A bacteriophage is a virus that infects bacteria. It is commonly employed by molecular biologists as a vector for genetic engineering experiments.

Isolating DNA from a bacteriophage can be done using the following protocol:

1. Retrieve a culture containing infected bacteria, along with extracellular phage progeny
2. Suspend the culture in solution and centrifuge, to isolate a bacterial pellet (which is discarded) from the phage-containing supernatant
3. Suspend the isolated phages in solution, and add polyethylene glycol (PEG) and NaCl to give precipitated phage particles
4. Centrifuge in a caesium-chloride gradient to give a pellet of phage particles, and then re-suspend the pellet
5. Use phenol or chloroform extract to remove protein particles
6. Use RNase to remove unwanted RNA
7. Use ethanol precipitation to recover phage DNA