Imagine that you are a student in Alfred Hershey and Martha...

Imagine that you are a student in Alfred Hershey and Martha Chase's lab in the late 1940 s. You are given five test tubes containing $E .$ coli bacteria infected with T2 bacteriophages that have been labeled with either $^{32} \mathrm{P}$ or $^{35} \mathrm{S}$. Unfortunately, you forget to mark the tubes and are now uncertain about which were labeled with $^{32} \mathrm{P}$ and which with $^{35} \mathrm{S}$. You place the contents of each tube in a blender and turn it on for a few seconds to shear off the phage protein coats. You then centrifuge the contents to separate the protein coats and the cells. You check for the presence of radioactivity and obtain the following results. Which tubes contained $E .$ coli infected with $^{32}$ P-labeled phage? Explain your answer. (TABLE CAN'T COPY)

Key Concepts

Hershey-Chase Experiment

This landmark experiment demonstrated that DNA is the genetic material by showing that only the DNA of the bacteriophage, and not its protein coat, entered the bacterial cells during infection. It was a critical step in identifying the molecular basis of heredity.

DNA Labeling with 32P

32P is a radioactive isotope that labels DNA because phosphorus is a fundamental component of the nucleotide backbone. In experiments, when bacteriophages are labeled with 32P, their DNA becomes radioactive. When these phages infect bacteria, the radioactive DNA is injected into the cells, which can be detected after separating the cellular material by centrifugation.

Protein Labeling with 35S

35S is a radioactive isotope that specifically labels proteins due to the presence of sulfur in certain amino acids. Phages labeled with 35S have their protein coats marked, but since the protein does not enter the cell during infection, the radioactivity remains with the phage remnants in the supernatant after centrifugation.

Centrifugation Separation Technique

Centrifugation is used to separate cellular components based on density. After blending the phage-infected bacteria to dislodge the protein coats, centrifugation separates the heavier bacterial cells (pellet) from the lighter protein fragments (supernatant). This separation enables detection of where the radioactive label is localized.

Interpretation of Radioactive Results

By determining which fraction (pellet or supernatant) contains the radioactivity, it is possible to distinguish between 32P-labeled and 35S-labeled phage infections. Radioactivity in the pellet indicates that DNA, labeled with 32P, was injected into the bacterial cells, making those tubes the ones containing 32P-labeled phage infections.

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