Restriction enzyme or restriction endonuclease is a protein found in various strains of bacteria that recognize specific sequences in DNA and cut them into fragments. The restriction enzyme acts as a defense mechanism as it destroys the bacteriophages or phage.

Phage is a bacterial virus that infects bacteria by injecting its DNA into the bacteria. The restriction enzymes restrict the replication of this viral DNA by splitting it into useless fragments. It can also be separated from bacteria and used as a tool in the recombinant DNA technology.

Types of Restriction Enzymes

The restriction enzymes are recognized by the molecular biologists Werner Arber, Hamilton O Smith, and Daniel Nathans. They classified restriction enzymes into four types and designated as type I, II, III, and IV. This classification is mainly based on their subunit composition, recognition site and cofactor requirements.

Type I: Type I restriction enzymes or REases are large and complex proteins with three subunits: HsdR, HsdM, and HsdS). HsdR is essential for restriction, HsdM for methyltransferase activity and HsdS is necessary for DNA sequence recognition. The type I REases are multifunctional and capable of both cleavage and modification activity. But the cofactors- Hydrolyzed Adenosine Triphosphate, Magnesium, and S-Adenosyl Methionine- are required for the functional activity of this restriction enzymes.

Type II: Type II restriction enzymes cleave DNA at specific sequences and they can produce discrete restriction fragments. They use only Magnesium as a cofactor for recognizing and cleaving DNA. They can use in the laboratory for cloning and DNA analysis. In the late 1990s, the subcategories of Type II restriction enzymes were discovered based on the variation from main characteristics of Type II REases. The subcategories include Type IIB, Type IIE, Type IIF, Type IIG, Type IIM, Type IIS and Type IIT.

Type III: Type III restriction enzymes are also the restriction and modification enzymes. They also require cofactors- Hydrolyzed Adenosine Triphosphate and S-Adenosyl Methionine- to perform restriction and methylation activities. These enzymes composed of two subunits and they are also multifunctional.

Type IV: Type IV restriction enzymes perform their restriction and modification activities with the cofactor S-Adenosyl Methionine. Type IV restriction enzymes are capable of identifying irregular DNA sequences and restrict the replication of them.

Uses of Restriction Enzymes in Biotechnology

Restriction enzymes are able to cleave double-stranded DNA into fragments at specific sequences to prevent the replication of viral DNA. This functional activity of restriction enzymes led to the extensive use of these enzymes in the Genetic Engineering. Restriction enzymes use as an important tool in the recombinant DNA technology to generate recombinant DNA molecules, which are molecules that consist of genes from two different organisms. Typically, the DNA obtained from the bacteria called plasmid join to the DNA from another gene of interest for cloning. Restriction enzymes have been widely used for identifying humans and other species. It also has applications in modern genetics. They can also be used to map the entire genome and to confirm the identity of a particular DNA fragment.