Colloidal particle motion as a diagnostic of DNA conformational transitions

Tethered particle motion is an experimental technique to monitor conformational changes in single molecules of DNA in real time, by observing the position fluctuations of a micrometer-size particle attached to the DNA. This article reviews some recent work on theoretical problems inherent in the interpretation of TPM experiments, both in equilibrium and dynamical aspects.Comment: 19pp. Accepted for publication in Curr Op Colloid Interf Scienc

Fluctuating Membranes with Tilt Order

Thermal fluctuations are important for amphiphilic bilayer membranes since typical bending stiffnesses can be a few $k_B T$. The rod-like constituent molecules are generically tilted with respect to the local normal for packing reasons. We study the effects of fluctuations on membranes with nematic order, a simplified idealization with the same novel features as realistic tilt order. We find that nematic membranes lie in the same universality class as hexatic membranes, {\it i.e.} the couplings that distinguish nematic from hexatic order are marginally irrelevant. Our calculation also illustrates the advantages of conformal gauge, which brings great conceptual and technical simplifications compared to the more popular Monge gauge.Comment: 13 pages; print this using plain Te

Dynamically-Stabilized Pores in Bilayer Membranes

Zhelev and Needham have recently created large, quasi-stable pores in artificial lipid bilayer vesicles [Biochim. Biophys. Acta 1147 (1993) 89]. Initially created by electroporation, the pores remain open for up to several seconds before quickly snapping shut. This result is surprising in light of the large line tension for holes in bilayer membranes and the rapid time scale for closure of large pores. We show how pores can be dynamically stabilized via a new feedback mechanism. We also explain quantitatively the observed sudden pore closure as a tangent bifurcation. Finally we show how Zhelev and Needham's experiment can be used to measure accurately the pore line tension, an important material parameter. For their SOPC/CHOL mixture we obtain a line tension of 2.6 10^{-6} erg/cm.Comment: 7 pages RevTeX, 3 included eps figures (published version). Postscript also available at http://dept.physics.upenn.edu/~moro

Entropic Elasticity of Twist-Storing Polymers

We investigate the statistical mechanics of a torsionally constrained polymer. The polymer is modeled as a fluctuating rod with bend stiffness A kT and twist stiffness C kT. In such a model, thermal bend fluctuations couple geometrically to an applied torque through the relation Lk = Tw + Wr. We explore this coupling and find agreement between the predictions of our model and recent experimental results on single lambda-DNA molecules. This analysis affords an experimental determination of the microscopic twist stiffness (averaged over a helix repeat). Quantitative agreement between theory and experiment is obtained using C=109 nm. The theory further predicts a thermal reduction of the effective twist rigidity induced by bend fluctuations. Finally, we find a small reflection of molecular chirality in the experimental data and interpret it in terms of a twist-stretch coupling of the DNA duplex.Comment: 37 pages RevTeX, 2 postscript figures. Revisions include the analysis of new data and an investigation of non-perturbative effects. Postscript also available at http://www.physics.upenn.edu/~moro

Hitchhiking Through the Cytoplasm

We propose an alternative mechanism for intracellular cargo transport which results from motor induced longitudinal fluctuations of cytoskeletal microtubules (MT). The longitudinal fluctuations combined with transient cargo binding to the MTs lead to long range transport even for cargos and vesicles having no molecular motors on them. The proposed transport mechanism, which we call ``hitchhiking'', provides a consistent explanation for the broadly observed yet still mysterious phenomenon of bidirectional transport along MTs. We show that cells exploiting the hitchhiking mechanism can effectively up- and down-regulate the transport of different vesicles by tuning their binding kinetics to characteristic MT oscillation frequencies

ENGAGING STUDENTS IN RESEARCH: THE USE OF STRUCTURED PROFESSIONAL DIALOGUE

Graduate students frequently have difficulty defining, developing, and resolving research problems in a manner consistent with the agricultural economics community. Here, we report on a seminar designed to assist graduate students integrate subject matter courses into independent research proposals through participation in professional dialogue. Professional dialogue involves developing arguments to explain and resolve questions where the explanations are supported by warranted evidence and appropriately qualified. The premise of the seminar is that more active problem solvers are developed using professional dialogue to sharpen critical thinking and writing skills.Teaching/Communication/Extension/Profession,

An LQR controller design approach for a Large Gap Magnetic Suspension System (LGMSS)

Two control approaches for a Large Gap Magnetic Suspension System (LGMSS) are investigated and numerical results are presented. The approaches are based on Linear Quadratic Regulator (LQR) control theory and include a nonzero set point regulator with constant disturbance input and an integral feedback regulator. The LGMSS provides five degree of freedom control of a cylindrical suspended element which is composed of permanent magnet material. The magnetic actuators are air core electromagnets mounted in a planar way

Food Co-ops in America: Communities, Consumption, and Economic Democracy

Review of: "Food Co-ops in America: Communities, Consumption, and Economic Democracy" by Philip Nelson

Purpose and potential for commodity exchanges in African economies:

This paper reviews the purpose and potential of commodity exchanges in Africa. Drawing from the existing literature and using indicative empirics, it examines the conditions that enable successful exchanges, highlights the special challenges to setting up exchanges in Africa, and reviews alternatives to domestic exchanges. We argue that many critical preconditions for the successful establishment of commodity exchanges in Africa remain binding in the short to medium term. The development of commodity exchanges in the region is impeded by the relatively small size of domestic commodity markets, the weak physical and communication infrastructure, a lack of supportive legal and regulatory environments, and the likelihood of policy interventions, particularly in the staple cereals market. Meanwhile, the demand for a domestic commodity exchange for export crops may be limited due to the availability of well-established exchanges abroad and functioning auction floors. The paper highlights three points: (a) efforts to launch exchanges in Africa should realistically assess whether basic conditions for success can be met, (b) if the pre-conditions cannot be met, the use of existing exchanges abroad or the development of regional exchanges may be more feasible than the establishment of national commodity exchanges, and (c) the goals of risk management and reduced transaction costs might be achieved more effectively by improving market fundamentals through investments in transportation, information services, or other financial institutions.commodity exchanges, Risk management, Market development,