provide the "nonbiologists at the symposium with a sense of how the genome knows where its genes are and how it expresses those genes."

Possibly no other scientific discovery in the second half of the 20th century has had the impact on science and culture that elucidation of DNA's structure and function has had. The field of molecular biology has exploded into the forefront of the life sciences, and as its practitioners rapidly develop applications from these insights, new horizons appear continuously. The working elements of genetics, called genes, can now be duplicated and manufactured, and then reintroduced into living organisms, which generally accept them and follow their new instructions. Recombinant DNA technology and gene therapy promise to change not only our view of medicine, but also society's fundamental sense of control over its biological fate, perhaps even its evolution.

HOW DNA WORKS

Despite the significance of modern genetics, many of its fundamentals are still not widely understood. A summary of what has been learned about DNA might serve as a useful introduction to the discussion on transcription and gene expression:

  • The heritable genetic information for all life comes in the form of a molecule called DNA. A full set of a plant's or an animal's DNA is located inside the nucleus of every cell in the organism. Intrinsic to the structure of the DNA molecule are very long strings composed of so-called base pairs, of which there are four types. A gene is a segment of this string that has a particular sequence of the four base pairs, giving it a unique character. Genes are linked one after another, and the string of DNA is carried on complex structures called chromosomes, of which there are 23 pairs in humans. Researchers put the number of discrete genes in humans at about 100,000. To clarify the concept of DNA, Douglas Hanahan from the University of California, San Francisco, invoked the metaphor of a magnetic tape, "which looks the same throughout, but has within it (or can have) discrete songs composed of information." A gene can thus be likened to a particular song.

  • The general outline of this picture was known by the early 1950s, but even the electron microscope had not revealed exactly how the DNA molecule was structured. When British biophysicist Francis Crick and American molecular biologist James Watson first proposed the double-helix structure for DNA, a thunderclap echoed throughout molecular biology and biochemistry. Much more than just a



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