JOINING FOREIGN DNA AND PLASMID
The foreign DNA and the plasmid both are cut with the same enzyme or enzymes. This leaves the same sticky ends on the plasmid and on the foreign DNA. The two types of molecules are mixed so that their sticky ends pair.

Often the plasmid is cut in the middle of a marker gene, deactivating the marker. For example, if the cut site is in the middle of a lacZ gene, the plasmid loses its ability to turn bacterial colonies blue.