The main difference between type I and type II restriction enzymes is, type I restriction enzymes are very complex as compared to type II restriction enzymes and apart from that, type I restriction enzymes are made up of three non identical sub units. While on the other hand, type II restriction enzymes are made up of two identical sub units.Nucleases which cut into DNA at an internal position is known as restriction enzyme. The point where these restriction enzymes cut i sin the middle of DNA but not at the end. For this property of restriction enzymes, they are known as endonucleases. These restriction are also called molecular scissors, molecular knives, molecular scalpels or restriction enzymes. Recognition site or restriction site is the region where these restriction enzymes cut the DNA. This restriction site have at least 4-8 base pairs.There are three groups of these restriction enzymes, which are as follows:
- Type I restriction endonuclease
- Type II restriction endonuclease
- Type III restriction endonuclease
Type I Restriction Enzymes
Type I enzymes are multi sub units and are complex combination of modification and restriction enzymes. These enzymes cut DNA at random points but far from their recognition sequences. These enzymes were known to be very rare but now after analysis of sequenced genomes we have found out that they are common. Type I restriction enzymes do not make gel banding patterns or discrete restriction fragments, therefore they are not used practically as much as type II enzymes but they do have particular biochemical importance.
Type II Restriction Enzymes
Type II restriction enzymes within their recognition site or close to their recognition sites, at defined positions. Apart from that, they do produce discrete gel banding patterns and discrete fragments. They are of much practical value and are used very commonly. They are widely available commercially. Most of these enzymes bind to DNA as homodimers and they also recognize DNA sequences which are symmetrical. Some of these restriction enzymes also recognize asymmetrical DNA sequences because they bind as heterodimers. Some enzymes also recognize sequences which are continuous. In theses sequences, two half sites are adjacent of recognition sequence. Some of these enzymes also recognize sequences which are discontinuous, in which half sites are separated from each other. The cleavage which occurs in DNA leaves 5 phosphate on one side and 3 hydroxyl on other side. They are smaller with sub units which are in the range of around 200-350. Most common type II enzymes are type IIS. They cleave outside their recognition site to one side. They are normal in size and they recognize sequences which are asymmetric and continuous. They have two domains, one for DNA and the other one for cleavage of DNA. For most parts, they bind DNA as monomers. DNA strands which contain various recognition sites, type II recognition enzymes are very active on them. Another type of type II enzymes are type IIG. They have restriction and modification enzymes, moreover, they are large.
- Type I restriction enzymes are more complex as compared to type II restriction enzymes.
- Type I recognition enzymes are made up of three non identical sub units. Type II recognition enzymes are made up of two identical sub units.
- Molecular weight of type I restriction enzymes is 400000 daltons. Molecular weight of type II restriction enzymes is 20,000 to 100000 daltons.
- Site of cutting of type I restriction enzymes is 1000 nucleotides away from recognition site. Site of cutting of type II restriction enzymes is in the same recognition site.
- Sequence of cutting is non specific in type I restriction enzymes. Sequence of cutting is specific in type II restriction enzymes.
- Type I enzymes are responsible for protecting DNA by methylation and type II enzymes have no methylation activity.
- For activation of Type I enzymes, ATP, Mg2+ and adenosyl methionine are needed. For activation of type II enzymes, only Mg2+ is required.
- Type I enzyme has endonuclease and methylase activity, both. Type II only have restriction activity, it does not have endonuclease or methylase activity.
- Type I enzymes were identified before type II enzymes.
- Type I were identified in two different strains of E.Coli.
- Examples of type I enzymes are, Ecok and EcoB. Examples of type II enzymes are, Hind II and EcoRI.