Restriction endonucleases: they are a subset of endonuclease enzymes that cut dsdna at specific dna sequences identified by the enzyme itself, endonucleolytic cleavage of dna to give specific double-stranded fragments with ter(cid:373)i(cid:374)al 5"-phosphates. Important because they allowed the cut and pasting of dna: came from bacteria, bacteria evolved re for protection from phages (virus) It restricts the growth and replication of phages: but host dna must be protected from restriction. 4 classes of restriction endonucleases: type i: require atp, sam and cut away from recognition site, type ii: most useful for molecular biology, requires mg2+ but not atp or sam, cuts at. Ecori is a dimer with two mg2+ ions that cleaves dna recognition site: type iii: characteristics similar to type i, sam not required, type iv: uses modified (methylated) dna as substrate. Dna restriction by type ii res: catalyzes endonucleolytic cleavage of. Dna to give specific double-stranded frag(cid:373)e(cid:374)ts with ter(cid:373)i(cid:374)al 5" phosphates: 4 subtypes of type ii enzymes: