This means that after creating recombinant DNA molecules in a test tube, recombinant molecules must be introduced into host cells for propagation. This process is called, transformation. Generally, this step leads to thousands of colonies, with each colony consisting of identical cells each containing identical recombinant DNA molecules. In the process of creating recombinant DNA molecules, it is necessary to select for those colonies that contain recombinant DNA molecules with the gene of interest. This is because several different types of DNA molecules are generated during genetic engineering, including many that do not carry the desired gene.
Selection involves three steps.
First, it is necessary to determine which host cells actually received a plasmid vector during transformation. Second, one must determine which plasmids actually have a DNA insert of some type. Thus, colonies with recombinant plasmid vectors can be distinguished from those with non-recombinants based on the color of the colony. Finally, it is necessary to determine which recombinant plasmids actually contain the gene sequence of interest. DNA hybridization , specific antibodies , polymerase chain reaction, or selecting for the actual phenotype are methods frequently used at this step.