3,593 research outputs found
Evolutionary dynamics of adult stem cells: Comparison of random and immortal strand segregation mechanisms
This paper develops a point-mutation model describing the evolutionary
dynamics of a population of adult stem cells. Such a model may prove useful for
quantitative studies of tissue aging and the emergence of cancer. We consider
two modes of chromosome segregation: (1) Random segregation, where the daughter
chromosomes of a given parent chromosome segregate randomly into the stem cell
and its differentiating sister cell. (2) ``Immortal DNA strand''
co-segregation, for which the stem cell retains the daughter chromosomes with
the oldest parent strands. Immortal strand co-segregation is a mechanism,
originally proposed by Cairns (J. Cairns, {\it Nature} {\bf 255}, 197 (1975)),
by which stem cells preserve the integrity of their genomes. For random
segregation, we develop an ordered strand pair formulation of the dynamics,
analogous to the ordered strand pair formalism developed for quasispecies
dynamics involving semiconservative replication with imperfect lesion repair
(in this context, lesion repair is taken to mean repair of postreplication
base-pair mismatches). Interestingly, a similar formulation is possible with
immortal strand co-segregation, despite the fact that this segregation
mechanism is age-dependent. From our model we are able to mathematically show
that, when lesion repair is imperfect, then immortal strand co-segregation
leads to better preservation of the stem cell lineage than random chromosome
segregation. Furthermore, our model allows us to estimate the optimal lesion
repair efficiency for preserving an adult stem cell population for a given
period of time. For human stem cells, we obtain that mispaired bases still
present after replication and cell division should be left untouched, to avoid
potentially fixing a mutation in both DNA strands.Comment: 9 pages, 3 figure
A Dorsal Hand Vein Recognition-based on Local Gabor Phase Quantization with Whitening Transformation
The hand vein pattern is a biometric feature in which the actual pattern is the shape of the vein network and its characteristics are the vein features. This paper investigates a new approach for dorsal hand vein pattern identification from grey level dorsal hand vein information. In this study Gabor filter quadrature pair is employed to compute locally in a window for every pixel position to extract the phase information. The phases of six frequency coefficients are quantized and it is used to form a descriptor code for the local region. These local descriptors are decorrelated using whitening transformation and a histogram is generated for every pixel which describes the local pattern. Experiments are evaluated on North China University of Technology dorsal hand vein image dataset with minimum distance classifier and the results are analyzed for recognition rate, run time and equal error rate. The proposed method gives 100 per cent recognition rate and 1 per cent EER for fusion of both left and right hands.Defence Science Journal, 2014, 64(2), pp. 159-167. DOI: http://dx.doi.org/10.14429/dsj.64.465
A new type of charged defect in amorphous chalcogenides
We report on density-functional-based tight-binding (DFTB) simulations of a
series of amorphous arsenic sulfide models. In addition to the charged
coordination defects previously proposed to exist in chalcogenide glasses, a
novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic
site in a seesaw configuration and a three-fold coordinated sulfur site in a
planar trigonal configuration, was found in several models. The
valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under
HOMO-to-LUMO electronic excitation. This structural transformation is
accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests
that such transformations could contribute to photo-darkening in these
materials.Comment: 5 pages, 2 figure
Observation of sub-natural linewidths for cold atoms in a magneto-optic trap
We have studied the absorption of a weak probe beam through cold rubidium
atoms in a magneto-optic trap. The absorption spectrum shows two peaks with the
smaller peak having linewidth as small as 28% of the natural linewidth. The
modification happens because the laser beams used for trapping also drive the
atoms coherently between the ground and excited states. This creates
``dressed'' states whose energies are shifted depending on the strength of the
drive. Linewidth narrowing occurs due to quantum coherence between the dressed
states. The separation of the states increases with laser intensity and
detuning, as expected from this model.Comment: 8 pages, 4 figure
The statistics of particle velocities in dense granular flows
We present measurements of the particle velocity distribution in the flow of
granular material through vertical channels. Our study is confined to dense,
slow flows where the material shears like a fluid only in thin layers adjacent
to the walls, while a large core moves without continuous deformation, like a
solid. We find the velocity distribution to be non-Gaussian, anisotropic, and
to follow a power law at large velocities. Remarkably, the distribution is
identical in the fluid-like and solid-like regions. The velocity variance is
maximum at the core, defying predictions of hydrodynamic theories. We show
evidence of spatially correlated motion, and propose a mechanism for the
generation of fluctuational motion in the absence of shear.Comment: Submitted to Phys. Rev. Let
Cosmological Implications of the Very High Redshift GRB 050904
We report near simultaneous multi-color (RIYJHK) observations made with the
MAGNUM 2m telescope of the gamma ray burst GRB 050904 detected by the SWIFT
satellite. The spectral energy distribution shows a very large break between
the I and J bands. Using intergalactic transmissions measured from high
redshift quasars we show that the observations place a 95% confidence lower
limit of z=6.18 on the object, consistent with a later measured spectroscopic
redshift of 6.29 obtained by Kawai et al. (2005) with the Subaru telescope. We
show that the break strength in the R and I bands is consistent with that
measured in the quasars. Finally we consider the implications for the star
formation history at high redshift.Comment: Accepted for publication in the Astrophysical Journal. Expanded
introduction and discussio
Control of Surficial Slides by Different Erosion Control Techniques
Many natural and embankment slopes fall due to the erosion of the top surficial soil mantle. Denudation of vegetation from soil slopes combined with the further steepening of slopes due to natural and man-made causes such as cuts result in such a type of failure. Essentially, the corrective measures appropriate for stabilizing these slopes comprise erosion control by establishing vegetation on the slope. The methods of vegetative turfing include asphalt mulch, coir/jute netting, geogrids and stone apron techniques. The paper sets forth the case history giving details of some of the relatively new techniques for erosion control on a hill road in India and evaluates the relative performance and the relative economics of these methods
India\u27s First Venture Relating to Subsurface Drainage by Horizontal Drains
Landslides in the Nilgiris district in 1978-79 1 caused heavy damages to property and human life. The field studies indicated that in certain spots, the build up of excess hydrostatic pressure, caused by direct and indirect ingress of water into the soil mantle, is the main cause for certain landslides in the region. The paper attempts to give the total case history relating to a pioneering venture of its kind towards the correction of landslides undertaken in India. The case history assumes special significance in as much as the horizontal drains successfully installed for the first time in this country, provided the needed ocular demonstration of the efficacy of horizontal drains to the practicing engineers in India
Analytic models of plausible gravitational lens potentials
Gravitational lenses on galaxy scales are plausibly modelled as having
ellipsoidal symmetry and a universal dark matter density profile, with a Sersic
profile to describe the distribution of baryonic matter. Predicting all lensing
effects requires knowledge of the total lens potential: in this work we give
analytic forms for that of the above hybrid model. Emphasising that complex
lens potentials can be constructed from simpler components in linear
combination, we provide a recipe for attaining elliptical symmetry in either
projected mass or lens potential. We also provide analytic formulae for the
lens potentials of Sersic profiles for integer and half-integer index. We then
present formulae describing the gravitational lensing effects due to
smoothly-truncated universal density profiles in cold dark matter model. For
our isolated haloes the density profile falls off as radius to the minus fifth
or seventh power beyond the tidal radius, functional forms that allow all
orders of lens potential derivatives to be calculated analytically, while
ensuring a non-divergent total mass. We show how the observables predicted by
this profile differ from that of the original infinite-mass NFW profile.
Expressions for the gravitational flexion are highlighted. We show how
decreasing the tidal radius allows stripped haloes to be modelled, providing a
framework for a fuller investigation of dark matter substructure in galaxies
and clusters. Finally we remark on the need for finite mass halo profiles when
doing cosmological ray-tracing simulations, and the need for readily-calculable
higher order derivatives of the lens potential when studying catastrophes in
strong lenses.Comment: 24 pages, 10 figures, matches published versio
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