An ampicillin-resistant, tetracycline-resistant plasmid, pBR322, is cleaved with Pstl, w...

An ampicillin-resistant, tetracycline-resistant plasmid, pBR322, is cleaved with Pstl, which cleaves within the ampicillin resistance gene. The cut plasmid is ligated with Pstl-digested Drosophila DNA to prepare a genomic library, and the mixture is used to transform E. coli K12.

(a) Which antibiotic should be added to the medium to select cells that have incorporated a plasmid?

(b) If recombinant cells were plated on medium containing ampicillin or tetracycline and medium with both antibiotics, on which plates would you expect to see growth of bacteria containing plasmids with Drosophila DNA inserts?

(c) How can you explain the presence of colonies that are resistant to both antibiotics?