Bulletin of the American Physical Society
74th Annual Meeting of the Southeastern Section
Volume 52, Number 13
Thursday–Saturday, November 8–10, 2007; Nashville, Tennessee
Session EA: Biophysics |
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Chair: Shane Hutson, Vanderbilt University Room: Scarritt-Bennett Center Laskey Great Hall |
Thursday, November 8, 2007 3:45PM - 4:15PM |
EA.00001: Nitric Oxide Scavenging by Hemoglobin in Health, Disease, and Therapeutics Invited Speaker: Nitric oxide (NO) is the endothelium-derived relaxing factor (EDRF). It is made in endothelial cells lining blood vessels and diffuses to smooth muscle cells where it leads to muscle relaxation, vessel dilatation, and increased blood flow and also plays a large role in controlling platelet aggregation and inflammation. Hemoglobin (Hb), the oxygen carrying molecule in the blood, reacts at nearly diffusion limited rates with nitric oxide to (in some reactions) form nitrate ands thereby destroy NO activity. The presence of such large amounts of such a potent NO scavenger in the blood challenges the idea that NO is indeed the EDRF. Encapsulation in red blood cells in healthy individuals limits NO scavenging by Hb. Biophysical experiments will be described exploring and evaluating these mechanisms. Other studies will be described discussing how red cells break open (lyse) in pathological situations and the cell-free Hb reduces NO bioavailability. Finally, methods to restore NO bioavailability through therapeutics will be discussed. [Preview Abstract] |
Thursday, November 8, 2007 4:15PM - 4:45PM |
EA.00002: DNA Looping, Supercoiling and Tension Invited Speaker: In complex organisms, activation or repression of gene expression by proteins bound to enhancer or silencer elements located several kilobases away from the promoter is a well recognized phenomenon. However, a mechanistic understanding of any of these multiprotein interactions is still incomplete. Part of the difficulty in characterizing long-range interactions is the complexity of the regulatory systems and also an underestimation of the effect of DNA supercoiling and tension. Supercoiling is expected to promote interactions between DNA sites because it winds the DNA into compact plectonemes in which distant DNA segments more frequently draw close. The idea that DNA is also under various levels of tension is becoming more widely accepted. Forces that stretch the double helix in vivo are the electrostatic repulsion among the negatively charged phosphate groups along the DNA backbone, the action of motor enzymes perhaps acting upon a topologically constrained sequence of DNA or chromosome segregation during cell mitosis following DNA replication. Presently, little is known about the tension acting on DNA in vivo, but characterization of how physiological regulatory processes, such as loop formation, depend on DNA tension in vitro will indicate the stretching force regimes likely to exist in vivo. In this light, the well studied CI protein of bacteriophage l, which was recently found to cause a of 3.8 kbp loop in DNA, is an ideal system in which to characterize long-range gene regulation. The large size of the loop lends itself to single-molecule techniques, which allow characterization of the dynamics of CI-mediated l DNA looping under controlled levels of supercoiling and tension. Such experiments are being used to discover the principles of long-range interactions in l and in more complex systems. [Preview Abstract] |
Thursday, November 8, 2007 4:45PM - 5:15PM |
EA.00003: Nanofluidic DNA analysis - applications and physics Invited Speaker: DNA stretching in nanofluidic channels that are round 100 nm in diameter and 100's of microns long is an emerging technique for the genetic analysis of long nucleic acid molecules. We will explain why nanofluidic stretching differs from other single-molecule techniques, in particular how the ability to measure individuality is greatly enhanced by the fundamentally different averaging properties. We will present an overview of the basic physics that enables this exciting new technique, and discuss proof-of-principle experiments that have demonstrated how genetic information can be gathered by the technique. In order to unlock the full potential of multi-step analyses, we have begun to develop a toolbox for connecting nanochannels into networks, and control the motion of single molecules by creating a spatially and temporally modulated energy landscape. As part of this ``nanoplumbing'' approach, we have demonstrated nanofluidic switches that can be activated by application of an external a.c. electric field requiring only two external electrodes. Finally, I will discuss recent results which show that stretched DNA can undergo a phase transition-like collapse under application of an a.c. field, and discuss possible mechanisms. We have observed giant electrostriction of 75\% and more, comparable with high-performance artificial muscles. [Preview Abstract] |
Thursday, November 8, 2007 5:15PM - 5:45PM |
EA.00004: Cellular dynamics and embryonic morphogenesis Invited Speaker: The elongated body axis is a characteristic feature of many multicellular animals. Axis elongation occurs largely through cell rearrangements that are coordinated across a large cell population and driven by an asymmetric distribution of cytoskeletal and junctional proteins [1]. To visualize cellular dynamics during this process, we performed time-lapse confocal imaging of cell behavior in the Drosophila embryo. These studies revealed that rearranging cells display a steady increase in topological disorder that is accompanied by the formation of transient structures where 5-11 cells meet [2,3]. These multicellular rosettes form and resolve in a directional fashion to produce a local change in the aspect ratio of the cellular assembly, contributing to an overall change in tissue structure. We propose that higher-order rosette structures link local cell interactions to global tissue reorganization during morphogenesis. [1] J. Zallen and E. Wieschaus, Developmental Cell 6, 343 (2004). [2] J. Zallen and R. Zallen, J. Phys.: Condens. Matter 16, S5073 (2004). [3] J. Blankenship et al., Developmental Cell 11, 459 (2006). [Preview Abstract] |
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