Abstract

Without doubt, the greatest achievement in biology over the past millennium has been the elucidation of the mechanism of heredity. Heredity is surely the strangest of physiological processes: Organisms encapsulate instructions for creating a member of their species in their gametes, these instructions are passed on to a fertilized egg, and then they unfold spontaneously to give rise to offspring. The ancient Greeks puzzled over these remarkable phenomena. Hippocrates imagined that instructional particles were gathered together from throughout the adult body, having been shaped by experience, while Aristotle believed that the instructions were constant and inherent in the gametes. But philosophers could do no more than speculate for the ensuing 2000 years, because there was no way to probe the physical nature of these instructions. How the nature of heredity came to be understood over the past 200 years is an extraordinary tale of scientific progress. In dizzying succession, biologists found that the heredity instructions followed specific rules of transmission, resided in the chromosomes contained in the nucleus, were embodied in the molecule DNA, were written in a precise genetic code, and could be read out in their entirety to specify organismic shape and

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