Selection of the Transformed Host Cell

The transformation process always produces a mixed population of cells: some are successfully transformed (recombinant), and others are not (non-recombinant). To isolate the recombinant cells from the non-recombinant ones, the marker genes carried by the plasmid vector are put to use. For example, the pBR322 plasmid vector carries two different marker genes -- one for ampicillin resistance and one for tetracycline resistance. When the restriction enzyme PstI is used to insert the foreign DNA, it knocks out the ampicillin-resistance gene on the plasmid in the process; as a result, a cell that has taken up the recombinant plasmid becomes sensitive to ampicillin, while a non-recombinan …