restriction enzyme
Think of a pair of molecular scissors that only snip where it sees one exact phrase, ignoring the rest of the text entirely. A restriction enzyme is just such a tool: a protein that scans DNA and cuts it only at a specific short sequence it recognizes, leaving the strand intact everywhere else.
The recognition sites are usually short, often four to eight base pairs long, and many are palindromes — they read the same on both strands in opposite directions. Depending on the enzyme, the cut produces either blunt ends or staggered ends with single-stranded overhangs. Hundreds of different restriction enzymes are known, each with its own recognition sequence, which lets researchers cut DNA at predictable, chosen places.
Restriction enzymes are natural: bacteria use them as a defense system, chopping up the DNA of invading viruses while protecting their own genome by chemically marking it. Their discovery gave molecular biologists the precise cutting tools that made recombinant DNA possible, and they remain a workhorse of the lab even alongside newer methods.
To clone a gene, a researcher cuts both the gene's DNA and the plasmid with the same restriction enzyme, so their matching ends will fit together like puzzle pieces.
Cutting two DNAs with one enzyme gives complementary ends ready to be joined.
The enzyme EcoRI recognizes the sequence GAATTC and cuts between the G and the A on each strand, leaving identical four-base sticky ends that can re-pair with any other DNA cut by the same enzyme.