The substance needed to join the insulin gene to bacterial DNA is restriction enzymes and DNA ligase.

Here's how it works:

1. Restriction enzymes: These enzymes act like molecular scissors, cutting DNA at specific sequences. They are used to cut both the insulin gene and the bacterial DNA at specific sites.

2. DNA ligase: This enzyme acts like molecular glue, joining the cut ends of DNA molecules together. It's used to join the insulin gene fragment into the bacterial DNA, creating a recombinant DNA molecule.

In summary: Restriction enzymes cut the DNA, and DNA ligase pastes the pieces together to create a hybrid DNA molecule containing the insulin gene.