Library (biology)

Terminology

The term "library" can refer to a population of organisms, each of which carries a DNA molecule inserted into a cloning vector, or alternatively to the collection of all of the cloned vector molecules.

cDNA libraries

A cDNA library represents a sample of the mRNA purified from a particular source (either a collection of cells, a particular tissue, or an entire organism), which has been converted back to a DNA template by the use of the enzyme reverse transcriptase. It thus represents the genes that were being actively transcribed in that particular source under the physiological, developmental, or environmental conditions that existed when the mRNA was purified. cDNA libraries can be generated using techniques that promote "full-length" clones or under conditions that generate shorter fragments used for the identification of "expressed sequence tags".

cDNA libraries are useful in reverse genetics, but they only represent a very small (less than 1%) portion of the overall genome in a given organism.

Applications of cDNA libraries include:

• Discovery of novel genes
• Cloning of full-length cDNA molecules for in vitro study of gene function
• Study of the repertoire of mRNAs expressed in different cells or tissues
• Study of alternative splicing in different cells or tissues

Genomic libraries

A genomic library is a set of clones that together represents the entire genome of a given organism. 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. For most practical purposes, the tissue source of the genomic DNA is unimportant because each cell of the body contains virtually identical DNA (with some exceptions).

Applications of genomic libraries include:

• Determining the complete genome sequence of a given organism (see genome project)
• Serving as a source of genomic sequence for generation of transgenic animals through genetic engineering
• Study of the function of regulatory sequences in vitro
• Study of genetic mutations in cancer tissues

Synthetic mutant libraries

In contrast to the library types described above, a variety of artificial methods exist for making libraries of variant genes. Variation throughout the gene can be introduced randomly by either error-prone PCR, DNA shuffling to recombine parts of similar genes together, or transposon-based methods to introduce indels. Alternatively, mutations can be targeted to specific codons during de novo synthesis or saturation mutagenesis to construct one or more point mutants of a gene in a controlled way. This results in a mixture of double stranded DNA molecules which represent variants of the original gene.

The expressed proteins from these libraries can then be screened for variants which exhibit favorable properties (e.g. stability, binding affinity or enzyme activity). This can be repeated in cycles of creating gene variants and screening the expression products in a directed evolution process.