Answer: Enzyme.
Step-by-step explanation: Scientist use restriction enzyme to "clip" DNA for use in gene therapy. This enzyme helps in cleaving the DNA by either sticky ends or blunt ends so that the gene of interest can be incorporated in the host DNA.
This clipping of a DNA segment is done for using the DNA for producing vital products. The products can be therapeutic, agricultural, commercial et cetera.
The small fragment of the DNA( host cell) is replaced by the DNA of the organism having gene of interest, the gene of interest gets incorporated into the DNA(host) and starts replicating.
This replication produces the desired products in the host cell which is extracted out of the cell to be used by us.