Transformation: Bacterial cells are normally not efficient at taking up foreign DNA. However, treatment with calcium or other ions can make bacteria competent and greatly increase the efficiency. Transformation involves mixing competent bacterial cells with plasmid DNA.

Selection of Transformed Bacteria

Once the vector is in the bacterium, it is necessary to identify those cells that have been transformed with a plasmid. This is usually accomplished by selection with antibiotic resistance (coded by one of the genes on the plasmid). Bacteria without the plasmid die when the particular antibiotic is added to the medium. Cells with a plasmid are resistant and can grow. Cells that have foreign DNA in the cloning site can be distinguished by loss of function for the marker gene on the plasmid.