1,860 research outputs found
Particle interactions and lattice dynamics: Scenarios for efficient bidirectional stochastic transport?
Intracellular transport processes driven by molecular motors can be described
by stochastic lattice models of self-driven particles. Here we focus on
bidirectional transport models excluding the exchange of particles on the same
track. We explore the possibility to have efficient transport in these systems.
One possibility would be to have appropriate interactions between the various
motors' species, so as to form lanes. However, we show that the lane formation
mechanism based on modified attachment/detachment rates as it was proposed
previously is not necessarily connected to an efficient transport state and is
suppressed when the diffusivity of unbound particles is finite. We propose
another interaction mechanism based on obstacle avoidance that allows to have
lane formation for limited diffusion. Besides, we had shown in a separate paper
that the dynamics of the lattice itself could be a key ingredient for the
efficiency of bidirectional transport. Here we show that lattice dynamics and
interactions can both contribute in a cooperative way to the efficiency of
transport. In particular, lattice dynamics can decrease the interaction
threshold beyond which lanes form. Lattice dynamics may also enhance the
transport capacity of the system even when lane formation is suppressed.Comment: 25 pages, 17 figures, 2 table
ATPγS stalls splicing after B complex formation but prior to spliceosome activation.
The ATP analog ATPγS inhibits pre-mRNA splicing in vitro, but there have been conflicting reports as to which step of splicing is inhibited by this small molecule and its inhibitory mechanism remains unclear. Here we have dissected the effect of ATPγS on pre-mRNA splicing in vitro. Addition of ATPγS to splicing extracts depleted of ATP inhibited both catalytic steps of splicing. At ATPγS concentrations ≥0.5 mM, precatalytic B complexes accumulate, demonstrating a block prior to or during the spliceosome activation stage. Affinity purification of the ATPγS-stalled B complexes (B(ATPγS)) and subsequent characterization of their abundant protein components by 2D gel electrophoresis revealed that B(ATPγS) complexes are compositionally more homogeneous than B complexes previously isolated in the presence of ATP. In particular, they contain little or no Prp19/CDC5L complex proteins, indicating that these proteins are recruited after assembly of the precatalytic spliceosome. Under the electron microscope, B(ATPγS) complexes exhibit a morphology highly similar to B complexes, indicating that the ATPγS-induced block in the transformation of the B to B(act) complex is not due to a major structural defect. Likely mechanisms whereby ATPγS blocks spliceosome assembly at the activation stage, including inhibition of the RNA helicase Brr2, are discussed. Given their more homogeneous composition, B complexes stalled by ATPγS may prove highly useful for both functional and structural analyses of the precatalytic spliceosome and its conversion into an activated B(act) spliceosomal complex
Spontaneous symmetry breaking in a two-lane model for bidirectional overtaking traffic
First we consider a unidirectional flux \omega_bar of vehicles each of which
is characterized by its `natural' velocity v drawn from a distribution P(v).
The traffic flow is modeled as a collection of straight `world lines' in the
time-space plane, with overtaking events represented by a fixed queuing time
tau imposed on the overtaking vehicle. This geometrical model exhibits platoon
formation and allows, among many other things, for the calculation of the
effective average velocity w=\phi(v) of a vehicle of natural velocity v.
Secondly, we extend the model to two opposite lanes, A and B. We argue that the
queuing time \tau in one lane is determined by the traffic density in the
opposite lane. On the basis of reasonable additional assumptions we establish a
set of equations that couple the two lanes and can be solved numerically. It
appears that above a critical value \omega_bar_c of the control parameter
\omega_bar the symmetry between the lanes is spontaneously broken: there is a
slow lane where long platoons form behind the slowest vehicles, and a fast lane
where overtaking is easy due to the wide spacing between the platoons in the
opposite direction. A variant of the model is studied in which the spatial
vehicle density \rho_bar rather than the flux \omega_bar is the control
parameter. Unequal fluxes \omega_bar_A and \omega_bar_B in the two lanes are
also considered. The symmetry breaking phenomenon exhibited by this model, even
though no doubt hard to observe in pure form in real-life traffic, nevertheless
indicates a tendency of such traffic.Comment: 50 pages, 16 figures; extra references adde
The critical exponents of the two-dimensional Ising spin glass revisited: Exact Ground State Calculations and Monte Carlo Simulations
The critical exponents for of the two-dimensional Ising spin glass
model with Gaussian couplings are determined with the help of exact ground
states for system sizes up to and by a Monte Carlo study of a
pseudo-ferromagnetic order parameter. We obtain: for the stiffness exponent
, for the magnetic exponent
and for the chaos exponent . From Monte Carlo simulations we
get the thermal exponent . The scaling prediction is
fulfilled within the error bars, whereas there is a disagreement with the
relation .Comment: 8 pages RevTeX, 7 eps-figures include
Towards a realistic microscopic description of highway traffic
Simple cellular automata models are able to reproduce the basic properties of
highway traffic. The comparison with empirical data for microscopic quantities
requires a more detailed description of the elementary dynamics. Based on
existing cellular automata models we propose an improved discrete model
incorporating anticipation effects, reduced acceleration capabilities and an
enhanced interaction horizon for braking. The modified model is able to
reproduce the three phases (free-flow, synchronized, and stop-and-go) observed
in real traffic. Furthermore we find a good agreement with detailed empirical
single-vehicle data in all phases.Comment: 7 pages, 7 figure
An empirical test for cellular automaton models of traffic flow
Based on a detailed microscopic test scenario motivated by recent empirical
studies of single-vehicle data, several cellular automaton models for traffic
flow are compared. We find three levels of agreement with the empirical data:
1) models that do not reproduce even qualitatively the most important empirical
observations,
2) models that are on a macroscopic level in reasonable agreement with the
empirics, and 3) models that reproduce the empirical data on a microscopic
level as well.
Our results are not only relevant for applications, but also shed new light
on the relevant interactions in traffic flow.Comment: 28 pages, 36 figures, accepted for publication in PR
Phase diagrams of in Double Exchange Model with added antiferromagnetic and Jahn-Teller interaction
The phase diagram of the multivalent manganites , in
space of temperature and doping , is a challenge for the theoretical
physics. It is an important test for the model used to study these compounds
and the method of calculation. To obtain theoretically this diagram for
, we consider the two-band Double Exchange Model for manganites with
added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to
calculate Curie and N\'{e}el temperatures we derive an effective Heisenberg
model for a vector which describes the local orientation of the total
magnetization of the system. The exchange constants of this model are different
for different space directions and depend on the density of electrons,
antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well
known phase transitions from A-type antiferromagnetism to ferromagnetism at low
and C-type antiferromagnetism to G-type antiferromagnetism at large , we
argue that the antiferromagnetic exchange constants should depend on the
lattice direction. We show that ferromagnetic to A-type antiferromagnetic
transition results from the Jahn-Teller distortion. Accounting adequately for
the magnon-magnon interaction, Curie and N\'{e}el temperatures are calculated.
The results are in very good agreement with the experiment and provide values
for the model parameters, which best describe the behavior of the critical
temperature for .Comment: 13 pages, 5 figure
Manipulation of drugs to achieve the required dose is intrinsic to paediatric practice but is not supported by guidelines or evidence
Background: A lack of age-appropriate formulations can make it difficult to administer medicines to children. A manipulation of the dosage form may be required to achieve the required dose. This study aimed to describe medicines that are manipulated to achieve the required dose in paediatric practice.Method: A structured, undisguised observational study and postal survey. The observational study investigated drug manipulations occurring in clinical practice across three sites. The questionnaire, administered to a sample of paediatric nurses throughout the UK, surveyed manipulations conducted and nurses' experiences and views.Results: The observational study identified 310 manipulations, of which 62% involved tablets, 21% were intravenous drugs and 10% were sachets. Of the 54 observed manipulations 40 involved tablets with 65% of the tablets being cut and 30% dispersed to obtain a smaller dose. 188 manipulations were reported by questionnaire respondents, of these 46% involved tablets, 12% were intravenous drugs, and 12% were nebuliser solutions. Manipulations were predominantly, but not exclusively, identified in specialist clinical areas with more highly dependent patients. Questionnaire respondents were concerned about the accuracy of the dose achieved following manipulations and the lack of practice guidance.Conclusion: Manipulations to achieve the required dose occur throughout paediatric in-patient settings. The impact of manipulations on the efficacy of the drugs, the accuracy of the dose and any adverse effects on patients is not known. There is a need to develop evidence-based guidance for manipulations of medicines in children
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