bacteriophage λ cloning vector: The bacteriophage λ cloning vector has a middle segment responsible for insertion/excision (I/E Region) and this region can be replaced with the foreign DNA with the help of two BamHI sites present on the either side of I/E region. Hence, in a cloning strategy described in Figure 18.3, foreign DNA is put into the vector and then allowed to infect the bacteria. In the presence of I/E region, phage will integrate into the bacterial chromosome and continue lysogeny cycle. But when I/E region is disrupted or replaced with the foreign DNA, it will continue lytic cycle and form plaque. The examples of bacteriophage λ based vector are EMBL3, EMBL4 and their vector map is shown in Figure 18.4.

Figure 18.3:  bacteriophage λ cloning system.