Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H2: Recent Advances in Cell and Single Molecule Manipulation |
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Sponsoring Units: DCMP Chair: Omar Saleh, University of California, Santa Barbara Room: Oregon Ballroom 202 |
Tuesday, March 16, 2010 8:00AM - 8:36AM |
H2.00001: Mobile magnetic traps for cell manipulation Invited Speaker: Highly localized magnetic field gradients in the vicinity of patterned ferromagnetic wires and disks provide a planar template to assemble labeled cells or magnetic micro-/ nano-particles onto designed arrays. By combining the platform with externally controlled weak ($<$100 Oe) fields, cells are transported across surfaces with programmable directed forces that are gentle enough to not produce damage. In addition to manipulating immunomagnetically labeled biological cells, magnetic microspheres that act as magnetically actuated miniature force transmitting probes navigate fluid-borne unlabeled cells with micrometer precision. The versatility of this approach is evident when the magnetic forces are tuned, enabling the Brownian motion of microscopic objects be controlled. Central to these observations are the simple methods to create, with nanoscale precision, highly confined field gradients. In addition to the convenience of optical microscope observation and advantage of suppressing randomizing thermal fluctuations of fluid-borne cells, development of such mobile magnetic traps will provide real-time analysis of living cells through direct manipulation that offers much more accurate selection than data-averaging over a population of cells. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 9:12AM |
H2.00002: Unraveling chromatin structure using magnetic tweezers Invited Speaker: The compact, yet dynamic organization of chromatin plays an essential role in regulating gene expression. Although the static structure of chromatin fibers has been studied extensively, the controversy about the higher order folding remains. The compaction of eukaryotic DNA into chromatin has been implicated in the regulation of all DNA processes. To understand the relation between gene regulation and chromatin structure it is essential to uncover the mechanisms by which chromatin fibers fold and unfold. We used magnetic tweezers to probe the mechanical properties of individual nucleosomes and chromatin fibers consisting of a single, well-defined array of 25 nucleosomes. From these studies five major features appeared upon forced extension of chromatin fibers: the elastic stretching of chromatin's higher order structure, the breaking of internucleosomal contacts, unwrapping of the first turn of DNA, unwrapping of the second turn of DNA, and the dissociation of histone octamers. These events occur sequentially at the increasing force. Neighboring nucleosomes stabilize DNA folding into a nucleosome relative to isolated nucleosomes. When an array of nucleosomes is folded into a 30 nm fiber, representing the first level of chromatin condensation, the fiber stretched like a Hookian spring at forces up to 4 pN. Together with a nucleosome-nucleosome stacking energy of 14 kT this points to a solenoid as the underlying topology of the 30 nm fiber. Surprisingly, linker histones do not affect the length or stiffness of the fibers, but stabilize fiber folding up to forces of 7 pN. The stiffness of the folded chromatin fiber points at histone tails that mediate nucleosome stacking. Fibers with a nucleosome repeat length of 167 bp instead of 197 bp are significantly stiffer, consistent with a two-start helical arrangement. The extensive thermal breathing of the chromatin fiber that is a consequence of the observed high compliance provides a structural basis for understanding the balance between chromatin condensation and transparency for DNA transactions. The kinetics of force induced nucleosome unstacking was resolved using a Hidden Markov analysis. Overall, our results reveal a highly dynamic structure that combines high level of compaction of DNA with transient accessibility. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:48AM |
H2.00003: Single molecule experiments using magnetic tweezers Invited Speaker: |
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