In the Hershey-Chase experiments, which component enters the bacterial cell after infection?

• A. DNA
• B. Phage protein
• C. Lipids
• D. RNA

Answer: (A)

The important idea is that genetic information for the phage is carried by DNA, not by the protein coat. In the Hershey-Chase experiments, they labeled DNA with radioactive phosphorus and labeled phage proteins with radioactive sulfur. After infection, the radioactive DNA was found inside the bacterial cells and in the progeny phages, whereas the labeled proteins remained outside. This shows the genome—the DNA—enters the cell to drive production of new virus particles, while the protein shell stays outside. Therefore, the component entering the cell is DNA. Lipids and RNA do not play that entering role here, and, for this phage, RNA is not the genetic material, so it would not be expected to enter and direct replication.