SELECTION OF RECOMBINANT

• Selection of recombinants formed can be done by one of the following methods.

1. Inactivation of antibiotics

• 1. If a foreign DNA ligates at the Bam HI site of tetracycline resistance gene in the vector pBR322, the recombinant plasmid loses the tetracycline resistance due to insertion of foreign DNA.
• 2. It can still be selected out from non – recombinant ones by plating the transormants on ampicillin containing medium.
• 3. The transformants growing on ampicillin containing medium are then transferred on to a medium containing tetracycline.
• 4. The recombinants can grow in ampicillin containing medium but not on that containing tetracycline.
• 5. Non – recombinants can grow on the medium containing both the antibiotics and thus recombinants are selected.

2. Insertional inactivation

• Here, a recombinant DNA is inserted within the coding sequence of an enzyme β  galactosidase, which results into inactivation of the enzyme.