Plasmids function independently of other bacterial DNA and can enter cells of other organisms, carrying genetic information with them that then merges with that of the host cell. To transfer genes from one cell to another, researchers use an enzyme to cut a gap in a bacterial plasmid. They then insert a gene from the donor DNA strand into the plasmid. Because the cut ends of both the plasmid and the donor gene segment are chemically 'sticky,' they attach to each other or recombine to form a plasmid containing the new gene. The recombined plasmid now carries the gene that can result in the expression of the new trait when transferred into a plant cell by the plasmid.
Plasmids are also important as the basis of gene cloning in biotechnology.
Not every attempted transfer will result in the successful combining of the desired gene with the DNA of the new host, and in only a few combinations will the new gene be located so that it is expressed properly without disrupting the function of the rest of the host's genes. Thus, a great many cells have to be transformed in order to get a few that function properly. When the altered bacteria multiply, the plasmid containing the desired gene is also copied or cloned in every cell. Because bacteria grow quickly, this method makes it possible to make millions of copies of the desired gene in a short period. Using the appropriate enzymes, the plant genes are then extracted again from the bacteria and inserted into the plasmids of A. tumefaciens.