A restriction enzyme or restriction endonuclease is an enzyme which cuts DNA at particular recognition nucleotide sequences with Type II restriction enzymes cutting double-stranded DNA known as restriction sites. Like enzymes, make in archaea and bacteria, are thought to have evolved to give a defense mechanism against occupy viruses. Within a bacterial host the restriction enzymes selectively cut up foreign DNA in a procedure called as restriction; host DNA is methylated by a modification enzyme (a methylase) to protect it from the restriction enzyme's activity. Cooperatively, these two procedures form the restriction modification system. For cut the DNA a restriction enzyme create two incisions, once by every sugar-phosphate backbone of the DNA double helix.