Question

Why were \(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\) chosen for use in the Hershey-Chase experiment? Discuss the rationale and conclusions of this experiment.

Short answer

Answer: The Hershey-Chase experiment provided evidence that DNA, not protein, is the genetic material responsible for transferring genetic information during the life cycle of a bacteriophage. They used radioactive isotopes of phosphorus and sulfur to label DNA and proteins, respectively, and found that DNA entered the E. coli host cells, not the proteins.

Step-by-step solution

1. Introduction to the Hershey-Chase Experiment

The Hershey-Chase experiment was a groundbreaking study conducted in 1952 by Alfred Hershey and Martha Chase. This experiment provided evidence that DNA, not protein, was the genetic material. They used bacteriophages (viruses that infect bacteria) as their model system, with E. coli as the host bacteria.

2. Selection of Radioactive Isotopes

Hershey and Chase used two different radioactive isotopes in their experiment: \(^{32} \mathrm{P}\) (phosphorus-32) and \(^{35} \mathrm{S}\) (sulfur-35). These isotopes were chosen because they can be easily detected in the laboratory and they are found primarily in different macromolecules: - \(^{32} \mathrm{P}\) is found abundantly in DNA because the phosphorus forms the backbone of the DNA molecule via the phosphodiester bonds. Proteins, on the other hand, contain a very small amount of phosphorus. - \(^{35} \mathrm{S}\) is found in proteins since it is present in the sulfur-containing amino acids methionine and cysteine. DNA does not contain sulfur.

3. Hershey-Chase Experiment Design

The experiment was designed to use the radioactive isotopes as labels for DNA and protein, allowing Hershey and Chase to track their location. The bacteriophages were grown in a culture medium with either \(^{32} \mathrm{P}\) (to label DNA) or \(^{35} \mathrm{S}\) (to label protein). The labeled bacteriophages were then allowed to infect E. coli cells. After the infection process, they centrifuged the mixture to separate bacteriophages' empty shells from the E. coli cells. Finally, they measured the radioactivity in both fractions to determine whether DNA or protein entered the host cells.

4. Results

Hershey and Chase found that the bacteria infected with bacteriophages labeled with \(^{32} \mathrm{P}\) contained a high level of radioactivity, indicating that the labeled DNA had entered the cells. Conversely, bacteria infected with bacteriophages labeled with \(^{35} \mathrm{S}\) showed significantly lower radioactivity levels, suggesting that the labeled protein remained outside the host cells.

5. Conclusions

The results of the Hershey-Chase experiment confirmed that DNA, and not protein, was responsible for transferring genetic information during the life cycle of the bacteriophage. This provided strong evidence in support of the idea that DNA is the genetic material, helping to lay the foundation for the molecular biology revolution that followed.

Key concepts

Genetic Material

The Hershey-Chase experiment is pivotal in biology as it established DNA as the carrier of genetic information. Before this experiment, scientists debated whether proteins or DNA were responsible for heredity. DNA, or deoxyribonucleic acid, is composed of nucleotides that harbor the genetic instructions for the development and functioning of all known organisms and many viruses.

Each nucleotide contains a phosphate group, a sugar group, and a nitrogenous base. The sequence of these bases encodes the necessary information to build and maintain an organism, similar to how letters form words and sentences. DNA's double-helix structure, discovered by Watson and Crick shortly after the Hershey-Chase experiment, further emphasizes its role as genetic material, with the pairing between bases enabling the replication of genetic data.

Radioactive Isotopes

Radioactive isotopes, also known as radioisotopes, are atoms that emit radiation as they decay. They are essential tools in biological research for tracking and identifying molecular pathways. Scientists like Hershey and Chase leveraged radioactive isotopes for their ability to label molecules unambiguously.

In the Hershey-Chase experiment, they used (^{32}P) and (^{35}S) to distinctly label DNA and proteins, respectively. These isotopes emit radiation detectable with special equipment, making it possible to follow the path of these molecules during bacteriophage infection of bacteria. The choice of isotopes was crucial because it allowed the researchers to demonstrate that only the DNA entered the bacteria and directed the subsequent production of progeny phages.

DNA Structure

The understanding of DNA's structure underpins much of modern biology. It is composed of two long strands that twist around each other to form a double helix. Each strand is a sequence of nucleotides, consisting of one of four nitrogenous bases—adenine (A), thymine (T), guanine (G), or cytosine (C)—a sugar molecule (deoxyribose), and a phosphate group.

The phosphate groups create a phosphate-deoxyribose backbone from which the bases extend. In the Hershey-Chase experiment, phosphate played a critical role since it is abundant in DNA, which allowed (^{32}P) to effectively label the DNA. This structure is key to the function of DNA, as the specific pairing of the bases (A with T, C with G) enables the replication of the genetic code.

Bacteriophage

Bacteriophages, often referred to as phages, are viruses that infect and replicate within bacteria. They are composed of a protein coat, known as a capsid, which encases the genetic material—either DNA or RNA. The Hershey-Chase experiment harnessed bacteriophages as a model to understand the flow of genetic information.

Since bacteriophages inject their genetic material into a host cell to replicate, Hershey and Chase could analyze which component of the phage—the DNA or the protein—entered the bacterial cells. Their findings, that the bacteria only became radioactive when the phages' DNA was labeled, demonstrated unequivocally that the DNA was the genetic material being transmitted, while the protein coat remained on the outside. This crucial experiment lent support to the idea that bacteriophages could serve as a simple and powerful model system for studying molecular genetics.