Tuesday, April 18, 2006

Movement of chromosome in nucleus visualized

Movement of chromosome in nucleus visualized, EurekAlert, 17-Apr-2006, Jeanne Galatzer-Levy, University of Illinois at Chicago [Graphic: San Diego State University College of Sciences, "Actin Myosin Crossbridge 3D Animation." Note: This is not a model of the cell nucleus actin- myosin motor referred to in the article, but presumably the principle is the same.] The cell is understood to be highly organized, with specialized areas for different functions and molecular motors shuttling components around. Researchers from the University of Illinois' Chicago and Urbana-Champaign campuses now offer the first imaging evidence from live cells of ongoing organization and transport within the cell nucleus. ... But scientists have had no way to track chromosome movement inside the nucleus or to determine whether the location of the chromosomes was the result of random diffusion or if they are moved around by molecular motors. In a study published in the April 17 issue of Current Biology, UIC and UIUC researchers show that chromosomes in the cell nucleus are capable of directed, long-range movement that depends on actin and myosin, the major molecular motor complex in the cytoplasm. Chien-Hui Chuang studied a chromosome that is normally found in an inactive state near the nuclear periphery and moves to the interior of the nucleus when it receives an activating signal. "The movement following activation was radically different from the rapid, but short-range, diffuse movement previously observed in these nuclei," said Dr. Andrew Belmont .... It was clear that this was directed movement that required a motor, Belmont said, because the chromosome was moving in a nearly straight line perpendicular to the nuclear envelope. .... "It looked nothing like the random, but localized, bouncing around that had been previously observed," he said. Belmont's group collaborated with Primal de Lanerolle who had discovered a type of myosin in the nucleus. Most myosin molecules are found in the cytoplasm, where they interact with actin filaments to do physical work. Because these molecules can contract muscles or move things around, they are called molecular motors. .... When the researchers introduced a mutant form of myosin protein to the nucleus, the movement slowed. Introducing a mutant actin that does not form filaments stopped the movement, while the introduction of an actin mutant that enhances filament formation accelerated the movement. In addition, when a drug that inhibits actin/myosin interactions was added to the cells, the chromosome movement was stopped completely. These experiments conclusively established that actin and myosin are involved in this chromosome movement. "While we have known for a long time that actin is present in the nucleus and we had shown that myosin is also present in the nucleus, nobody really knew if they worked together," said de Lanerolle. "There has been tantalizing evidence of organization in the nucleus--active genes found in the central region associated with nucleus complexes of transcription machinery necessary for gene expression, while inactive genes are found at the periphery," Belmont said. "For the first time, we have been able to observe an active mechanism for directed long-range chromosome movements that depend directly or indirectly on actin and myosin." ... [Also at ScienceDaily. Yet another vital function of the cell under the control of molecular machines! How could (and why would) a `blind watchmaker' be so far-sighted? And so clever?]

Stephen E. Jones, BSc (Biol).
"Problems of Evolution"

"Hunt Morgan and other geneticists have shown that certain variations can be brought about by the action of heat and electricity, radium and X-rays, and that these variations are inherited-at least for some generations. And here it appeared that Nature had something tangible to select, and thus produce new species. No doubt in Nature lightning is common, and cosmic rays are apparently raining on the earth all the time, but there is no evidence whatever that such agencies have ever produced a new species. In fact, it seems that even if such mutations are selected by man, they only last for a few generations, and gradually the animals revert to the normal." (Broom R., "Finding the Missing Link," [1950], Greenwood Press: Westport CT, Second edition, 1951, Reprinted, 1975, pp.98-99)

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