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Every NEET aspirant learns that Hershey and Chase used radioactive phosphorus and sulfur to show DNA is the genetic material. Fewer know what their numbers actually were, or how carefully they worded their conclusion. This post tells the story from the original paper, works through the percentages they reported, and shows why the experiment's design was cleverer than the textbook diagram suggests.
Who were Hershey and Chase?
Alfred Hershey's Nobel biography records that he was born on 4 December 1908 in Owosso, Michigan, and earned a PhD at Michigan State College in 1934. He taught and did research at Washington University School of Medicine until 1950, when he joined the Department of Genetics of the Carnegie Institution of Washington at Cold Spring Harbor, New York. He died on 22 May 1997.
Martha Chase was his co-author. Their paper, Independent functions of viral protein and nucleic acid in growth of bacteriophage, appeared in the Journal of General Physiology on 20 September 1952 with both names and the Cold Spring Harbor address. In 1969 Hershey shared the Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria "for their discoveries concerning the replication mechanism and the genetic structure of viruses".
What question were they asking?
A bacteriophage is a virus that infects bacteria. Phage T2 is little more than DNA wrapped in protein. When it infects a cell, more phage are made inside. The question was which part goes in and directs that: the protein or the DNA.
The paper's first finding set things up. Osmotic shock broke T2 particles into two parts: a protein fraction holding nearly all the phage sulfur, which could still attach to bacteria, and the phage DNA, released into solution. The authors concluded that the sulfur-containing protein forms a protective coat, responsible for attaching the virus to bacteria.
How did the radioactive labels work?
The NCERT Class 12 chapter on molecular inheritance explains the trick. DNA contains phosphorus but not sulfur. Protein contains sulfur but not phosphorus. So phage grown on a medium with radioactive phosphorus have labelled DNA, and phage grown with radioactive sulfur have labelled protein. NCERT notes that these were two separate batches of phage.
After the labelled phage attached to E. coli, the mixture was agitated in a blender to shear off whatever remained outside the cells, then spun in a centrifuge to separate the bacteria from the phage material.
What did they actually measure?
The paper's summary gives the numbers:
A Waring blender released 75 percent of the phage sulfur, but only 15 percent of the phage phosphorus, into solution.
The infected cells could still produce new phage after blending.
Progeny phage from sulfur-labelled parents carried less than 1 percent of the parental radioactivity.
Progeny from phosphorus-labelled parents carried 30 percent or more of the parental phosphorus.
Worked example: following the counts
Suppose each batch of phage carries 20,000 counts per minute of radioactivity. These are made-up totals; the percentages are from Hershey and Chase (1952).
Sulfur batch, after blending. 75 percent of 20,000 = 15,000 counts in the liquid. The other 5,000 stay with the cells.
Phosphorus batch, after blending. 15 percent of 20,000 = 3,000 counts in the liquid. 17,000 stay with the cells.
Progeny. Sulfur batch: under 1 percent of 20,000 is under 200 counts in the new phage. Phosphorus batch: 30 percent or more of 20,000 is at least 6,000 counts.
What do the numbers mean? A quarter of the sulfur stayed with the cells after blending, so blending alone was not perfectly clean. The paper's own interpretation is that most of the phage sulfur stays at the cell surface. The decisive evidence is step 3: material from the DNA passes to the next generation of phage, while almost none of the protein does.
Common mistakes:
Saying radioactive phosphorus labels protein. It labels DNA.
Saying both labels were in the same phage. NCERT describes separate batches.
Saying "no" sulfur entered and "all" phosphorus did. The real figures were percentages, and the progeny data matter as much as the blender data.
If you want the physics behind radioactive labels, our guide to radioactive decay and half-life covers it.
How careful was their conclusion?
The paper ends with restraint. The sulfur-containing protein, the authors wrote, "probably has no function in the growth of intracellular phage. The DNA has some function. Further chemical inferences should not be drawn from the experiments presented." They also noted they had not determined whether sulfur-free material other than DNA entered the cell.
NCERT describes the experiment as the unequivocal proof that DNA is the genetic material. That is how it is remembered, and it built on the 1944 work told in our post on Avery, MacLeod and McCarty. But Hershey and Chase themselves claimed only what their counts supported.
What can a student take from their way of working?
Choose a marker that can only mean one thing. Sulfur for protein, phosphorus for DNA.
Measure, then interpret. They reported percentages, not just "yes" or "no".
Look at the next generation. What reaches the progeny is stronger evidence than what sticks to the cell.
Do not claim more than the data allow. Their last sentence is a model of this.
When a question twists the set-up, for example by asking what happens if the blending step is skipped, reason it through rather than recall a diagram. The Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer. For the whole chapter, see our guide to the molecular basis of inheritance.
Redo the worked example with 50,000 counts per batch and write one sentence on what each result shows.
