The Hershey-Chase Experiment Explained

In 1952, at Cold Spring Harbor Laboratory on Long Island, Alfred Hershey and Martha Chase ran a bacteriophage through a kitchen blender and settled one of the most consequential open questions in biology: is genetic material made of DNA or protein? Their result, published in the Journal of General Physiology that same year, showed that when a virus infects a bacterial cell, it is DNA that gets injected inside to direct the infection, while the protein shell stays outside. That finding did not single-handedly invent the idea that DNA carries heredity, but it delivered the sharpest, most direct piece of evidence the field had yet seen, and it helped clear the runway for Watson and Crick's structural discovery the following year.

The protein-versus-DNA debate that the experiment was built to resolve

For much of the first half of the twentieth century, most biologists who thought seriously about the physical basis of heredity assumed the answer would turn out to be protein. The reasoning was not careless. Proteins are built from 20 different amino acids that can be strung together in an enormous number of sequences and folded into an enormous number of three-dimensional shapes, and biologists reasoned that a molecule capable of encoding the staggering variety of inherited traits seen across living things would need that kind of combinatorial richness. DNA, by comparison, seemed almost suspiciously plain: a long chain built from only four repeating chemical units, called nucleotide bases (adenine, thymine, guanine, and cytosine), strung together in a pattern that, in the early twentieth century, some chemists mistakenly believed simply repeated in a fixed, boring order. A molecule that monotonous did not look, on its face, like it could hold the instructions for an entire organism.

That assumption had already taken a serious hit eight years earlier. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty, working at the Rockefeller Institute, published experiments showing that a purified DNA extract from one strain of Streptococcus pneumoniae could transform a harmless strain of the same bacterium into a disease-causing one, permanently and heritably. Avery's team had gone to considerable lengths to rule out protein contamination in their DNA extracts, using enzymes that degrade protein, RNA, and DNA separately to show that only the DNA-degrading enzyme destroyed the transforming activity. It is one of the more underappreciated ironies in the history of molecular biology that this result, now recognized as a foundational discovery, was met with real skepticism at the time. Many biologists, including some prominent geneticists, suspected the transforming activity might still be traceable to some trace protein contaminant that had escaped detection, a possibility that felt more intuitively likely to them than "plain" DNA doing the job. The field needed a second, independently designed experiment, ideally using a completely different biological system, before the DNA hypothesis would be widely accepted as settled rather than merely plausible.

That is the gap Hershey and Chase's work filled, not by repeating Avery's transformation experiment, but by attacking the same underlying question from an entirely different angle, using a virus instead of a transformable bacterium and a labeling strategy that let them track two different molecules in the same experiment simultaneously.

Why a bacteriophage made the perfect experimental system

Hershey and Chase worked with bacteriophage T2, a virus, sometimes shortened simply to "phage," that infects the common gut bacterium Escherichia coli. Bacteriophages are viruses that infect bacteria specifically, and T2 belongs to a well-studied family of phages that had already become a workhorse organism in the emerging field that researchers were starting to call molecular biology by the early 1950s. Structurally, T2 is built from two components and essentially nothing else: an outer protein coat, shaped roughly like a lunar lander with a head and a tail, and an interior core of DNA packed tightly inside that head.

What made T2 experimentally useful was its infection mechanism. Rather than being engulfed whole by the bacterial cell the way some viruses are, a T2 phage attaches to the outside of an E. coli cell using its tail fibers, punctures the bacterial cell wall, and injects its internal contents into the cell's interior while its outer protein shell remains stuck to the exterior of the bacterium, essentially acting like an empty syringe left behind after an injection. That mechanical detail is what made the experiment possible at all. If a phage's protein coat and DNA had entered the bacterial cell together as a single unit, there would have been no clean way to separate "what went in" from "what stayed out," and no way to determine which of the two components was actually responsible for directing the bacterial cell to start manufacturing new phage particles.

Hershey and Chase exploited one more piece of basic biochemistry: DNA contains phosphorus as a structural part of its backbone but contains essentially no sulfur, while protein contains sulfur, present in the amino acids cysteine and methionine, but no phosphorus. That chemical asymmetry meant the two molecules could be tagged independently and unambiguously with radioactive isotopes, a technique known as radioactive labeling, in which atoms of an element are replaced with a radioactive version of the same element so their location can later be tracked with a radiation detector.

How the labeling and the blender step actually worked

Hershey and Chase grew two separate batches of T2 phage under different labeling conditions. One batch was grown in bacteria cultured in a medium containing radioactive phosphorus-32, so that as new phage particles assembled inside those bacteria, phosphorus-32 was incorporated into the phage DNA. A second, entirely separate batch was grown in a medium containing radioactive sulfur-35, so that sulfur-35 became incorporated into the amino acids that make up the phage protein coat. Because of the sulfur-phosphorus asymmetry described above, each isotope labeled one and only one component of the virus: phosphorus-32 marked DNA and nothing else, sulfur-35 marked protein and nothing else.

Each labeled phage batch was then allowed to infect a fresh, unlabeled culture of E. coli for a short period, just long enough for the phages to attach to the bacterial surface and inject their contents. At that point came the step that made the experiment famous well beyond biology departments: the infected bacterial suspension was agitated at high speed in a Waring blender, the same general kind of household kitchen blender used for food preparation, adapted for laboratory use. The blending was not applied randomly or for dramatic effect; it delivered a calibrated dose of shear force strong enough to physically strip the empty phage protein shells off the outer surface of the bacterial cells without rupturing the bacteria themselves.

After blending, the mixture was spun in a centrifuge, a device that separates particles by mass and density by spinning them at high speed. The heavier, intact bacterial cells settled to the bottom as a pellet, while the smaller, detached phage protein shells stayed suspended in the surrounding liquid, called the supernatant. This centrifugation step turned an biological question, "which component entered the cell?", into a purely physical, measurable one: whichever isotope ended up in the bacterial pellet had gone inside the cell, and whichever isotope ended up in the supernatant had stayed outside.

Experimental batch	Radioactive isotope used	What it uniquely labeled	Where the label ended up after blending and centrifuging
Phosphorus-32 labeled phage	³²P	Viral DNA only (phosphorus is absent from protein)	Found predominantly in the bacterial pellet, indicating DNA had entered the cell
Sulfur-35 labeled phage	³⁵S	Viral protein coat only (sulfur is absent from DNA)	Found predominantly in the supernatant liquid, indicating protein stayed outside the cell

A clean chemical asymmetry between the two molecules, phosphorus in DNA and sulfur in protein, let one physical separation step answer a question biologists had debated for decades on theoretical grounds alone.

In the sulfur-35 batches, the great majority of the radioactive protein label was recovered in the surrounding liquid after blending and centrifuging rather than inside the bacterial cells, while the phosphorus-32 DNA label stayed predominantly associated with the bacterial pellet. Hershey and Chase reported the separation was not perfect in either direction, a small amount of each isotope showed up on the "wrong" side, but the split was large and consistent enough across repeated trials to point clearly toward DNA as the molecule that entered the cell (Hershey and Chase, *Journal of General Physiology*, 1952).

Why was the blender step so important to the experiment's design?

Without a way to physically remove the phage protein shells still clinging to the bacterial cell exterior, researchers had no way to distinguish material that had genuinely been injected into the cell from material that was simply still attached outside it. The blender supplied just enough mechanical shear to strip off the external shells while leaving the bacterial cells intact, and the subsequent centrifugation step then let phosphorus and sulfur be measured separately in the pellet versus the supernatant.

Did Hershey and Chase get a perfectly clean 100 percent split between DNA and protein?

No, and this is a detail sometimes smoothed over in retellings. A small percentage of sulfur-35 remained associated with the bacterial pellet, and a small percentage of phosphorus-32 turned up in the supernatant, meaning the separation was strong but not absolute. Hershey and Chase treated this imperfection honestly in their published paper rather than presenting the result as flawless, and the strength of the conclusion rested on the size and consistency of the majority split, not on a perfect 100-to-0 outcome.

What the result actually demonstrated, and what it didn't

The core finding was that radioactively labeled DNA entered the bacterial cells during infection while radioactively labeled protein remained outside, and that infected bacteria went on to produce new, fully formed phage particles, complete with both DNA and protein, using material that was overwhelmingly derived from the phosphorus-labeled DNA fraction rather than the sulfur-labeled protein fraction. That pattern strongly supported the conclusion that DNA, not protein, was the genetic material actually responsible for directing the bacterial cell to manufacture new virus particles.

It is worth being precise here rather than overstating the result, since Hershey and Chase themselves were notably cautious in their original paper. The experiment demonstrated that the bulk of the injected material driving new phage production was DNA. It did not, by itself, prove that literally zero protein of any kind entered the cell, and Hershey's own writing in the 1952 paper reflects that caution: he described the results as consistent with the idea that DNA plays the primary hereditary role but stopped short of claiming the experiment offered absolute, unqualified proof. That kind of restraint is itself a useful lesson in how careful experimentalists communicate results that are strong but not mathematically airtight.

Combined with the earlier Avery-MacLeod-McCarty transformation work from 1944, the Hershey-Chase result gave the scientific community two independently designed experiments, using two very different biological systems, pointing at the same conclusion. That convergence mattered enormously for how quickly the field's consensus shifted. A single striking result can always be attributed to some unrecognized experimental artifact; two unrelated experimental designs converging on the same answer is much harder to explain away, and by the early 1950s the weight of evidence had shifted decisively toward DNA as the molecule of heredity. That shift set the immediate scientific context in which James Watson and Francis Crick determined DNA's double-helix structure in 1953, a discovery that answered the next obvious question once DNA's role as the genetic material was accepted: given that DNA carries heredity, what physical structure lets it store and copy that information?

The Nobel Prize, and what it was actually awarded for

Alfred Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria. It is worth stating precisely what that prize recognized, because it is a common and understandable point of confusion: the Nobel committee's citation honored the three scientists "for their discoveries concerning the replication mechanism and the genetic structure of viruses," a broad body of work on bacteriophage biology built up over roughly two decades, not the 1952 blender experiment specifically as an isolated achievement (Nobel Prize in Physiology or Medicine, 1969).

Delbrück, Luria, and Hershey are often grouped together as founders of what became known as the "phage group," a loose, highly collaborative community of researchers centered on using bacteriophages as a simplified model system for studying the basic mechanics of genetics and viral reproduction. Delbrück and Luria had done foundational quantitative work in the early 1940s, including the Luria-Delbrück fluctuation test, which provided some of the first rigorous statistical evidence that bacterial resistance to viral infection arises from random, spontaneous mutation rather than being induced by the virus itself, a finding with clear conceptual kinship to how natural selection later came to be understood in the context of antibiotic resistance. Hershey's contributions, including but not limited to the 1952 blender experiment, extended that broader phage-genetics research program. The Nobel Prize recognized this larger, cumulative body of work across all three laureates, and readers should be cautious of any account that frames the prize as having been awarded specifically and only for the blender experiment.

Limitations, caveats, and what the experiment didn't settle

A rigorous accounting of the Hershey-Chase experiment has to include what it left unresolved, not just what it proved. First, as already noted, the isotope separation was strong but imperfect, leaving room for later, more refined experiments and imaging techniques to further confirm the result with cleaner data. Second, the experiment was conducted in a single bacteriophage-bacterium system, T2 infecting E. coli; extending the conclusion to all viruses, let alone to genetic material broadly across all of life, required additional evidence from other systems accumulated over subsequent years, including further confirmation from Avery-style transformation work in other bacterial species.

Third, and this is a point genuinely worth appreciating rather than treating as a footnote, the Hershey-Chase experiment answered "which molecule carries the instructions," but it said essentially nothing about how that molecule physically stores and copies information. That mechanistic question, how a molecule built from just four repeating chemical units could encode something as complex as a full set of hereditary instructions, remained wide open until Watson and Crick's 1953 structural model showed how a double helix with complementary base pairing could both store information in the sequence of its bases and replicate that information with high fidelity. The blender experiment and the double-helix discovery are often taught back-to-back for good reason: one established what the genetic material was, and the other explained how that material could actually do the job.

The takeaway

The Hershey-Chase experiment used two isotopes, phosphorus-32 to label bacteriophage DNA and sulfur-35 to label its protein coat, then physically sheared infected bacteriophages off E. coli cells with a kitchen blender to show that DNA, not protein, was the component that entered the bacterial cell and directed the production of new virus particles. Building on Oswald Avery's earlier 1944 transformation experiments, this result helped shift the scientific consensus firmly toward DNA as the molecule of heredity, clearing conceptual ground for Watson and Crick's double-helix discovery the following year. Alfred Hershey's later 1969 Nobel Prize, shared with Max Delbrück and Salvador Luria, recognized that broader body of phage-genetics work rather than the blender experiment in isolation, a distinction worth keeping straight given how often the two get collapsed into one in popular retellings.

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FAQ

Q: What question did the Hershey-Chase experiment answer?

It answered whether DNA or protein carries the genetic instructions a bacteriophage injects into a bacterial cell during infection, a question that had divided biologists for decades because DNA's comparatively simple four-base chemical structure led many to doubt it could be complex enough to encode heredity, while protein's 20-amino-acid alphabet seemed structurally richer.

Q: How did Hershey and Chase distinguish DNA from protein in their experiment?

They exploited a chemical asymmetry: DNA contains phosphorus but no sulfur, while protein contains sulfur but no phosphorus. By growing separate bacteriophage batches with radioactive phosphorus-32 or radioactive sulfur-35, they could track exactly which molecule entered infected bacterial cells using a radiation detector after physically separating the cells from detached phage shells.

Q: Why did the experiment use a kitchen blender?

A Waring blender provided just enough mechanical shear force to strip the empty phage protein shells off the outside of infected bacterial cells without destroying the bacteria themselves, allowing Hershey and Chase to physically separate, via centrifugation, genetic material that had entered the cell from material still attached outside.

Q: What did the Hershey-Chase results actually show?

Radioactive phosphorus, marking phage DNA, was found predominantly inside the bacterial pellet after blending and centrifuging, while radioactive sulfur, marking phage protein, remained predominantly in the surrounding liquid. The split was strong but not perfect, and it strongly indicated DNA was the molecule injected into the cell to direct new virus production.

Q: Did the Hershey-Chase experiment prove DNA is the genetic material by itself?

Not entirely on its own. Hershey and Chase were notably cautious in their 1952 paper, describing the result as strong support rather than absolute proof, since isotope separation was imperfect. The finding gained its full persuasive weight in combination with Oswald Avery's independent 1944 bacterial transformation experiments, which reached the same conclusion through a completely different biological system.

Q: What was Alfred Hershey's 1969 Nobel Prize actually awarded for?

Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria "for their discoveries concerning the replication mechanism and the genetic structure of viruses," a citation covering roughly two decades of collaborative bacteriophage-genetics research, not the 1952 blender experiment alone.

Q: How did this experiment influence the discovery of DNA's structure?

By providing strong direct evidence that DNA, not protein, carries genetic information, the Hershey-Chase result helped focus scientific attention on understanding DNA's physical structure and how it could store and copy hereditary information, setting the stage for James Watson and Francis Crick's double-helix model in 1953, just one year later.