For a long time, the fundamental question of what carries genetic information remained unanswered. Scientists understood that genetic material dictated an organism’s traits and could be passed down through generations. However, the precise molecule responsible for heredity was a subject of considerable debate.
The Experiment’s Core Question
Before the Hershey-Chase experiment, the scientific community largely believed that proteins were the carriers of genetic information. Proteins exhibit remarkable complexity and diversity, composed of 20 different amino acids, which seemed fitting for the vast array of inherited traits. In contrast, DNA was often dismissed as too simple a molecule, with early hypotheses suggesting a simple, repetitive structure.
Genetic material needed to be stable, capable of self-replication, and able to undergo changes for evolution. Proteins appeared to fulfill these requirements due to their versatility and functional roles. Despite earlier evidence hinting at DNA’s involvement, a definitive proof was lacking, making the question of DNA versus protein a central scientific puzzle.
Experimental Design and Methodology
To resolve the debate, Alfred Hershey and Martha Chase conducted their experiments in 1952, utilizing bacteriophages as their model system. Bacteriophages are viruses that specifically infect bacteria, and they were an ideal choice due to their simple composition of only DNA and protein. These viruses inject their genetic material into a host bacterium, hijacking its machinery to produce new viruses, while their outer structures remain outside.
The researchers employed radioactive isotopes to selectively label the two distinct components of the bacteriophage. Sulfur-35 (³⁵S) was used to label proteins, as sulfur is present in amino acids found in proteins but not in DNA. Conversely, phosphorus-32 (³²P) labeled DNA, as phosphorus is a component of the DNA backbone but absent from proteins. By growing separate batches of bacteriophages in media containing these isotopes, Hershey and Chase created phages with either radioactive protein or radioactive DNA.
The experimental procedure involved three main steps: infection, blending, and centrifugation. First, the radioactively labeled bacteriophages infected Escherichia coli (E. coli) bacteria. After a short incubation, the mixture was agitated to detach the viral protein coats adhering to the outside of the bacterial cells, while keeping the bacterial cells intact. Finally, the mixture was centrifuged, causing the heavier bacterial cells to form a pellet at the bottom of the tube, while lighter, detached phage particles remained in the liquid supernatant. This separation allowed the scientists to determine which radioactive component, DNA or protein, had entered the bacterial cells.
The Groundbreaking Results
When bacteriophages labeled with sulfur-35 (³⁵S) infected the bacteria, the vast majority of the radioactivity remained in the supernatant after blending and centrifugation. This indicated that the protein coats of the viruses had stayed outside the bacterial cells and did not enter the host cells to direct viral replication.
In contrast, when bacteria were infected with bacteriophages whose DNA was labeled with phosphorus-32 (³²P), the radioactivity was found predominantly within the bacterial pellet. This demonstrated that the viral DNA had successfully entered the bacterial cells. Furthermore, as new generations of phages were produced inside these bacteria, the ³²P radioactivity was passed on to the progeny viruses.
The distinct results from the two experimental groups provided definitive evidence. The presence of radioactive DNA inside the infected bacterial cells, and its subsequent transmission to new viral particles, proved that DNA, not protein, was the genetic material responsible for directing the synthesis of new viruses.
Profound Scientific Impact
The Hershey-Chase experiment, published in 1952, provided conclusive evidence that DNA is the genetic material. This built upon earlier suggestive work, such as the Avery-MacLeod-McCarty experiment, which also pointed to DNA’s role in heredity but had not achieved widespread acceptance. The clear and elegant design of the Hershey-Chase experiment, using bacteriophages, made its results more readily accepted by the scientific community, settling the debate between DNA and protein.
The establishment of DNA as the carrier of genetic information marked a turning point in biology. It paved the way for subsequent groundbreaking research, including the elucidation of the DNA double helix structure by James Watson and Francis Crick in 1953. Their discovery, which revealed how DNA could store and replicate information, was directly informed by the understanding that DNA was the hereditary molecule.
This foundational understanding propelled the expansion of molecular biology. Scientists could now focus on how DNA functions, leading to discoveries about DNA replication, gene expression, and the mechanisms of heredity. The Hershey-Chase experiment’s legacy is immense, as it provided the framework for modern genetics, genetic engineering, and the comprehensive study of genomes, including the human genome project.