DEOXYRIBONUCLEIC ACID, OR DNA, exploded onto the criminal justice scene a mere twenty years ago. Its arrival was stunning. Right away it seemed that the police had a tool that could cull order out of the chaos of crime scenes and quickly separate the guilty from the innocent.

Understanding the structure of DNA is one of the most remarkable scientific advances of the past fifty years. The applications that will result from this breakthrough have only begun to be known. The tiny DNA molecule spells out the fate of each cell in every living organism. Whether that cell becomes part of an ant or an anteater, a daisy or a rose, or even a liver or a heart depends on the unique arrangements of the rungs of this molecule that is shaped like a spiraling, twisted ladder. And even as scientists continue to struggle to understand exactly how...