Gene Cloning, the process of isolating a single gene from among all the different genes in an organism to determine its structure. The process usually involves preparing a gene library, or gene bank—a collection of living bacteria that consists of all the genes in an organism with each individual bacterium containing one of the genes. This is done by cutting up an individual’s DNA. Alternatively, a library of all the DNA sequences expressed in a particular cell can be created by making complementary DNA (cDNA) copies of the messenger RNA (mRNA) found in such a cell (see Molecular Biology). In either case, the DNA fragments are linked to a vector—either a bacterial virus known as a bacteriophage or a circular DNA molecule known as a plasmid—that is then introduced into bacteria so that each individual bacterium takes only one copy of the vector and therefore receives only one DNA fragment.

Libraries prepared in this way can be screened to identify the individual bacterium containing the particular gene that researchers wish to study. This bacterium is then picked up and grown to produce a clone of identical bacteria. The vector containing the inserted DNA replicates whenever the bacterial cell divides, which results in the production of enough cloned insert DNA to determine the structure of the gene. In this manner the genes encoding particular proteins, or the genes whose inactivation by mutation results in a specific disease, can be studied in detail. Specifically, the sequence can be determined and the nature of the mutation that results in disease can be identified.

Subsequently, the gene can be expressed in the bacterial cell to produce the appropriate protein, which can be used to treat diseases such as diabetes (see insulin) or dwarfism (see growth hormone). It has recently become possible to introduce entire cloned functional genes into individuals, treating the disease more directly. Such gene therapy procedures with cloned DNA are likely to be increasingly used in the future.