DNA library is a collection of DNA fragments
DNA library is a collection of DNA fragments that is stored and propagated in a population of micro-organisms through the process of molecular cloning.
A genomic library is a collection of the total genomic DNA from a single organism
There are two important types of libraries:
- Genomic libraries
- cDNA libraries.
A genomic library is composed of DNA from a nuclear genome fragmented by restriction endonuclease digestion and recombined with vector DNA. Each recombinant plasmid (or clone) has the same vector sequence, but has a different region of the genome (sometimes referred to as the insert) with which it is recombined. A complete genomic library would contain every region of the genome represented as a clone.
A cDNA (complementary DNA) library is composed of the "expressed" genome. It is made to DNA copies of the mRNA expressed in a particular cell. The mRNA is isolated from a specific tissue or cell type and is copied into DNA using the enzyme reverse transcriptase. Reverse transcriptase is normally encoded by double – stranded RNA viruses. It will copy RNA into DNA. The cDNA libraries are specific for the tissue from which the mRNA is isolated.
The distribution of sequences in the cDNA library is also tissue specific. A gene that is highly expressed will be copied into DNA more often and therefore compose a larger fraction of the library than a gene that is poorly expressed. The cDNAs, since they are made from mature mRNAs, will only include the exons and not the introns. In fact, early comparisons of cDNAs and genomic sequences first alerted scientists to the existence of exons and introns.