ABSTRACT
DNA libraries are sets of DNA clones (a clone is a genetically distinct individual or set of identical individuals), each of which has been derived from the insertion of a different fragment into a vector followed by propagation in the host. Genomic libraries are prepared from random fragments of genomic DNA. However, genomic libraries may be an inefficient method of finding a gene, particularly in large eukaryotic genomes, where much of the DNA is noncoding (see Topic D4). The alternative is to use as the source of the library the mRNA from a cell or tissue which is known to express the gene. DNA copies (cDNA) are synthesized from the mRNA by reverse transcription and are then inserted into a vector to form a cDNA library. cDNA libraries are efficient for cloning a gene sequence, but yield only the coding region, and not the surrounding genomic sequences.
