Genetic engineering is the altering of the genetic material of living cells in order to make them capable of producing new substances or performing new functions, like getting a micro-organism to produce human insulin or a sheep to produce a human blood-clotting protein in its milk. The technique became possible during the 1950s when scientists discovered the structure of deoxyribonucleic acid (DNA) molecules and how DNA stores and transmit genetic information. Largely as the result of the pioneering work of James Watson (1928-) and Francis Crick (1916-), scientists found that the sequence of nitrogen bases that make up any specific DNA molecule codes for the manufacture of specific chemical compounds. That sequence acts, therefore, as an "instruction manual" that directs all cell functions. Certain practical consequences of that discovery became almost immediately apparent. Suppose that the base sequence T-G-G-C-T-A-C-T on a DNA molecule carries the instruction...