A collection of cloned DNA fragments representing the entire genetic material of an organism. This facilitates screening and isolation of any particular gene. DNA libraries are created by fractionating the genomic DNA into fragments using restriction enzymes and/or physical methods. These fragments are cloned (see gene cloning) and the host cells containing the recombinant fragments are centrifuged and frozen; alternatively, the phage vectors are maintained in culture. Individual genes in the library are identified using specific gene probes with the Southern blotting technique or, via their protein products, using Western blotting. DNA libraries are thus repositories of raw material for use in genetic engineering. A large genome, such as that of humans, is most conveniently cloned using vectors that can accommodate large fragments of DNA, such as yeast artificial chromosomes, maintained in cell culture. Libraries containing complementary DNA (cDNA libraries) are created essentially by first isolating the messenger RNA molecules expressed in a particular cell or tissue, and then using reverse transcriptase to produce DNA copies. These are joined to plasmid or phage vectors and cloned to create the library. A cDNA library thus represents only part of the genome, namely that which is being expressed at a particular time by the cell type in question.
Subjects: Biological Sciences.