Chapter 8, Problem 1DAA

Hershey–Chase Experiments The graph shown in FIGURE 8.5 is reproduced from an original 1952 publication by Hershey and Chase. Bacteriophage were labeled with radioactive tracers and allowed 10 infect bacteria. The virus–bacteria mixtures were then whirled in a blender to dislodge any viral components attached to the exterior of the bacteria. Afterward, radioactivity from the tracers was measured.

FIGURE 8.5 Detail of Alfred Hershey and Martha Chase’s 1952 publication describing their experiments with bacteriophage.

“Infected bacteria” refers to the percentage of bacteria that survived the blender.

Before blending what percentage of each isotope. ³⁵S and ³²P, was extracellular (outside the bacteria)?

Summary Introduction

To explain: The percentages of $\text{S}$  and $\text{P}$  isotopes that were present extracellularly before blending.

Concept introduction: DNA is the organic molecule that carries information from one generation to another. Several experiments were to be conducted to prove that DNA is the genetic material. One such experiment is the Hershey–Chase experiment commonly known as the blender experiment. Bacteriophages are viruses that infect the bacteria and take up their translation machinery for their life cycle. This property was exploited to prove that DNA is a genetic material along with the help of radioactive isotopes.

Explanation of Solution

The bacteriophage has three components: a capsid made of proteins and genetic material made of nucleic acids. The nucleic acids have phosphorus, and proteins have sulfur. The radioactive isotopes of these are selected that is, $\text{S}$ and $\text{P}$. In the experiment, the sulfur and phosphorus are labeled as radioactive tracers and were then allowed to infect the bacteria. The virus–bacteria mixture was mixed in a blender to remove the viral ghost shell from the extracellular side of the bacteria. Following this, the amount of radioactive tracers that is $\text{S}$ and $\text{P}$, was measured.

A graph was plotted with running time in the blender as X-axis and percentage of radioisotopes and infected bacteria in the Y-axis. Three scatter plots were drawn one for each radioactive tracer $\text{S}$ and $\text{P}$ one for infected bacteria.

Before blending the mixture that is, at 0 minute the percentage of radioactive tracers were measured. The percentages of $\text{S}$ and $\text{P}$ before blending are 5% and 16%, respectively.

Conclusion

Before blending, the percentage of $\text{S}$ is 5% and $\text{P}$ is 16%.