Why were ³²P and ³⁵S chosen for use in the Hershey–Chase experiment? Discuss the rationale and conclusions of this experiment.

Verified step by step guidance

1. Understand the biological molecules involved: DNA contains phosphorus but no sulfur, while proteins contain sulfur but no phosphorus. This difference allows selective labeling of DNA and protein.
2. Recognize why ³²P and ³⁵S isotopes were chosen: ³²P was used to label DNA because phosphorus is a component of the DNA backbone, and ³⁵S was used to label proteins because sulfur is found in some amino acids but not in DNA.
3. Review the experimental design: Hershey and Chase grew bacteriophages (viruses that infect bacteria) in media containing either ³²P or ³⁵S, so that the DNA or protein of the phages would be radioactively labeled, respectively.
4. Analyze the results: After allowing the labeled phages to infect bacteria, they used a blender to separate phage protein coats from bacterial cells and then measured radioactivity in the bacteria and the supernatant to determine which molecule entered the bacterial cells.
5. Conclude from the experiment: The presence of ³²P inside the bacteria and the absence of ³⁵S indicated that DNA, not protein, is the genetic material responsible for directing viral replication.

Key Concepts

Radioactive Isotope Labeling

Radioactive isotopes like ³²P and ³⁵S are used as tracers to track molecules in biological experiments. ³²P labels DNA because phosphorus is present in the DNA backbone but not in proteins, while ³⁵S labels proteins since sulfur is found in amino acids but not in DNA. This selective labeling allows scientists to distinguish between DNA and protein during the experiment.

Hershey–Chase Experiment Design

The Hershey–Chase experiment used bacteriophages labeled with ³²P in their DNA and ³⁵S in their protein coats to infect bacteria. After infection, blending and centrifugation separated phage coats from bacterial cells, revealing which radioactive label entered the bacteria. This design tested whether DNA or protein carried genetic information.

Conclusions of the Hershey–Chase Experiment

The experiment concluded that DNA, not protein, is the genetic material because only ³²P-labeled DNA entered the bacterial cells and directed viral replication. This provided strong evidence that DNA carries hereditary information, fundamentally advancing molecular genetics.