161 research outputs found
Conformational mechanism for the stability of microtubule-kinetochore attachments
Regulating the stability of microtubule(MT)-kinetochore attachments is
fundamental to avoiding mitotic errors and ensure proper chromosome segregation
during cell division. While biochemical factors involved in this process have
been identified, its mechanics still needs to be better understood. Here we
introduce and simulate a mechanical model of MT-kinetochore interactions in
which the stability of the attachment is ruled by the geometrical conformations
of curling MT-protofilaments entangled in kinetochore fibrils. The model allows
us to reproduce with good accuracy in vitro experimental measurements of the
detachment times of yeast kinetochores from MTs under external pulling forces.
Numerical simulations suggest that geometrical features of MT-protofilaments
may play an important role in the switch between stable and unstable
attachments
Ensemble Inequivalence and the Spin-Glass Transition
We report on the ensemble inequivalence in a many-body spin-glass model with
integer spin. The spin-glass phase transition is of first order for certain
values of the crystal field strength and is dependent whether it was derived in
the microcanonical or the canonical ensemble. In the limit of infinitely
many-body interactions, the model is the integer-spin equivalent of the
random-energy model, and is solved exactly. We also derive the integer-spin
equivalent of the de Almeida-Thouless line.Comment: 19 pages, 7 figure
MECHANISM OF CATIONIC LACTAM POLYMERIZATION
The authors suggested a new mechanism for the interpretation of cationic lactam
polymerization according to which in the chain propagation reaction. through the appropriate
intermediates. compounds belonging to various polymer homologous series are formed and
additional polymerization processes are superimposed onto the original ones. On the basis of the
new mechanism a kinetic model has been developed by the computer simulation of which rate
and equilibrium constants could be determined. The latter enabled. a good quantitative
description of polymerization
Ensemble Inequivalence in the Spherical Spin Glass Model with Nonlinear Interactions
We investigate the ensemble inequivalence of the spherical spin glass model
with nonlinear interactions of polynomial order . This model is solved
exactly for arbitrary and is shown to have first-order phase transitions
between the paramagnetic and spin glass or ferromagnetic phases for .
In the parameter region around the first-order transitions, the solutions give
different results depending on the ensemble used for the analysis. In
particular, we observe that the microcanonical specific heat can be negative
and the phase may not be uniquely determined by the temperature.Comment: 15 pages, 10 figure
An Emergent Space for Distributed Data with Hidden Internal Order through Manifold Learning
Manifold-learning techniques are routinely used in mining complex
spatiotemporal data to extract useful, parsimonious data
representations/parametrizations; these are, in turn, useful in nonlinear model
identification tasks. We focus here on the case of time series data that can
ultimately be modelled as a spatially distributed system (e.g. a partial
differential equation, PDE), but where we do not know the space in which this
PDE should be formulated. Hence, even the spatial coordinates for the
distributed system themselves need to be identified - to emerge from - the data
mining process. We will first validate this emergent space reconstruction for
time series sampled without space labels in known PDEs; this brings up the
issue of observability of physical space from temporal observation data, and
the transition from spatially resolved to lumped (order-parameter-based)
representations by tuning the scale of the data mining kernels. We will then
present actual emergent space discovery illustrations. Our illustrative
examples include chimera states (states of coexisting coherent and incoherent
dynamics), and chaotic as well as quasiperiodic spatiotemporal dynamics,
arising in partial differential equations and/or in heterogeneous networks. We
also discuss how data-driven spatial coordinates can be extracted in ways
invariant to the nature of the measuring instrument. Such gauge-invariant data
mining can go beyond the fusion of heterogeneous observations of the same
system, to the possible matching of apparently different systems
Ensemble equivalence in spin systems with short-range interactions
We study the problem of ensemble equivalence in spin systems with short-range
interactions under the existence of a first-order phase transition. The
spherical model with nonlinear nearest-neighbour interactions is solved exactly
both for canonical and microcanonical ensembles. The result reveals apparent
ensemble inequivalence at the first-order transition point in the sense that
the microcanonical entropy is non-concave as a function of the energy and
consequently the specific heat is negative. In order to resolve the paradox, we
show that an unconventional saddle point should be chosen in the microcanonical
calculation that represents a phase separation. The XY model with non-linear
interactions is also studied by microcanonical Monte Carlo simulations in two
dimensions to see how this model behaves in comparison with the spherical
model.Comment: 17 pages, 19 figures, revised versio
Quantum dot-based multiphoton fluorescent pipettes for targeted neuronal electrophysiology
Targeting visually identified neurons for electrophysiological recording is a fundamental neuroscience technique; however, its potential is hampered by poor visualization of pipette tips in deep brain tissue. We describe quantum dot-coated glass pipettes that provide strong two-photon contrast at deeper penetration depths than those achievable with current methods. We demonstrated the pipettes' utility in targeted patch-clamp recording experiments and single-cell electroporation of identified rat and mouse neurons in vitro and in vivo
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