<Tr> <Th> PMC </Th> <Td> articles </Td> </Tr> <Tr> <Th> PubMed </Th> <Td> articles </Td> </Tr> <Tr> <Th> NCBI </Th> <Td> proteins </Td> </Tr> <P> Typical type II restriction enzymes differ from type I restriction enzymes in several ways . They form homodimers, with recognition sites that are usually undivided and palindromic and 4--8 nucleotides in length . They recognize and cleave DNA at the same site, and they do not use ATP or AdoMet for their activity--they usually require only Mg as a cofactor . These enzymes cleave the phosphodiester bond of double helix DNA . It can either cleave at the center of both strands to yield a blunt end, or at a staggered position leaving overhangs called sticky ends . These are the most commonly available and used restriction enzymes . In the 1990s and early 2000s, new enzymes from this family were discovered that did not follow all the classical criteria of this enzyme class, and new subfamily nomenclature was developed to divide this large family into subcategories based on deviations from typical characteristics of type II enzymes . These subgroups are defined using a letter suffix . </P>

What two kinds of enzymes are used in sequencing dna