A restriction digest is a molecular biology procedure used to prepare DNA for analysis or other processing. Also known as DNA fragmentation, it uses a restriction enzyme to selectively cleave strands of DNA into shorter segments, which are more suitable for analytical techniques such as chromatography. Restriction digests are performed in Genetic fingerprinting techniques using restriction fragment length polymorphisms, and are used in many other DNA manipulations.
A given restiction enzyme cuts DNA segments that only within a specific nucleotide sequence, and always makes its incisions in the same way. These recognition sequences are typically only four to twelve nucleotides long. Because there are only so many ways to arrange the four nucleotides--A,C,G and T--into a four or eight or twelve nucleotide sequence, recognition sequences tend to "crop up" by chance in any long sequence. Furthermore, restriction enzymes specific to hundreds of distinct sequences have been identified and synthesized for sale to laboratories. As a result, potential "restriction sites" appear in almost any gene or chromosome. Meanwhile, the sequences of some artificial plasmids include a "linker" that contains dozens of restriction enzyme recognition sequences within a very short segment of DNA. So no matter the context in which a gene naturally appears, there is probably a pair of restriction enzymes that can snip it out, and which will produce ends that enable the gene to be spliced into a plasmid (i.e. which will enable what molecular biologists call "cloning" of the gene).
See also: DNA sequencing