DNA libraries:

A DNA library is a group of cloned DNA fragments in a cloning vector which can be fined for a DNA of interest.  If the target is to isolate particular gene sequences, two kinds of library are useful:

- Genomic DNA libraries: A genomic DNA library is made from the genomic DNA of an organism. For instance, a mouse genomic library could be made through digesting mouse nuclear DNA with a restriction nuclease to produce a huge number of various DNA fragments but all with identical cohesive ends.

Figure:  A simple method of DNA cloning using a plasmid vector.

The DNA fragments would then be ligated into linearized plasmid vector molecules or into an appropriate virus vector.  This library would hold all of the nuclear DNA sequences of the mouse and could be searched for any particular mouse gene of interest.  Every clone in the library is known as a genomic DNA clone. Not every genomic DNA clone would hold a complete gene since in several cases the restriction enzyme will have cut at least once within the gene. Therefore some clones will contain only a category of a gene.

- cDNA libraries: A cDNA library is made through using the reverse transcriptase of a retrovirus  to synthesize cDNA (complementary  DNA)  copies  of the total mRNA from a cell (or perhaps a subfraction of it). A single-stranded cDNA is converted into double-stranded DNA and inserted into the vector.  Each clone in the library is known as a cDNA clone. Unlike a complete genomic library that holds all of the nuclear DNA sequences of an organism a cDNA library contain only sequences which are expressed as mRNA. Various tissues of an animal, which express some genes in general but also various different genes, will therefore yield different cDNA libraries.