DNA Libraries

Genomic DNA is not always the source of the fragments in a DNA library. A second major class of libraries uses cDNA which is generated by copying the messenger RNA from an organism or tissue of interest. Because it reflects the mRNA content of a biological system (or cell type) at a particular time and under particular conditions, a cDNA library can be considered a "snapshot" of gene expression in that system. This information can be of great value in understanding when and how certain genes are expressed in an organism or cell type. Additionally, cDNA, unlike genomic DNA, lacks introns and other noncoding segments of sequence and is relatively straightforward to clone and express. This greatly facilitates the analysis of gene products (proteins) in eukaryotes.

Creating a cDNA library is similar to creating a genomic DNA library, except that the starting material for cDNA libraries is mRNA, not DNA. The enzyme reverse transcriptase is used to copy the mRNA to DNA. The DNA fragments are then cloned into vectors (typically plasmids) by ligation and moved into a host organism, as with genomic libraries.

Often, cDNA libraries are constructed using plasmid vectors with sequences that allow the cloned cDNA fragments to be expressed as proteins. Such "expression libraries" can be searched with protein-finding tools such as antibodies, and then the gene coding for the protein can be isolated. cDNA libraries are also used for expressed sequence tag (EST) analysis, in which small portions of many cDNAs are sequenced to provide an overview of gene expression in a particular sample.

DNA libraries play important roles in modern molecular biology research. The many genome-sequencing projects that are revolutionizing our understanding of genetics are entirely dependent on genomic DNA library techniques. cDNA libraries are invaluable in the study of gene expression and protein function, and for EST analysis. Continued progress in the development of library techniques and a continued interest in their applications suggest that these tools will remain an important part of the field for years to come.