4,201 research outputs found
Which way up? Recognition of homologous DNA segments in parallel and antiparallel alignment
Homologous gene shuffling between DNA promotes genetic diversity and is an
important pathway for DNA repair. For this to occur, homologous genes need to
find and recognize each other. However, despite its central role in homologous
recombination, the mechanism of homology recognition is still an unsolved
puzzle. While specific proteins are known to play a role at later stages of
recombination, an initial coarse grained recognition step has been proposed.
This relies on the sequence dependence of the DNA structural parameters, such
as twist and rise, mediated by intermolecular interactions, in particular
electrostatic ones. In this proposed mechanism, sequences having the same base
pair text, or are homologous, have lower interaction energy than those
sequences with uncorrelated base pair texts; the difference termed the
recognition energy. Here, we probe how the recognition energy changes when one
DNA fragment slides past another, and consider, for the first time, homologous
sequences in antiparallel alignment. This dependence on sliding was termed the
recognition well. We find that there is recognition well for anti-parallel,
homologous DNA tracts, but only a very shallow one, so that their interaction
will differ little from the interaction between two nonhomologous tracts. This
fact may be utilized in single molecule experiments specially targeted to test
the theory. As well as this, we test previous theoretical approximations in
calculating the recognition well for parallel molecules against MC simulations,
and consider more rigorously the optimization of the orientations of the
fragments about their long axes. The more rigorous treatment affects the
recognition energy a little, when the molecules are considered rigid. However
when torsional flexibility of the DNA molecules is introduced, we find
excellent agreement between analytical approximation and simulation.Comment: Paper with supplemental material attached. 41 pages in all, 4 figures
in main text, 3 figures in supplmental. To be submitted to Journa
An analysis of type F2 software measurement standards for profile surface texture parameters
This paper reports on an in-depth analysis of ISO 5436 part 2 type F2 reference software for the calculation of profile surface texture parameters that has been performed on the input, implementation and output results of the reference software developed by the National Physical Laboratory (NPL), the National Institute of Standards and Technology (NIST) and Physikalisch-Technische Bundesanstalt (PTB). Surface texture parameters have been calculated for a selection of 17 test data files obtained from the type F1 reference data sets on offer from NPL and NIST. The surface texture parameter calculation results show some disagreements between the software methods of the National Metrology Institutes. These disagreements have been investigated further, and some potential explanations are given
Intelligent sampling for the measurement of structured surfaces
Uniform sampling in metrology has known drawbacks such as coherent spectral aliasing and a lack of efficiency in terms of measuring time and data storage. The requirement for intelligent sampling strategies has been outlined over recent years, particularly where the measurement of structured surfaces is concerned. Most of the present research on intelligent sampling has focused on dimensional metrology using coordinate-measuring machines with little reported on the area of surface metrology. In the research reported here, potential intelligent sampling strategies for surface topography measurement of structured surfaces are investigated by using numerical simulation and experimental verification. The methods include the jittered uniform method, low-discrepancy pattern sampling and several adaptive methods which originate from computer graphics, coordinate metrology and previous research by the authors. By combining the use of advanced reconstruction methods and feature-based characterization techniques, the measurement performance of the sampling methods is studied using case studies. The advantages, stability and feasibility of these techniques for practical measurements are discussed
Unitary relations in time-dependent harmonic oscillators
For a harmonic oscillator with time-dependent (positive) mass and frequency,
an unitary operator is shown to transform the quantum states of the system to
those of a harmonic oscillator system of unit mass and time-dependent
frequency, as well as operators. For a driven harmonic oscillator, it is also
shown that, there are unitary transformations which give the driven system from
the system of same mass and frequency without driving force. The transformation
for a driven oscillator depends on the solution of classical equation of motion
of the driven system. These transformations, thus, give a simple way of finding
exact wave functions of a driven harmonic oscillator system, provided the
quantum states of the corresponding system of unit mass are given.Comment: Submitted to J. Phys.
A dynamical system approach to higher order gravity
The dynamical system approach has recently acquired great importance in the
investigation on higher order theories of gravity. In this talk I review the
main results and I give brief comments on the perspectives for further
developments.Comment: 6 pages, 1 figure, 2 tables, talk given at IRGAC 2006, July 200
Enhancement of superhorizon scale inflationary curvature perturbations
We show that there exists a simple mechanism which can enhance the amplitude
of curvature perturbations on superhorizon scales during inflation, relative to
their amplitude at horizon crossing. The enhancement may occur even in a
single-field inflaton model, and occurs if the quantity becomes
sufficiently small, as compared to its value at horizon crossing, for some time
interval during inflation. We give a criterion for this enhancement in general
single-field inflation models.Comment: 5 pages RevTeX file with 2 figures incorporated v2:Contains important
O(k^2) correctio
Shear dynamics in Bianchi I cosmologies with R^n-gravity
We give the equations governing the shear evolution in Bianchi spacetimes for
general f(R)-theories of gravity. We consider the case of R^n-gravity and
perform a detailed analysis of the dynamics in Bianchi I cosmologies which
exhibit local rotational symmetry. We find exact solutions and study their
behaviour and stability in terms of the values of the parameter n. In
particular, we found a set of cosmic histories in which the universe is
initially isotropic, then develops shear anisotropies which approaches a
constant value.Comment: 25 pages LaTeX, 6 figures. Revised to match the final version
accepted for publication in CQ
Observational constraints on the spectral index of the cosmological curvature perturbation
We evaluate the observational constraints on the spectral index , in the
context of the CDM hypothesis which represents the simplest viable
cosmology. We first take to be practically scale-independent. Ignoring
reionization, we find at a nominal 2- level . If
we make the more realisitic assumption that reionization occurs when a fraction
to 1 of the matter has collapsed, the 2- lower bound is
unchanged while the 1- bound rises slightly. These constraints are
compared with the prediction of various inflation models. Then we investigate
the two-parameter scale-dependent spectral index, predicted by running-mass
inflation models, and find that present data allow significant scale-dependence
of , which occurs in a physically reasonable regime of parameter space.Comment: ReVTeX, 15 pages, 5 figures and 3 tables, uses epsf.sty Improved
treatment of reionization and small bug fixed in the constant n case; more
convenient parameterization and better treatment of the n dependence in the
CMB anisotropy for the running mass case; conclusions basically unchanged;
references adde
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