11,612 research outputs found
Stochastic kinetics of ribosomes: single motor properties and collective behavior
Synthesis of protein molecules in a cell are carried out by ribosomes. A
ribosome can be regarded as a molecular motor which utilizes the input chemical
energy to move on a messenger RNA (mRNA) track that also serves as a template
for the polymerization of the corresponding protein. The forward movement,
however, is characterized by an alternating sequence of translocation and
pause. Using a quantitative model, which captures the mechanochemical cycle of
an individual ribosome, we derive an {\it exact} analytical expression for the
distribution of its dwell times at the successive positions on the mRNA track.
Inverse of the average dwell time satisfies a ``Michaelis-Menten-like''
equation and is consistent with the general formula for the average velocity of
a molecular motor with an unbranched mechano-chemical cycle. Extending this
formula appropriately, we also derive the exact force-velocity relation for a
ribosome. Often many ribosomes simultaneously move on the same mRNA track,
while each synthesizes a copy of the same protein. We extend the model of a
single ribosome by incorporating steric exclusion of different individuals on
the same track. We draw the phase diagram of this model of ribosome traffic in
3-dimensional spaces spanned by experimentally controllable parameters. We
suggest new experimental tests of our theoretical predictions.Comment: Final published versio
Distribution of dwell times of a ribosome: effects of infidelity, kinetic proofreading and ribosome crowding
Ribosome is a molecular machine that polymerizes a protein where the sequence
of the amino acid residues, the monomers of the protein, is dictated by the
sequence of codons (triplets of nucleotides) on a messenger RNA (mRNA) that
serves as the template. The ribosome is a molecular motor that utilizes the
template mRNA strand also as the track. Thus, in each step the ribosome moves
forward by one codon and, simultaneously, elongates the protein by one amino
acid. We present a theoretical model that captures most of the main steps in
the mechano-chemical cycle of a ribosome. The stochastic movement of the
ribosome consists of an alternating sequence of pause and translocation; the
sum of the durations of a pause and the following translocation is the time of
dwell of the ribosome at the corresponding codon. We derive the analytical
expression for the distribution of the dwell times of a ribosome in our model.
Whereever experimental data are available, our theoretical predictions are
consistent with those results. We suggest appropriate experiments to test the
new predictions of our model, particularly, the effects of the quality control
mechanism of the ribosome and that of their crowding on the mRNA track.Comment: 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 it. The definitive publisher authenticated version
is available online at DOI:10.1088/1478-3975/8/2/02600
Instability of dilute granular flow on rough slope
We study numerically the stability of granular flow on a rough slope in
collisional flow regime in the two-dimension. We examine the density dependence
of the flowing behavior in low density region, and demonstrate that the
particle collisions stabilize the flow above a certain density in the parameter
region where a single particle shows an accelerated behavior. Within this
parameter regime, however, the uniform flow is only metastable and is shown to
be unstable against clustering when the particle density is not high enough.Comment: 4 pages, 6 figures, submitted to J. Phys. Soc. Jpn.; Fig. 2 replaced;
references added; comments added; misprints correcte
Probing the superconducting ground state of the noncentrosymmetric superconductors CaTSi3 (T = Ir, Pt) using muon-spin relaxation and rotation
The superconducting properties of CaTSi3 (where T = Pt and Ir) have been
investigated using muon spectroscopy. Our muon-spin relaxation results suggest
that in both these superconductors time-reversal symmetry is preserved, while
muon-spin rotation data show that the temperature dependence of the superfluid
density is consistent with an isotropic s-wave gap. The magnetic penetration
depths and upper critical fields determined from our transverse-field muon-spin
rotation spectra are found to be 448(6) and 170(6) nm, and 3800(500) and
1700(300) G, for CaPtSi3 and CaIrSi3 respectively. The superconducting
coherence lengths of the two materials have also been determined and are 29(2)
nm for CaPtSi3 and 44(4) nm for CaIrSi3.Comment: 6 pages, 7 figure
Anomalous transport in disordered exclusion processes with coupled particles
We consider one-dimensional asymmetric exclusion processes with a simple
attractive interaction, where the distance between consecutive particles is not
allowed to exceed a certain limit and investigate the consequences of this
coupling on the transport properties in the presence of random-force type
disorder by means of a phenomenological random trap picture. In the
phase-separated steady state of the model defined on a finite ring, the
properties of the density profile are studied and the exponent governing the
decay of the current with the system size in the biased phase is derived. In
case all consecutive particles are coupled with each other and form a closed
string, the current is found to be enhanced compared to the model without
coupling, while if groups of consecutive particles form finite strings, the
current is reduced. The motion of a semi-infinite string entering an initially
empty lattice is also studied. Here, the diffusion of the head of the string is
found to be anomalous, and two phases can be distinguished, which are
characterised by different functional dependences of the diffusion exponent on
the bias. The obtained results are checked by numerical simulation.Comment: 20 pages, 11 figure
Two-dimensional Burgers Cellular Automaton
A two-dimensional cellular automaton(CA) associated with a two-dimensional
Burgers equation is presented. The 2D Burgers equation is an integrable
generalization of the well-known Burgers equation, and is transformed into a 2D
diffusion equation by the Cole-Hopf transformation. The CA is derived from the
2D Burgers equation by using the ultradiscrete method, which can transform
dependent variables into discrete ones. Some exact solutions of the CA, such as
shock wave solutions, are studied in detail.Comment: Latex2.09, 17 pages including 7 figure
Asymmetric exclusion process with next-nearest-neighbor interaction: some comments on traffic flow and a nonequilibrium reentrance transition
We study the steady-state behavior of a driven non-equilibrium lattice gas of
hard-core particles with next-nearest-neighbor interaction. We calculate the
exact stationary distribution of the periodic system and for a particular line
in the phase diagram of the system with open boundaries where particles can
enter and leave the system. For repulsive interactions the dynamics can be
interpreted as a two-speed model for traffic flow. The exact stationary
distribution of the periodic continuous-time system turns out to coincide with
that of the asymmetric exclusion process (ASEP) with discrete-time parallel
update. However, unlike in the (single-speed) ASEP, the exact flow diagram for
the two-speed model resembles in some important features the flow diagram of
real traffic. The stationary phase diagram of the open system obtained from
Monte Carlo simulations can be understood in terms of a shock moving through
the system and an overfeeding effect at the boundaries, thus confirming
theoretical predictions of a recently developed general theory of
boundary-induced phase transitions. In the case of attractive interaction we
observe an unexpected reentrance transition due to boundary effects.Comment: 12 pages, Revtex, 7 figure
Weakly coupled, antiparallel, totally asymmetric simple exclusion processes
We study a system composed of two parallel totally asymmetric simple
exclusion processes with open boundaries, where the particles move in the two
lanes in opposite directions and are allowed to jump to the other lane with
rates inversely proportional to the length of the system. Stationary density
profiles are determined and the phase diagram of the model is constructed in
the hydrodynamic limit, by solving the differential equations describing the
steady state of the system, analytically for vanishing total current and
numerically for nonzero total current. The system possesses phases with a
localized shock in the density profile in one of the lanes, similarly to
exclusion processes endowed with nonconserving kinetics in the bulk. Besides,
the system undergoes a discontinuous phase transition, where coherently moving
delocalized shocks emerge in both lanes and the fluctuation of the global
density is described by an unbiased random walk. This phenomenon is analogous
to the phase coexistence observed at the coexistence line of the totally
asymmetric simple exclusion process, however, as a consequence of the
interaction between lanes, the density profiles are deformed and in the case of
asymmetric lane change, the motion of the shocks is confined to a limited
domain.Comment: 14 pages, 15 figures, to appear in Phys. Rev.
Micellar Aggregates of Gemini Surfactants: Monte Carlo Simulation of a Microscopic Model
We propose a "microscopic" model of gemini surfactants in aqueous solution.
Carrying out extensive Monte Carlo simulations, we study the variation of the
critical micellar concentration (CMC) of these model gemini surfactants with
the variation of the (a) length of the spacer connecting the two hydrophilic
heads, (b) length of the hydrophobic tail and (c) the bending rigidity of the
hydrocarbon chains forming the spacer and the tail; some of the trends of
variation are counter-intuitive but are in excellent agreement with the
available experimental results. Our simulations also elucidate the dependence
of the shapes of the micellar aggregates and the magnitude of the CMC on the
geometrical shape and size of the surfactant molecules and the electrical
charge on the hydrophilic heads
Extinction of the N=20 neutron-shell closure for 32Mg examined by direct mass measurements
The 'island of inversion' around Mg is one of the most important
paradigm for studying the disappearance of the stabilizing 'magic' of a shell
closure. We present the first Penning-trap mass measurements of the exotic
nuclides Na and Mg, which allow a precise determination of
the empirical shell gap for Mg. The new value of 1.10(3) MeV is the
lowest observed shell gap for any nuclide with a canonical magic number.Comment: 6 pages, 4 figures, submitted to Physical Review
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