Evidence 2 — Bacteriophages (Hershey and Chase)

• A bacteriophage (phage) is a virus that infects bacteria.
• Its particle is a DNA molecule enclosed in a protein coat, with a tail-like extension that attaches to the bacterial cell wall.
• After attachment, the bacterium bursts open after a time (about 32 minutes in the book's account), releasing many new phages.
• So something passed from phage to bacterium that carried the instructions for making new phages.

Hershey and Chase exploited a chemical difference:

• DNA contains phosphorus but no sulphur.
• Protein contains sulphur but (almost) no phosphorus.

• They grew one batch of phages whose protein was labelled with radioactive sulphur (³⁵S) and another batch whose DNA was labelled with radioactive phosphorus (³²P).
• They let each batch infect bacteria, then separated the phage coats from the bacteria (by agitation and centrifugation) and measured radioactivity inside and outside the bacterial cells.

• Result: nearly all the labelled DNA (³²P) entered the bacteria; the labelled protein (³⁵S) stayed outside.
• Conclusion: only the viral DNA enters, and it directs the bacterium to build new viruses.
• Hence the genes of the phage are DNA.