112 research outputs found
Hard-sphere limit of soft-sphere model for granular materials: Stiffness dependence of steady granular flow
Dynamical behavior of steady granular flow is investigated numerically in the
inelastic hard sphere limit of the soft sphere model. We find distinctively
different limiting behaviors for the two flow regimes, i.e., the collisional
flow and the frictional flow. In the collisional flow, the hard sphere limit is
straightforward; the number of collisions per particle per unit time converges
to a finite value and the total contact time fraction with other particles goes
to zero. For the frictional flow, however, we demonstrate that the collision
rate diverges as the power of the particle stiffness so that the time fraction
of the multiple contacts remains finite even in the hard sphere limit although
the contact time fraction for the binary collisions tends to zero.Comment: 10 figures. Typos removed. to appear in Phys. Rev.
Linear stability analysis of rapid granular flow on a slope and density wave formation
The linear stability of rapid granular flow on a slope under gravity against
the longitudinal perturbation is analyzed using hydrodynamic equations. It is
demonstrated that the steady flow uniform along the flow direction becomes
unstable against the long-wavelength perturbations longitudinal to the flow
direction for certain parameter ranges to form the density wave, in contrast
with the finite wavelength instability against the transverse perturbation
(Forterre & Pouliquen 2002). It is shown that the instability can be understood
as the the long-wave instability of the kinematic waves in a quasi-one
dimensional system. The results are compared with our previous molecular
dynamics simulations (Mitarai & Nakanishi 2001), where the spontaneous density
wave formation has been found.Comment: 25 pages, 15 figures, submitted to J. Fluid Mech.; larger parameter
range investigated, discussions revise
Control of ribosome traffic by position-dependent choice of synonymous codons
Messenger RNA encodes a sequence of amino acids by using codons. For most
amino acids there are multiple synonymous codons that can encode the amino
acid. The translation speed can vary from one codon to another, thus there is
room for changing the ribosome speed while keeping the amino acid sequence and
hence the resulting protein. Recently, it has been noticed that the choice of
the synonymous codon, via the resulting distribution of slow- and
fast-translated codons, affects not only on the average speed of one ribosome
translating the messenger RNA (mRNA) but also might have an effect on nearby
ribosomes by affecting the appearance of "traffic jams" where multiple
ribosomes collide and form queues. To test this "context effect" further, we
here investigate the effect of the sequence of synonymous codons on the
ribosome traffic by using a ribosome traffic model with codon-dependent rates,
estimated from experiments. We compare the ribosome traffic on wild type
sequences and sequences where the synonymous codons were swapped randomly. By
simulating translation of 87 genes, we demonstrate that the wild type
sequences, especially those with a high bias in codon usage, tend to have the
ability to reduce ribosome collisions, hence optimizing the cellular investment
in the translation apparatus. The magnitude of such reduction of the
translation time might have a significant impact on the cellular growth rate
and thereby have importance for the survival of the species.Comment: 12 pages, 6 Figures. This is an author-created, un-copyedited version
of an article accepted for publication in Physical Biology. IOP Publishing
Ltd is not responsible for any errors or omissions in this version of the
manuscript or any version derived from i
Disturbance accelerates the transition from low- to high- diversity state in a model ecosystem
The effect of disturbance on a model ecosystem of sessile and mutually
competitive species [Mathiesen et al. Phys. Rev. Lett. 107, 188101 (2011);
Mitarai et al. Phys. Rev. E 86, 011929 (2012) ] is studied. The disturbance
stochastically removes individuals from the system, and the created empty sites
are re-colonized by neighbouring species. We show that the stable
high-diversity state, maintained by occasional cyclic species interactions that
create isolated patches of meta-populations, is robust against small
disturbance. We further demonstrate that finite disturbance can accelerate the
transition from the low- to high-diversity state by helping creation of small
patches through diffusion of boundaries between species with stand-off
relation.Comment: 8 pages, 8 figures. Typos correcte
Dynamic competition between transcription initiation and repression: Role of nonequilibrium steps in cell-to-cell heterogeneity
Transcriptional repression may cause transcriptional noise by a competition
between repressor and RNA polymerase binding. Although promoter activity is
often governed by a single limiting step, we argue here that the size of the
noise strongly depends on whether this step is the initial equilibrium binding
or one of the subsequent unidirectional steps. Overall, we show that
nonequilibrium steps of transcription initiation systematically increase the
cell-to-cell heterogeneity in bacterial populations. In particular, this allows
also weak promoters to give substantial transcriptional noise.Comment: 5 pages, 3 fiugres. Figure and text update
Ribosome collisions and Translation efficiency: Optimization by codon usage and mRNA destabilization
Individual mRNAs are translated by multiple ribosomes that initiate
translation with a few seconds interval. The ribosome speed is codon dependant,
and ribosome queuing has been suggested to explain specific data for
translation of some mRNAs in vivo. By modelling the stochastic translation
process as a traffic problem, we here analyze conditions and consequences of
collisions and queuing. The model allowed us to determine the on-rate (0.8 to
1.1 initiations per sec) and the time (1 sec) the preceding ribosome occludes
initiation for Escherichia coli lacZ mRNA in vivo. We find that ribosome
collisions and queues are inevitable consequences of a stochastic translation
mechanism that reduce the translation efficiency substantially on natural
mRNAs. The cells minimize collisions by having its mRNAs being unstable and by
a highly selected codon usage in the start of the mRNA. The cost of mRNA
breakdown is offset by the concomitant increase in translational efficiency.Comment: 5 figures, 3 table
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