DNA library

A DNA library is exactly what the name suggests: a collection, like a shelf of books, where each book is a different cloned fragment of DNA, and together they represent the whole of something — an organism's genome or all the genes it was expressing. Instead of holding one gene, you hold a whole searchable archive.

To build one, the source DNA is broken into many fragments, each fragment is inserted into a vector, and the recombinant vectors are introduced into host cells. Each host cell carries one fragment, so the population of cells, taken together, holds the entire collection. A genomic library is made from chromosomal DNA and represents the full genome, while a cDNA library is made from messenger RNA (via reverse transcriptase) and represents only the genes that were active in the source tissue.

The point of a library is retrieval. Researchers can screen it with a probe or by sequencing to find the particular clone that carries a gene of interest, then grow that clone to get pure copies of the gene. Before whole genomes could be sequenced cheaply, libraries were the main way to fish a single gene out of a vast genome; they remain useful for cloning, screening, and functional studies.