Restriction endonucleases are found in a wide range of bacterial species warener arber discovered thet their biologicals function is to recognize and cleave foreign DNA,such DNA is said to be restricted. In the host cell’s DNA, the sequence that would be recognised by the restriction endonuclease in protected from digestion by methylation of the DNA,catalysed by specific methylase. The restiction endonulclease and the corresponding methylase in a bacterium are referred to as restriction modificaton system.there are three types of restriction endonuclease, designated I,II, and III. Type I and III are Generally large multi submit complex containing both the endonuclease and methylase activities.
Type 1 :-
These RE cleave DNA at random sites that can be more than 1000 base pairs from the recognition sequences from the 5’-end of the sequence TCA located with in recognition sites .its require ATP.
Type 2 :-
Type 2 RE were first isolated by humition smith are simpler, require no ATP,and cleavage of DNA with recognition sequences itself. Only type 2 restriction endonuclease are used for restriction mapping and gene cloning in view of thier cleavage only at specific sites.
Type 3 :-
These endonuclease cleave the DNA in a about 25 base pair from the recognition sequence. It move along the DNA in a rection that require the energy Of ATP.
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