The main difference between type I and type II restriction enzymes is that type I restriction enzymes are very complex compared to type II restriction enzymes and apart from that, type I restriction enzymes they are made up of three non-identical subunits. While, on the other hand, type II restriction enzymes are composed of two identical subunits. The nucleas that cut DNA at an internal position are known as restriction enzymes. The point where these restriction enzymes cut is the middle of the DNA but not the end. Because of this property of restriction enzymes, it is known as endonucleases. These restrictions are also called molecular scissors, molecular knives, molecular scalpels, or restriction enzymes. The recognition site or restriction site is the region where these restriction enzymes cut DNA. This restriction site is at least 4-8 base pairs.

• Restriction endonuclease type I
• Type II restriction endonuclease
• Type III restriction endonuclease

Type I restriction enzymes

Type I enzymes are multiple subunits and are a complex combination of restriction and modification enzymes. These enzymes cut DNA at random points but away from their recognition sequences. These enzymes were known to be very rare, but now, after analyzing sequenced genomes, we discover that they are common. Type I restriction enzymes do not form discrete restriction fragment or gel band patterns, therefore they are not used nearly as much as type II enzymes, but they are of particular biochemical importance.

Type II restriction enzymes within their recognition site or near their recognition sites, at defined positions. Apart from that, they produce discrete gel band patterns and discrete fragments. They have a lot of practical value and are used very frequently. They are widely available commercially. Most of these enzymes bind to DNA as homodimers and also recognize DNA sequences that are symmetric. Some of these restriction enzymes also recognize asymmetric DNA sequences because they bind as heterodimers. Some enzymes also recognize sequences that are continuous. In these sequences, two half sites are adjacent to the recognition sequence. Some of these enzymes also recognize sequences that are discontinuous, in which the half sites are separated from each other. The cleavage that occurs in DNA leaves 5 phosphate on one side and 3 hydroxyl on the other side. They are smaller with subunits that are in the range of around 200-350. The most common type II enzymes are type IIS. They open to the side outside of their recognition site. They are of normal size and recognize asymmetric and continuous sequences. They have two domains, one for DNA and the other for DNA cleavage. In most parts, they bind to DNA as monomers. DNA strands that contain several recognition sites, type II recognition enzymes are very active in them. Another type of type II enzyme is type IIG. They have restriction and modification enzymes, plus they’re great. They set off to the side outside of their recognition site. They are of normal size and recognize asymmetric and continuous sequences. They have two domains, one for DNA and the other for DNA cleavage. In most parts, they bind to DNA as monomers. DNA strands that contain several recognition sites, type II recognition enzymes are very active in them. Another type of type II enzyme is type IIG. They have restriction and modification enzymes, plus they’re great. They open to the side outside of their recognition site. They are of normal size and recognize asymmetric and continuous sequences. They have two domains, one for DNA and the other for DNA cleavage. In most parts, they bind to DNA as monomers. DNA strands that contain several recognition sites, type II recognition enzymes are very active in them. Another type of type II enzyme is type IIG. They have restriction and modification enzymes, plus they’re great. Another type of type II enzyme is type IIG. They have restriction and modification enzymes, plus they’re great. Another type of type II enzyme is type IIG. They have restriction and modification enzymes, plus they’re great.

1. Type I restriction enzymes are more complex compared to type II restriction enzymes.
2. Type I recognition enzymes are composed of three non-identical subunits. Type II recognition enzymes are made up of two identical subunits.
3. The molecular weight of type I restriction enzymes is 400,000 daltons. The molecular weight of type II restriction enzymes is 20,000 to 100,000 daltons.
4. The type I restriction enzyme cleavage site is 1000 nucleotides from the recognition site. The type II restriction enzyme cleavage site is at the same recognition site.
5. The cleavage sequence is not specific for type I restriction enzymes. The cleavage sequence is specific for type II restriction enzymes.
6. Type I enzymes are responsible for protecting DNA by methylation and type II enzymes have no methylation activity.
7. For the activation of type I enzymes, ATP, Mg2 + and adenosyl methionine are needed. For the activation of type II enzymes, only Mg2 + is required.
8. Type I enzyme has endonuclease and methylase activity, both. Type II only has restriction activity, it has no endonuclease or methylase activity.
9. Type I enzymes were identified earlier than type II.
10. Type I was identified in two different strains of E. Coli.
11. Examples of type I enzymes are Ecok and EcoB. Examples of type II enzymes are Hind II and EcoRI.