Why do restriction enzymes cut DNA at specific restriction sites?

• A. Different base sequences in DNA mean restriction enzymes cut only at specific sequences.
• B. Restriction enzymes cut at random locations.
• C. Restriction enzymes cut only GC-rich regions.
• D. Restriction enzymes cut at every phosphodiester bond.

Answer: (A)

Restriction enzymes cut DNA only at specific sequences because they are sequence-specific endonucleases. Each enzyme has an active site that recognizes a particular pattern of bases—often a palindromic sequence—so it binds and cuts only when that exact DNA motif is present. Because DNA base sequences vary, different enzymes recognize different motifs and therefore cut at defined locations rather than at random. They do not cut every bond, nor do they preferentially target GC-rich regions in general; their action depends on matching their specific recognition sequence. This precise targeting is what lets scientists predict and manipulate fragment sizes in cloning and DNA analysis.