In 1952, Alfred Hershey and Martha Chase started with a virus called a bacteriophage to settle one of the most heated debates in biology: the chemical identity of the genetic material. That's why at the time, the scientific community was deeply divided. In practice, proteins, with their complex structures and 20 distinct amino acids, seemed the obvious candidates for carrying hereditary information. DNA, by contrast, was viewed by many as a "stupid molecule"—too simple, too repetitive, and structurally monotonous to encode the complexity of life. The Hershey-Chase experiments, often referred to as the Waring Blender experiments, provided the definitive, elegant proof that DNA, not protein, is the molecule of inheritance.
The Historical Context: Protein vs. DNA
Before 1952, the prevailing "tetranucleotide hypothesis" suggested DNA was merely a structural scaffold—a repetitive polymer of four bases (adenine, thymine, guanine, cytosine) incapable of storing specific information. Proteins, with their infinite variability, were the darlings of the genetic theory.
This perspective began to shift in 1944 when Oswald Avery, Colin MacLeod, and Maclyn McCarty published their landmark work on Streptococcus pneumoniae. Now, they demonstrated that a "transforming principle" extracted from heat-killed virulent bacteria could convert non-virulent strains into virulent ones. Crucially, they showed this principle was destroyed by enzymes that digest DNA (DNase) but not by enzymes that digest protein (protease) or RNA (RNase) Not complicated — just consistent..
Despite the rigor of the Avery-MacLeod-McCarty experiment, skepticism persisted. The scientific world needed a completely different experimental system—one that physically separated the genetic material from the protein coat during the act of infection. Critics argued that trace protein contamination in the DNA preparations might be the actual transforming agent. This is precisely where Hershey and Chase entered the picture.
This is where a lot of people lose the thread.
Why the Bacteriophage Was the Perfect Tool
Hershey and Chase started with a virus called a bacteriophage (specifically, the T2 phage infecting Escherichia coli) for strategic reasons. Here's the thing — bacteriophages, or "phages," are viruses that infect bacteria. Their structure is remarkably simple: a protein shell (capsid) enclosing a core of nucleic acid.
This simplicity offered a unique advantage. During infection, the phage attaches to the bacterial cell wall and injects its genetic material into the cytoplasm, leaving the empty protein coat (ghost) attached to the exterior. This physical separation of protein and nucleic acid during the infection process provided a natural "labeling" mechanism that no chemical purification could achieve Practical, not theoretical..
To build on this, phages replicate rapidly inside bacteria, producing hundreds of progeny within minutes. This allowed Hershey and Chase to track exactly which molecular component—protein or DNA—entered the host cell and directed the production of new viral particles That alone is useful..
The Ingenious Use of Radioactive Isotopes
The core brilliance of the Hershey-Chase experiment lay in the use of radioactive isotopes to differentially label the two candidate molecules. They exploited the distinct elemental composition of DNA and protein:
- Phosphorus-32 (³²P) labels DNA: DNA contains phosphorus in its phosphate backbone (sugar-phosphate-sugar). Proteins contain virtually no phosphorus.
- Sulfur-35 (³⁵S) labels Protein: Proteins contain sulfur in the amino acids methionine and cysteine. DNA contains no sulfur.
By growing separate batches of phages in media containing either ³²P or ³⁵S, they created two distinct populations of "tagged" viruses:
- Batch A: Phages with radioactive DNA (³²P labeled) and non-radioactive protein.
- Batch B: Phages with radioactive Protein (³⁵S labeled) and non-radioactive DNA.
The Experimental Protocol: The Waring Blender Step
The experimental procedure was a masterclass in kinetic control. It unfolded in several precise stages:
1. Infection (Adsorption and Injection)
The labeled phages were mixed with unlabeled E. coli bacteria. The mixture was incubated for a short period—just long enough for the phages to attach to the cell walls and inject their genetic material (typically 8 to 10 minutes). During this phase, the viral DNA enters the cytoplasm, while the protein capsid remains outside That alone is useful..
2. Shearing (The Blender Treatment)
This is the most famous step. To separate the empty protein coats (ghosts) from the bacterial cells, Hershey and Chase used a Waring Blender. They subjected the infected suspension to high-speed shear forces Simple as that..
- The mechanical agitation violently stripped the empty protein coats off the bacterial surfaces.
- Crucially, the bacterial cells themselves remained intact, keeping any injected material safely inside.
3. Centrifugation (Separation)
The blended slurry was then spun in a centrifuge Simple, but easy to overlook..
- Pellet (Bottom): Heavy bacterial cells (containing any injected genetic material).
- Supernatant (Top): Lighter viral protein coats (ghosts) and any free phages that didn't infect.
4. Measurement of Radioactivity
Finally, they measured the radioactivity in both the pellet and the supernatant using a Geiger counter Easy to understand, harder to ignore..
The Results: A Clear Verdict
The data was unambiguous and decisive:
| Labeled Component | Radioactivity in Pellet (Bacteria) | Radioactivity in Supernatant (Ghosts) |
|---|---|---|
| ³²P (DNA) | High (~70-80%) | Low (~20-30%) |
| ³⁵S (Protein) | Low (~1-5%) | High (~80-90%) |
Interpretation:
- When DNA was labeled (³²P), the radioactivity entered the bacterial cells (pellet). This meant DNA was injected into the host.
- When Protein was labeled (³⁵S), the radioactivity stayed outside in the supernatant (ghosts). This meant protein did not enter the host cell in significant amounts.
The conclusion was inescapable: The genetic material—the molecule that enters the cell and directs the production of new viruses—is DNA.
The Second Phase: Progeny Production
Hershey and Chase didn't stop at entry. Now, they performed a second set of experiments to confirm that the injected DNA was functionally active. They allowed the infection to proceed to completion (lysis of the cell) and analyzed the progeny phages.
They found that the progeny phages released from bacteria infected with ³²P-labeled parents contained ³²P in their DNA. Conversely, progeny from ³⁵S-labeled parents contained no ³⁵S in their protein coats (the sulfur had been diluted into the bacterial metabolic pool and not specifically incorporated into new viral protein in a tagged manner) Not complicated — just consistent..
Some disagree here. Fair enough It's one of those things that adds up..
This proved that the injected DNA retained its identity, directed the synthesis of new viral components (including protein coats), and was packaged into the next generation of viruses. Protein served only as a delivery vehicle—a protective container discarded upon arrival.
Scientific Impact and Legacy
The publication of "Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage" in The Journal of General Physiology (1952) was a watershed moment.
- Confirmation of Avery: It independently verified the Avery-MacLeod-McCarty conclusion using a totally different organism (viruses vs. bacteria) and a physical separation method rather than biochemical purification. The "protein contamination" counter-argument was rendered moot.
- Catalyst for the Double Helix: The results landed on the desks of James Watson and Francis Crick (and Rosalind Franklin and Maurice Wilkins) at a critical juncture. Knowing for certain that DNA was the genetic molecule intensified the race to solve its structure. Watson later recalled that the Hershey-Chase experiment made the structure of DNA "the most important problem in biology."
- Foundation of Molecular Biology: This experiment established the central dogma's starting point:
