The Hershey-Chase Experiment

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

In this lesson we will discuss the Hershey-Chase experiment, a simple but powerful experiment that helped prove that DNA was genetic material. Hershey and Chase used a phage, or a virus that infects bacteria, and E. coli to answer their question.

Hershey and Chase

When you hear the name 'Hershey,' what springs to mind? Yeah, I know what you're thinking. And we're not talking about that one. Although, if you think it would help you concentrate, go ahead and grab a bite of chocolate while I tell you a little story. See, there's another famous Hershey out there, one who helped to significantly advance biology and genetics in the mid-1900s. And that Hershey had a sidekick named Chase. Together, this team used a simple experiment to help prove that our hereditary, or genetic, material is DNA. You might not realize this was ever a controversy, but it definitely was. Before we get to the details of the experiment, we should first go over a couple of concepts related to genetic material.

Genetic Material

Genetic material is the material, or the kind of molecule, that carries the information for building an entire organism. We also more commonly refer to this as hereditary material, or the material that gets passed on to offspring during reproduction. Today, it might be common for us to think of genetic material, then think of genes, and then immediately think of DNA. But all things in science must be proven, and there was a time when it was not yet clear whether the genetic material was in fact DNA or if maybe it was actually protein. Hershey and Chase came up with an idea to help settle this dispute.

Bacteriophages are Viruses that Infect Bacteria

The crux of the Hershey-Chase experiment was the use of a bacteriophage, or a phage for short. A bacteriophage is a virus that infects bacteria. Phages have a simple structure consisting of a protein shell or coat that encloses phage DNA or RNA.

The structure of the bacteriophage used by Hershey and Chase
Overview of phage structure.

Hershey and Chase had a bacterium, E. coli, and a bacteriophage, called T2. These were the experimental tools they used to prove that DNA was the genetic material. Their entire experiment hinged on the ability of that phage to infect E. coli cells.

The Typical Phage Lifecycle

Let's take a look at what we mean when we say that the T2 phage will infect E. coli.

The typical phage lifecycle begins when the phage docks on the outside of the bacterial cell. The protein shell is able to loosely attach to binding sites on the surface of the bacterial cell. After the phage lands there it injects its DNA into the bacterial cell. At this point the bacterial cell is basically hijacked. It stops carrying out its normal bacterial functions and starts using the phage DNA to produce the parts to build new phages.

Then, the bacterial cell begins producing all the proteins needed for the phage coat and begins making copies of the DNA to package up inside the protein coats. After the new phages are built the bacterial cell bursts open, in a process called lysis, and the new phage are released so they can go on to infect another cell.

Overview of the phage lifecycle including (1) phage attachment to a bacterial cell, (2) injection of genetic material, and (3) release of newly assembled phage. Hershey and Chase were investigating if the genetic material was protein (gray boxes) or DNA (green).
Phage Lifecycle

Now the trick is that at the time, think the the early 1950s, scientists weren't quite sure if it was phage proteins or phage DNA that was being injected into the bacterial cell and used to produce new phages. In other words, scientists weren't sure if the phage genetic material was protein or DNA. While we know now that it was DNA, remember at the time there was some controversy. That is where Hershey and Chase came in. They decided that since the structure of the phage was so simple, consisting of just the protein coat and the enclosed DNA, they could use a couple simple tricks to figure out if the genetic material was DNA or protein.

The Hershey-Chase Experiment

So let's try to restate the question that Hershey and Chase were investigating and the basic experimental approach.

The Question: Is the phage genetic material DNA or protein?

The Experiment: Using E. coli and the T2 phage, track whether it is protein or DNA that is injected into E. coli and used as the genetic material to build new phages.

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