Final answer:
Scientists can join together two DNA fragments by using sticky ends. The process involves annealing the complementary sticky ends and using DNA ligase to permanently join the fragments.
Step-by-step explanation:
When DNA is cut with a restriction enzyme, it leaves overhangs called sticky ends. These sticky ends can hydrogen bond with complementary overhangs on another piece of DNA. Annealing, the process of forming hydrogen bonds between complementary sequences, allows the sticky ends to come together. DNA ligase, an enzyme involved in DNA replication, permanently joins the DNA fragments when the sticky ends are aligned. This process allows scientists to splice any DNA fragment between the two ends of a plasmid DNA that has been cut with the same restriction enzyme.