By Kay Tang – Updated Aug 30, 2022

Early History

In the 1960s, scientists Werner Arber and Stewart Linn discovered that certain enzymes in E. coli could block viral replication by cleaving DNA. They identified a class of enzymes—later termed restriction nucleases—that cut DNA at random positions, highlighting the need for a more precise tool.

Breakthrough Discovery

In 1968, H. O. Smith, K. W. Wilcox and T. J. Kelley isolated the first well‑characterised restriction enzyme, HindIII, at Johns Hopkins University. HindIII cuts DNA at a specific 6‑base‑pair sequence, a discovery that opened the door to the systematic use of restriction enzymes in molecular biology. Since then, over 900 enzymes have been identified from 230 bacterial strains, providing a vast toolkit for scientists.

Mapping DNA

Restriction enzymes enable genome mapping through a technique called Restriction Fragment Length Polymorphism (RFLP). By cutting DNA at known recognition sites, researchers generate fragments of characteristic lengths that can be separated by gel electrophoresis. RFLP has proven invaluable for DNA typing, forensic analysis, and studying genetic variation in populations.

Generating Recombinant DNA

The cornerstone of genetic engineering is the creation of recombinant DNA molecules. In practice, a plasmid vector is cut with a restriction enzyme, and a gene of interest—often derived from a different organism—is inserted. The compatible sticky ends produced by Type II enzymes are joined by DNA ligase, forming a stable hybrid chromosome that can be propagated in bacteria.

Types of Restriction Enzymes

Restriction enzymes are categorised into three main classes:

• Type I recognise a specific sequence but cleave only one strand and require a separate enzyme to cut the second strand, releasing nucleotides at the cut site.
• Type II cut both strands at or near the recognition sequence, producing either blunt or sticky ends that are ideal for cloning.
• Type III cleave both strands at a defined distance from the recognition site, a property useful for certain analytical applications.

Sources: Dolan DNA Learning Center, Access Excellence, Medicine Encyclopedia, University of Strathclyde.