Restriction enzymes act like “molecular scissors.” They generate either blunt ends or sticky ends depending on how they cut the DNA. Sticky ends are especially useful because they can easily join with complementary DNA fragments cut by the same enzyme. This specificity enables researchers to insert genes into plasmids or other vectors accurately, which is essential in genetic engineering and biotechnology.

Restriction enzymes, also called restriction endonucleases, recognize specific DNA sequences (usually palindromic) and cut the DNA at or near those sites. For example, EcoRI recognizes the sequence GAATTC and cuts between G and A. This property allows scientists to cut DNA at precise locations, which is useful for cloning, gene mapping, and recombinant DNA experiments.

Another way of looking at this is that restriction endonucleases (REN) are designed to cleave foreign DNA that makes its way into a bacterium. Like all enzymes, it recognizes a substrate, which in this case is a specific sequence of DNA, and cleaves the DNA on both strands. Thus a phage that infects a bacterium must get past the host defense system, i.e., the REN, in order to replicate. RENs don't work by themselves, but in combination with a methylating enzyme (methyltrnasferase) that adds a methyl group (CH3-) to adenine and cytosine bases within or near the REN recognition site. The host methylates the cut sites, which prevents the REN from cleaving the host DNA. Neat trick. Interestingly, phage that do survive the REN themselves become methylated and thus resistant to the hosts REN.

In biotechnology, RENs are isolated and used for cutting and pasting of DNA for cloning. It was one of the last major tools to be discovered and applied in the 1970s for cloning to succeed.