Since the discovery of the DNA double helix (in 1953 by Watson and Crick), technologies have been developed that allow scientists to manipulate the genome of bacteria to produce human hormones, as well as the genome of crop plants to achieve high yield and enhanced flavor. The universality of the genetic code has allowed DNA isolated from a specific organism to be transferred and incorporated in another organism, transforming bacterial, yeast, plant, and animal cells. This transformation ability is the essence of recombinant DNA technology. Recombinant DNA has been used to make medically useful proteins that would otherwise have been difficult to obtain in necessary amounts, or to engineer plants to be herbicide or insect resistant. This article presents a science activity which focuses on mapping DNA using restriction enzymes. This activity can help students gain a better understanding about DNA and its manipulation. The activity is designed for high school and college biology students and addresses the following "National Science Education Standards:" 9-12 The Cell and 9-12 Molecular Basis of Heredity. The mapping of linear and circular DNA molecules is explained along with tips for teaching recombinant DNA technology in the classroom. DNA mapping promotes the formal reasoning and, concomitantly, the cognitive development of biology students. (Contains 8 figures.)