Restriction enzymes are synthesised by eubacterias as a defence mechanism. These are specifically endonucleases and cleave double stranded DNA where they find their specific restriction sites. Restriction enzymes are of three types: type I, type II and type III. Of which only type II restriction endonucleases restricts DNA within the recognition site and thus are used as molecular scissors for genetic engineering. Sites recognisable by the restriction enzymes are also present in the host genome but they are methylated for protection against their own enzyme, so that they do not undergo self digestion.