The Hershey-Chase Experiment 🧬

students, imagine trying to solve a mystery about life’s instructions: what molecule carries the genetic code? In the 1940s and 1950s, scientists knew that chromosomes contained DNA and proteins, but many still thought proteins were the most likely genetic material because they are so chemically diverse. The Hershey-Chase experiment gave strong evidence that DNA, not protein, is the genetic material in bacteriophages, which are viruses that infect bacteria. This lesson will help you understand the experiment, the evidence behind it, and why it matters for IB Biology SL and the broader topic of Unity and Diversity.

Learning objectives:

Explain the main ideas and terminology behind the Hershey-Chase Experiment.
Apply IB Biology SL reasoning related to experimental evidence.
Connect the experiment to the broader topic of Unity and Diversity.
Summarize how the experiment fits within Unity and Diversity.
Use evidence and examples related to the experiment in biology contexts.

What problem were Hershey and Chase trying to solve? 🔬

Before this experiment, scientists already knew that living organisms share a basic unity: all cells use nucleic acids, proteins, lipids, and carbohydrates in similar ways. But there was still a major question about diversity in heredity: which molecule stores the information that can be passed from one generation to the next? students, this question mattered because heredity explains how traits are inherited and how life continues with variation.

The bacteriophage used in the experiment had a very simple structure. It consisted mainly of a protein coat surrounding DNA. When the virus infects a bacterium, it attaches to the cell and injects material into it so the virus can make more copies of itself. Hershey and Chase asked a key question: when the virus infects the bacterium, does the protein coat enter the cell, or does the DNA enter the cell and direct the production of new viruses?

This was a smart experimental design because the two main components of the virus could be labeled separately. Since DNA contains phosphorus but very little sulfur, and proteins contain sulfur but not phosphorus in significant amounts, the scientists used radioactive isotopes to trace each molecule.

How did the experiment work? 🧪

Hershey and Chase used two different radioactive labels:

$^{32}\text{P}$ to label DNA, because DNA contains phosphorus in its phosphate backbone.
$^{35}\text{S}$ to label protein, because proteins can contain sulfur in amino acids such as cysteine and methionine.

They grew viruses in media containing one of these isotopes so the virus particles became labeled. Then they let the labeled viruses infect bacteria.

After infection, they used a blender to shake off the virus protein coats from the outside of the bacterial cells. Then they used centrifugation to separate the bacteria from the viral coats. The bacterial cells formed a pellet at the bottom of the tube, while the lighter viral coats stayed in the liquid supernatant.

This procedure let the scientists ask: which radioactive label ended up inside the bacteria?

Results of the experiment

When the viruses were labeled with $^{32}\text{P}$, most of the radioactivity was found in the bacterial pellet.
When the viruses were labeled with $^{35}\text{S}$, most of the radioactivity was found in the supernatant, not in the bacterial pellet.

This showed that DNA entered the bacterial cells, while protein stayed outside.

The data strongly supported the idea that DNA carries genetic information in bacteriophages. Since the DNA entered the cell and directed the formation of new viruses, it had to contain the instructions for making more phages.

Why was the experiment convincing? 📊

The experiment was convincing because it used a clear comparison between two possible hereditary molecules. students, a good scientific experiment needs a testable question, a controlled method, and evidence that supports one explanation over another.

Several features made the Hershey-Chase experiment strong:

Specific labeling: $^{32}\text{P}$ labeled DNA and $^{35}\text{S}$ labeled protein.
Separation method: blender treatment removed attached protein coats.
Centrifugation: bacteria could be separated from viral remains.
Direct measurement: radioactivity showed where each molecule was located.

The logic was simple but powerful. If protein were the genetic material, the sulfur label should have entered the bacteria. Instead, it stayed outside. Because the phosphorus label entered the bacteria, DNA was identified as the molecule carrying hereditary information in the virus.

This is an example of using evidence to support a conclusion in biology. The conclusion was not based on guesswork; it was based on the pattern of radioactive labels after infection.

Key terms you should know 📚

To understand this topic clearly, students, it helps to know these terms:

Bacteriophage: a virus that infects bacteria.
Genetic material: the molecule that stores hereditary information.
Isotope: atoms of the same element with different numbers of neutrons.
Radioactive isotope: an isotope that emits radiation and can be detected.
$^{32}\text{P}$: radioactive phosphorus used to label DNA.
$^{35}\text{S}$: radioactive sulfur used to label protein.
Supernatant: the liquid above the solid pellet after centrifugation.
Pellet: the solid material collected at the bottom after centrifugation.
Infection: the process by which the virus enters or injects material into a host cell.

These terms connect to wider IB Biology ideas about molecular structure and function. DNA, proteins, and viruses all show how the chemical basis of life supports biological processes.

How does this fit into Unity and Diversity? 🌍

The Hershey-Chase experiment belongs in the topic of Unity and Diversity because it shows both shared biological principles and differences among life forms.

Unity

All living things rely on nucleic acids to store and pass on information. The experiment helped confirm that DNA is the genetic material in viruses, which fits with the broader pattern that nucleic acids are central to heredity across many organisms. Even though viruses are not cells, they still depend on the same basic chemical language found in all life: nucleic acids, proteins, and specific molecular interactions.

Diversity

Viruses are very different from cells. They do not carry out metabolism on their own and must use a host cell to reproduce. The bacteriophage model shows a unique form of biological diversity, because it is a simple infectious particle rather than a full cellular organism. Yet even this simple system uses DNA as information storage, showing a shared molecular basis despite structural differences.

This makes the experiment an excellent example of how biology studies both common patterns and variation. The same DNA molecule can function as genetic material in very different systems.

Applying IB Biology reasoning to the experiment 🧠

students, IB Biology often asks you to interpret evidence rather than just memorize facts. Here is how to reason through this experiment like a scientist.

If you are given data from a similar investigation, ask:

What is being labeled?

DNA or protein?

Where is the label found after infection and separation?

In the bacteria or in the liquid?

What does that location suggest?

If it enters the bacteria, that molecule is likely involved in heredity.

A good exam-style conclusion might be: the radioactive phosphorus was found in the bacterial pellet, showing that DNA entered the cells during infection. The radioactive sulfur remained in the supernatant, showing that protein did not enter the cells. Therefore, DNA is the genetic material in bacteriophages.

You may also be asked why the experiment used viruses rather than whole organisms. The answer is that bacteriophages are simple enough to study, and their components can be labeled more easily than in complex cells.

Why does this experiment still matter? 🌟

The Hershey-Chase experiment is a classic example of how science builds knowledge through evidence. It helped confirm that DNA carries genetic information, which led to major progress in genetics, molecular biology, and biotechnology. Without knowing that DNA is the hereditary material, later discoveries about replication, transcription, and translation would have been harder to understand.

It also shows that scientific ideas can be tested in a direct way. By carefully isolating one variable at a time, scientists can uncover what actually happens inside biological systems. That approach is central to IB Biology SL and to real scientific research.

Conclusion

students, the Hershey-Chase experiment is a landmark study because it showed that DNA, not protein, is the genetic material in bacteriophages. By using radioactive isotopes $^{32}\text{P}$ and $^{35}\text{S}$, Hershey and Chase demonstrated that DNA enters bacterial cells during infection while protein remains outside. This experiment is important in Unity and Diversity because it reveals a shared chemical basis of heredity across living systems, while also showing the diversity of viral structures and life cycles. It is a powerful example of how biological knowledge comes from careful observation, controlled experiments, and evidence-based reasoning.

Study Notes

The Hershey-Chase experiment tested whether DNA or protein is the genetic material in bacteriophages.
Bacteriophages are viruses that infect bacteria.
$^{32}\text{P}$ was used to label DNA because DNA contains phosphorus.
$^{35}\text{S}$ was used to label protein because proteins can contain sulfur.
After infection, a blender removed viral protein coats from the outside of bacteria.
Centrifugation separated bacterial cells into a pellet and viral remains into the supernatant.
Radioactive phosphorus was found in the pellet, showing that DNA entered the bacteria.
Radioactive sulfur stayed in the supernatant, showing that protein did not enter the bacteria.
The conclusion was that DNA is the genetic material in bacteriophages.
This experiment supports the idea that nucleic acids store hereditary information.
It connects to Unity and Diversity by showing a common molecular basis of heredity and the diversity of viral forms.
In exams, focus on the method, the results, and the conclusion supported by evidence.