4.2 Cosmid Vectors

All of the lambda DNA sequence may be removed, except for the cos sites to allow larger pieces of DNA to be inserted. A site for the origin of replication (ori) and perhaps an antibiotic resistance gene also are retained.


Since no viral genes are included in the vector, they replicate as plasmids in E coli. The amplified DNA or its gene product may be isolated from the infected cells.

Figure 32. Assembly of lambda phage in the test tube

This method allows insertion of larger pieces of DNA and also has the advantage of efficient infection of cells by the bacteriophage particles. Recall that transformation of cells by plasmids alone is relatively inefficient (perhaps 1 in 1000 molecules) and is usually limited to inserted DNA fragments of up to 15 kb.

Figure 33. Incorporation of DNA into lambda--derrived particles