DNA Libraries

‘DNA library’ is the term used to describe a collection of recombinant clones or DNA molecules generated from a specific source of DNA. There are two main types of DNA library which are very distinct in their origin and purpose. DNA from a nucleated cell, whatever the tissue source, from a specific organism is used to make a ‘genomic’ library. The idea of a general genomic library is to produce a set of clones that contain enough DNA fragments so that the entire genome of the organism is represented. Variations of genomic DNA libraries such as chromosome-specific libraries that were prepared from chromosomes sorted by flow cytometry10 were employed in the Human Genome Project in an attempt to shorten the path between the starting DNA and generation of the genome map. The second type is the cDNA library, which is made from  mRNA that has been reverse transcribed by the enzyme reverse transcriptase. Reverse transcriptase produces complementary DNA (or cDNA) fragments which are then cloned into a vector. Therefore, unlike a genomic DNA library, a cDNA library is representative of the expressed genes in a particular cell or tissue type. Thus a skeletal muscle cDNA library contains sequences expressed in the muscle tissue at the time the mRNA was harvested. cDNA libraries are particularly useful for cloning sequences where there is biological information. For example, it is known that mammalian skeletal muscle produces high levels of  phosphoglucomutase (PGM1) enzyme activity, hence PGM1 cDNA clones are expected (and found) to be well represented in skeletal muscle cDNA libraries.