Alfred Hershey and Martha Chase were pioneering scientists who conducted groundbreaking experiments in 1952 that addressed a fundamental question in biology. Their work aimed to determine the molecule responsible for carrying genetic information. At the time, the scientific community debated whether genetic traits were passed down through proteins or deoxyribonucleic acid (DNA). Their findings significantly advanced the understanding of heredity.
The Mystery of Heredity
Before Hershey and Chase’s contributions, the scientific community faced a significant puzzle regarding the identity of the genetic material. While DNA had been discovered in 1869, many scientists initially believed that proteins were the carriers of hereditary information. This belief stemmed from the perceived complexity and diversity of proteins, which are composed of 20 different amino acids capable of forming a vast array of structures and functions. Proteins were known to perform a wide range of cellular tasks, including acting as enzymes, structural components, and signaling molecules, making them seemingly capable of encoding the diverse traits observed in organisms.
Conversely, DNA was thought to be a simpler molecule, composed of only four types of nucleotides: adenine, thymine, cytosine, and guanine. The “tetranucleotide hypothesis,” an incorrect theory, proposed that these four nucleotides occurred in a fixed, repeating sequence, suggesting a monotonous structure too simple to carry complex genetic information. Despite earlier evidence from the Avery-MacLeod-McCarty experiment in 1944 suggesting DNA’s role in transformation, a degree of skepticism persisted within the scientific community, setting the stage for Hershey and Chase’s decisive investigation.
The Hershey-Chase Experimental Approach
To resolve the debate, Hershey and Chase designed an experiment utilizing bacteriophages, which are viruses that specifically infect bacteria. These viruses are relatively simple, consisting only of a protein coat surrounding a nucleic acid core, which in this case is DNA. Their approach involved selectively labeling the protein and DNA components of the bacteriophages with different radioactive isotopes.
In one set of experiments, bacteriophages were grown in a medium containing radioactive sulfur-35 (35S). Sulfur is present in proteins but not in DNA, so this isotope acted as a specific marker for the viral protein coats. For the other set, bacteriophages were grown in a medium containing radioactive phosphorus-32 (32P). Phosphorus is a component of DNA but not typically found in proteins, thus labeling the viral DNA.
The labeled bacteriophages were then allowed to infect Escherichia coli (E. coli) bacteria. During infection, bacteriophages attach to the outside of the bacterial cell and inject their genetic material into the host. After a brief infection period, the researchers used a blender to agitate the mixture. This blending step was designed to shear off the external viral coats that remained attached to the outside of the bacterial cells, effectively separating them from the bacteria.
Following blending, the mixture was subjected to centrifugation, a process that spins samples at high speeds. Centrifugation separates components based on their density, causing the heavier bacterial cells to form a pellet at the bottom of the tube, while the lighter viral particles and detached protein coats remained in the liquid supernatant. By analyzing the radioactivity in the pellet (containing bacteria) and the supernatant (containing viral remnants), Hershey and Chase could determine which viral component had entered the bacterial cells.
Unveiling the Genetic Material
The results of the Hershey-Chase experiment provided clear evidence regarding the nature of the genetic material. When bacteriophages labeled with radioactive phosphorus-32 (32P) were used, the majority of the radioactivity was found within the bacterial pellet after centrifugation. This indicated that the DNA, marked by the 32P, had entered the bacterial cells during the infection process.
In contrast, when bacteriophages labeled with radioactive sulfur-35 (35S) were used, most of the radioactivity remained in the supernatant, outside the bacterial cells. This demonstrated that the protein coats, marked by the 35S, did not enter the bacteria. The observation that radioactive phosphorus (DNA) was found inside the bacteria, while radioactive sulfur (protein) largely stayed outside, led to a direct conclusion. Hershey and Chase concluded that DNA, not protein, was the genetic material injected by the bacteriophages into the bacterial cells to direct the production of new viruses.
The Enduring Impact
The Hershey-Chase experiment provided definitive evidence that DNA is the molecule of heredity. Their findings solidified DNA’s role as the carrier of genetic information, effectively resolving a major scientific debate that had persisted for decades.
This confirmation that DNA is the genetic material paved the way for subsequent groundbreaking research in molecular biology. It directed scientific focus towards understanding DNA’s structure and function. This foundational work was instrumental in setting the stage for James Watson and Francis Crick’s elucidation of the DNA double helix structure in 1953, a discovery that further revolutionized biology and opened new avenues for genetic research.