Q.Give an account of Hershey and Chase experiment. What did it conclusively prove? If both DNA and proteins contained phosphorus and sulphur do you think the result would have been the same?

The Hershey-Chase experiment used bacteriophages—viruses that infect bacteria—to settle the debate over whether DNA or protein is the genetic material. They grew two batches of phage: one in a medium containing radioactive phosphorus (P-32), which labels DNA because DNA contains phosphorus, and another in a medium containing radioactive sulphur (S-35), which labels protein because protein contains sulphur. Each batch of labelled phage was allowed to infect separate cultures of E. coli bacteria.

After infection, the cultures were blended and centrifuged. The heavier bacterial cells formed a pellet at the bottom, while lighter phage coats remained in the supernatant. In the P-32 (DNA-labelled) experiment, most radioactivity was found inside the bacterial pellet. In the S-35 (protein-labelled) experiment, most radioactivity stayed in the supernatant with the empty phage coats. This showed that only DNA entered the bacterial cell during infection, while protein remained outside.

The experiment conclusively proved that DNA is the genetic material—the molecule that carries hereditary information from one generation to the next. …

The question of what carries hereditary information was one of the most hotly debated in early 20th-century biology. By the 1940s, scientists knew that chromosomes contained both DNA and protein, but most believed protein was the more likely candidate for the genetic material — it was complex, diverse, and seemed far more capable of carrying vast amounts of information than the seemingly simple, repetitive DNA molecule. The Hershey-Chase experiment, performed in 1952 by Alfred Hershey and Martha Chase, was designed to settle this debate once and for all.

Hershey and Chase worked with a bacteriophage — a virus that infects bacteria. Specifically, they used the T2 phage, which attacks E. coli. A bacteriophage is essentially a protein coat wrapped around a core of DNA. When it infects a bacterium, it injects its genetic material into the host cell, leaving the empty protein shell outside. The injected material then hijacks the bacterium's machinery to produce hundreds of new phages. The key question was: which component — the protein coat or the DNA core — actually enters the cell and directs this reproduction?

To answer this, Hershey and Chase used a clever radioactive labelling strategy. They knew that DNA contains phosphorus but not sulphur, while protein contains sulphur but not phosphorus. So they grew two separate batches of phages: one batch was grown in a medium containing radioactive phosphorus (³²P), which got incorporated into the DNA of those phages. The other batch was grown in a medium containing radioactive sulphur (³⁵S), which got incorporated into the protein coats.

The actual experiment proceeded in two parallel steps. First, the phages labelled with radioactive phosphorus (³²P) were allowed to infect E. coli cells. After a short time, the infected bacteria were agitated in a blender to shear off any phage parts that remained attached to the outside of the bacterial cells. The mixture was then centrifuged. The heavier bacterial cells settled at the bottom as a pellet, while the lighter phage debris remained in the supernatant. When they measured the radioactivity, they found that most of the ³²P (the DNA label) was inside the bacterial pellet — meaning the DNA had entered the cells.

In the second parallel experiment, they repeated the exact same procedure using phages labelled with radioactive sulphur (³⁵S). This time, when they blended and centrifuged, the vast majority of the ³⁵S (the protein label) was found in the supernatant, not in the bacterial pellet. The protein coats had remained outside the cells and were sheared off by the blender.

Reframe the whole experiment as a lesson in experimental design rather than a fact to memorise: the only thing that made the result interpretable was that phosphorus and sulphur were each found in exactly one of the two candidate molecules, so each radioactive label pointed unambiguously to one molecule. Asking "what property must a tracer have to give an unambiguous answer" leads straight to the same conclusion the second pa …