A Feeling for the Organism: The Life and Work of Barbara McClintock
It would be churlish indeed to argue that Watson and Crick’s elucidation of the double helical structure of DNA in 1953 was anything less than one of the great scientific achievements of modern history. Yet, in a curious way, this discovery differed from other revolutionary events in science by its doubly conservative nature. First, it seemed to confirm the canonical view of genetical systems as rows of beads (genes) on strings (chromosomes), and its implication that evolution proceeds slowly and gradually, based ultimately on the generation of new genetic variation (mutation) by pinpoint changes within the beads (substitution of one nucleic acid base for another yielding a different amino acid in the translated protein).
Second, it represented the greatest modern triumph of the standard methodology that has ruled orthodox science ever since Descartes: reductionism, with its particular claim (in this case) that the complex forms of completed organisms could be explained, ultimately, as the translated products of an inherited program coded by a simple mechanism of four bases variously arranged in groups of three. Had not Watson built his double helix as a tinker-toy machine based on the sizes and fits of constituent parts? And had not Crick, soon afterward, proclaimed the “central dogma” (his words) of the new biology—that DNA makes RNA and RNA makes protein, in a oneway flow of information, a unidirectional process of mechanical construction?
It is a credit to the power of Watson and Crick’s model and to the fruitfulness of good science in general that, thirty years later, this Cartesian view of molecular genetics has been superseded, as a second revolution transmutes our view of inheritance and development. The genome, a cell’s compendium of genetic information, is not a stationary set of beads on strings, subject to change only by substituting one bead for another. The genome is fluid and mobile, changing constantly in quality and quantity, and replete with hierarchical systems of regulation and control.
Genes come in pieces, and the shuffling of their segments can produce new combinations. Some genes can excise themselves from a chromosome and move to other locations in the genome; if these “transposable elements” operate as regulators to turn adjacent genes on and off, their movement to other places (and near different genes) can have major effects on the timing and control of development. Other genes make copies of themselves, and these identical daughters can then either reside next to their parent or move to other chromosomes. In this way, hundreds or thousands of copies of the same gene may be repeated within an organism’s genetic program. The multiple copies of this “gene family” may then diverge in function, thus providing a solution to the old conundrum of how anything novel can evolve if all genes make products necessary for an organism’s construction and well-being. (Original copies may continue to make the required product, while new copies are “free” to alter and experiment.)
If these processes replace a static with a more …
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