Q.In the Hershey-Chase experiment, why was blending and centrifugation used after phage infection, and what did the distribution of radioactivity between the pellet and the supernatant reveal in each labelled batch?
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Alfred Hershey and Martha Chase's 1952 experiment with bacteriophage T2 provided the conclusive proof that DNA, not protein, is the genetic material, finally overcoming lingering doubt from biologists who felt protein's greater chemical diversity made it a more plausible carrier of hereditary information. Because nucleic acids are rich in phosphorus (but contain no sulphur) while proteins are rich in sulphur, in the amino acids cysteine and methionine, (but contain no phosphorus), Hershey and Chase separately labelled one batch of T2 phage with radioactive phosphorus-32 (marking DNA) and another batch with radioactive sulphur-35 (marking protein). Each labelled phage batch was allowed to infect unlabelled E. coli; the culture was then agitated in a blender to shear off phage material still attached to the outside of the bacteria, and centrifuged to separate the heavier bacterial pellet from the lighter phage debris in the supernatant. Radioactive phosphorus (32P) was found alm …
Blending sheared off phage material still attached outside the bacteria; centrifugation then separated the heavier bacterial pellet from the lighter supernatant. …
Step 1. After allowing a short period for infection, each culture was violently blended, shearing off any phage material still attached to the OUTSIDE of the bacterial cells.
Step 2. The mixture was then centrifuged, which pellets the heavier bacterial cells at the bottom of the tube while lighter material (sheared-off empty phage coats) remains in the supernatant above.
Step 3. In the 35S (protein-labelled) culture, almost all the radioactivity was found in the supernatant — the protein coats had never entered the bacteria and were simply sheared off outside.
Step 4. In the 32P (DNA-labelled) culture, almost all the radioactivity was found in the pellet — the phage DNA had entered the bacterial cells. …
Connect the physical step (blending removes only what's OUTSIDE the cell; centrifuging separates by weight) to what each fra …
- Assuming blending destroyed the bacteria themselves — it only sheared off material stuck to the OUTER surface, …
- CBSE 2026Set A1 markMCQQ.What did Hershey and Chase conclude on the basis of their experiments?(a) Protein from the virus enters the bacteria(b) DNA from the virus enters the bacteria(c) RNA from the virus enters the bacteria(d) Transforming principle
›Reveal solutionSolution
Hershey and Chase showed that only viral DNA (not protein) enters the bacterium, so DNA is the genetic material.
Using bacteriophages labelled with radioactive phosphorus (32P in DNA) or radioactive sulphur (35S in protein), Hershey and Chase infected bacteria and then separated the phage coats by blending and centrifuging. Radioactive DNA (32P) was found inside the bacteria, while the 35S-labelled prote …
- CBSE 2026Set ANNUAL1 markMCQQ.Name the scientist who used viruses to prove that DNA is a hereditary material :(a) Watson and Crick(b) Hershey and Chase(c) Meselson and Stahl(d) Avery, Macleod and Macarty
›Reveal solutionSolution
Alfred Hershey and Martha Chase (1952) used bacteriophage viruses (radioactively labelled) to prove that DNA, not protein, is the genetic material.
Hershey and Chase grew one batch of bacteriophage (T2 phage, which infects E. coli) with radioactive phosphorus (³²P, which labels DNA) and another batch with radioactive sulphur (³⁵S, which labels protein). They allowed the labelled phages to infect fresh bacteria, then agitated the culture in a blender to separate the phage coats (which remain outside the bacterium) from the bacterial cells (which contain the injected material), followed by centrifugation.
- Bacteria infected with ³²P-labelled phage became radioactive (DNA entered the cell).
- Bacteria infected with ³⁵S-labelled phage did NOT become radioactive (protein stayed outside). …
- CBSE 2026Set ANNUAL1 markMCQQ.Hershey and Chase's experiment with bacteriophage showed that :(a) DNA contains radioactive sulphur.(b) Protein gets into the bacterial cells.(c) Viruses undergo transformation.(d) DNA is the genetic material.
›Reveal solutionSolution
By separately radiolabelling phage DNA and phage protein and tracking which one entered bacterial cells and produced new phage progeny, Hershey and Chase conclusively demonstrated that DNA (not protein) is the hereditary material.
Alfred Hershey and Martha Chase, in 1952, worked with the T2 bacteriophage, a virus that infects the bacterium Escherichia coli, consisting only of a protein coat surrounding a DNA core. They grew one batch of phages in a medium containing radioactive phosphorus (32P), which gets incorporated into DNA (since DNA, but not protein, contains phosphorus), and another batch in a medium with radioactive sulphur (35S), which gets incorporated into protein (since protein, but not DNA, contains sulphur amino acids). After allowing each labelled phage batch to infect separate E. coli cultures, they used a blender to shear off the empty phage coats from the bacterial cell surfaces and centrifuged the mixture to separate the coats (in the supernatant) from the infected bacterial cells (in the pellet). They found that the 32P (DNA) label was mostly found inside the bacterial cells, wh …
- CBSE 2025Set 57/5/11 markMCQQ.The correct depiction of the centrifugation step of the experiment conducted by Alfred Hershey and Martha Chase on using radioactive labelled phages to prove that DNA is the genetic material is : (A) [Left] No Radioactive (35S) detected in cells + Radioactive (35S) detected in supernatant; [Right] Radioactive (32P) detected in cells + No Radioactivity detected in supernatant (B) [Left] Radioactive (35S) detected in cells + Radioactive (35S) detected in supernatant; [Right] No Radioactive (32P) detected in cells + No Radioactivity detected in supernatant (C) [Left] No Radioactive (35S) detected in cells + No Radioactivity detected in supernatant; [Right] Radioactive (32P) detected in cells + Radioactive (35S) detected in supernatant (D) [Left] Radioactive (35S) detected in cells + No Radioactivity detected in supernatant; [Right] No Radioactive (32P) detected in cells + Radioactive (35S) detected in supernatant
›Reveal solutionSolution
In the Hershey-Chase experiment, after centrifugation, the cells contained radioactive phosphorus (³²P) but no radioactive sulfur (³⁵S), while the supernatant contained radioactive sulfur (³⁵S) but no radioactive phosphorus (³²P), proving DNA is the genetic material.
Before we delve into the ingenious experiment that settled a fundamental question in biology, let's briefly recall the structure of DNA itself. DNA, the blueprint of life, is a double helix composed of nucleotides. A crucial aspect of its structure, discovered later by Watson and Crick, is complementary base pairing, where adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This precise pairing is vital for DNA's ability to store and transmit genetic information accurately. However, for many years, scientists were unsure if it was DNA or protein that carried this genetic information.
The scientific community in the mid-20th century was divided. Proteins, with their complex and diverse structures, seemed like strong candidates for carrying genetic information. DNA, on the other hand, was considered too simple, a mere repeating polymer. It took a series of elegant experiments to definitively prove DNA's role. Among the most conclusive was the work of Alfred Hershey and Martha Chase in 1952.
Hershey and Chase designed an experiment using bacteriophages (viruses that infect bacteria) to determine whether DNA or protein was the genetic material. Their approach was brilliant because bacteriophages inject their genetic material into a host bacterium to replicate, leaving most of their structural components outside.
NoteBacteriophages are ideal for this experiment because they consist primarily of DNA and protein. They essentially act like tiny syringes, injecting their genetic material into a bacterial cell.
The core of their experiment involved selectively labeling the DNA and protein components of the bacteriophages with different radioactive isotopes:
- Radioactive Phosphorus (32P): DNA contains phosphorus but no sulfur. By growing phages in a medium containing 32P, their DNA became radioactively labeled.
- Radioactive Sulfur (35S): Proteins contain sulfur but no phosphorus. By growing phages in a medium containing 35S, their protein coats became radioactively labeled.
They then allowed these labeled phages to infect E. coli bacteria. After a short period of infection, they subjected the mixture to a crucial step: blending. This blending step was designed to agitate the solution and detach the viral protein coats (capsids) from the surface of the bacterial cells. The idea was to separate the material that entered the cell from the material that remained outside.
The final and most critical step, which directly addresses your question, was centrifugation. Centrifugation is a process that uses centrifugal force to separate components of a mixture based on their density.
- When the blended mixture was centrifuged, the heavier bacterial cells, which had been infected, settled at the bottom of the test tube, forming a pellet.
- The lighter viral particles and detached protein coats remained suspended in the liquid above, forming the supernatant.
Now, let's consider the two experimental setups and their expected outcomes:
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Experiment 1: Phages labeled with 35S (protein label)
- If protein was the genetic material, it would enter the bacterial cells, and the radioactivity would be found in the pellet (with the cells).
- If protein was not the genetic material, it would remain outside the cells (or detach during blending) and be found in the supernatant (with the viral coats).
- Result: Hershey and Chase found that the vast majority of the 35S radioactivity was detected in the supernatant, not in the bacterial cells. This indicated that protein did not enter the cells to direct the synthesis of new viruses.
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Experiment 2: Phages labeled with 32P (DNA label)
- If DNA was the genetic material, it would enter the bacterial cells, and the radioactivity would be found in the pellet (with the cells).
- If DNA was not the genetic material, it would remain outside the cells and be found in the supernatant.
- Result: Hershey and Chase observed that almost all the 32P radioactivity was detected in the bacterial cells (the pellet). Furthermore, these cells, when allowed to grow, produced new phages that also contained 32P, confirming that the DNA had indeed directed the synthesis of new viral particles. …
- CBSE 2024Set E1 markMCQQ.What was the conclusion of Hershey and Chase experiment?(a) Genetic material of bacteria is DNA(b) Genetic material of virus is DNA(c) Genetic material of bacteria is RNA(d) Genetic material of virus is RNA
›Reveal solutionSolution
Hershey and Chase showed that the genetic material of the bacteriophage (virus) is DNA, not protein.
Alfred Hershey and Martha Chase (1952) worked with the T2 bacteriophage, which has only DNA and a protein coat:
- They labelled the DNA with radioactive phosphorus (32P) in one batch and the protein coat with radioactive sulphur (35S) in another. …
- CBSE 2024Set ANNUAL1 markMCQQ.The unequivocal proof that DNA is the genetic material, came from the experiments of which scientists.(a) Schleiden and Schwann(b) Mathew Meselson and Franklin Stahl(c) Francois Jacob and Jacque Monod(d) Alfred Hershey and Martha Chase
›Reveal solutionSolution
The Hershey-Chase experiment used radioactively labelled bacteriophage DNA and protein to show that only DNA enters the bacterium and directs progeny virus formation.
Hershey and Chase grew one batch of bacteriophages with radioactive phosphorus (³²P, labelling DNA) and another batch with radioactive sulphur (³⁵S, labelling protein). Each batch was allowed to infect E. coli, then agitated in a blender to separate the phage coats (which remained outside the bacterial cell) from the bacterial cells. Only the ³²P (DNA) label was found inside the bacteria and in the progeny phages, while the ³⁵S (protein) label stayed in the supernatant with the empty phage coats. This proved tha …
- CBSE 2020Set ANNUAL1 markQ.What is a Bacteriophage?
›Reveal solutionSolution
Bacteriophages are bacteria-infecting viruses, famous for proving that DNA (not protein) is the genetic material.
A bacteriophage ('bacteria eater') is a virus that specifically infects bacteria. Structurally it has a protein coat (capsid) enclosing its genetic material, which may be DNA or RNA. On infection, the phage attaches to the bacterial cell surface and injects its genetic material into the host, while the protein coat stays outside; the injected nucleic acid then hijacks the bacterial machinery to make new phage particles. This behaviour was exploited by Hershey and Chase (1952) in their T2 bacteriophage experiment, which used radio …
- CBSE 2019Set HE1 markQ.Fill in the blank: Virus that infect bacteria is called ______.
›Reveal solutionSolution
A virus that specifically infects bacterial cells is called a bacteriophage.
Viruses are obligate intracellular parasites that infect a wide range of hosts — plants, animals and bacteria. A virus that infects and replicates within bacteria is called a bacteriophage (literally, 'bacteria eater'). The most extensively studied bacteriophages are those that infect the bacterium Escherichia coli, such as the T-even phages (T2, T4). Bacteriophages played a historic role in molecular biology — the Hershey–Chase blender experiment (1952) used a bacteriophage (T2) …
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