1,450 research outputs found
Constraints, Histones, and the 30 Nanometer Spiral
We investigate the mechanical stability of a segment of DNA wrapped around a
histone in the nucleosome configuration. The assumption underlying this
investigation is that the proper model for this packaging arrangement is that
of an elastic rod that is free to twist and that writhes subject to mechanical
constraints. We find that the number of constraints required to stabilize the
nuclesome configuration is determined by the length of the segment, the number
of times the DNA wraps around the histone spool, and the specific constraints
utilized. While it can be shown that four constraints suffice, in principle, to
insure stability of the nucleosome, a proper choice must be made to guarantee
the effectiveness of this minimal number. The optimal choice of constraints
appears to bear a relation to the existence of a spiral ridge on the surface of
the histone octamer. The particular configuration that we investigate is
related to the 30 nanometer spiral, a higher-order organization of DNA in
chromatin.Comment: ReVTeX, 15 pages, 18 figure
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Mites Parasitic on Australasian and African Spiders Found in the Pet Trade; a Redescription of Ljunghia pulleinei Womersley
Parasitic mites associated with spiders are spreading world-wide through the trade in tarantulas and other pet species.
Ljunghia pulleinei Womersley, a mesostigmatic laelapid mite originally found in association with the mygalomorph spider
Selenocosmia stirlingi Hogg (Theraphosidae) in Australia, is redescribed and illustrated on the basis of specimens from the
African theraphosid spider Pterinochilus chordatus (Gersta¨cker) kept in captivity in the British Isles (Wales). The mite is known
from older original descriptions of Womersley in 1956; the subsequent redescription of Domrow in 1975 seems to be
questionable in conspecificity of treated specimens with the type material. Some inconsistencies in both descriptions are
recognised here as intraspecific variability of the studied specimens. The genus Arachnyssus Ma, with species A. guangxiensis
(type) and A. huwenae, is not considered to be a valid genus, and is included in synonymy with Ljunghia Oudemans. A new
key to world species of the genus Ljunghia is provided
Physical theory of the twentieth century and contemporary philosophy
It has been shown that the criticism of Pauli as well as of Susskind and
Glogover may be avoided if the standard quantum-mechanical mathematical model
has been suitably extended. There is not more any reason for Einstein's
citicism, either, if in addition to some new results concerning Bell's
inequalities and Belifante's argument are taken into account. The ensemble
interpretation of quantum mechanics (or the hidden-variable theory) should be
preferred, which is also supported by the already published results of
experiments with three polarizers. Greater space in the text has been devoted
also to the discussion of epistemological problems and some philosophical
consequences.Comment: 12 page
Gap soliton dynamics in an optical lattice as a parametrically driven pendulum
A long wavelength optical lattice is generated in a two-level medium by
low-frequency contrapropagating beams. Then a short wave length gap soliton
generated by evanescent boundary instability (supratransmission) undergoes a
dynamics shown to obey the Newton equation of the parametrically driven
pendulum, hence presenting extremely rich, possibly chaotic, dynamical
behavior. The theory is sustained by numerical simulations and provides an
efficient tool to study soliton trajectories
Exposure of cultured hepatocytes to cyclic AMP enhances the vasopressin-mediated stimulation of inositol phosphate production
Activation of membrane protein kinase C by glucagon and Ca2+-mobilizing hormones in cultured rat hepatocytes. Role of phosphatidylinositol and phosphatidylcholine hydrolysis
Dynamic imaging using Motion-Compensated SmooThness Regularization on Manifolds (MoCo-SToRM)
We introduce an unsupervised motion-compensated reconstruction scheme for
high-resolution free-breathing pulmonary MRI. We model the image frames in the
time series as the deformed version of the 3D template image volume. We assume
the deformation maps to be points on a smooth manifold in high-dimensional
space. Specifically, we model the deformation map at each time instant as the
output of a CNN-based generator that has the same weight for all time-frames,
driven by a low-dimensional latent vector. The time series of latent vectors
account for the dynamics in the dataset, including respiratory motion and bulk
motion. The template image volume, the parameters of the generator, and the
latent vectors are learned directly from the k-t space data in an unsupervised
fashion. Our experimental results show improved reconstructions compared to
state-of-the-art methods, especially in the context of bulk motion during the
scans
Sequence Effects on DNA Entropic Elasticity
DNA stretching experiments are usually interpreted using the worm-like chain
model; the persistence length A appearing in the model is then interpreted as
the elastic stiffness of the double helix. In fact the persistence length
obtained by this method is a combination of bend stiffness and intrinsic bend
effects reflecting sequence information, just as at zero stretching force. This
observation resolves the discrepancy between the value of A measured in these
experiments and the larger ``dynamic persistence length'' measured by other
means. On the other hand, the twist persistence length deduced from
torsionally-constrained stretching experiments suffers no such correction. Our
calculation is very simple and analytic; it applies to DNA and other polymers
with weak intrinsic disorder.Comment: LaTeX; postscript available at
http://dept.physics.upenn.edu/~nelson/index.shtm
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