Another common type of libraries used to isolate genes is the cDNA library, which represents complementary DNA (hence cDNA) copies of the mRNA population present within a particular tissue. Such cDNA libraries are often preferable to genomic libraries as a source of cloned genes
1. because the obtained clone is a different representation of the coding sequences, without introns or other noncoding sequences found in genomic DNA, and
2. because the the use of a particular mRNA source often enriches substantially for the sequences derived from a given gene known to be expressed selectively in that tissue. For example, the beta-globin gene is represented at only one part per million in a human genomic library, but it is a major mRNA transcript in red blood cells. Thus a cDNA library prepared from red blood cells represents an excellent cloning source to isolate globin gene cDNAs. Similarly, a liver or muscle cDNA library is a good source of clones for genes known to be expressed preferentially or exclusively in those tissues.
A number of methods have been developed to clone cDNAs, all of which rely on the enzyme reverse transcriptase, a RNA-dependent DNA polymerase that can synthesize a cDNA strand from an RNA template. Reverse transcriptase requires a primer to initiate DNA synthesis. Usually, a sequence of polythymine residues (oligo-dT) is used; this short homopolymer binds to the polyA tail at the 3' end of mRNA molecules and thus primers synthesis of a complementary copy. This single-stranded cDNA is then converted to a double-stranded molecule by one of the several available methods, and the double-stranded cDNA can then be ligated into a suitable vector, usually a plasmid or a bacteriophage, to create a cDNA library representing all of the original mRNA transcripts found in the starting cell type or tissue. Some cleverly engineered vectors contain transcription and translation signals adjacent to the cloning cDNA in E.coli or in yeast. Such so-called expression vectors can then be screened with antibodies raised against, for example the protein whose gene one is trying to isolate.
Representive cDNA libraries from different tissues or different times of development are valuable resources for gene cloning. Many such libraries are now widely available.