![]() The researchers used optical tweezers to probe the barrier encountered by enzymes like RNA polymerase that prompt DNA in nucleosomes to uncoil from histones. We're trying to understand the mechanical barrier presented by the nucleosome and, further, how the nucleosome structure is modified to clear the way for information transfer." Nucleosomes seem to stand in the way of a genetic information-transfer process that happens millions of times a day in the cells of our bodies. "But structural and biophysical details of the system have not been clear. But, if we released the fiber before the histones were detached, the nucleosomes were able to reassemble themselves and the whole process could be repeated." "Of course the basic plan - nucleosomes and higher-order structures condensing DNA into a manageable space - has been known for some time," Wang says. Describing the release of DNA from a single nucleosome, he says: "When we pulled on an individual chromatin fiber with increasing force, low force initially released 76 base pairs of DNA per nucleosome, then higher forces yielded 80 more base pairs with the histones still bound to the DNA, followed by histone detachment at still higher forces. ![]() The three stages became apparent when a nucleosome was uncoiled as the DNA was stretched with increasing force, says Brent Brower-Toland, lead author of the article and a research associate in Cornell's Laboratory of Atomic and Solid State Physics. Wang, assistant professor of physics at Cornell, who led the scientific team, said the researchers are proposing a three-stage model for the way in which nucleosomal units in chromatin open to reveal their DNA to enzymes like RNA polymerase. The complex of DNA plus histones in cells of higher organisms is called chromatin. Chromosomal DNA is packaged into the compact structure of the nuclesome with the help of specialized proteins called histones. 99, Issue 4), "Mechanical Disruption of Individual Nucleosomes Reveals a Reversible Multistage Release of DNA." It marks the first direct observation of the dynamic structure of individual nucleosomes. The report by physicists and biologists at Cornell University and the University of Massachusetts appears in the current Proceedings of the National Academy of Sciences (Vol. ![]() By using optical tweezers to pull individual strands of chromatin - the DNA-protein complex that chromosomes are made of - researchers have seen for the first time how information in fundamental genetic packaging units, called nucleosomes, might become accessible to molecules that "read" it. ![]()
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