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.
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Analogy
Molecular scissors that cut only at one exact word.
Example
EcoRI site: GAATTCCuts between G and Aon 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.