use plasmid vector with antibiotic resistant gene
vector contains 2 marker genes:
marker gene 1 = identify if cell has taken up plasmid or not. cells with plasmid gain antibiotic resistance
grown on medium, non recombinant cells will die
marker gene 2 = identify if cell has taken up hybrid vector.
foreign DNA in marker gene 2. cells with hybrid gene cant make this DNA
cell with hybrid vector not resistant. so will die
ampicillin test on replica plate
colonies that grow on first plate but not on replica (ampicillin) plate are recombinant.