A genomic library is a comprehensive collection of cloned DNA fragments derived from a genome. Each part of the genome is represented in the library several times, and the number of times it is represented on average is called the coverage of a library. The library can be screened for the presence of the sequence of interest by radioactively labelling the DNA (usually between 100 and 500 nucleotides long) and using this as a probe to identify the clone that contains the selected sequence. The clone selected can then be grown in bacteria to produce large amounts of clone DNA, which can be studied. If for example, the sequence of interest was part of a gene, by using this sequence as a probe, the clone containing, hopefully, the whole gene could be isolated.

The DNA fragments are generated from genomic DNA by treatment by restriction enzymes--enzymes that cut...