1,085 research outputs found
Statistical Physics of Self-Replication
Self-replication is a capacity common to every species of living thing, and
simple physical intuition dictates that such a process must invariably be
fueled by the production of entropy. Here, we undertake to make this intuition
rigorous and quantitative by deriving a lower bound for the amount of heat that
is produced during a process of self-replication in a system coupled to a
thermal bath. We find that the minimum value for the physically allowed rate of
heat production is determined by the growth rate, internal entropy, and
durability of the replicator, and we discuss the implications of this finding
for bacterial cell division, as well as for the pre-biotic emergence of
self-replicating nucleic acids.Comment: 4+ pages, 1 figur
Mean field approaches to the totally asymmetric exclusion process with quenched disorder and large particles
The process of protein synthesis in biological systems resembles a one
dimensional driven lattice gas in which the particles (ribosomes) have spatial
extent, covering more than one lattice site. Realistic, nonuniform gene
sequences lead to quenched disorder in the particle hopping rates. We study the
totally asymmetric exclusion process with large particles and quenched disorder
via several mean field approaches and compare the mean field results with Monte
Carlo simulations. Mean field equations obtained from the literature are found
to be reasonably effective in describing this system. A numerical technique is
developed for computing the particle current rapidly. The mean field approach
is extended to include two-point correlations between adjacent sites. The
two-point results are found to match Monte Carlo simulations more closely
Sufficient conditions for the anti-Zeno effect
The ideal anti-Zeno effect means that a perpetual observation leads to an
immediate disappearance of the unstable system. We present a straightforward
way to derive sufficient conditions under which such a situation occurs
expressed in terms of the decaying states and spectral properties of the
Hamiltonian. They show, in particular, that the gap between Zeno and anti-Zeno
effects is in fact very narrow.Comment: LatEx2e, 9 pages; a revised text, to appear in J. Phys. A: Math. Ge
Evanescent channels and scattering in cylindrical nanowire heterostructures
We investigate the scattering phenomena produced by a general finite-range
nonseparable potential in a multi-channel two-probe cylindrical nanowire
heterostructure. The multi-channel current scattering matrix is efficiently
computed using the R-matrix formalism extended for cylindrical coordinates.
Considering the contribution of the evanescent channels to the scattering
matrix, we are able to put in evidence the specific dips in the tunneling
coefficient in the case of an attractive potential. The cylindrical symmetry
cancels the "selection rules" known for Cartesian coordinates. If the
attractive potential is superposed over a non-uniform potential along the
nanowire, then resonant transmission peaks appear. We can characterize them
quantitatively through the poles of the current scattering matrix. Detailed
maps of the localization probability density sustain the physical
interpretation of the resonances (dips and peaks). Our formalism is applied to
a variety of model systems like a quantum dot, a core/shell quantum ring or a
double-barrier, embedded into the nanocylinder.Comment: RevTeX, 16 pages, 19 figures, 2 table
First proof of concept of remote attendance for future observation strategies between Wettzell (Germany) and ConcepciĂłn (Chile)
Current VLBI observations are controlled and attended locally at the radio telescopes on the basis of pre-scheduled session files. Operations have to deal with system specific station commands and individual setup procedures. Neither the scheduler nor the correlator nor the data-analyst gets real-time feedback about system parameters during a session. Changes in schedules after the start of a session by remote are impossible or at least quite difficult. For future scientific approaches, a more flexible mechanism would optimize the usage of resources at the sites. Therefore shared-observation control between world-wide telescope s, remote attendance/control as well as completely unattended-observations could be useful, in addition to the classic way to run VLBI observations. To reach these goals, the Geodetic Observatory Wettzell in cooperation with the Max-Planck-Institute for Radio Astronomy (Bonn) have developed a software extension to the existing NASA Field System for remote control. It uses the principle of a remotely accessible, autonomous process cell as server extension to the Field System on the basis of Remote Procedure Calls (RPC). Based on this technology the first completely remote attended and controlled geodetic VLBI session between Wettzell, Germany and ConcepciĂłn, Chile was successfully performed over 24 hours. This first test was extremely valuable for gathering information about the differences between VLBI systems and measuring the performance of internet connections and automatic connection re-establishments. During the 24h-session, the network load, the number of sent/received packages and the transfer speed were monitor ed and captured. It was a first reliable test for the future wishes to control several telescopes with one graphical user interface on different data transfer rates over large distances in an efficient way. In addition, future developments for an authentication and user role management will be realized within the upcoming NEXPReS project
Instabilities in complex mixtures with a large number of components
Inside living cells are complex mixtures of thousands of components. It is
hopeless to try to characterise all the individual interactions in these
mixtures. Thus, we develop a statistical approach to approximating them, and
examine the conditions under which the mixtures phase separate. The approach
approximates the matrix of second virial coefficients of the mixture by a
random matrix, and determines the stability of the mixture from the spectrum of
such random matrices.Comment: 4 pages, uses RevTeX 4.
Unstructured regions in IRE1α specify BiP-mediated destabilisation of the luminal domain dimer and repression of the UPR
Funder: Medical Research Council; FundRef: http://dx.doi.org/10.13039/501100000265Funder: European Molecular Biology Organization; FundRef: http://dx.doi.org/10.13039/100004410Coupling of endoplasmic reticulum (ER) stress to dimerisation-dependent activation of the UPR transducer IRE1 is incompletely understood. Whilst the luminal co-chaperone ERdj4 promotes a complex between the Hsp70 BiP and IRE1’s stress-sensing luminal domain (IRE1LD) that favours the latter’s monomeric inactive state and loss of ERdj4 de-represses IRE1, evidence linking these cellular and in vitro observations is presently lacking. We report that enforced loading of endogenous BiP onto endogenous IRE1α repressed UPR signalling in CHO cells and deletions in the IRE1α locus that de-repressed the UPR in cells, encode flexible regions of IRE1LD that mediated BiP-induced monomerisation in vitro. Changes in the hydrogen exchange mass spectrometry profile of IRE1LD induced by ERdj4 and BiP confirmed monomerisation and were consistent with active destabilisation of the IRE1LD dimer. Together, these observations support a competition model whereby waning ER stress passively partitions ERdj4 and BiP to IRE1LD to initiate active repression of UPR signalling
The cytoplasm of living cells: A functional mixture of thousands of components
Inside every living cell is the cytoplasm: a fluid mixture of thousands of
different macromolecules, predominantly proteins. This mixture is where most of
the biochemistry occurs that enables living cells to function, and it is
perhaps the most complex liquid on earth. Here we take an inventory of what is
actually in this mixture. Recent genome-sequencing work has given us for the
first time at least some information on all of these thousands of components.
Having done so we consider two physical phenomena in the cytoplasm: diffusion
and possible phase separation. Diffusion is slower in the highly crowded
cytoplasm than in dilute solution. Reasonable estimates of this slowdown can be
obtained and their consequences explored, for example, monomer-dimer equilibria
are established approximately twenty times slower than in a dilute solution.
Phase separation in all except exceptional cells appears not to be a problem,
despite the high density and so strong protein-protein interactions present. We
suggest that this may be partially a byproduct of the evolution of other
properties, and partially a result of the huge number of components present.Comment: 11 pages, 1 figure, 1 tabl
Unstructured regions in IRE1α specify BiP-mediated destabilisation of the luminal domain dimer and repression of the UPR
Coupling of endoplasmic reticulum stress to dimerisation‑dependent activation of the UPR transducer IRE1 is incompletely understood. Whilst the luminal co-chaperone ERdj4 promotes a complex between the Hsp70 BiP and IRE1's stress-sensing luminal domain (IRE1LD) that favours the latter's monomeric inactive state and loss of ERdj4 de-represses IRE1, evidence linking these cellular and in vitro observations is presently lacking. We report that enforced loading of endogenous BiP onto endogenous IRE1α repressed UPR signalling in CHO cells and deletions in the IRE1α locus that de-repressed the UPR in cells, encode flexible regions of IRE1LD that mediated BiP‑induced monomerisation in vitro. Changes in the hydrogen exchange mass spectrometry profile of IRE1LD induced by ERdj4 and BiP confirmed monomerisation and were consistent with active destabilisation of the IRE1LD dimer. Together, these observations support a competition model whereby waning ER stress passively partitions ERdj4 and BiP to IRE1LD to initiate active repression of UPR signalling
A Kohn-Sham system at zero temperature
An one-dimensional Kohn-Sham system for spin particles is considered which
effectively describes semiconductor {nano}structures and which is investigated
at zero temperature. We prove the existence of solutions and derive a priori
estimates. For this purpose we find estimates for eigenvalues of the
Schr\"odinger operator with effective Kohn-Sham potential and obtain
-bounds of the associated particle density operator. Afterwards,
compactness and continuity results allow to apply Schauder's fixed point
theorem. In case of vanishing exchange-correlation potential uniqueness is
shown by monotonicity arguments. Finally, we investigate the behavior of the
system if the temperature approaches zero.Comment: 27 page
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