A1.2.14 Evidence from the Hershey–Chase experiment for DNA as the genetic material (HL only) Notes

How the Hershey-Chase Experiment Demonstrated DNA as the Genetic Material

1. Scientists in the 1950s were debating a critical question: What molecule carries the genetic instructions for life?
2. Is it protein, known for its complexity and versatility? Or is it DNA, a simpler molecule often underestimated?
3. This debate ended with a groundbreaking experiment by Alfred Hershey and Martha Chase that changed our understanding of genetics.

The Experiment: Tracing DNA and Protein with Radioisotopes

1. Hershey and Chase devised an ingenious experiment to distinguish between DNA and protein as the genetic material.
2. Their approach relied on two key tools:
   1. Radioactive Isotopes:
      1. Phosphorus-32 ($^{32}P$)was used to label DNA because DNA contains phosphorus (in its phosphate backbone) but no sulfur.
      2. Sulfur-35 ($^{35}S$)was used to label proteins because proteins contain sulfur (in amino acids like cysteine) but no phosphorus.
   2. Bacteriophages:
      1. They grew bacteriophages in media containing either $^{32}P$ or $^{35}S$, ensuring that the DNA in one group of phages was radioactive and the protein coat in the other group was radioactive.

1. Labeling with radioisotopes
   1. Viruses were grown in medium containing radioactive ³²P (phosphorus) → labeled the DNA.
   2. Other viruses were grown in medium containing radioactive ³⁵S (sulfur) → labeled the protein coat.
2. Infection of bacteria
   1. Radioactively labeled viruses infected E. coli cells.
3. Blender step
   1. A blender shook the virus–bacteria mixture, dislodging virus coats from bacterial surfaces.
4. Centrifugation step
   1. Bacteria (larger) formed a pellet at the bottom of the tube.
   2. Viral protein coats (smaller) remained in the supernatant.

The Results: Following the Radioactive Trail

1. In the$^{32}P$-labeled DNA group, the radioactivity was found in the bacterial pellet, indicating that DNA had entered the bacterial cells.
2. In the$^{35}S$-labeled protein group, the radioactivity remained in the supernatant, showing that the protein coat did not enter the cells.

The Conclusion: DNA is the Genetic Material

1. Hershey and Chase's experiment provided definitive evidence that DNA, not protein, is the molecule of inheritance.
2. This breakthrough laid the foundation for modern molecular biology, transforming our understanding of genetics and heredity.

How New Technologies Enabled the Hershey-Chase Experiment

1. The Hershey-Chase experiment was made possible by a critical technological innovation: the use of radioisotopes as molecular tracers.
2. This advancement highlights the transformative role of technology in scientific discovery.

Why Were Radioisotopes Essential?

1. Radioisotopes enabled Hershey and Chase to selectively label DNA and protein, allowing them to track the movement of each molecule during the experiment.
2. Without this technology, distinguishing between the two components of the bacteriophage would have been impossible.