FIGURE 12–3 Hershey-Chase Experiment Alfred Hershey and Martha Chase used different radioactive markers to label the DNA and proteins of bacteriophages. The bacteriophages injected only DNA, not proteins, into bacterial cells.

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The Hershey-Chase Experiment Hershey and Chase studied a bacteriophage that was composed of a DNA core and a protein coat. They wanted to determine which part of the virus—the protein coat or the DNA core—entered the bacterial cell. Their results would either support or disprove Avery's finding that genes were made of DNA.

The pair grew viruses in cultures containing radioactive isotopes of phosphorus-32 (³²P) and sulfur-35 (³⁵S). This was a clever strategy, because proteins contain almost no phosphorus, and DNA contains no sulfur. Therefore, these radioactive substances could be used as markers, enabling the scientists to tell which molecules actually entered the bacteria, carrying the genetic information of the virus. If they found radioactivity from ³⁵S in the bacteria, it would mean that the virus's protein coat had been injected into the bacteria. If they found ³²P, then the DNA core had been injected.

The two scientists mixed the marked viruses with bacterial cells. They waited a few minutes for the viruses to inject their genetic material. Next, they separated the viruses from the bacteria and tested the bacteria for radioactivity. Figure 12–3 shows the steps in this experiment. What were the results? Nearly all the radioactivity in the bacteria was from phosphorus (³²P), the marker found in DNA. Hershey and Chase concluded that the genetic material of the bacteriophage was indeed DNA, not protein. Hershey and Chase's experiment with bacteriophages confirmed Avery's results, convincing many scientists that DNA was the genetic material found in genes—not just in viruses and bacteria, but in all living cells.

In Your Notebook Identify the independent and dependent variables in the Hershey-Chase experiment, and list some possible control variables.

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