Optical tweezers show how DNA uncoils

ITHACA, N.Y. -- 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.

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. 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. Chromosomal DNA is packaged into the compact structure of the nuclesome with the help of specialized proteins called histones. The complex of DNA plus histones in cells of higher organisms is called chromatin.

Michelle D. 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 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. 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. 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

Contact: Roger Segelken
Cornell University News Service

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