Q.How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?

Hershey and Chase used radioactive isotopes to tag the two molecules differently. They labelled DNA with radioactive phosphorus-32, because phosphorus is present in DNA but absent in proteins. They labelled protein with radioactive sulphur-35, because sulphur is present in proteins but absent in DNA.

• They allowed the labelled bacteriophages to infect E. coli bacteria.
• After infection, they used a blender to shear off the viral coats from the bacterial surface.
• They then centrifuged the mixture: the heavier bacterial cells formed a pellet at the bottom, while the lighter viral coats stayed in the supernatant. …

The question of whether DNA or protein carried hereditary information was a central puzzle in mid-20th-century biology. Proteins, with their 20 amino acids and complex structures, seemed the more likely candidate to many scientists. DNA, by contrast, appeared chemically simple — just four nucleotides. The Hershey-Chase experiment, published in 1952, settled this debate with elegant, clean logic.

Alfred Hershey and Martha Chase worked with bacteriophages — viruses that infect bacteria. A bacteriophage is essentially a protein coat wrapped around a core of DNA. When it attacks a bacterium, the phage attaches to the bacterial surface and injects its genetic material. The empty protein shell (called the "ghost") remains outside. The key question was: which part — the protein coat or the DNA inside — actually enters the bacterium and directs the production of new phages?

To answer this, Hershey and Chase used radioactive isotopes as labels. They grew phages in two different media: one containing radioactive sulfur-35, and another containing radioactive phosphorus-32. Why these two? Sulfur is present in the amino acids cysteine and methionine, so it labels proteins. Phosphorus is a component of the DNA backbone (in the phosphate groups), so it labels DNA. Proteins contain no phosphorus, and DNA contains no sulfur — so the labels were perfectly specific.

The experiment proceeded in two parallel tracks. In one set, phages had their proteins labelled with radioactive sulfur. In the other, phages had their DNA labelled with radioactive phosphorus. Each batch of labelled phages was allowed to infect separate cultures of E. coli bacteria. After a short time — enough for infection but before the bacteria burst open — the mixtures were agitated in a blender. This mechanical shearing detached any phage parts that remained stuck to the outside of the bacterial cells.

Then the mixtures were centrifuged. The heavier bacterial cells formed a pellet at the bottom of the tube, while the lighter phage ghosts and any detached fragments remained in the supernatant (the liquid above the pellet).

Here is what they observed:

• In the sulfur-labelled experiment (protein label), most of the radioactivity was found in the supernatant — outside the bacteria. The bacterial pellet contained very little radioactivity. This meant the protein coat never entered the cell; it stayed attached to the outside and was stripped off by the blender.

• In the phosphorus-labelled experiment (DNA label), most of the radioactivity was found in the bacterial pellet — inside the cells. The supernatant had very little radioactivity. This meant the DNA had entered the bacteria. …

Frame it as a prediction-and-elimination test rather than a narrative recount: if protein carried the genetic information, the S-35 label (in protein) should appear inside the bacteria after infection; if DNA did, the P-32 label (in DNA) should. Since only P-32 turned up inside the bacter …