# Restriction enzyme

A bacterial enzyme that cuts DNA at a specific short sequence. Many leave overhanging single-strand ends ('sticky ends') that can pair with matching ends from other DNA.

Biology (university) · Part 05: Gene regulation and biotechnology · Card 4 of 8 · Last updated 2026-10-19

## Analogy

Molecular scissors that cut only at one exact word.

## Example

**Sticky ends**

EcoRI site: GAATTC
Cuts between G and A
on each strand.
Leaves 4 bases hanging.

## In practice

EcoRI cuts at GAATTC. Any DNA cut with EcoRI can be joined to any other DNA cut with EcoRI.

## Practice

### Flashcards

- **What does a restriction enzyme do?** It cuts DNA at a specific short sequence.
- **Which enzyme joins DNA pieces together in cloning?** DNA ligase.

### Interview questions

**Describe the steps of producing human insulin in bacteria using recombinant DNA.**

- Cut gene and plasmid with same restriction enzyme
- Join with DNA ligase
- Insert plasmid into bacteria
- Bacteria multiply and express the gene
- Collect and purify the protein

## Questions people ask

### Why don't bacteria cut their own DNA?

Bacteria add methyl tags to their own DNA at the recognition sites, which stops their enzymes cutting there. The enzymes evolved as a defence, to chop up the DNA of invading viruses that lack these tags. Scientists borrowed them as tools for cutting DNA at chosen places.

### What are sticky ends?

Some restriction enzymes cut the two DNA strands in staggered places, leaving short single-stranded overhangs. Overhangs from two different DNA molecules cut with the same enzyme can base-pair with each other. DNA ligase then seals the joins, so pieces from different organisms can be combined.

## Related concepts

- [Recombinant DNA](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-5)
- [Polymerase chain reaction](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-6)
- [Gel electrophoresis](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-7)
- [CRISPR-Cas9](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-8)

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