Restriction enzymes Type II –

Restriction enzymes type II identifies the palindromic sequences on the DNA and cut the DNA strand at certain specific site. Types of Restriction Enzymes Restriction enzymes are traditionally classified into four types on the basis of subunit composition, cleavage position, sequence specificity and cofactor requirements. However, amino acid sequencing has uncovered extraordinary variety among restriction enzymes and revealed that at the molecular level there are many more than four different types. Type I enzymes are complex, multisubunit, combination restriction-andmodification enzymes that cut DNA at random far from their recognition sequences. Type II enzymes cut DNA at defined positions close to or within their recognition sequences. They produce discrete restriction fragments and distinct gel banding patterns, and they are the only class used in the laboratory for DNA analysis and gene cloning. Rather than forming a single family of related proteins, Type II enzymes are a collection of unrelated proteins of many different sorts. Type II enzymes frequently differ so utterly in amino acid sequence from one another, and indeed from every other known protein, that they exemplify the class of rapidly evolving proteins that are often indicative of involvement in host-parasite interactions. The most common Type II enzymes are those like HhaI, HindIII and NotI that cleave DNA within their recognition sequences. Enzymes of this kind are the principle ones available commercially.. Type III enzymes are also large combination restriction-andmodification enzymes. They cleave outside of their recognition sequences and require two such sequences in opposite orientations within the same DNA molecule to accomplish cleavage; they rarely give complete digests. Type IV enzymes recognize modified, typically methylated DNA and are exemplified by the McrBC and Mrr systems of E. coli.