Restriction Enzyme
A restriction enzyme (restriction endonuclease) is a bacterial enzyme that cuts double-stranded DNA at or near a specific short nucleotide sequence called a recognition site, typically 4–8 base pairs in length. Bacteria produce these enzymes as part of a restriction-modification defence system to degrade foreign viral DNA that lacks the bacterium's own methylation marks. In molecular biology, type II restriction enzymes such as EcoRI and HindIII are indispensable tools for cutting DNA predictably to produce defined fragments with either blunt ends or "sticky" (cohesive) overhangs, enabling gene cloning, restriction mapping, and DNA fingerprinting.
Common Restriction Enzymes and Their Recognition Sequences
| Enzyme | Source Organism | Recognition Sequence (5'→3') | Cut Type |
|---|---|---|---|
| EcoRI | E. coli RY13 | G↓AATTC | 4-nt 5' sticky end |
| HindIII | H. influenzae Rd | A↓AGCTT | 4-nt 5' sticky end |
| BamHI | B. amyloliquefaciens H | G↓GATCC | 4-nt 5' sticky end |
| SmaI | S. marcescens | CCC↓GGG | Blunt end |
| NotI | N. otitidis-caviarum | GC↓GGCCGC | 4-nt 5' sticky end (8-cutter) |
| PstI | P. stuartii 164 | CTGCA↓G | 4-nt 3' sticky end |
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Related Terms
Gene Cloning
Gene cloning is the process of making multiple identical copies of a specific gene or DNA fragment by inserting it into a vector (such as a plasmid or bacteriophage) and replicating it inside a host organism, typically Escherichia coli. The technique involves cutting both the target DNA and the vector with the same restriction enzyme to generate compatible sticky ends, ligating them with DNA ligase, transforming the recombinant construct into host cells, and selecting colonies that contain the insert. Gene cloning is foundational to modern biotechnology and medicine, enabling the production of recombinant proteins (such as insulin), gene therapy constructs, diagnostic probes, and transgenic organisms.
Gel Electrophoresis
Gel electrophoresis is a laboratory technique used to separate macromolecules — primarily DNA, RNA, or proteins — by size and charge as they migrate through a porous gel matrix under the influence of an electric field. Negatively charged nucleic acids migrate toward the positive electrode (anode), with smaller fragments travelling faster and further than larger ones, producing a pattern of bands that can be visualised by staining with ethidium bromide or SYBR Green and exposing to UV light. Gel electrophoresis is one of the most widely used techniques in molecular biology, underpinning applications from forensic DNA profiling and paternity testing to restriction mapping, PCR product verification, and Southern blotting.
Reverse Transcriptase
Reverse transcriptase (RT) is an RNA-dependent DNA polymerase enzyme that synthesises a complementary DNA (cDNA) strand using an RNA template, a process called reverse transcription that is the reverse of the normal transcription step in the central dogma. It was discovered independently by Howard Temin and David Baltimore in 1970, a finding that won them the Nobel Prize in Physiology or Medicine in 1975 and fundamentally altered the understanding of genetic information flow. Reverse transcriptase is encoded by retroviruses (including HIV) and retrotransposons, and is an essential biotechnology tool used to create cDNA libraries for cloning, gene expression analysis, and RT-PCR diagnostics.
From English "restriction" (the function of restricting phage growth in bacteria, described by Werner Arber in the 1960s) + Latin "enzyme" (from Greek "en" in + "zyme" leaven). Werner Arber, Daniel Nathans, and Hamilton Smith shared the 1978 Nobel Prize in Physiology or Medicine for discovering restriction enzymes and their applications.