Question

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

Short answer

In the Hershey-Chase experiment, radioactive isotopes \(^{32} \mathrm{P}\) (phosphorus-32) and \(^{35} \mathrm{S}\) (sulfur-35) were chosen because they could be specifically incorporated into DNA and protein, respectively, allowing the researchers to trace these components during the experiment. The conclusions of the experiment demonstrated that DNA, rather than protein, was the genetic material responsible for transmitting heredity, as the radioactivity from \(^{32} \mathrm{P}\)-labeled DNA was found inside bacterial cells, while \(^{35} \mathrm{S}\)-labeled protein remained outside the cells, not contributing to the transmission of genetic information.

Step-by-step solution

Importance of \(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\) in the Hershey-Chase experiment

In the Hershey-Chase experiment, Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) as a model system to understand the nature of genetic material. The bacteriophages were grown in the presence of radioactive isotopes \(^{32} \mathrm{P}\) (phosphorus-32) and \(^{35} \mathrm{S}\) (sulfur-35), which were incorporated into the DNA and protein of the viruses, respectively. The rationale behind using these isotopes was that: 1. \(^{32} \mathrm{P}\) is a radioactive isotope of phosphorus, an element found exclusively in the DNA backbone (not found in proteins). Therefore, it provides a unique way to label and trace DNA during the experiment. 2. \(^{35} \mathrm{S}\) is a radioactive isotope of sulfur, an element found in some amino acids (cysteine and methionine) that make up proteins. Since sulfur is not found in DNA, it allowed the researchers to specifically label the protein components of the virus. By studying the transmission of these radioactive isotopes into bacterial cells, Hershey and Chase could determine if it was the DNA or the protein that carried the genetic information.

Rationale of the Hershey-Chase experiment

The rationale of the Hershey-Chase experiment was to determine which part of the bacteriophage - the protein coat or the DNA - was responsible for carrying genetic information. To do this, they labeled the DNA and proteins of the bacteriophage with radioactive isotopes, allowed the bacteriophages to infect the bacterial cells, and then analyzed the presence and distribution of the radioactive isotopes in the infected cells. The steps were as follows: 1. Grow bacteriophages in the presence of radioactive isotopes (\(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\)) to incorporate these isotopes into the DNA and proteins, respectively. 2. Separate the labeled bacteriophages into two groups: one containing DNA labeled with \(^{32} \mathrm{P}\) and the other containing protein labeled with \(^{35} \mathrm{S}\). 3. Allow each group of labeled bacteriophages to infect bacterial cells. 4. Use a blender to separate the bacteriophages and bacterial cells, creating a mixture of viral protein coats and bacterial pellet. 5. Centrifuge the mixture to separate the bacterial cells (that contain the genetic material) from the viral protein coats. 6. Measure the radioactivity in the bacterial pellet and the supernatant (containing the protein coats) to determine which component carried the genetic information.

Conclusions of the Hershey-Chase experiment

The conclusions of the Hershey-Chase experiment were as follows: 1. They observed that the bacterial pellet, which contained the genetic material, was significantly more radioactive when the bacteriophages had been labeled with \(^{32} \mathrm{P}\), indicating that the DNA had entered the bacterial cells. 2. In contrast, when bacteriophages were labeled with \(^{35} \mathrm{S}\), the radioactivity was primarily found in the supernatant, containing the viral protein coats. This indicated that the proteins remained outside the bacterial cells and did not contribute to the transmission of genetic information. Thus, the Hershey-Chase experiment demonstrated that DNA, rather than protein, was the genetic material responsible for transmitting heredity. This crucial finding laid the foundation for our understanding of molecular biology and the central role of DNA in genetics.

Key concepts

Radioisotopes

The Hershey-Chase experiment cleverly utilized radioisotopes to differentiate between DNA and proteins in bacteriophages. By incorporating radioisotopes such as \(^{32} \text{P}\) and \(^{35} \text{S}\), the researchers could easily trace where each component went during infection.

• \(^{32} \text{P}\) tags phosphorus, which is present in DNA. This isolation was important because phosphorus isn't found in proteins, allowing researchers to effectively track DNA.


• \(^{35} \text{S}\) attaches to sulfur, an element in some protein amino acids, but absent in DNA. This contrast helped scientists label and follow the proteins.

These isotopes were vital in determining the genetic material, as they helped visualize the molecular journey inside the bacterial cells. Without such precise labeling, distinguishing between DNA and proteins would have been impossible.

Bacteriophages

Bacteriophages, often just called phages, are viruses that infect bacteria. They were the ideal choice for the Hershey-Chase experiment because of their simple structure, made solely of DNA and protein.

• The structure of a phage consists of a protein coat encasing its DNA or RNA genetic material.


• Their life cycle involves attaching to a bacterial cell, injecting their genetic material, and hijacking the host's machinery to reproduce.

Their simplicity means that any new bacterial cell containing phage DNA indicates DNA is the primary genetic material. By observing whether the phage protein coat or DNA entered the bacterial cell, Hershey and Chase could conclude which component carried genetic information.

DNA as Genetic Material

The Hershey-Chase experiment was groundbreaking in establishing DNA as the genetic material. Before this, proteins were suspected because of their complexity and variety. However, the experiment proved otherwise.

• When phages with \(^{32} \text{P}\)-labeled DNA infected bacteria, the radioactive label was found in the bacterial cells. This indicated that DNA entered and was responsible for carrying genetic information.


• Conversely, phages with \(^{35} \text{S}\)-labeled proteins retained radioactivity outside the cells, confirming proteins were not the transmission vehicle of genetic traits.

These results established DNA's pivotal role, overturning previous beliefs and aiding our understanding of genetic inheritance. It set the stage for future discoveries in molecular biology, including the structure of DNA itself.