The Hershey-Chase experiment, conducted in 1952, definitively proved that DNA, not protein, carries the genetic information, thereby establishing its fundamental role as the hereditary material essential for life.
The Pivotal Hershey-Chase Experiment
Before the mid-20th century, the scientific community debated whether DNA or protein was the molecule responsible for heredity. While Avery, MacLeod, and McCarty's work in 1944 provided strong evidence for DNA, many scientists still favored protein due to its perceived greater complexity. Alfred Hershey and Martha Chase performed a groundbreaking experiment using a simple biological system: viruses that infect bacteria.
Why Viruses?
They chose to study the T2 bacteriophage, a type of virus that specifically infects Escherichia coli (E. coli) bacteria. This virus offered a clear advantage because it consists only of DNA and protein. When a bacteriophage infects a bacterium, it injects its genetic material into the host cell, which then reprograms the cell to produce more viruses. The key question was: what exactly was being injected?
Experimental Design
Hershey and Chase designed their experiment to track both DNA and protein during the infection process using radioactive isotopes.
Key Components Labeled
| Component | Labeled With | Why This Isotope? |
|---|---|---|
| DNA | Radioactive Phosphorus ($^{32}$P) | Phosphorus is a key component of DNA, but not protein. |
| Protein | Radioactive Sulfur ($^{35}$S) | Sulfur is found in specific amino acids (methionine, cysteine) that make up protein, but not in DNA. |
The Steps of the Experiment
- Labeling: Two batches of T2 bacteriophages were prepared.
- One batch was grown in a medium containing $^{32}$P, so their DNA became radioactively labeled.
- The other batch was grown in a medium containing $^{35}$S, so their proteins became radioactively labeled.
- Infection: Each batch of labeled viruses was then allowed to infect separate cultures of E. coli bacteria. The viruses attach to the bacterial surface and inject their genetic material.
- Blending: After a short period, the cultures were agitated in a blender. This step was crucial to dislodge the viral coats (capsids), which remained outside the bacterial cells, from the infected bacteria.
- Centrifugation: The mixture was then centrifuged at high speed. This separates the heavier bacterial cells (pellet at the bottom) from the lighter viral coats and uninfected viruses (supernatant liquid).
- Measurement: The radioactivity in both the pellet (bacteria) and the supernatant (viral coats) was measured for each experimental batch.
Key Findings and Implications
The results of the Hershey-Chase experiment were conclusive and provided compelling evidence for DNA as the genetic material:
- In the $^{32}$P experiment: The vast majority of the $^{32}$P radioactivity was found in the bacterial pellet, indicating that the DNA had entered the host cells. Furthermore, this radioactivity was passed on to the next generation of viruses produced within the bacteria.
- In the $^{35}$S experiment: Most of the $^{35}$S radioactivity remained in the supernatant, associated with the viral coats outside the bacterial cells. Very little $^{35}$S was detected inside the bacteria, and it was not passed on to the new viruses.
Conclusion: These findings clearly demonstrated that DNA, not protein, carried the genetic instructions necessary for viral replication. The material that entered the bacterial cells and directed the synthesis of new viruses was DNA. This experiment was a landmark achievement, solidifying DNA's role as the molecule of heredity and paving the way for further research into its structure and function.
