Phage or Bacteriophage is a virus that infects bacteria. Different altered forms of these phages are used by molecular biologists in DNA cloning, where they are convenient vectors because of their infectious nature.

Structure of a Bacteriophage

The transfection efficiency into the bacterial host is usually two orders of magnitude greater for phage over that of plasmids.

 The bacteriophages most used are derived from two “wild” phages, called M13 and lambda (λ).

Lambda phages are used to clone segments of DNA in the range of around 10-20 kb. They are lytic phages, i.e., they replicate by lysing their host cell and releasing more phages.

On a bacterio-logical plate, this results in a small clear zone – a plaque. Some lambda vectors have also been developed which are expression vectors (q.v.).

The M13 system can grow inside a bacterium, so that it does not destroy the cell it infects but causes it to make new phages continuously. It is a single-stranded DNA phage, and is used for the Sanger di-deoxy DNA sequencing method.

Both of these phages grow on Escherichia coli as a host bacterium.

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