1,481 research outputs found
Novel steady state of a microtubule assembly in a confined geometry
We study the steady state of an assembly of microtubules in a confined
volume, analogous to the situation inside a cell where the cell boundary forms
a natural barrier to growth. We show that the dynamical equations for growing
and shrinking microtubules predict the existence of two steady states, with
either exponentially decaying or exponentially increasing distribution of
microtubule lengths. We identify the regimes in parameter space corresponding
to these steady states. In the latter case, the apparent catastrophe frequency
near the boundary was found to be significantly larger than that in the
interior. Both the exponential distribution of lengths and the increase in the
catastrophe frequency near the cell margin is in excellent agreement with
recent experimental observations.Comment: 8 pages, submitted to Phys. Rev.
Transport enhancement from incoherent coupling between one-dimensional quantum conductors
We study the non-equilibrium transport properties of a highly anisotropic
two-dimensional lattice of spin-1/2 particles governed by a Heisenberg XXZ
Hamiltonian. The anisotropy of the lattice allows us to approximate the system
at finite temperature as an array of incoherently coupled one-dimensional
chains. We show that in the regime of strong intrachain interactions, the weak
interchain coupling considerably boosts spin transport in the driven system.
Interestingly, we show that this enhancement increases with the length of the
chains, which is related to superdiffusive spin transport. We describe the
mechanism behind this effect, compare it to a similar phenomenon in single
chains induced by dephasing, and explain why the former is much stronger
Weak measurement takes a simple form for cumulants
A weak measurement on a system is made by coupling a pointer weakly to the
system and then measuring the position of the pointer. If the initial
wavefunction for the pointer is real, the mean displacement of the pointer is
proportional to the so-called weak value of the observable being measured. This
gives an intuitively direct way of understanding weak measurement. However, if
the initial pointer wavefunction takes complex values, the relationship between
pointer displacement and weak value is not quite so simple, as pointed out
recently by R. Jozsa. This is even more striking in the case of sequential weak
measurements. These are carried out by coupling several pointers at different
stages of evolution of the system, and the relationship between the products of
the measured pointer positions and the sequential weak values can become
extremely complicated for an arbitrary initial pointer wavefunction.
Surprisingly, all this complication vanishes when one calculates the cumulants
of pointer positions. These are directly proportional to the cumulants of
sequential weak values. This suggests that cumulants have a fundamental
physical significance for weak measurement
Actin disassembly by cofilin, coronin, and Aip1 occurs in bursts and is inhibited by barbed-end cappers
Turnover of actin filaments in cells requires rapid actin disassembly in a cytoplasmic environment that thermodynamically favors assembly because of high concentrations of polymerizable monomers. We here image the disassembly of single actin filaments by cofilin, coronin, and actin-interacting protein 1, a purified protein system that reconstitutes rapid, monomer-insensitive disassembly (Brieher, W.M., H.Y. Kueh, B.A. Ballif, and T.J. Mitchison. 2006. J. Cell Biol. 175:315–324). In this three-component system, filaments disassemble in abrupt bursts that initiate preferentially, but not exclusively, from both filament ends. Bursting disassembly generates unstable reaction intermediates with lowered affinity for CapZ at barbed ends. CapZ and cytochalasin D (CytoD), a barbed-end capping drug, strongly inhibit bursting disassembly. CytoD also inhibits actin disassembly in mammalian cells, whereas latrunculin B, a monomer sequestering drug, does not. We propose that bursts of disassembly arise from cooperative separation of the two filament strands near an end. The differential effects of drugs in cells argue for physiological relevance of this new disassembly pathway and potentially explain discordant results previously found with these drugs
Motile cilia defects in diseases other than primary ciliary dyskinesia:The contemporary diagnostic and research role for transmission electron microscopy
Ultrastructural studies have underpinned the cell biological and clinical investigations of the varied roles of motile cilia in health and disease, with a long history since the 1950s. Recent developments from transmission electron microscopy (TEM; cryo-electron microscopy, electron tomography) have yielded higher resolution and fresh insights into the structure and function of these complex organelles. Microscopy in ciliated organisms, disease models, and in patients with ciliopathy diseases has dramatically expanded our understanding of the ubiquity, multisystem involvement, and importance of cilia in normal human development. Here, we review the importance of motile cilia ultrastructural studies in understanding the basis of diseases other than primary ciliary dyskinesia
An Explicit Bound for Dynamical Localisation in an Interacting Many-Body System
We characterise and study dynamical localisation of a finite interacting
quantum many-body system. We present explicit bounds on the disorder strength
required for the onset of localisation of the dynamics of arbitrary ensemble of
sites of the XYZ spin-1/2 model. We obtain these results using a novel form of
the fractional moment criterion, which we establish, together with a
generalisation of the self-avoiding walk representation of the system Green's
functions, called path-sums. These techniques are not specific to the XYZ model
and hold in a much more general setting. We further present bounds for two
observable quantities in the localised regime: the magnetisation of any
sublattice of the system as well as the linear magnetic response function of
the system. We confirm our results through numerical simulations.Comment: 35 pages; 5 figure
Properties of the kinetochore in vitro. II. Microtubule capture and ATP-dependent translocation.
Quantitative predictions on auxin-induced polar distribution of PIN proteins during vein formation in leaves
The dynamic patterning of the plant hormone auxin and its efflux facilitator
the PIN protein are the key regulator for the spatial and temporal organization
of plant development. In particular auxin induces the polar localization of its
own efflux facilitator. Due to this positive feedback auxin flow is directed
and patterns of auxin and PIN arise. During the earliest stage of vein
initiation in leaves auxin accumulates in a single cell in a rim of epidermal
cells from which it flows into the ground meristem tissue of the leaf blade.
There the localized auxin supply yields the successive polarization of PIN
distribution along a strand of cells. We model the auxin and PIN dynamics
within cells with a minimal canalization model. Solving the model analytically
we uncover an excitable polarization front that triggers a polar distribution
of PIN proteins in cells. As polarization fronts may extend to opposing
directions from their initiation site we suggest a possible resolution to the
puzzling occurrence of bipolar cells, such we offer an explanation for the
development of closed, looped veins. Employing non-linear analysis we identify
the role of the contributing microscopic processes during polarization.
Furthermore, we deduce quantitative predictions on polarization fronts
establishing a route to determine the up to now largely unknown kinetic rates
of auxin and PIN dynamics.Comment: 9 pages, 4 figures, supplemental information included, accepted for
publication in Eur. Phys. J.
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