关键词： genomic integrity   DNA repair   nonhomologous end-joining   super-resolution microscopy   single-molecule FRET  

摘要： Nonhomologous end-joining (NHEJ) is a major repair pathway for DNA double-strand breaks (DSBs), involving synapsis and ligation of the broken strands. We describe the use of in vivo and in vitro single-molecule methods to define the organization and interaction of NHEJ repair proteins at DSB ends. Super-resolution fluorescence microscopy allowed the precise visualization of XRCC4, XLF, and DNA ligase IV filaments adjacent to DSBs, which bridge the broken chromosome and direct rejoining. We show, by single-molecule FRET analysis of the Ku/XRCC4/XLF/DNA ligase IV NHEJ ligation complex, that end-to-end synapsis involves a dynamic positioning of the two ends relative to one another. Our observations form the basis of a new model for NHEJ that describes the mechanism whereby filament-forming proteins bridge DNA DSBs in vivo. In this scheme, the filaments at either end of the DSB interact dynamically to achieve optimal configuration and end-to-end positioning and ligation.

关键词： Research Article  

摘要： Geometric and mechanical properties of individual cells and interactions among neighboring cells are the basis of formation of tissue patterns. Understanding the complex interplay of cells is essential for gaining insight into embryogenesis, tissue development, and other emerging behavior. Here we describe a cell model and an efficient geometric algorithm for studying the dynamic process of tissue formation in 2D (e.g. epithelial tissues). Our approach improves upon previous methods by incorporating properties of individual cells as well as detailed description of the dynamic growth process, with all topological changes accounted for. Cell size, shape, and division plane orientation are modeled realistically. In addition, cell birth, cell growth, cell shrinkage, cell death, cell division, cell collision, and cell rearrangements are now fully accounted for. Different models of cell-cell interactions, such as lateral inhibition during the process of growth, can be studied in detail. Cellular pattern formation for monolayered tissues from arbitrary initial conditions, including that of a single cell, can also be studied in detail. Computational efficiency is achieved through the employment of a special data structure that ensures access to neighboring cells in constant time, without additional space requirement. We have successfully generated tissues consisting of more than 20,000 cells starting from 2 cells within 1 hour. We show that our model can be used to study embryogenesis, tissue fusion, and cell apoptosis. We give detailed study of the classical developmental process of bristle formation on the epidermis of D. melanogaster and the fundamental problem of homeostatic size control in epithelial tissues. Simulation results reveal significant roles of solubility of secreted factors in both the bristle formation and the homeostatic control of tissue size. Our method can be used to study broad problems in monolayered tissue formation. Our software is publicly availab

关键词： Living systems: Biological Matter  

摘要： The understanding of morphogenesis in living organisms has been renewed by tremendous progress in experimental techniques that provide access to cell scale, quantitative information both on the shapes of cells within tissues and on the genes being expressed. This information suggests that our understanding of the respective contributions of gene expression and mechanics, and of their crucial entanglement, will soon leap forward. Biomechanics increasingly benefits from models, which assist the design and interpretation of experiments, point out the main ingredients and assumptions, and ultimately lead to predictions. The newly accessible local information thus calls for a reflection on how to select suitable classes of mechanical models. We review both mechanical ingredients suggested by the current knowledge of tissue behaviour, and modelling methods that can help generate a rheological diagram or a constitutive equation. We distinguish cell scale (“intra-cell”) and tissue scale (“inter-cell”) contributions. We recall the mathematical framework developed for continuum materials and explain how to transform a constitutive equation into a set of partial differential equations amenable to numerical resolution. We show that when plastic behaviour is relevant, the dissipation function formalism appears appropriate to generate constitutive equations; its variational nature facilitates numerical implementation, and we discuss adaptations needed in the case of large deformations. The present article gathers theoretical methods that can readily enhance the significance of the data to be extracted from recent or future high throughput biomechanical experiments.

关键词： mechanical properties   single layer MoS2   molecular dynamics  

摘要： Based on the benchmark created by the first principle calculations, the effectiveness of different empirical potential functions, including CVFF1, CVFF2, SW and REBO potentials, on describing the mechanical behavior of single layer MoS2 (SLMoS2) are evaluated. The mechanical properties including the elastic modulus E, Poisson's ratio nu, nonlinear elastic modulus D, failure stress and ultimate strain are considered. It is found that under a small deformation, the REBO and CVFF2 potentials can provide an effective description of the elastic behavior of SLMoS2;whereas under a large deformation, the SW potential gives a more accurate stress value and the CVFF2 potential can only predict the right value under biaxial tension. After modifying the cut-off distances in the REBO potential, the failure strain can be reasonably predicted by the REBO potential;whereas the failure stress will be overestimated by about 20-40% for biaxial and uniaxial tension. The present study can provide help on accurately understanding the mechanical behavior of SLMoS2.

关键词： indium tin oxide   mechanical activation   chemical preparation   grain boundaries   thermal properties   decomposition kinetics  

摘要： Isothermal decomposition process of chemically transforming indium tin oxide（ITO） powders into indium（III） hydroxide powders was investigated. Two types of powders were analyzed, i.e., non-activated and mechanically activated. It has been found that in the case of activated sample, shorter induction periods appear, which permits growth of smaller crystals, while in the case of non-activated sample, long induction periods appear, characterized by the growth of larger crystals. DAEM approach has shown that decomposition processes of non-activated and mechanically activated samples can be described by contracting volume model with a linear combination of two different density distribution functions of apparent activation energies（Ea）, and with first-order model, with a single symmetrical density distribution function of Ea, respectively. It was established that specific characteristics of particles not only affect the mechanism of decomposition processes, but also have the significant impact on thermodynamic properties.

关键词： 双连杆机械臂   运动链   动态模型  

摘要： 该文描述了出现在双连杆机械臂动态参数模型中的问题,并对其性能进行了评估。创建了机械臂的运动模型,连接在绝对空间中链接位移与夹持器中心位置,解决了链接位置的正向运动问题。同时得到一组非线性函数,建立了机械臂的广义坐标和笛卡尔坐标之间的连接。使用Denavit-Hartenberg方法对运动链进行编码。作为解决逆运动学问题的结果,获得一个给定的位置和夹持器输出链路方向的广义坐标方程系统。在数学软件MATLAB(Simulink)中分析得到系统动力学的模型。该文的结论通过数学实验进行证实。

关键词： 滚动轴承   振动机械   点蚀   故障诊断   力学模型  

摘要： 针对振动机械滚动轴承内、外圈单点点蚀故障,通过分析轴承工作状况及钢球通过缺陷时接触变形量和弹性接触力的变化,建立了振动机械轴承内、外圈单点点蚀故障的动力学模型,并通过数值仿真得到了振动机械轴承内、外圈单点点蚀故障频谱。以振动筛为载体进行试验研究,验证了理论模型的正确性及有效性。理论分析与试验结果一致表明:振动机械轴承外圈单点点蚀故障时的故障包络谱中有较为明显的调制现象,内圈单点点蚀故障时的包络谱有轻微的调制现象,与旋转机械轴承内外圈单点点蚀故障谱有明显的区别。

关键词： mechanical activation   agglomerate   powder mixture   structure   layer thickness   interphase surface   titanium   nickel  

摘要： An experimental study of the process of mechanical activation (MA) of a nickel and titanium powder mixture has been carried out. It is revealed that the MA time significantly affects the physical-chemical and structural transformations of the grinded mixture. The reason for such an effect is the change of the composition and structure of the layered agglomerates formed during MA from the mixture components. The kinetic constant characterizing the rate of growth of the interphase surface in the agglomerates during mechanical treatment of the powder mixture is estimated using the method of inverse problem. Theoretical calculations of the layer thickness and interphase surface in the agglomerate are carried out.

关键词： Carbon nanotube   Damping   Mechanical properties   Molecular dynamics   Polycarbonate   Thermal conductivity  

摘要： The present study aims at examining the mechanical properties of multi-walled carbon nanotubes-polycarbonate composites (MWCNT-PC), through a molecular dynamics (MD) simulation. Composites of MWCNT-PC were modeled using Materials Studio 5.5 software. Multiwall carbon nanotubes (MWCNTs) compositions in polycarbonate (PC) were varied by weight from 0.5% to 10% and also by volume from 2% to 16%. Forcite module in Materials Studio was used for finding mechanical properties. A marked increase in the elastic modulus (up to 89%) has been observed, even with the addition of a small quantity (up to 2 weight %) of MWCNTs. Also, upon addition of about 2 volume % of MWCNTs, the elastic modulus increases by almost 10%. The increase in mechanical properties is found to supplement earlier experimental investigations of these composites using nano-indentation techniques. Better load transfer property of MWCNTs, larger surface area and interaction between reinforcement with base matrix are the suggested reasons for this increase in mechanical properties. (C) 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

关键词： acrylamides   B3LYP   density functional theory   propagation kinetics   quantum mechanical modeling  

摘要： In this study propagation kinetics of free radical polymerization of N-methylacrylamide (NMAAm) is studied with density functional theory calculations. The propagation rate constant ratio of N,N-dimethylacrylamide (DMAAm) and NMAAm (k(NMAAm)/k(DMAAm)) is evaluated via model reactions at dimeric stage. The most favorable modes of addition is shown to be determined by the steric effects and hydrogen bonding interactions between the reactive fragments. Gauche and trans orientations are preferred as the least energetic additions in pro-meso and pro-racemo attacks, respectively. Benchmark studies with various density functionals (M05-2X, M06-2X, MPWB1K, BMK) combined with 6-311+G(3df,2p) basis set assess the calculations. The k(NMAAm)/k(DMAAm) ratio obtained in this study is in line with the experimental value. The addition reaction barrier via dimeric associates in case of NMAAm does not yield significant difference than the barrier via monomeric species.