Figure 18.4 :  Vector map of bacteriophage λ based vector EMBL3 and EMBL4

Cosmids: Cosmids vectors offer additional advantage over bacteriophage λ based cloning vector as they have bacterial origin of replication. They are chimeric cloning vectors and consist of region from a bacterial plasmid and bacteriophage λ. They contains flanking cos site and hence after their delivery into the host cell, it adopts circular form. As cosmids contain bacterial origin of replication, it can be maintained in bacteria as such. In addition, it has antibiotic resistance gene (tetracycline) and allow selection of transformed host cells. The cloning strategy follows the similar mechanism as discussed before for bacteriophage λ based vector and it is outlined in Figure 18.5. The example of cosmid vector is pLFR-5.