How did Hershey and Chase prove that DNA is the genetic material?

Hershey and Chase conducted an elegant experiment in 1952 using bacteriophages (viruses that infect bacteria) to demonstrate that DNA is the genetic material responsible for heredity, rather than proteins.

Here’s a summary of their experiment:

Bacteriophages and Labeling: They used two different isotopic labels to separately tag the DNA and proteins of the bacteriophage T2 virus. Phosphorus-32 (
32
32
P) was used to label the DNA, as it is incorporated into the phosphate groups of DNA nucleotides. Sulfur-35 (
35
35
S) was used to label the proteins, as it is incorporated into the sulfur-containing amino acids (cysteine and methionine) of proteins.

Infection of Bacteria: They separately infected two batches of Escherichia coli (E. coli) bacteria with these labeled T2 phages:

In one batch, the phage DNA was labeled with
32
32
P.
In the other batch, the phage proteins were labeled with
35
35
S.
Blender Experiment: After allowing time for the viruses to infect the bacteria and replicate, they used a kitchen blender to separate the phage particles from the bacterial cells. This step was crucial because it physically separated the phage particles that remained outside the bacteria (containing mostly empty phage coats and labeled proteins) from the bacterial cells (containing the progeny phages that would have replicated within the cells).

Centrifugation: The mixture from the blender was then centrifuged to separate the heavier bacterial cells from the lighter phage particles.

Radioactive Analysis: Hershey and Chase then measured the radioactivity in both the bacterial cells and the phage particles.

Results Interpretation:

They found that the radioactivity (indicative of the labeled material) was mainly present within the bacterial cells when
32
32
P was used (labeling DNA).
Conversely, when
35
35
S was used (labeling proteins), the radioactivity remained mostly in the phage particles outside the bacterial cells.
Conclusion: From this experiment, Hershey and Chase concluded that it was the DNA of the phage, not the protein, that entered the bacterial cells and directed the synthesis of new phage particles inside the cells. This provided strong evidence that DNA, rather than protein, is the genetic material responsible for the inheritance and replication of genetic information in cells. This experiment played a significant role in solidifying the understanding that DNA is the molecule that carries genetic information in living organisms.