Restriction Enzyme

Endonucleases cut the DNA at specific sites (generally) when the bases are not protected (modified, usually by methylation). Bacteria synthesize restriction enzymes as a defense against invading foreign DNAs such as phages and foreign plasmids. They may facilitate recombination and transposition. Three major types have been identified. Type II enzymes are used most widely for genetic engineering. Type II enzymes have separate endonuclease and methylase proteins. Their structure is simple, cleave at the recognition site(s); the recognition sites are short (4–8 bp) and frequently palindromic, they require Mg²⁺ for cutting, the methylation donor is SAM. So far more than 3,000 Type II restriction endonucleases have been identified. Type II proteins, unless they have a similar pattern of cleavage, have relatively low sequence homologies.

Type III enzymes carry out restriction and modification with the help of two proteins which share a polypeptide. They have two different subunits.

Cleavage...