Which type of enzyme is used to cut the Factor IX gene from human DNA in Stage 1 of the described genetic engineering process?

• A. Ligase
• B. Restriction endonuclease
• C. DNA polymerase
• D. Reverse transcriptase

Answer: (B)

The key idea is how scientists precisely cut out a specific gene from human DNA. In Stage 1, you need to excise the Factor IX gene so it can be moved into a vector for cloning. Restriction endonucleases are the enzymes that do this: they recognize specific DNA sequences and cleave the DNA backbone at those sites, generating defined fragments with ends that can later be joined to a vector. This precise cutting is essential for isolating the gene without altering other parts of the genome.

Ligase, by contrast, is the enzyme that would join DNA fragments together after they’ve been cut; DNA polymerase builds new DNA strands, and reverse transcriptase creates DNA from an RNA template. So the enzyme best suited for cutting out the gene is restriction endonuclease.