Alfred Hershey and Martha Chase conducted an experiment in 1952, advancing the understanding of heredity. At that time, the precise molecule responsible for carrying genetic information within living organisms remained uncertain. Their work clarified this fundamental biological question.
The Search for Genetic Material
Prior to Hershey and Chase’s research, scientists debated the identity of the genetic material. Scientists considered both proteins and nucleic acids as potential candidates. Proteins, with their complex and diverse structures, seemed capable of encoding vast biological information. In contrast, DNA, initially thought to be a simpler molecule, was sometimes considered primarily involved in phosphorus storage.
Earlier experiments by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944 suggested DNA’s role in genetic transformation. However, their findings were not universally accepted, leaving skepticism about whether DNA or proteins carried the blueprints of life. This unresolved question set the stage for Hershey and Chase’s experiment.
The Bacteriophage Experiment
To resolve the debate, Hershey and Chase designed an experiment using T2 bacteriophages, viruses that infect Escherichia coli bacteria. Bacteriophages were ideal because they possess a simple structure: a protein outer coat and an internal DNA core. These viruses infect bacteria by attaching to the host cell and injecting genetic material, compelling the bacterial machinery to produce new viruses.
Hershey and Chase used radioactive isotopes to distinguish viral protein and DNA. In one preparation, they labeled bacteriophage proteins with sulfur-35 (³⁵S). Sulfur is present in proteins but not in DNA, making ³⁵S a specific marker for the protein coat. In a separate preparation, they labeled bacteriophage DNA with phosphorus-32 (³²P). Phosphorus is a component of DNA’s backbone but is absent from proteins, allowing ³²P to specifically track the viral DNA. The viruses incorporated these isotopes when grown in radioactive media.
The experiment proceeded through three main steps: infection, blending, and centrifugation. First, radioactively labeled bacteriophages infected unlabeled E. coli bacteria. Next, the mixture was agitated in a blender. This mechanical shearing action detached the empty viral protein coats from the infected bacteria. Finally, the mixture underwent centrifugation, a process that separates components based on density. The heavier bacterial cells, containing injected genetic material, formed a pellet; the lighter viral particles and detached protein coats remained in the supernatant.
Unveiling DNA’s Role
Analysis of the radioactive labels after centrifugation yielded clear results. When bacteriophage proteins were labeled with sulfur-35, nearly all the radioactivity remained in the supernatant, outside the bacterial cells. This indicated that the viral protein coat did not enter the host bacterium during infection.
Conversely, when bacteriophage DNA was labeled with phosphorus-32, the radioactivity was predominantly found within the bacterial pellet. This demonstrated that the viral DNA entered the bacterial cells. Furthermore, subsequent generations of viruses produced by the infected bacteria also contained ³²P, confirming that DNA carried the genetic instructions for viral replication. These findings provided evidence that DNA, not protein, was the hereditary material injected into the host cell to direct the synthesis of new viral particles.
Shaping Modern Biology
The findings from the Hershey-Chase experiment had a profound influence on the field of biology. Their work provided compelling and widely accepted evidence that DNA served as the molecule of heredity. This settled the long-standing debate and directed scientific inquiry towards understanding the structure and function of DNA.
The confirmation of DNA’s role opened new avenues for research and paved the way for breakthroughs. Just one year later, in 1953, James Watson and Francis Crick elucidated the double helix structure of DNA, a discovery that was significantly informed by the understanding that DNA was the genetic material. The Hershey-Chase experiment thus laid foundational groundwork for the emergence of molecular biology, accelerating studies into DNA replication, gene expression, and the mechanisms of inheritance. Their experimental design and conclusive results remain a classic example of scientific inquiry that profoundly reshaped the understanding of life itself.