The Hershey-Chase Experiment

Scientists:

Aim of the Experiment:

To determine whether DNA or protein is the genetic material that enters the host bacterial cell during bacteriophage infection.

Experimental Organism:

Bacteriophage T2 - a virus that infects Escherichia coli (E. coli)

Structure of T2 Bacteriophage:

• Outer protein coat (capsid) - contains sulphur (S) in amino acids cysteine and methionine
• Inner DNA core - contains phosphorus (P) in its phosphate backbone
• Sulphur is absent from DNA; Phosphorus is absent from proteins

Radioactive Labels Used:

Experimental Setup - Two Parallel Sets

Set A - Labeling the DNA:

• T2 phages were grown in a culture medium containing radioactive ³²P
• Result: Phage DNA becomes radioactively labeled (³²P-DNA phages)

Set B - Labeling the Protein:

• T2 phages were grown in a culture medium containing radioactive ³⁵S
• Result: Phage protein coat becomes radioactively labeled (³⁵S-protein phages)

The Hershey-Chase experiment

Observations:

Conclusion / Inference:

DNA, not protein, is injected into the host bacterial cell during phage infection. Since the injected material directs the synthesis of new phage particles inside the bacterium, DNA is the genetic material responsible for heredity.

• In 1952, Alfred Hershey and Martha Chase proved that DNA is the genetic material using bacteriophages and E. coli bacteria.
• They used radioactive isotopes: ³²P to label DNA and ³⁵S to label proteins.
• Viruses grown in ³²P medium had radioactive DNA, while those grown in ³⁵S medium had radioactive protein.
• These labelled viruses were allowed to infect E. coli, and then blending and centrifugation were done to separate viral coats.
• Only bacteria infected with ³²P-labelled viruses became radioactive, showing that DNA entered the bacterial cells.
• Bacteria infected with ³⁵S-labelled viruses were not radioactive, proving proteins did not enter the cells; hence, DNA is the genetic material.