An ampicillin-resistant, tetracycline-resistant plasmid, pBR322, is cleaved with PstI, which cleaves within the ampicillin resistance gene. The cut plasmid is ligated with PstI digested Drosophila DNA to prepare a genomic library, and the mixture is used to transform E. coli K12. Which Antibiotic should be added to the medium to select cells that have incorporated a plasmid

A plasmid is a small, circular, double-stranded DNA molecule, which is physically separated from chromosomal DNA and is able to replicate independently. Moreover, plasmid transformation is the process by which exogenous DNA is introduced into a cell (usually a bacterial host cell). In this case, Escherichia coli K12 cells were transformed with the pBR322 plasmids that encode both ampicillin and tetracycline resistance genes. According to data, plasmids were firstly cleaved with the PstI type II restriction endonuclease within the ampicillin resistance gene, thereby these plasmids lose the ampicillin-resistance gene that allows selecting E. coli K12 cells by their resistance to ampicillin. In consequence, tetracycline is the only one selective marker that will indicate that the bacteria have incorporated the plasmid.