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Restriction endonucleases (restriction enzymes) are bacterial enzymes that cut DNA at or near specific recognition sequences. So the answer to “what is a restriction enzyme” is: a sequence-recognising DNA cutter, which bacteria use against foreign DNA and which labs use for cloning and DNA analysis. This page is a reference for naming and classification; for the bench steps see the restriction enzyme digest setup guide.
What is a restriction endonuclease?
An endonuclease cleaves bonds inside a nucleic acid chain rather than trimming it from an end. A restriction endonuclease is one that does this at a defined recognition site. In bacteria the role is defence: the enzyme cuts unmethylated foreign DNA, while the host’s own DNA is protected by methylation (Thermo Fisher, Restriction enzyme basics). The protecting modification is made by a DNA methyltransferase, covered in the methyltransferase guide.
The same properties make these enzymes useful for gene cloning, where a vector’s multiple cloning site is a cluster of recognition sequences for different enzymes. Type II enzymes leave a 5′ phosphate and a 3′ hydroxyl at each cut end, and most need Mg2+ for the reaction (Thermo Fisher).
Types I to IV: how the classes differ
Restriction enzymes are grouped into four types. The table below sets out the components, cofactor and cut position of each type (Thermo Fisher; Roberts et al., 2003).
| Type | Components and cofactor | Where it cuts |
|---|---|---|
| Type I | Multi-subunit (R, M and S subunits); cuts and methylates; uses ATP | A variable distance from the recognition site |
| Type II | Mg2+ for most reactions; the companion methyltransferase is usually a separate protein | At a fixed position within or close to the recognition sequence |
| Type III | Two subunits (Mod and Res); uses ATP | A set distance from one site; cutting needs two recognition sites in inverse orientation |
| Type IV | Acts only on modified DNA, for example methylated DNA | About 30 bp from the site (Thermo Fisher) |
What are Type III restriction enzymes and what are their characteristics?
Type III enzymes have Mod and Res subunits, use ATP, recognise a two-part sequence in inverse orientation and cut a set distance from one of the sites. That need for two oppositely oriented sites separates them from Type II enzymes, which cut at a fixed position inside or beside a single site.
Type 2 restriction endonuclease
Type II is the class most associated with bench work because its cut position is fixed relative to the site. Within it, Type IIP enzymes recognise symmetric sequences and make fixed, symmetrical cuts (Roberts et al., 2003); Thermo Fisher notes that Type IIP is the most prevalent among commercially available enzymes.
What is a palindromic restriction site?
A palindromic restriction site is a symmetric recognition sequence, the kind recognised by Type IIP enzymes. Roberts et al. note that such sequences are often termed palindromes, and that the enzymes make fixed, symmetrical cuts at them. CCCGGG, the site recognised by SmaI, is an example: the complementary strand reads the same 5′ to 3′ (Thermo Fisher).
What are isoschizomers (and neoschizomers)?
Isoschizomers are restriction enzymes that recognise the same sequence with the same specificity (Thermo Fisher; Roberts et al.). Neoschizomers are the subset that recognise the same sequence but cut at different positions.
| Pair | Relationship | Source detail |
|---|---|---|
| HpaII and MspI | Isoschizomers, not neoschizomers | Roberts et al. (2003) |
| SmaI and XmaI (or Cfr9I) | Neoschizomers; both recognise CCCGGG | SmaI leaves blunt ends, XmaI/Cfr9I leave a 5′ overhang (Thermo Fisher) |
Sharing a recognition sequence does not make two enzymes interchangeable. Thermo Fisher notes that isoschizomers can still differ in site preferences, reaction conditions, sensitivity to methylation and star activity, so read the supplier datasheet for the specific enzyme before swapping one for another.
How to read a restriction enzyme name
- Letters from the source organism. HincII: H for the genus (Haemophilus), “in” for the species (influenzae), “d” for the strain (serotype d).
- Roman numeral. It distinguishes different enzymes from the same strain.
- R. and M. prefixes. Under the nomenclature of Roberts et al., R. marks a restriction enzyme and M. its methyltransferase; italics are no longer used, and “methyltransferase” is preferred to “methylase”.
Where this matters for lab staff
Two checks follow from the sources. First, when a protocol names an enzyme, identify its type and whether it is a Type IIP palindromic cutter before assuming fixed, symmetrical cuts. Second, when a substitute is proposed, confirm whether it is an isoschizomer or a neoschizomer and compare conditions, methylation sensitivity and star activity on the datasheet, because a shared site does not guarantee the same behaviour or the same end structure (blunt versus overhang).
Common misunderstandings
- “All restriction enzymes cut at a fixed spot in a palindrome.” That describes Type IIP. Types I and III cut at a distance from the site, and Type IV targets modified DNA.
- “Isoschizomer and neoschizomer mean the same thing.” Neoschizomers are a subset that cut at different positions; HpaII and MspI are isoschizomers but not neoschizomers.
- “Same site means same reaction conditions.” Isoschizomers may differ in conditions, methylation sensitivity and star activity.
FAQ
What do endonuclease enzymes do?
They cut a nucleic acid chain at internal bonds. Restriction endonucleases do so at specific recognition sequences, and in bacteria they cut unmethylated foreign DNA as a defence.
What is a restriction enzyme?
It is another name for a restriction endonuclease: an enzyme that recognises a DNA sequence and cleaves at or near it.
What is a palindromic restriction site?
A symmetric recognition sequence, as recognised by Type IIP enzymes.
What are isoschizomers?
Enzymes with the same recognition sequence and specificity, which can still differ in reaction conditions, methylation sensitivity and star activity.
Sources
- Thermo Fisher Scientific, Restriction enzyme basics, checked 7 November 2026.
- Roberts RJ et al. (2003), A nomenclature for restriction enzymes, DNA methyltransferases, homing endonucleases and their genes, Nucleic Acids Research 31(7):1805, checked 7 November 2026.
