Proteins use a lock and key system to bind to DNA
(PHYSorg.com Biology)

You can think of DNA as a string of letters-As, Cs, Ts, and Gs-that together spell out the information needed for the construction and function of cells. Each cell in your body shares the same DNA. So, for cells to take on their differing roles, they must be able to turn on and off specific genes with precise control. The genes active in a brain cell, for instance, are different than those active in a skin cell.