39,036 research outputs found
Helix untwisting and bubble formation in circular DNA
The base pair fluctuations and helix untwisting are examined for a circular
molecule. A realistic mesoscopic model including twisting degrees of freedom
and bending of the molecular axis is proposed. The computational method, based
on path integral techniques, simulates a distribution of topoisomers with
various twist numbers and finds the energetically most favorable molecular
conformation as a function of temperature. The method can predict helical
repeat, openings loci and bubble sizes for specific sequences in a broad
temperature range. Some results are presented for a short DNA circle recently
identified in mammalian cells.Comment: The Journal of Chemical Physics, vol. 138 (2013), in pres
CMB Anomalies from Relic Anisotropy
Most of the analysis of the Cosmic Microwave Background relies on the
assumption of statistical isotropy. However, given some recent evidence
pointing against isotropy, as for instance the observed alignment of different
multipoles on large scales, it is worth testing this assumption against the
increasing amount of available data. As a pivot model, we assume that the
spectrum of the primordial perturbations depends also on their directionality
(rather than just on the magnitude of their momentum, as in the standard case).
We explicitly compute the correlation matrix for the temperature anisotropies
in the simpler case in which there is a residual isotropy between two spatial
directions. As a concrete example, we consider a different initial expansion
rate along one direction, and the following isotropization which takes place
during inflation. Depending on the amount of inflation, this can lead to broken
statistical isotropy on the largest observable scales.Comment: 6 pages, 2 .ps figure
Optimal Clustering under Uncertainty
Classical clustering algorithms typically either lack an underlying
probability framework to make them predictive or focus on parameter estimation
rather than defining and minimizing a notion of error. Recent work addresses
these issues by developing a probabilistic framework based on the theory of
random labeled point processes and characterizing a Bayes clusterer that
minimizes the number of misclustered points. The Bayes clusterer is analogous
to the Bayes classifier. Whereas determining a Bayes classifier requires full
knowledge of the feature-label distribution, deriving a Bayes clusterer
requires full knowledge of the point process. When uncertain of the point
process, one would like to find a robust clusterer that is optimal over the
uncertainty, just as one may find optimal robust classifiers with uncertain
feature-label distributions. Herein, we derive an optimal robust clusterer by
first finding an effective random point process that incorporates all
randomness within its own probabilistic structure and from which a Bayes
clusterer can be derived that provides an optimal robust clusterer relative to
the uncertainty. This is analogous to the use of effective class-conditional
distributions in robust classification. After evaluating the performance of
robust clusterers in synthetic mixtures of Gaussians models, we apply the
framework to granular imaging, where we make use of the asymptotic
granulometric moment theory for granular images to relate robust clustering
theory to the application.Comment: 19 pages, 5 eps figures, 1 tabl
Quantum Corrections to the Cosmological Evolution of Conformally Coupled Fields
Because the source term for the equations of motion of a conformally coupled
scalar field, such as the dilaton, is given by the trace of the matter energy
momentum tensor, it is commonly assumed to vanish during the radiation
dominated epoch in the early universe. As a consequence, such fields are
generally frozen in the early universe. Here we compute the finite temperature
radiative correction to the source term and discuss its consequences on the
evolution of such fields in the early universe. We discuss in particular, the
case of scalar tensor theories of gravity which have general relativity as an
attractor solution. We show that, in some cases, the universe can experience an
early phase of contraction, followed by a non-singular bounce, and standard
expansion. This can have interesting consequences for the abundance of thermal
relics; for instance, it can provide a solution to the gravitino problem. We
conclude by discussing the possible consequences of the quantum corrections to
the evolution of the dilaton.Comment: 24 pages, 7 figure
Grazing impact on soil chemical and biological properties under different plant cover types in a mountain area of Southern Italy.
Grazing can contribute to soil degradation by compaction due to roaming of livestock and loss of herbaceous cover, affecting also soil microbial community. Aim of this study was to assess grazing impact on soil microbial community and nutrient status under different plant cover types (i.e., fernery, chestnut wood, garigue). Grazed and ungrazed soils were analysed for water holding capacity, pH, organic carbon, N, S, K, Mg, Fe, Mn, Zn and Cu content, microbial biomass, fungal mycelium and potential respiration. Moreover, some ecophysiological indices, as microbial quotient, coefficient of endogenous mineralization (CEM), metabolic quotient (qCO2) and fungal fraction of microbial carbon were calculated. The results of present study showed that a moderate intensity grazing had low or no impact on chemical characteristics of soils and affects microbial community mainly in grazed areas with lower vegetation cover and lower content of nutrient and organic carbon, compared to areas with a thick layer of vegetation
Slow-roll Inflation for Generalized Two-Field Lagrangians
We study the slow-roll regime of two field inflation, in which the two fields
are also coupled through their kinetic terms. Such Lagrangians are motivated by
particle physics and by scalar-tensor theories studied in the Einstein frame.
We compute the power spectra of adiabatic and isocurvature perturbations on
large scales to first order in the slow-roll parameters. We discuss the
relevance of the extra coupling terms for the amplitude and indexes of the
power spectra. Beyond the consistency condition which involves the amplitude of
gravitational waves, additional relations may be found in particular models
based on such Lagrangians: as an example, we find an additional general
consistency condition in implicit form for Brans-Dicke theory in the Einstein
frame.Comment: 17 pages, 1 figure, accepted for publication in Phys. Rev.
J-factors of short DNA molecules
The propensity of short DNA sequences to convert to the circular form is
studied by a mesoscopic Hamiltonian method which incorporates both the bending
of the molecule axis and the intrinsic twist of the DNA strands. The base pair
fluctuations with respect to the helix diameter are treated as path
trajectories in the imaginary time path integral formalism. The partition
function for the sub-ensemble of closed molecules is computed by imposing chain
ends boundary conditions both on the radial fluctuations and on the angular
degrees of freedom. The cyclization probability, the J-factor, proves to be
highly sensitive to the stacking potential, mostly to its nonlinear parameters.
We find that the J-factor generally decreases by reducing the sequence length (
N ) and, more significantly, below N = 100 base pairs. However, even for very
small molecules, the J-factors remain sizeable in line with recent experimental
indications. Large bending angles between adjacent base pairs and anharmonic
stacking appear as the causes of the helix flexibility at short length scales.Comment: The Journal of Chemical Physics - May 2016 ; 9 page
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