716 research outputs found
Embedded Ribbons of Graphene Allotropes: An Extended Defect Perspective
Four fundamental dimer manipulations can be used to produce a variety of
localized and extended defect structures in graphene. Two-dimensional templates
result in graphene allotropes, here viewed as extended defects, which can
exhibit either metallic or semiconducting electrical character. \emph{Embedded
allotropic ribbons}--i.e. thin swaths of the new allotropes--can also be
created within graphene. We examine these ribbons and find that they maintain
the electrical character of their parent allotrope even when only a few atoms
in width. Such extended defects may facilitate the construction of monolithic
electronic circuitry.Comment: 24 pages, 21 figure
Electron Bernstein waves in spherical tokamak plasmas with "magnetic wells"
In addition to traditional regimes with monotonously increasing magnetic
field, regimes with "magnetic wells" also occur in spherical tokamaks (STs).
The magnetic field profile inversion modifies significantly the whole picture
of the wave propagation and damping. Since the magnetic wells may become quite
common with further improvement of ST performance, analysis of such
configurations is of interest for assessment of EBW plasma heating an CD
perspectives. In this paper the basic features of the EBWs propagation and
damping for the second cyclotron harmonic in a slab model are considered.Comment: Proc. of 13-th Joint Workshop on ECE and ECRH, N.Novgorod, Russia May
17-20, 2004, 8 pages, 4 fig
Epigenetic Chromatin Silencing: Bistability and Front Propagation
The role of post-translational modification of histones in eukaryotic gene
regulation is well recognized. Epigenetic silencing of genes via heritable
chromatin modifications plays a major role in cell fate specification in higher
organisms. We formulate a coarse-grained model of chromatin silencing in yeast
and study the conditions under which the system becomes bistable, allowing for
different epigenetic states. We also study the dynamics of the boundary between
the two locally stable states of chromatin: silenced and unsilenced. The model
could be of use in guiding the discussion on chromatin silencing in general. In
the context of silencing in budding yeast, it helps us understand the phenotype
of various mutants, some of which may be non-trivial to see without the help of
a mathematical model. One such example is a mutation that reduces the rate of
background acetylation of particular histone side-chains that competes with the
deacetylation by Sir2p. The resulting negative feedback due to a Sir protein
depletion effect gives rise to interesting counter-intuitive consequences. Our
mathematical analysis brings forth the different dynamical behaviors possible
within the same molecular model and guides the formulation of more refined
hypotheses that could be addressed experimentally.Comment: 19 pages, 5 figure
Plasma Physics
Contains research objectives and reports on four research projects.U.S. Atomic Energy Commission (Contract AT(30-1)-1842)U.S. Atomic Energy Commission Contract W-7405-Eng-3
OPERating ON Chromatin, a Colorful Language where Context Matters
Histones, the fundamental packaging elements of eukaryotic DNA, are highly decorated with a diverse set of post-translational modifications (PTMs) that are recognized to govern the structure and function of chromatin. Ten years ago, we put forward the histone code hypothesis, which provided a model to explain how single and/or combinatorial PTMs on histones regulate the diverse activities associated with chromatin (e.g. gene transcription). At that time, there was a limited understanding of both the number of PTMs that occur on histones as well as the proteins that place, remove and interpret them. Since the conception of this hypothesis, the field has witnessed an unprecedented advance in our understanding of the enzymes that contribute to the establishment of histone PTMs, as well as the diverse effector proteins that bind them. While debate continues as to whether histone PTMs truly constitute a strict “code”, it is becoming clear that PTMs on histone proteins function in elaborate combinations to regulate the many activities associated with chromatin. In this special issue, we celebrate the 50th anniversary of the landmark publication of the lac operon with a review that provides a current view of the histone code hypothesis, the lessons we have learned over the last decade, and the technologies that will drive our understanding of histone PTMs forward in the future
Microwave Gaseous Disharges
Contains reports on seven research projects.Atomic Energy Commission under Contract AT(30-1)184
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