Bacteriophages, viruses that infect bacteria, are useful in the study of how genes function. The attributes of bacteriophages include their small size and simplicity of genetic organization.

The most intensively studied bacteriophage is the phage called lambda. It is an important model system for the latent infection of mammalian cells by retroviruses, and it has been widely used for cloning purposes. Lambda is the prototype of a group of phages that are able to infect a cell and redirect the cell to become a factory for the production of new virus particles. This process ultimately results in the destruction of the host cell (lysis). This process is called the lytic cycle. On the other hand, lambda can infect a cell, direct the integration of its genome into the DNA of the host, and then reside there. Each time the host genome replicates, the viral genome undergoes...