Cutting of DNA at Specific Locations

Restriction enzymes cut DNA at specific sites, but the cutting is only one step in a larger process. The actual digestion is a controlled laboratory procedure, and the results must be checked before moving forward.

A restriction enzyme digestion is performed by incubating purified DNA molecules with the chosen restriction enzyme under optimal conditions — the right temperature, buffer, and salt concentration — for that specific enzyme. The reaction is not instantaneous; it requires time for the enzyme to locate and cut every recognition site.

To check whether the digestion has worked properly, scientists use agarose gel electrophoresis. DNA is a negatively charged molecule, so when placed in an electric field, it moves towards the positive electrode (the anode). The agarose gel acts like a sieve: smaller DNA fragments travel faster and farther than larger ones, so the digested DNA separates into distinct bands. This allows you to see if the cutting was complete and whether the fragments are of the expected sizes.

The same restriction enzyme digestion is performed separately on the vector DNA (the carrier DNA, such as a plasmid). Both the source DNA (containing the gene of interest) and the vector DNA are cut with the same restriction enzyme. This is crucial because the same enzyme produces complementary sticky ends on both pieces of DNA. …