Bacteriophages -

Bacteriophages are viruses which infect bacteria. These are commonly called phages. They are more complicated than plasmids. In addition to having an origin of replication, phage DNA contains genes coding for proteins that form a protective shell around the DNA. But, like plasmids, phages lack the machinery necessary to actually make proteins; consequently, they reproduce only inside living bacterial cells.

Many phages are like miniature hypodermic syringes. The phage DNA is wrapped into a tight ball inside a head like structure made of protein. A tail, also made of protein, is attached to the head. When such a phage particle comes into contact with a bacterial cell, the phage tail adheres to the cell wall and the DNA is squirted out of the head, through the tail, into the bacterium.

Many phages also have genes for their own DNA replication machinery. If a DNA fragment is spliced into a phage DNA molecule without destroying important phage genes, the phage will reproduce the fragment along with its own DNA when it infects a bacterial cell.

There are different kinds of phages such as lambda, M13, etc., which are useful as vectors in gene cloning.