The HersheyandChase experiment provided strong evidence that DNA, rather than protein, is the genetic material transmitted by a virus to a host cell. It showed that viral DNA entered bacterial cells and directed the production of new viruses.
This is Additional Higher Level (AHL) content in IB Biology subtopic A1.2, Nucleic acids.
Hershey and Chase studied bacteriophages, viruses that infect bacteria. A bacteriophage consists mainly of DNA enclosed in a protein coat, so it was useful for testing whether DNA or protein carried hereditary information.
They prepared two groups of bacteriophages with different radioactive labels:
| Bacteriophage component | Radioactive label and reason |
|---|---|
| DNA | Phosphorus-32 (), because DNA contains phosphorus in its phosphate backbone |
| Protein coat | Sulfur-35 (), because some amino acids contain sulfur, whereas DNA does not |
Each group infected Escherichia coli. After infection, the mixtures were agitated in a blender to detach phage coats from the bacterial surfaces and then centrifuged. The heavier bacteria formed the pellet, while detached phage coats remained in the supernatant.
Most was detected in the bacterial pellet and later in newly produced bacteriophages. Most remained in the supernatant. Therefore, DNA entered the cells and supplied the information needed to produce new viruses, while most of the protein coat remained outside.
A common misconception is that this experiment proved protein has no biological role. It did not: proteins perform many essential functions. The experiment specifically provided evidence that DNA was the hereditary material in these bacteriophages.
Exam technique: For an “explain” question, link each observation to the conclusion: in the pellet shows DNA entered bacteria; in the supernatant shows protein remained outside; therefore DNA carried the genetic information.