25,004 research outputs found
Robustness of cell cycle control and flexible orders of signaling events.
The highly robust control of cell cycles in eukaryotes enables cells to undergo strictly ordered G1/S/G2/M phases and respond adaptively to regulatory signals; however the nature of the robustness remains obscure. Specifically, it is unclear whether events of signaling should be strictly ordered and whether some events are more robust than others. To quantitatively address the two questions, we have developed a novel cell cycle model upon experimental observations. It contains positive and negative E2F proteins and two Cdk inhibitors, and is parameterized, for the first time, to generate not only oscillating protein concentrations but also periodic signaling events. Events and their orders reconstructed under varied conditions indicate that proteolysis of cyclins and Cdk complexes by APC and Skp2 occurs highly robustly in a strict order, but many other events are either dispensable or can occur in flexible orders. These results suggest that strictly ordered proteolytic events are essential for irreversible cell cycle progression and the robustness of cell cycles copes with flexible orders of signaling events, and unveil a new and important dimension to the robustness of cell cycle control in particular and to biological signaling in general
Energy Spectra of Anti-nucleons in Finite Nuclei
The quantum vacuum in a many-body system of finite nuclei has been
investigated within the relativistic Hartree approach which describes the bound
states of nucleons and anti-nucleons consistently. The contributions of the
Dirac sea to the source terms of the meson-field equations are taken into
account up to the one-nucleon loop and one-meson loop. The tensor couplings for
the - and -meson are included in the model. The overall nucleon
spectra of shell-model states are in agreement with the data. The calculated
anti-nucleon spectra in the vacuum differ about 20 -- 30 MeV with and without
the tensor-coupling effects.Comment: 4 pages, to appear in the Proceedings of MENU 2004 (Beijing, Aug. 29
-- Sept. 4, 2004
Microdistribution of oxygen in silicon and its effects on electronic properties
The effects of interstitial oxygen on the electrical characteristics of Czochralski-grown silicon crystals were investigated for the first time on a microscale. It was found that the generation of thermal donors is not a direct function of the oxygen concentration. It was further found that the minority carrier life-time decreases with increasing oxygen concentration, on a microscale in as-grown crystals. It was thus shown, again for the first time, that oxygen in as grown crystals is not electronically inert as generally believed. Preannealing at 1200 C commonly employed in device fabrication, was found to suppress the donor generation at 450 C and to decrease the deep level concentrations
Solvable senescence model with positive mutations
We build upon our previous analytical results for the Penna model of
senescence to include positive mutations. We investigate whether a small but
non-zero positive mutation rate gives qualitatively different results to the
traditional Penna model in which no positive mutations are considered. We find
that the high-lifespan tail of the distribution is radically changed in
structure, but that there is not much effect on the bulk of the population. Th
e mortality plateau that we found previously for a stochastic generalization of
the Penna model is stable to a small positive mutation rate.Comment: 3 figure
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P2-type Na2/3Ni1/3Mn2/3O2 Cathode Material with Excellent Rate and Cycling Performance for Sodium-Ion Batteries
P2-type Na2/3Ni1/3Mn2/3O2 is an air-stable cathode material for sodium-ion batteries. However, it suffers irreversible P2-O2 phase transition in 4.2-V plateau and shows poor cycling stability and rate capability within this plateau. To evaluate the practicability of this material in 2.3–4.1 V voltage range, single-crystal micro-sized P2-type Na2/3Ni1/3Mn2/3O2 with high rate capability and cycling stability is synthesized via polyvinylpyrrolidone (PVP)-combustion method. The electrochemical performance is evaluated by galvanostatic charge-discharge tests. The kinetics of Na+ intercalation/deintercalation is studied detailly with potential intermittent titration technique (PITT), galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV). The discharge capacity at 0.1 C in 2.3–4.1 V is 87.6 mAh g−1. It can deliver 91.5% capacity at 40 C rate and keep 89% after 650 cycles at 5C. The calculated theoretical energy density of full cell with hard carbon anode is 210 Wh kg−1. The moderate energy density associated with high power density and long cycle life is acceptable for load adjustment of new-energy power, showing the prospect of practical application
Star-forming regions of the Aquila rift cloud complex. I. NH3 tracers of dense molecular cores
(Abridged) Aims. In the present part of our survey we search for ammonia
emitters in the Aquila rift complex which trace the densest regions of
molecular clouds. Methods. From a CO survey carried out with the Delingha 14-m
telescope we selected ~150 targets for observations in other molecular lines.
Here we describe the mapping observations in the NH3(1,1) and (2,2) inversion
lines of the first 49 sources performed with the Effelsberg 100-m telescope.
Results. The NH3(1,1) and (2,2) emission lines are detected in 12 and 7
sources, respectively. Among the newly discovered NH3 sources, our sample
includes the following well-known clouds: the starless core L694-2, the Serpens
cloud Cluster B, the Serpens dark cloud L572, the filamentary dark cloud L673,
the isolated protostellar source B335, and the complex star-forming region
Serpens South. Angular sizes between 40" and 80" (~0.04-0.08 pc) are observed
for compact starless cores but as large as 9' (~0.5 pc) for filamentary dark
clouds. The measured kinetic temperatures of the clouds lie between 9K and 18K.
From NH3 excitation temperatures of 3-8K we determine H2 densities with typical
values of ~(0.4-4) 10^4 cm^-3. The masses of the mapped cores range between
~0.05 and ~0.5M_solar. The relative ammonia abundance, X= [NH3]/[H2], varies
from 10^-7 to 5 10^-7 with the mean = (2.7+/-0.6) 10^-7 (estimated from
spatially resolved cores assuming the filling factor eta = 1). In two clouds,
we observe kinematically split NH3 profiles separated by ~1 km/s. The splitting
is most likely due to bipolar molecular outflows for one of which we determine
an acceleration of <~ 0.03 km/s/yr. A starless core with significant rotational
energy is found to have a higher kinetic temperature than the other ones which
is probably caused by magnetic energy dissipation.Comment: 28 pages, 22 figures, 6 tables, accepted for publication in A&
Phase transition in the 3 Kelvin phase in the eutectic Sr2RuO4-Ru
The inhomogeneous 3-Kelvin (3K) phase of the eutectic Sr2RuO4 with Ru
inclusions nucleates superconductivity at the interface between Ru and Sr2RuO4.
The structure of the interface state and its physical properties are examined
here. Two superconducting phases are identified between the transitions to the
bulk phase at 1.5K and to the 3K phase. The nucleation of the 3K phase results
in a state conserving time reversal symmetry, which generates an intrinsically
frustrated superconducting network in samples with many Ru inclusions. At a
lower temperature (>1.5K), a discontinuous (first order) transition to an
interface state breaking time reversal symmetry is found leading to an
unfrustrated network phase. It is shown that this phase transition located at a
temperature between 1.5 and 3K would yield the anomalous property that the
critical current in such a network depends on the sign of the current,
reproducing recent experimental observations.Comment: This paper has been withdrawn by the authors. 5 pages, 6 figure
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