A kerfuffle over whether or not the University of California at Berkeley was a place for serious scientists led directly to the creation of our sibiling publication, The Journal of General Physiology. Dr Flexner’s old Professor, Jacques Loeb, was banned from the pages of The American Journal of Physiology when he moved from the University of Chicago to the West Coast in 1902 (and thus “no longer represented a ‘major’ institution”). Eight years later, Flexner recruited Loeb to the Rockefeller Institute, and eight years after that, in 1918, Loeb launched The Journal of General Physiology.
So it’s numerologically appropriate that eight years after Avery and colleagues demonstrated in JEM that DNA was the “transforming principle”, Alfred Hershey and Martha Chase should publish their own work on the chemical nature of the hereditary material of viruses in JGP. These are the beloved Waring Blendor (sic) experiments*- Hershey had found the setting where phage** (and phage “ghosts”, i.e. protein shells) clinging to the bacteria would be spun-off without bursting the bacteria themselves.
T2 phage (illustration by Madalena Parreira).
The key to understanding Hershey and Chase’s experimental setup is quite simple: proteins contain sulfur & DNA doesn’t; the reverse is true of phosphorous. Growing phage in the presence of radioactive sulfur-35 or phosphorus-32 isotopes allows tracking of the different compounds’ fate simply by measuring radiation in bacterial cultures. By shaking the bacteria in the blender (or blendor), and then spinning the whole mix in a centrifuge, Hershey and Chase could use a Geiger counter to find which viral molecules were passed on the next generations (via synthesis in the bacteria). They conclude that:
“The experiments reported in this paper show that one of the first steps in the growth of T2 is the release from its protein coat of the nucleic acid of the virus particle, after which the bulk of the sulfur-containing protein has no further function.”
or, in other words
“We have concluded above that the bulk of the sulfur-containing protein of the resting phage particle takes no part in the multiplication of phage, and in fact does not enter the cell. It follows that little or no sulfur should be transferred from parental phage to progeny.”
Though Hershey & Chase’s work is often referred to as “definitive” confirmation of the Avery group’s work, it is worth noting that the levels of contamination in their work vastly exceed those published in 1944- a fact the authors do not attempt to conceal:
“The radiochemical purity of the preparations is somewhat uncertain, owing to the possible presence of inactive phage particles and empty phage membranes.”
“The following experiments show that this is readily accomplished by strong shearing forces applied to suspensions of infected cells, and further that infected cells from which 80 per cent of the sulfur of the parent virus has been removed remain capable of yielding phage progeny.”
Even the best-case scenario
“The experiments described below show that this expectation is correct, and that the maximal transfer is of the order 1 per cent”
was unimaginable in Avery’s meticulous, painstaking biochemical work, and contamination had led Hershey to the wrong conclusion only a year before:
“The properties described explain a mistaken preliminary report (Hershey et al., 1951) of the transfer of S 35 from parental to progeny phage.”
Perhaps the most salient aspect of Hershey and Chase’s landmark study, what immediately stands out when reading the original manuscript and the rationale behind the experimental design, is that these experiments were not set up to test if DNA was the genetic material at all. They were optimized to examine if protein could be. But the idea of DNA as the genetic material was much more acceptable in 1952, and the use of radioactive isotopes labeling was very appealing in the heyday of the Atomic Age. So for many scientists at the time, Hershey and Chase’s 1952 JGP classic marked the definitive acceptance of deoxyribonucleic acid as the chemical agent of heredity.
* Matthew Cobb kills this romantic image: “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 (…) the apparatus used by Hershey and Chase was a highly specialized, unstylish bronze-coloured piece of laboratory equipment’”. Dr Cobb may or may not be available to come to your kid’s party and tell him there is no Santa.
** A group of viruses that infect bacteria (short for bacteriophage).