Restriction enzymes occur naturally in bacteria, but they’re also really useful tools in molecular biology. This is because they cut DNA at known sequences. This tool can be used in many ways.
Previously we looked at the symmetry of the EcoRI recognition site. This diagram shows how EcoRI cuts the DNA (I’ve put the lower strand upside down because I think it shows the symmetry better than the usual way of writing these letters up the right way):
The DNA is cut between the G and A in both strands, leaving AATT-overhangs that are only one DNA stand. (I’ve written the dash after the T because that’s where it joins to the double-stranded bit).
Single-stranded DNA is “sticky”. If it finds a complementary single-strand of DNA the two strands can stick together.
Any DNA cut with this enzyme has the same “sticky end”, i.e. AATT-. So any piece of DNA cut with EcoRI could potentially join to any other cut with the same enzyme. Another enzyme, a DNA ligase, can be used to seal the join.
This means you can be use restriction enzymes for cutting and joining DNA. This makes them very useful.
Bacterial DNA and mouse DNA that have been cut with EcoRI can be joined together.
A circular piece of bacterial DNA
can be opened with one cut
and another piece of DNA (that’s been cut with the same enzyme) added
This is the process that was used to make human insulin for diabetes patients. A human insulin gene was pasted into a circular bacterial plasmid, and this recombinant DNA plasmid is be grown in massive amounts inside bacteria in the laboratory.
Khan Academy has quite a detailed tutorial on restriction and ligation, including blunt ends and sticky ends.
There’s a more detailed description of how human insulin is produced in the laboratory at the DNA Learning Centre.