Production of pure plasmid DNA is a prerequisite for most laboratories engaged in recombinant DNA research. There are a number of procedures available for plasmid purification, all of which have common features; namely, (1 ) growth of plasmid-containing bacteria, (2 ) lysis of bacteria, (3 ) low-speed centrifugation to remove the majority of bacterial DNA and protein, (4) an ultracentrifugation step in cesium chloride and ethidium bromide to separate plasmid DNA from RNA and chromosomal DNA, and (5) recovery of plasmid DNA band (extraction of ethidium bromide and cesium chloride). Steps (4) and (5) of this procedure are expensive, time-consuming, and require expensive machinery [ultracentrifuge, rotor(s)]. In addition, incomplete extraction of ethidium bromide can result in plasmid DNA that is not manipulable by many enzymes commonly used in DNA research.
