The Hershey-Chase experiment, published in 1952, provided the evidence that finally convinced the scientific community that DNA, not protein, carries genetic information. Alfred Hershey and Martha Chase used viruses that infect bacteria to show that DNA is the molecule injected into cells during infection, while the protein coat stays behind. It remains one of the most elegant experiments in the history of molecular biology.
The Question the Experiment Answered
By the early 1950s, scientists knew that genes existed and that they were made of either DNA or protein, but there was genuine debate about which one actually carried hereditary information. Proteins seemed like strong candidates because they’re complex molecules built from 20 different amino acids, giving them enormous variety. DNA, by contrast, is built from just four chemical building blocks and seemed too simple to encode the instructions for life.
Eight years before Hershey and Chase published their work, Oswald Avery, Colin MacLeod, and Maclyn McCarty had already shown that DNA could transfer genetic traits between bacteria. Despite this powerful evidence, many influential scientists remained unconvinced. The skepticism lingered partly because the earlier experiment couldn’t completely rule out protein contamination as an explanation. What the field needed was a fundamentally different approach using different organisms, one that could physically separate DNA from protein and test each one independently.
Why Bacteriophages Were the Perfect Tool
Hershey and Chase chose to work with a virus called T2 bacteriophage, which infects the bacterium E. coli. This virus has a beautifully simple structure: a protein shell on the outside and DNA packed inside. When T2 infects a bacterium, it attaches to the cell surface and injects something into the cell. Whatever gets injected takes over the bacterium’s machinery and forces it to produce hundreds of new virus copies.
The key insight was that T2 naturally separates its components during infection. The outer shell stays attached to the bacterial surface while the genetic instructions go inside. If you could figure out which part, the protein shell or the DNA core, actually entered the cell, you’d know which molecule carried the genetic information. The challenge was telling the two apart once the infection started.
Tracking DNA and Protein With Radioactive Labels
Hershey and Chase solved the tracking problem with a clever bit of chemistry. DNA contains phosphorus but no sulfur. Protein contains sulfur but essentially no phosphorus. This meant they could use two different radioactive isotopes as molecular tags: phosphorus-32 to label DNA exclusively, and sulfur-35 to label protein exclusively.
They grew two batches of T2 phages. One batch was raised in a medium containing phosphorus-32, so the radioactive atoms were incorporated into the viruses’ DNA. The other batch was grown with sulfur-35, making their protein coats radioactive. Each batch carried a different glowing tag on a different molecular component, and the two tags would never overlap.
Blenders, Centrifuges, and Separation
With their labeled viruses ready, Hershey and Chase let each batch infect separate cultures of normal, non-radioactive bacteria. They gave the phages enough time to attach and begin the infection process. Then came the critical separation step.
They placed the infected bacteria into an ordinary Waring kitchen blender and stirred them. The shearing forces of the blender were strong enough to rip the empty virus shells off the outside of the bacterial cells, but gentle enough to leave the bacteria themselves intact. (A centrifuge would have been too violent and destroyed the cells.) After blending, they spun the samples in a centrifuge. The heavier bacteria formed a pellet at the bottom of the tube, while the lighter virus shells and loose material stayed suspended in the liquid above, called the supernatant.
Now they just needed to check where the radioactivity ended up.
What the Results Showed
The results were striking. When bacteria were infected with phages labeled with phosphorus-32 (the DNA tag), most of the radioactivity appeared in the bacterial pellet. The DNA had gone inside the cells. Even more telling, much of that radioactivity was passed on to the next generation of phages produced by the infected bacteria.
When bacteria were infected with phages labeled with sulfur-35 (the protein tag), practically no radioactivity was found inside the bacterial cells. The protein had stayed outside, clinging to the cell surface, and was stripped away by the blender. It never entered the cell and played no role in producing new viruses.
The conclusion was clear: the DNA component of the virus is injected into the bacterial cell, while the protein component remains outside. Since DNA alone was sufficient to hijack the cell and produce a new generation of complete viruses (with both DNA and protein), DNA had to be the molecule carrying genetic instructions.
Why This Experiment Succeeded Where Others Hadn’t
The Avery-MacLeod-McCarty experiment of 1944 had pointed to the same answer, but it relied on purifying DNA from bacteria and showing it could transform other bacteria. Critics argued that trace amounts of protein might have contaminated the purified DNA, and that protein could still be the real transforming agent. The experiment was chemically rigorous, but it couldn’t completely silence the doubters.
The Hershey-Chase experiment sidestepped this problem entirely. Rather than trying to purify one molecule away from another, it used a biological system that naturally separates DNA from protein during infection. The radioactive labels made the separation unambiguous. There was no question of contamination because the two labels tracked completely different elements, phosphorus and sulfur, that are exclusive to different molecules.
As Hershey himself later reflected, “some redundancy of evidence was needed to be convincing, and diversity of experimental materials was often crucial to discovery.” The fact that two completely different experimental systems, bacterial transformation and phage infection, both pointed to DNA was what finally settled the debate.
Recognition and Legacy
In 1969, Alfred Hershey shared the Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria for their collective discoveries about the biology of viruses. The Nobel citation recognized the phage work as confirming DNA as the bearer of genetic information. Martha Chase, who was a research assistant at the time of the experiment, was not included in the prize, a fact that has drawn attention from historians of science.
The experiment’s impact extended well beyond settling the DNA-versus-protein debate. It helped launch the era of molecular biology. Within a year of its publication, James Watson and Francis Crick proposed the double-helix structure of DNA, a discovery that would have carried far less urgency if the scientific community hadn’t already accepted that DNA was the molecule of heredity. The Hershey-Chase experiment gave scientists a reason to care deeply about DNA’s structure, because they now knew that understanding that structure meant understanding the physical basis of life itself.