872 research outputs found
On the Mechanism of Protein Synthesis
Our knowledge about the mechanism of protein synthesis, one of the most important biochemical problems, is still extremely limited. It is evident from the very nature of proteins, their complexity and specificity, that the problem of protein synthesis presents numerous aspects. Theories of a template method of protein synthesis in the sense of Langmuir-Schaefer or Pauling are designed to explain the specificity of proteins. This paper will deal with only one aspect of protein synthesis, the mode of formation of the amide (peptide) linkage, which is the most typical linkage of every protein. In view of the close connection between protein synthesis and growth, any advance in understanding the former can he expected to shed some light not only on normal growth hut also on abnormal growth as typified by cancer formation. It is therefore essential that ideas on the problem of protein synthesis be communicated whereby impetus will be given for research efforts along new avenues of attack. On the more different fronts this problem will be tackled, the greater is the chance of ultimately solving it
HLA-G: expression in human keratinocytes in vitro and in human skin in vivo
Classical, polymorphic major histocompatibility complex class I molecules are
expressed on most nucleated cells.They present peptides at the cell surface and,
thus, enable the immune system to scan peptides for their antigenicity. The
function of the other, nonclassical class I molecules in man is controversial.
HLA-G which has been shown by transfection experiments to be expressed at the
cell surface, is only transcribed in placental tissue and in the fetal eye.Therefore, a
role of HLA-G in the control of rejection of the allogeneic fetus has been
discussed. We found that HLA-G expression is induced in keratinocytes by
culture in vitro. Three different alternative splicing products of HLA-G can be
detected: a full length transcript, an mRNA lacking exon 3 and a transcript devoid
of exon 3 and 4. Reverse transcription followed by polymerase chain reaction also
revealed the presence of HLA-G mRNA in vivo in biopsies of either diseased or
healthy skin
Scaffolding Discourse in Asynchronous Learning Networks
Discourse, a form of collaborative learning, is fundamentally a communications process. This in-progress study adapts Clark and Brennanâs grounding in communications principles to investigate how to âscaffoldâ asynchronous discourse. Scaffolding is defined as providing support for the learner at his or her level until the support is no longer needed. This paper presents early results from an experimental study measuring learning effectiveness. In the experiment, content and process scaffolding are manipulated based on pedagogic principles. A major contribution of the study is building and testing a technologymediated, discourse-centered, teaching and learning model called the Asynchronous Learning Networks Cognitive Discourse Model (ALNCDM). As discourse is one of the most widely used online methods of teaching and learning, the results of the study are expected to add to the body of knowledge on how to structure asynchronous online discourse assignments for more effective student learning
The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence
We investigate the influence of coherent structures on particle acceleration
in the strongly turbulent solar corona. By randomizing the Fourier phases of a
pseudo-spectral simulation of isotropic MHD turbulence (Re ), and
tracing collisionless test protons in both the exact-MHD and phase-randomized
fields, it is found that the phase correlations enhance the acceleration
efficiency during the first adiabatic stage of the acceleration process. The
underlying physical mechanism is identified as the dynamical MHD alignment of
the magnetic field with the electric current, which favours parallel
(resistive) electric fields responsible for initial injection. Conversely, the
alignment of the magnetic field with the bulk velocity weakens the acceleration
by convective electric fields - \bfu \times \bfb at a non-adiabatic stage of
the acceleration process. We point out that non-physical parallel electric
fields in random-phase turbulence proxies lead to artificial acceleration, and
that the dynamical MHD alignment can be taken into account on the level of the
joint two-point function of the magnetic and electric fields, and is therefore
amenable to Fokker-Planck descriptions of stochastic acceleration.Comment: accepted for publication in Ap
Mathematics, statistics and archaeometry: the past 50 years or so
This review of developments in the use of mathematics and statistics in archaeometry over the past 50 years is partial, personal and 'broad-brush'. The view is expressed that it is in the past 30 years or so that the major developments have taken place. The view is also expressed that, with the exception of methods for analysing radiocarbon dates and increased computational power, mathematical and statistical methods that are currently used, and found to be useful in widespread areas of application such as provenance studies, don't differ fundamentally from what was being done 30 years ago
Forbush decreases and solar events seen in the 10 - 20GeV energy range by the Karlsruhe Muon Telescope
Since 1993, a muon telescope located at Forschungszentrum Karlsruhe
(Karlsruhe Muon Telescope) has been recording the flux of single muons mostly
originating from primary cosmic-ray protons with dominant energies in the 10 -
20 GeV range. The data are used to investigate the influence of solar effects
on the flux of cosmic-rays measured at Earth. Non-periodic events like Forbush
decreases and ground level enhancements are detected in the registered muon
flux. A selection of recent events will be presented and compared to data from
the Jungfraujoch neutron monitor. The data of the Karlsruhe Muon Telescope help
to extend the knowledge about Forbush decreases and ground level enhancements
to energies beyond the neutron monitor regime.Comment: 9 pages, 7 figures, in Press AS
On the Estimation of Solar Energetic Particle Injection Timing from Onset Times near Earth
We examine the accuracy of a common technique for estimating the start time
of solar energetic particle injection based on a linear fit to the observed
onset time versus 1/(particle velocity). This is based on a concept that the
first arriving particles move directly along the magnetic field with no
scattering. We check this by performing numerical simulations of the transport
of solar protons between 2 and 2000 MeV from the Sun to the Earth, for several
assumptions regarding interplanetary scattering and the duration of particle
injection, and analyzing the results using the inverse velocity fit. We find
that in most cases, the onset times align close to a straight line as a
function of inverse velocity. Despite this, the estimated injection time can be
in error by several minutes. Also, the estimated path length can deviate
greatly from the actual path length along the interplanetary magnetic field.
The major difference between the estimated and actual path lengths implies that
the first arriving particles cannot be viewed as moving directly along the
interplanetary magnetic field.Comment: 19 pages, 3 Postscript figures. Astrophys. J., in pres
?We all Black innit??: Analysing relations between African and African-Caribbean groups in Britain
âWe all Black innit?â examines âintra ethnicâ relationships between second and third generation members of British African and British African Caribbean groups, located in two key urban settings (London and Birmingham). Through unstructured interviews, it explores ways in which positive informal/formal ârace based coalitionsâ (Rogers 2004), have been forged, partly as a result of supporting and celebrating each otherâs contributions to professionality (e.g education and work), and popular culture (e.g music and sports). Moreover, it discusses how members of these generations have come to embrace difference and commonalities in terms of âhistories language and cultureâ (Hall 1988:5), and the role of âpan ethnicityâ in facilitating these positive relationships. There is, however, acknowledgement of âintra ethnicâ tensions existing between these groups, largely resulting from historical (and often negative) stereotypes of each otherâs cultural attributes (Mwakigale 2009, Fanon 1952), and competition for scare socio-economic resources, intensified by allocation along colour or ethnic lines (Malik 2012). The findings suggest that although this appears to have been a greater issue for migrant generations who arrived in Britain, especially between the post war era and the 1980s, to an extent it has impacted on âintra ethnicâ relations between the second and third generations. In the main, however, it would seem that the mutual respect between younger generations of British African and British African Caribbean people, has grown with time
Possible Tomography of the Sun's Magnetic Field with Solar Neutrinos
The data from solar neutrino experiments together with standard solar model
predictions are used in order to derive the possible profile of the magnetic
field inside the Sun, assuming the existence of a sizeable neutrino magnetic
moment and the resonant spin flavour mechanism. The procedure is based on the
relationship between resonance location and the energy dependent neutrino
suppression, so that a large neutrino suppression at a given energy is taken to
be connected to a large magnetic field in a given region of the Sun. In this
way it is found that the solar field must undergo a very sharp increase by a
factor of at least 6 - 7 over a distance no longer than 7 - 10% of the solar
radius, decreasing gradually towards the surface. The range in which this sharp
increase occurs is likely to be the bottom of the convective zone. There are
also indications in favour of the downward slope being stronger at the start
and more moderate on approaching the solar surface. Typical ranges for the
magnetic moment are from a few times 10^{-13}\mu_B to its laboratory upper
bounds while the mass square difference between neutrino flavours is of order
(0.6-1.9) x 10^{-8}eV^2.Comment: Several minor corrections performed, sunspot anticorrelation
discussed, references added, 29 pages including 8 figures in PostScrip
Screened Coulomb interactions in metallic alloys: II Screening beyond the single-site and atomic sphere approximations
A quantitative description of the configurational part of the total energy of
metallic alloys with substantial atomic size difference cannot be achieved in
the atomic sphere approximation: It needs to be corrected at least for the
multipole moment interactions in the Madelung part of the one-electron
potential and energy. In the case of a random alloy such interactions can be
accounted for only by lifting the atomic sphere and single-site approximations,
in order to include the polarization due to local environment effects.
Nevertheless a simple parameterization of the screened Coulomb interactions for
the ordinary single-site methods, including the generalized perturbation
method, is still possible. We obtained such a parameterization for bulk and
surface NiPt alloys, which allows one to obtain quantitatively accurate
effective interactions in this system.Comment: 24 pages, 2 figure
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