Bacteriophages as Vectors

Bacteriophages as Vectors - Bacteriophages provide another source of cloning vectors. Since usually phage has a linear DNA molecule, a single break will generate two fragments, which are later joined together with foreign DNA to general a chimeric phage particle.

The chimeric phage can be isolated after allowing it to infect bacteria and collecting progeny particles after a lytic cycle. The use of phage particles as vector imposes a limitation on the size of foreign DNA, which can be cloned, because the capacity of phage head is only limited, and if the size of foreign DNA is too long size of phage DNA may not be accommodated in phage head.

In order to overcome this problem, those segments of DNA, which do not contain essential genes, may be removed. Such a technique has been followed in phage lambda (A.) to create a smaller vector genome having single restriction site for the enzyme EcoRI.

Since the reduced size also fails to be adequately packed in phage head (there is also a requirement of a minimum size of DNA), this automatically provides a selection method, in which only the chimeric particles will be obtained in the phage progeny, and the vector particles lacking cloned segment will be eliminated due to its reduced size.