A virus is considered a noncellular organism consisting of DNA or RNA enclosed in a protein coat -- which normally includes some enzymes. All viruses are parasites capable of replicating only within a host cell.
Viruses which infect bacteria such as E. coli are known as bacteriophages. Experiments on them have greatly increased our understanding of prokaryotic genetics (recall the Hershey Chase experiment which labeled the bacteriophage's protein coat with radioactive sulfur and its DNA with radioactive phosphorus).
The long evolutionary battle between bacteria and viruses has produced the exquisite host-parasite specificity seen in their offensive and defensive mechanisms.
The Lytic Cycle
Once a virus gets the upper hand it commandeers the bacteria's own RNA polymerase to make viral proteins and enzymes some of which chop up the bacteria's DNA for the nucleotides necessary to make viral DNA.
Eventually the bacteria becomes nothing more than a bag full of viruses. The final stop in the viral life cycle is to lyse (rupture) the membrane surrounding the host cell releasing hundreds of new infectious particles. This process is called the lytic cycle.
Review the lytic cycle for bacteriophage T4.
One such virus that we will study is lambda.
The Lysogenic cycle
Some viruses have a second life cycle called the lysogenic cycle. After the virus enters the host cell it can choose to insert its DNA directly into the host's chromosome, a process called lysogeny.
The viral DNA is replicated along with the rest of the host's chromosome and is passed on to all the bacteria's progeny.
The virus remains dormant, producing only a repressor protein which prevents other viruses from attacking the cell until some environmental clue triggers the return to the lytic cycle
Review the lysogenic cycle of the bacteriophage lambda.
Visit this site for an excellent review of viral biology
Modified Feb. 10 2002