Restriction Enzymes

Restriction enzymes are endonucleases that recognize specific double-stranded DNA sequences and cleave the DNA in both strands, e.g.:

They have been identified in many prokaryotic organisms and are considered to be part of a defense system directed against foreign DNA. Site-specific endodeoxyribonucleases are given EC numbers, e.g., EC 3.1.21.4 for a Type II enzyme (see below) .