Types of Restriction Endonucleases

There are the following four distinct types of restriction endonucleases:-

Type 1st restriction endonucleases:- are complex endonucleases, and have recognition sequences of about 15 bp; they cleave the DNA about 1000 bp away from the 5'-end of the sequence "TCA" located within the recognition site, e.g, EcoK, EcoB, etc.

Type 2nd restriction endonucleases:- are remarkably stable and induce cleavage either, in most cases, than 350 different type 2nd endonucleases with over 100 different recognition sequences are known. They require Mg⁺² ions for cleavage. The first type 2nd enzyme to be isolated was HindII in 1970. Only type 2nd restriction endonucleases are used for restriction mapping and gene cloning in view of their cleavage only at specific sites.

Type 3rd restriction endonucleases:- are intermediate between the type 1st and 2nd enzymes; they cleave DNA in the immediate vicinity of their recognition sites, e.g, EcoP1, EcoP15, HindIII, ect. Type 1st and 3rd restriction enzyme are not used in gene cloning. The type 3rd enzymes recognize asymmetric target sites, and cleave the DNA duplex on one side of the recognition sequence up to 20 bp away.