their structure, and on the last day of 1952, they submitted “A Proposed Structure for the Nucleic Acids” to the Proceedings of the National Academy of Sciences.

Diagram of the Pauling-Corey structure for DNA, as published in PNAS.

Diagram of the Pauling-Corey structure for DNA, as published in PNAS.

The paper was uncharacteristic of Pauling. Instead of his usual confidence, he stated that the structure was “promising” but also “extraordinarily tight.” Pauling likewise noted that the model accounted only “moderately well” for the x-ray data, and that the atomic positions were “probably capable of further refinement.” As it turned out, Pauling wasn’t seeking perfection with his structure. In reality, he wanted to be the first to publish a roughly correct structure of DNA. Rather than having the final say, he wanted the first.

Once the article was published in February of 1953, it became more and more apparent that Pauling’s structure wasn’t even roughly correct. By this time, Pauling had already moved on to other projects, and was surprised at the fact that his paper was received so poorly. Once he caught wind of the talk surrounding his structure, he decided to return to the topic of DNA. Despite the negative reaction, Pauling still believed that his structure was essentially right. However, he soon received better nucleotide samples from Alex Todd, an organic chemist at Cambridge, and began a more rigorous approach to determining the structure of DNA.

Unfortunately, by this time it was too late. Upon the publication of Pauling’s unsatisfactory model, James Watson and Francis Crick were given the green light to pursue their own model of DNA. Before long, Pauling saw that the work they were doing was very promising. A few days after first seeing their structure, Pauling received an advance copy of the Watson and Crick manuscript. At this point, he still retained a fair amount of confidence in his own model, but acknowledged that there was now another possible model. In a letter to Watson and Crick written on March 27, 1953, Pauling noted

I think that it is fine that there are now two proposed structures for nucleic acid, and I am looking forward to finding out what the decision will be as to which is incorrect.

However, he had still not seen Rosalind Franklin’s data; Watson and Crick had. (Interestingly enough, Robert Corey had traveled to England in 1952 and viewed Franklin’s photographs. It is unknown whether or not he purposely failed to provide Pauling with the details of the images.)

This fact would soon change. In April of 1953, Pauling was to attend a conference on proteins in Belgium. On his way, he stopped in England to see the Watson and Crick model of DNA as well as Franklin’s photographs. After examining both, Pauling was finally convinced that his structure was wrong and that Watson and Crick had solved DNA.

Linus Pauling, although disappointed with the results, accepted his defeat graciously. He gave Watson and Crick full credit for their discovery and assisted them in tying up a few loose ends with their model. For Pauling, this event was a single failure in a sea of successes. In fact, the very next year, he would win the Nobel Prize in Chemistry – the first of his two Nobel Prizes. Despite his embarrassing mistakes, Pauling was to remain in good standing with the scientific community.

Please check back on Thursday for the conclusion of the DNA structure series – an examination of the correct structure deduced by Watson and Crick. For more information on DNA, please visit the website Linus Pauling and the Race for DNA. For more information on Linus Pauling, visit the Linus Pauling Online Portal.

The Martha Chase Effect

Martha Chase and Alfred Hershey, 1953.

Martha Chase and Alfred Hershey, 1953.

The Phenomenon

It pretty well goes without saying that the primary mission of the Oregon State University Libraries Special Collections is to preserve, describe and make available the Ava Helen and Linus Pauling Papers.  Beginning, more or less, with the Pauling centenary in 2001, the main focus of our Pauling-related work has been description and accessibility via the web.  In so doing, we have scanned over one terabyte of data and created, at minimum, tens of thousands of static html pages devoted to the life, work and legacy of Linus Pauling and, to a lesser extent, Ava Helen Pauling.

Knowing this, one might reasonably assume that the top search engine query channeling into the content that we have created would be “Linus Pauling,” or some variant therof.  A reasonable assumption indeed but, as it turns out, quite wrong.  In 2008, as in 2007 and 2006 (a close second in 2005), the top keyword query for those who found our content through search was…”Martha Chase.”

Martha Chase was a geneticist who, in collaboration with Alfred Hershey, made an important contribution to the DNA story as it played out in the early 1950s.  Prior to Chase and Hershey’s work, scientists were mixed on the question as to what, exactly, was the genetic material.  Many researchers, Pauling included, initially felt that the stuff of heredity was contained in proteins.  Others, of course, eventually theorized that DNA was the source of genetic information.  Using an ordinary blender as their primary tool, Hershey and Chase devised a famous experiment which proved conclusively that DNA did, in fact, carry the genetic code.

Diagram of the Hershey-Chase Blender Experiment.  Image by Eric Arnold.

Diagram of the Hershey-Chase Blender Experiment. Image by Eric Arnold.

The breadth of Chase-related content that we have digitized is infinitesimally-small relative to the “reams” devoted to Pauling — this page and this page are pretty much it.  And yet, in the context of search, Martha Chase is the top draw to our resources.  It would seem then, that in the marketplace for information — at least that which is retrieved by search — supply and demand for Martha Chase approach their equilibrium at the two pages devoted to her work on our “Linus Pauling and the Race for DNA” site.

Looking through the web statistics, the phenomenon is remarkably consistent.  Not only has “Martha Chase” been the top search query for our domain over, essentially, the past four years, it was also the top search query for our domain over the final week of 2008.  Indeed, the trend has strengthened to the point where today, those who conduct the simple “Linus Pauling” search in Google will note “martha chase” as a recommended search related to Pauling, though in reality the two had little or no interaction at all.

Learning from the Chase Effect

Looking forward, the Chase Effect has become something that we’re thinking more and more about as we begin to develop new projects for the web.  Our top objective will always be to document Pauling’s impact on any number of fields, but in so doing there likely exists a great deal of opportunity for serving different user groups from what might be called “Chaseian” corners of the web.

To use the old many-fish-in-the-sea analogy, there is a lot of content related to Pauling on the Internet, and though we are the primary contributor to this content, we do compete for pageviews with scads of other extremely diverse projects.  (Take a look at the results for the simple “Linus Pauling” Google search to see how diverse the content providers really are.)  So it’s pretty clear that the Pauling sea is quite large and filled with all manner of creatures.

By comparison, Martha Chase represents a much smaller body of water and, in particular, image searches for her — which is probably where the lion’s share of our successful Chase referrals come from — are dominated by the domain.

The idea for future work is to think of the Pauling Papers as a collection of collections in attempting to uncover more Martha Chases.

To an extent we have already, somewhat unwittingly, done this with certain of the Key Participants highlighted on our various documentary history websites.  The Harvey Itano Key Participants page, for example, is the second result returned by Google for “Harvey Itano” searches.  Erwin Chargaff‘s page is seventh,  Arnold Sommerfeld‘s page is eighth and Edward Condon‘s is tenth, to name a few more examples.  In each instance, by developing mini-portals related to specific colleagues important to Pauling’s work, we have created resources that help meet the information demand of a non-Pauling user base.

As we standardize our metadata platforms — upgrading older projects and maintaining the standard for new — and, in the process, increase our capacity to “remix” our digital objects, the idea of enhancing existing mini-portals and creating new ones will emerge as an important consideration for our digitization workflow.  This is something that we’ll be talking a lot more about in the months to come.