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Extra info for [Magazine] Scientific American. Vol. 268. No 5

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The workers measured the wobbling using very long baseline interferometry. Radio astronomers often rely on this technique to make highly precise measurements of stellar objects. Various tidal forces had been thought to be solely responsible for the earthÕs nutations. Such mechanisms include the friction generated as the solid surface of the earth rubs against the atmosphere and oceans as well as the gravitational interactions with the sun and the moon. BuÝett discovered, however, a component of the nutations that could not be explained by tidal forces.

If the intertwined molecules bond with one another, the result is a gel (c, d ). cross-linking (which limits the maximum extension of the polymer chains); thus, a strongly cross-linked gel will swell less than a weakly cross-linked one. If the polymer from which a gel is made contains charged groups (molecules that readily accept or give up electrons) along its backbone, then additional eÝects may come into play. The Þrst of these is the so-called polyelectrolyte eÝect. In pure water, a polymer containing charged groups will tend to expand its dimensions in order to minimize the repulsion between them.

Copyright 1993 Scientific American, Inc. SCIENTIFIC AMERICAN May 1993 57 NORMAL CONTROL CHROMOSOME HEAT-SHOCKED CHROMOSOME PUFFS in the polytene chromosomes of the fruit ßy Drosophila melanogaster (left) indicate local gene activity. As the ßy passes through developmental stages, the puÝing pattern changes. Abnormally high temperatures also stimulate certain puÝs to form, as shown above. These puÝs reßect the expression of genes for heatshock proteins belonging to the hsp 70 molecular family. TEMPERATURE (CELSIUS) 23° 26° 29° 31° 33° 35° 37° 38° MOLECULAR WEIGHT (DALTONS) 82,000 70,000 68,000 36,000 27,000 26,000 23,000 22,000 HEAT-SHOCK PROTEIN LEVELS rise in cells as the temperature increases.

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