Restriction endonucleases are a class of enzyme that cut DNA
molecules. Each enzyme recognizes a unique sequence of
nucleotides in the DNA strand, usually about 4-6 base-pairs
long. The sequences are palindromic in that the complimentary
DNA strand has the same sequence only in the reverse direction,
so both strands of DNA are cut at the same location. Restriction
enzymes are found in many different strains of bacteria where
their biological role is to participate in cell defense. These
enzymes “restrict” foreign (e.g. viral) DNA that enters the
cell, by destroying it. The host cell has a restriction-
modification system that methylates its own DNA at sites
specific for its respective restriction enzymes, thereby
protecting it from cleavage. Over 800 known enzymes have been
discovered that recognize over 100 different nucleotide
sequences. Restriction enzymes are used in biotechnology to cut
DNA into smaller strands in order to study fragment length
differences among individuals (Restriction Fragment Length Po