Plasmid

Because of their ability to move genes from cell to cell, plasmids have become versatile tools for both research and biotechnology. In the laboratory, researchers use plasmids to carry marker genes, allowing them to trace the plasmid's inheritance across host cells. Transferred or "cloned" genes are used to produce a variety of important medical, agricultural, or environmental products that can be economically used by humans.

Researchers have also engineered plasmids to be extremely efficient cloning vectors. To be used in this way, the plasmid must contain at least one origin of replication, a multiple cloning site (called a polylinker) where a variety of restriction enzymes can cut so that foreign DNA can be inserted, a selectable genetic marker, and transcription and translation signals recognized by the host cell, so that the expression of a cloned gene can be easily identified.

The foreign DNA is often inserted in such a way that the expression of the foreign gene is tied to the expression of a marker gene. For example, one of the most popular methods to show that a foreign DNA has been inserted and expressed in the host is by the insertional inactivation of the lac Z gene. In this case, the foreign DNA is inserted in the middle of the lac Z gene so that the gene becomes defective and the enzyme it codes for no longer works. The damaged enzyme therefore cannot cleave the artificial substrate Xgal to produce a blue color or blue colony, as it normally would, and white colonies of bacteria are produced. Therefore, the white colonies indicate that artificial DNA has been successfully cloned or recombined into the plasmid in the lac Z gene, whereas nonrecombinant colonies are blue. The white colonies can thus be easily isolated for further expansion and experimentation.

Under certain circumstances, recombinant DNA experiments using plasmids are considered to be hazardous, and the ease with which plasmids are acquired by bacteria has led them to be classed as biohazards. They are therefore subject to guidelines and may require registration and approval. A publication produced by the National Institutes of Health, titled Guidelines for Research Involving Recombinant DNA Molecules, is the definitive reference for recombinant DNA research in the United States and should be consulted when considering research, particularly biomedical research, involving plasmids.