关键词： Cell cycle and cell division   Imaginal discs   Mechanical tension   Notch signaling   Cloning   Drosophila melanogaster   Relaxation (physics)   Cytoskeleton  

摘要： Shaping the primordia during development relies on forces and mechanisms able to control cell segregation. In the imaginal discs of Drosophila the cellular populations that will give rise to the dorsal and ventral parts on the wing blade are segregated and do not intermingle. A cellular population that becomes specified by the boundary of the dorsal and ventral cellular domains, the so-called organizer, controls this process. In this paper we study the dynamics and stability of the dorsal-ventral organizer of the wing imaginal disc of Drosophila as cell proliferation advances. Our approach is based on a vertex model to perform in silico experiments that are fully dynamical and take into account the available experimental data such as: cell packing properties, orientation of the cellular divisions, response upon membrane ablation, and robustness to mechanical perturbations induced by fast growing clones. Our results shed light on the complex interplay between the cytoskeleton mechanics, the cell cycle, the cell growth, and the cellular interactions in order to shape the dorsal-ventral organizer as a robust source of positional information and a lineage controller. Specifically, we elucidate the necessary and sufficient ingredients that enforce its functionality: distinctive mechanical properties, including increased tension, longer cell cycle duration, and a cleavage criterion that satisfies the Hertwig rule. Our results provide novel insights into the developmental mechanisms that drive the dynamics of the DV organizer and set a definition of the so-called Notch fence model in quantitative terms.

关键词： ERRORS  

摘要： A correction to the article 'Kinetics and Mechanistic Aspects of the Heck Reaction Promoted by a CN-Palladacycle' that was published in the 2005 issue is presented.

关键词： Anisotropy   Basic-Leucine Zipper Transcription Factors/chemistry   Kinetics   Optical Tweezers   Protein Folding   Protein Structure, Secondary   Protein Structure, Tertiary   Saccharomyces cerevisiae/chemistry   Saccharomyces cerevisiae Proteins/chemistry   Stress, Mechanical   Thermodynamics  

摘要： Coiled coils are one of the most abundant protein structural motifs and widely mediate protein interactions and force transduction or sensation. They are thus model systems for protein engineering and folding studies, particularly the GCN4 coiled coil. Major single-molecule methods have also been applied to this protein and revealed its folding kinetics at various spatiotemporal scales. Nevertheless, the folding energy and the kinetics of a single GCN4 coiled coil domain have not been well determined at a single-molecule level. Here we used high-resolution optical tweezers to characterize the folding and unfolding reactions of a single GCN4 coiled coil domain and their dependence on the pulling direction. In one axial and two transverse pulling directions, we observed reversible, two-state transitions of the coiled coil in real time. The transitions equilibrate at pulling forces ranging from 6 to 12 pN, showing different stabilities of the coiled coil in regard to pulling direction. Furthermore, the transition rates vary with both the magnitude and the direction of the pulling force by greater than 1000 folds, indicating a highly anisotropic and topology-dependent energy landscape for protein transitions under mechanical tension. We developed a new analytical theory to extract energy and kinetics of the protein transition at zero force. The derived folding energy does not depend on the pulling direction and is consistent with the measurement in bulk, which further confirms the applicability of the single-molecule manipulation approach for energy measurement. The highly anisotropic thermodynamics of proteins under tension should play important roles in their biological functions.

关键词： Coupled physical-biogeochemical model   Mechanistic process model   Osmotic stress   Senegal River Estuary  

摘要： This paper presents the outline of an estuary modeling study encompassing a study of the biogeochemical model on its own. It includes a preliminary validation of the new mechanistic features using mesocosm data, and a study of a coupled physical and biogeochemical model of the Senegal River Estuary (SRE) from flood to low-water conditions. The originality of this work is that it distinguishes the marine and freshwater origins of some of the organisms modeled and includes additional mortality terms owing to the osmotic stress experienced by the freshwater organisms. The mechanistic model implemented in the Eco3M modelling tool, and partially designed for this study, succeeded in representing the dynamics of the planktonic food web in two mesocosm experiments and in the SRE under very different conditions using a single set of model parameters. Among the important results, it reveals that distinguishing between marine and freshwater origins provides substantial additional information on the distribution of organisms along the estuary. Finally, failure to take account of the osmotic stress and the origin of the organisms can give rise to substantial errors in the calculation of the biomass in the SRE. (C) 2011 Elsevier B.V. All rights reserved.

关键词： density functional theory   excited states   fluorescence   ground states   molecular electronic states   molecule-photon collisions   organic compounds   quantum chemistry   solvent effects   water  

摘要： We present a combination of time-dependent density functional theory with the quantum mechanical/molecular mechanical approach which can be applied to study nonadiabatic dynamical processes in molecular systems interacting with the environment. Our method is illustrated on the example of ultrafast excited state dynamics of indole in water. We compare the mechanisms of nonradiative relaxation and the electronic state lifetimes for isolated indole, indole in a sphere of classical water, and indole + 3H(2)O embedded in a classical water sphere. In the case of isolated indole, the initial excitation to the S-2 electronic state is followed by an ultrafast internal conversion to the S-1 state with a time constant of 17 fs. The S-1 state is long living (> 30 ps) and deactivates to the ground state along the N-H stretching coordinate. This deactivation mechanism remains unchanged for indole in a classical water sphere. However, the lifetimes of the S-2 and S-1 electronic states are extended. The inclusion of three explicit water molecules opens a new relaxation channel which involves the electron transfer to the solvent, leading eventually to the formation of a solvated electron. The relaxation to the ground state takes place on a time scale of 60 fs and contributes to the lowering of the fluorescence quantum yield. Our simulations demonstrate the importance of including explicit water molecules in the theoretical treatment of solvated systems. (C) 2011 American Institute of Physics. [doi:10.1063/1.3622563]

关键词： Actins/genetics   Actins/metabolism   Animals   Biomechanics   Cells, Cultured   Cytoskeleton/metabolism   Fibroblasts/cytology   Fibroblasts/metabolism   Green Fluorescent Proteins/genetics   Green Fluorescent Proteins/metabolism   Immunohistochemistry   Immunohistochemistry   Rats   Recombinant Fusion Proteins/genetics   Recombinant Fusion Proteins/metabolism  

摘要： Mechanical forces can regulate various functions in living cells. The cytoskeleton is a crucial element for the transduction of forces in cell-internal signals and subsequent biological responses. Accordingly, many studies in cellular biomechanics have been focused on the role of the contractile acto-myosin system in such processes. A widely used method to observe the dynamic actin network in living cells is the transgenic expression of fluorescent proteins fused to actin. However, adverse effects of GFP-actin fusion proteins on cell spreading, migration and cell adhesion strength have been reported. These shortcomings were shown to be partly overcome by fusions of actin binding peptides to fluorescent proteins. Nevertheless, it is not understood whether direct labeling by actin fusion proteins or indirect labeling via these chimaeras alters biomechanical responses of cells and the cytoskeleton to forces. We investigated the dynamic reorganization of actin stress fibers in cells under cyclic mechanical loading by transiently expressing either egfp-Lifeact or eyfp-actin in rat embryonic fibroblasts and observing them by means of live cell microscopy. Our results demonstrate that mechanically-induced actin stress fiber reorganization exhibits very different kinetics in EYFP-actin cells and EGFP-Lifeact cells, the latter showing a remarkable agreement with the reorganization kinetics of non-transfected cells under the same experimental conditions.

关键词： 机械动力部件   实时监控   S7-300PLC   ET200  

摘要： 煤矿井下的特殊环境使得对机械动力部的检测一直处于相对简单和原始的状态,因此,设计了一种基于S7-300PLC煤矿井下机械动力部工况实时监控系统,并详细介绍了其工作原理、组成硬件的选择和软件的应用。经过定点实验证明,通过应用该系统工作人员数与检测时间都大大减少,检测效率提高29%。

关键词： Actuating device   Actuating force   Agricultural equipment   Agricultural machines   Control devices   Definitions   Layout   Manipulating devices   Operating devices   Operating forces   Operation   Specification (approval)   Tractors  

标准号: NF U02-101-2011

关键词： 机械动力学   研究生   课程改革   创新   案例教学  

摘要： 机械动力学是基础理论与实践应用紧密结合的研究生专业课程,属于力学与机械工程的交叉学科,既有基础理论高度的抽象性与严密的科学性,又具有实践应用的广泛性与深入性。针对研究生积极参与实际课题的浓厚兴趣与强烈愿望,通过对机械动力学课程内容和教学模式的改革,引入案例教学法,革新考核方式,使学生掌握解决问题的方法并应用到自己的科研课题中。同时,也引导学生从实际现象中发现和提出新问题,在培养"研究型"学生的实践过程中取得了良好效果。