1,373 research outputs found
Mean First Passage Time in Periodic Attractors
The properties of the mean first passage time in a system characterized by
multiple periodic attractors are studied. Using a transformation from a high
dimensional space to 1D, the problem is reduced to a stochastic process along
the path from the fixed point attractor to a saddle point located between two
neighboring attractors. It is found that the time to switch between attractors
depends on the effective size of the attractors, , the noise, ,
and the potential difference between the attractor and an adjacent saddle point
as: ; the
ratio between the sizes of the two attractors affects . The
result is obtained analytically for small and confirmed by numerical
simulations. Possible implications that may arise from the model and results
are discussed.Comment: 14 pages, 3 figures, submitted to journal of physics
Monte Carlo simulations of pulse propagation in massive multichannel optical fiber communication systems
We study the combined effect of delayed Raman response and bit pattern
randomness on pulse propagation in massive multichannel optical fiber
communication systems. The propagation is described by a perturbed stochastic
nonlinear Schr\"odinger equation, which takes into account changes in pulse
amplitude and frequency as well as emission of continuous radiation. We perform
extensive numerical simulations with the model, and analyze the dynamics of the
frequency moments, the bit-error-rate, and the mutual distribution of amplitude
and position. The results of our numerical simulations are in good agreement
with theoretical predictions based on the adiabatic perturbation approach.Comment: Submitted to Physical Review E. 8 pages, 5 figure
Mechanics and dynamics of X-chromosome pairing at X inactivation
At the onset of X-chromosome inactivation, the vital process whereby female mammalian cells equalize X products with
respect to males, the X chromosomes are colocalized along their Xic (X-inactivation center) regions. The mechanism
inducing recognition and pairing of the Xâs remains, though, elusive. Starting from recent discoveries on the molecular
factors and on the DNA sequences (the so-called "pairing sites") involved, we dissect the mechanical basis of Xic
colocalization by using a statistical physics model. We show that soluble DNA-specific binding molecules, such as those
experimentally identified, can be indeed sufficient to induce the spontaneous colocalization of the homologous
chromosomes but only when their concentration, or chemical affinity, rises above a threshold value as a consequence of a
thermodynamic phase transition. We derive the likelihood of pairing and its probability distribution. Chromosome dynamics
has two stages: an initial independent Brownian diffusion followed, after a characteristic time scale, by recognition and
pairing. Finally, we investigate the effects of DNA deletion/insertions in the region of pairing sites and compare model
predictions to available experimental data
A long noncoding RNA influences the choice of the X chromosome to be inactivated
X chromosome inactivation (XCI) is the process of silencing one of the X chromosomes in cells of the female mammal which ensures dosage compensation between the sexes. Although theoretically random in somatic tissues, the choice of which X chromosome is chosen to be inactivated can be biased in mice by genetic element(s) associated with the so-called X-controlling element (Xce). Although the Xce was first described and genetically localized nearly 40 y ago, its mode of action remains elusive. In the approach presented here, we identify a single long noncoding RNA (lncRNA) within the Xce locus, Lppnx, which may be the driving factor in the choice of which X chromosome will be inactivated in the developing female mouse embryo. Comparing weak and strong Xce alleles we show that Lppnx modulates the expression of Xist lncRNA, one of the key factors in XCI, by controlling the occupancy of pluripotency factors at Intron1 of Xist. This effect is counteracted by enhanced binding of Rex1 in DxPas34, another key element in XCI regulating the activity of Tsix lncRNA, the main antagonist of Xist, in the strong but not in the weak Xce allele. These results suggest that the different susceptibility for XCI observed in weak and strong Xce alleles results from differential transcription factor binding of Xist Intron 1 and DxPas34, and that Lppnx represents a decisive factor in explaining the action of the Xce
First step towards the new SPIRAL2 project control system
International audienceThe Spiral2 project at Ganil aims to produce rare ion beams using a Uranium carbide target fission process. The accelerator consists of a RFQ followed by a superconducting cavity linac and is designed to provide high intensity primary beams (deuterons, protons or heavy ions). The accelerator should be commissioned by the end of 2011; then, the first tests aiming to produce exotic beams are planned one year later. The control system will result of the collaboration between several institutes among which the Saclay Dapnia division yet having a good experience and knowledge with Epics. So and also because of its widely used functionalities, Epics has been chosen as the basic framework for the accelerator control and people from the other laboratories belonging to the collaboration are progressively acquiring a first experience with Epics. The paper first explains the organisation of the collaboration then it describes the basic hardware and software choices for the project. Some preliminary implementations are therefore given. As the project is still in its beginning phase, the paper ends by listing some interrogations not yet solved for the control system definition and opened for discussion
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