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

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.

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.