In molecular biology, a library is a collection of molecules in a stable form that represents some aspect of an organism. Two common types of libraries are cDNA libraries (formed from Complementary DNA) and genomic libraries. The nucleotide sequences of interest are preserved as inserts to a plasmid or the genome of a bacteriophage that has been used to infect bacterial cells. A cDNA library represents all of the mRNA present in a particular tissue, which has been converted back to a DNA template by the use of the enzyme reverse transcriptase. It represents the genes that are transcribed in particular tissues under particular physiological, developmental, or environmental conditions. cDNA libraries are useful in reverse genetics, but should not be confused with a genomic library, as it does not represent the entire genome, only a very small (less than 1%) portion which is being transcribed. A genomic library is a set of clones, packaged in the same vector, that together represents all regions of the genome of the organism in question. The number of clones that constitute a genomic library depends on (1) the size of the genome in question and (2) the insert size tolerated by the particular cloning vector system. Usually a cDNA library is created when reproducing eukaryotic genomic material, whereas genomic libraries are often created when working with genomic target material from bacteria and viruses.
