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

Answer: The radioactive isotopes \(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\) were chosen as tracers in the Hershey-Chase experiment because \(^{32} \mathrm{P}\) could specifically label the DNA of the bacteriophages, while \(^{35} \mathrm{S}\) could label the proteins. The rationale was to determine whether proteins or DNA were responsible for carrying genetic information. The experiment concluded that DNA, not proteins, transmitted genetic material to the offspring phages, as the radioactive isotope \(^{32} \mathrm{P}\) was found in the progeny phages, but not the \(^{35} \mathrm{S}\). This result provided strong evidence supporting the DNA-centric view of inheritance.

Step-by-step solution

Background: Hershey-Chase Experiment

The Hershey-Chase experiment was a key study performed in 1952 by Alfred Hershey and Martha Chase to determine whether genetic information is transmitted through proteins or DNA. In their experiments, they used bacteriophages, viruses that infect bacteria, to investigate the transmission of genetic material during the infection process.

Choice of Tracers: \(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\)

The Hershey-Chase experiment used two radioactive isotopes, \(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\), as tracers to identify which component of the bacteriophage was transmitting genetic information. The choice for these isotopes was made based on the biochemical composition of proteins and DNA. Phosphorous (\(\mathrm{P}\)), particularly in phosphate groups, is a significant component of DNA but is absent in proteins. Therefore, by using the radioactive isotope \(^{32} \mathrm{P}\), Hershey and Chase could specifically label the DNA molecules within the bacteriophage. On the other hand, sulfur (\(\mathrm{S}\)) is present in some amino acids, like cysteine and methionine, which are building blocks of proteins. This isotope is essentially absent in DNA. Thus, by using the radioactive isotope \(^{35} \mathrm{S}\), Hershey and Chase could label the proteins of the bacteriophage.

Rationale of the Experiment

The goal of the Hershey-Chase experiment was to determine which molecule, DNA or protein, was responsible for carrying genetic information from the parental bacteriophage to the offspring phages during infection. If proteins were responsible for transferring genetic information, then \(^{35} \mathrm{S}\) would be found in the progeny phages. Conversely, if DNA were the genetic material, then \(^{32} \mathrm{P}\) would be detected in the offspring phages.

Conclusions of the Experiment

Hershey and Chase found that progeny phages contained the radioactive isotope \(^{32} \mathrm{P}\), but not the \(^{35} \mathrm{S}\). This indicated that DNA, not proteins, was the genetic material being transmitted to the next generation of bacteriophages. The experiment provided strong evidence in favor of the DNA-centric view of inheritance, which significantly advanced our understanding of the molecular basis of genetics and laid the groundwork for future research on DNA structure and replication.