How mRNA Is Created From DNA Inside every cell, DNA holds the full set of genetic instructions for building and running the body. But DNA never leaves the cell’s nucleus, so the cell needs a temporary copy of just the specific information it wants to use. That temporary copy is called messenger RNA, or mRNA. The process begins when a cell decides it needs a certain protein. An enzyme called RNA polymerase finds the matching section of DNA and pulls the two DNA strands apart. It then reads one of