Which statement about restriction endonucleases is true?

• A. It cuts DNA at specific recognition sequences to create fragments
• B. It unwinds the DNA double helix
• C. It ligates DNA fragments
• D. It copies DNA sequences

Answer: (A)

Restriction endonucleases act as molecular scissors, recognizing specific DNA sequences and cutting the backbone at those sites to generate DNA fragments. This precise cleavage is what allows us to cut DNA into defined pieces for cloning and analysis. They don’t unwind the double helix—that’s done by helicases during replication or repair. They don’t ligate DNA fragments—that’s the job of DNA ligase. And they don’t copy DNA sequences—that’s the role of DNA polymerases during replication or amplification. For example, EcoRI recognizes GAATTC and cuts to produce fragments with ends that can be joined to other DNA pieces when compatible.