Bulletin of the American Physical Society
79th Annual Meeting of the APS Southeastern Section
Volume 57, Number 16
Wednesday–Saturday, November 14–17, 2012; Tallahassee, Florida
Session GC: Biological Physics |
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Chair: Richard Haglund, Vanderbilt University Room: DoubleTree Adams-Park |
Friday, November 16, 2012 8:30AM - 9:00AM |
GC.00001: Measuring looping probability of short double-stranded DNA Invited Speaker: Harold Kim Bending of double-stranded DNA (dsDNA) is associated with fundamental biological processes such as genome packaging and gene regulation, and therefore studying sequence-dependent dsDNA bending is a key to understanding biological impact of DNA sequence beyond the genetic code. Average mechanical behavior of long dsDNA is well described by the wormlike chain model, but the behavior of dsDNA at length scales around or below its persistence length remains controversial. In this talk, I will explain how we can measure looping probability of dsDNA using a fluorescence technique called FRET (F\"orster Resonance Energy Transfer) and infer its elastic properties. I will also explain how we compare the experimental results against a discrete wormlike chain model which successfully explains the behavior of long dsDNA. I will show that the behavior of short dsDNA ($<$200 base pairs) cannot be described by the wormlike chain model, but demonstrates subelastic deformation mechanism. [Preview Abstract] |
Friday, November 16, 2012 9:00AM - 9:30AM |
GC.00002: Managing surroundings: cell adaptations to 2D and 3D surfaces that promote movement Invited Speaker: Erin Rericha In the canonical model of amoeboid cell migration, the bounds of motility are set by cell-surface adhesion; surface attraction is required for actin mediated extension of pseuodopods to push the cell center of mass forward, yet it must be sufficiently low to allow cells to de-adhere and retract their rear [1]. When collective migration occurs the balance of resistive adhesion forces with protrusive forces is presumably altered as cellcell contact provides an additional mass to push against [2]. Specific integrin binding sites are known to impact focal adhesions during individual migration [3]. In the absence of specific binding sites such as in the social amoeba Dictyostelium, we envision that hydrophobic attraction and electrostatics works in a similar manner [4]. We studied the ability of Dictyostelium discoideum, which migrate individually and collectively, to move on surfaces of varying hydrophobicity and charge. We found that these cells actively regulate their surface contact such that individuals adhere and migrate equally well on surfaces of dramatically varying properties without changing cell shape, indicating the cells ``sense'' the surface. To find the timing of this adaptation we examine the spreading behavior at the initial surface contact and when the cells transition from one surface to another. In 3D collagen networks cells modify the environment by degrading and crosslinking the surrounding network as well as secreting additional collagen. We present direct visualization of the early network modifications by cancer cell lines in an effort to determine the boundary conditions of the collagen on a cell embedded in a network. [Preview Abstract] |
Friday, November 16, 2012 9:30AM - 9:42AM |
GC.00003: Electrokinetic trapping of a single fluorescent nanobead Jason K. King, Brian K. Canfield, Lloyd M. Davis We demonstrate electrokinetic control and confinement of a single 40 nm fluorescent latex bead in 25{\%} glycerol-water solution. Fluorescent beads are excited with a diode laser and imaged by a custom forward-illumination microscope onto a low-light CCD. The sample is loaded between two pairs of electrodes arranged in a crossed configuration on separate planes that allow generation of an electric field of variable orientation and strength. These electrodes consist of sputtered platinum over chrome patterned onto {\#}1.5 microscope coverslips. Astigmatism is introduced to the focus of the microscope tube lens to modify the point spread function (PSF) as a function of axial position, allowing determination of the particle position in three dimensions. By collapsing the rows and columns of the acquired image to one-dimensional arrays and fitting a Gaussian to each, the planar position and width can be determined with sub-pixel precision. The axial position can be calculated from the measured PSF ellipticity combined with calibration measurements performed on immobilized fluorescent beads at fixed distances from the focal plane. With this information we can apply appropriate voltages to counteract Brownian motion and further characterize the setup for use in single-molecule trapping. [Preview Abstract] |
Friday, November 16, 2012 9:42AM - 9:54AM |
GC.00004: Single Emitter Localization using a Four-focus Confocal Fluorescence Microscope James A. Germann, Brian K. Canfield, Lloyd M. Davis We demonstrate that four spatially separated and temporally pulsed laser foci can be used to detect and localize a single fluorescent emitter to below the diffraction limit in a confocal microscope. Optical excitation is accomplished using LabVIEW Real-Time to control sequential pulsing of four laser diodes. The beams are coupled collinearly through three beam splitters and focused in a custom confocal microscope. The individual foci are positioned at the vertices of a micron-sized tetrahedron, which establish a Cartesian coordinate system. Fluorescence photons are counted by a single-photon avalanche diode and time-gated based on the pulse excitation sequence. Emitter location is estimated from the count rates generated at the four foci using Maximum Likelihood techniques. Preliminary results for tracking a fluorescently labeled nanoparticle in an aqueous/glycerol solution with a piezoelectric stage are presented. Future research with the four-focus microscope will concentrate on trapping a single fluorescent molecule. [Preview Abstract] |
Friday, November 16, 2012 9:54AM - 10:06AM |
GC.00005: ABSTRACT WITHDRAWN |
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