833 research outputs found
Angular correlations of galaxy distribution
We study the angular correlations of various galaxy catalogs (CfA1, SSRS1,
Perseus-Pisces, APM Bright Galaxies and Zwicky). We find that the angular
correlation exponent is rather than as
usually found by the standard correlation function . We
identify the problem in the artificial decay of . Moreover we
find that no characteristic angular scale is present in any of the analyzed
catalogs. Finally we show that all the available data are consistent with each
other and the angular distribution of galaxies is quite naturally compatible
with a fractal structure with .Comment: 16 pages, latex, 3 postscript figures. Accepted for publication in
Astrophysical Journal Letters. This paper is also available at
http://www.phys.uniroma1.it/DOCS/PIL/pil.htm
Isotropic-Nematic transition of long thin hard spherocylinders confined in a quasi-two-dimensional planar geometry
We present computer simulations of long thin hard spherocylinders in a narrow
planar slit. We observe a transition from the isotropic to a nematic phase with
quasi-long-range orientational order upon increasing the density. This phase
transition is intrinsically two dimensional and of the Kosterlitz-Thouless
type. The effective two-dimensional density at which this transition occurs
increases with plate separation. We qualitatively compare some of our results
with experiments where microtubules are confined in a thin slit, which gave the
original inspiration for this work.Comment: 8 pages, 10 figure
Influence of M-phase chromatin on the anisotropy of microtubule asters
In many eukaryotic cells going through M-phase, a bipolar spindle is formed by microtubules nucleated from centrosomes. These microtubules, in addition to being "captured" by kinetochores, may be stabilized by chromatin in two different ways: short-range stabilization effects may affect microtubules in close contact with the chromatin, while long-range stabilization effects may "guide" microtubule growth towards the chromatin (e.g., by introducing a diffusive gradient of an enzymatic activity that affects microtubule assembly). Here, we use both meiotic and mitotic extracts from Xenopus laevis eggs to study microtubule aster formation and microtubule dynamics in the presence of chromatin. In "low-speed" meiotic extracts, in the presence of salmon sperm chromatin, we find that short-range stabilization effects lead to a strong anisotropy of the microtubule asters. Analysis of the dynamic parameters of microtubule growth show that this anisotropy arises from a decrease in the catastrophe frequency, an increase in the rescue frequency and a decrease in the growth velocity. In this system we also find evidence for long-range "guidance" effects, which lead to a weak anisotropy of the asters. Statistically relevant results on these long-range effects are obtained in "high-speed" mitotic extracts in the presence of artificially constructed chromatin stripes. We find that aster anisotropy is biased in the direction of the chromatin and that the catastrophe frequency is reduced in its vicinity. In this system we also find a surprising dependence of the catastrophe and the rescue frequencies on the length of microtubules nucleated from centrosomes: the catastrophe frequency increase and the rescue frequency decreases with microtubule length
Stability and Dynamics of Crystals and Glasses of Motorized Particles
Many of the large structures of the cell, such as the cytoskeleton, are
assembled and maintained far from equilibrium. We study the stabilities of
various structures for a simple model of such a far-from-equilibrium organized
assembly in which spherical particles move under the influence of attached
motors. From the variational solutions of the manybody master equation for
Brownian motion with motorized kicking we obtain a closed equation for the
order parameter of localization. Thus we obtain the transition criterion for
localization and stability limits for the crystalline phase and frozen
amorphous structures of motorized particles. The theory also allows an estimate
of nonequilibrium effective temperatures characterizing the response and
fluctuations of motorized crystals and glasses.Comment: 5 pages, 3 figure
A first principle (3+1) dimensional model for microtubule polymerization
In this paper we propose a microscopic model to study the polymerization of
microtubules (MTs). Starting from fundamental reactions during MT's assembly
and disassembly processes, we systematically derive a nonlinear system of
equations that determines the dynamics of microtubules in 3D. %coexistence with
tubulin dimers in a solution. We found that the dynamics of a MT is
mathematically expressed via a cubic-quintic nonlinear Schrodinger (NLS)
equation. Interestingly, the generic 3D solution of the NLS equation exhibits
linear growing and shortening in time as well as temporal fluctuations about a
mean value which are qualitatively similar to the dynamic instability of MTs
observed experimentally. By solving equations numerically, we have found
spatio-temporal patterns consistent with experimental observations.Comment: 12 pages, 2 figures. Accepted in Physics Letters
Absence of long-range diffusion of OmpA in E. coli is not caused by its peptidoglycan binding domain.
BACKGROUND: It is widely believed that integral outer membrane (OM) proteins in bacteria are able to diffuse laterally in the OM. However, stable, immobile proteins have been identified in the OM of Escherichia coli. In explaining the observations, a hypothesized interaction of the immobilized OM proteins with the underlying peptidoglycan (PG) cell wall played a prominent role. RESULTS: OmpA is an abundant outer membrane protein in E. coli containing a PG-binding domain. We use FRAP to investigate whether OmpA is able to diffuse laterally over long-range (> ~100Â nm) distances in the OM. First, we show that OmpA, containing a PG binding domain, does not exhibit long-range lateral diffusion in the OM. Then, to test whether PG interaction was required for this immobilization, we genetically removed the PG binding domain and repeated the FRAP experiment. To our surprise, this did not increase the mobility of the protein in the OM. CONCLUSIONS: OmpA exhibits an absence of long-range (> ~100Â nm) diffusion in the OM that is not caused by its PG binding domain. Therefore, other mechanisms are needed to explain this observation, such as the presence of physical barriers in the OM, or strong interactions with other elements in the cell envelope
Formation of helical membrane tubes around microtubules by single-headed kinesin KIF1A
The kinesin-3 motor KIF1A is in charge of vesicular transport in neuronal axons. Its single-headed form is known to be very inefficient due to the presence of a diffusive state in the mechanochemical cycle. However, recent theoretical studies have suggested that these motors could largely enhance force generation by working in teams. Here we test this prediction by challenging single-headed KIF1A to extract membrane tubes from giant vesicles along microtubule filaments in a minimal in vitro system. Remarkably, not only KIF1A motors are able to extract tubes but they feature a novel phenomenon: tubes are wound around microtubules forming tubular helices. This finding reveals an unforeseen combination of cooperative force generation and self-organized manoeuvreing capability, suggesting that the diffusive state may be a key ingredient for collective motor performance under demanding traffic conditions. Hence, we conclude that KIF1A is a genuinely cooperative motor, possibly explaining its specificity to axonal trafficking.Peer ReviewedPostprint (published version
Collision induced spatial organization of microtubules
The dynamic behavior of microtubules in solution can be strongly modified by
interactions with walls or other structures. We examine here a microtubule
growth model where the increase in size of the plus-end is perturbed by
collisions with other microtubules. We show that such a simple mechanism of
constrained growth can induce ordered structures and patterns from an initially
isotropic and homogeneous suspension. First, microtubules self-organize locally
in randomly oriented domains that grow and compete with each other. By imposing
even a weak orientation bias, external forces like gravity or cellular
boundaries may bias the domain distribution eventually leading to a macroscopic
sample orientation.Comment: Submitted to Biophysical Journa
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