48 research outputs found
Condensation - warming up to mitotic DNA architecture
During interphase, chromatin is in a state of least condensation and most accessible to transcription factors. When cells enter mitosis, replicated chromosomes are compacted, and sister chromatids are cohered to form specific mitotic architectures, which are essential for appropriate chromosome segregation. Disruption of the formation, regulation and maintenance of mitotic chromosome structure results in aneuploidy, which is tightly correlated with severe developmental maladies, aging and tumorigenesis. To understand how cells achieve mitotic condensed DNA architectures, we focus on the regulation of helicase activity and also the impact of site-specific condensation events. We report that helicase the Chl1 acts a novel regulator of mitotic chromosome condensation through cohesin-based mechanisms, revealing an exciting interface between native DNA structure that relies on helicase activity and higher-ordered chromosome compaction that requires cohesin complex. We also report for the first time that the condensed rDNA locus retains great plasticity during mitosis and responds to elevated temperature through a novel hypercondensation activity. This hyperthermic-induced rDNA hypercondensation is based on heat shock chaperone Hsp82, revealing a new role for chaperones in regulating mitotic DNA architecture
Semantic Object Parsing with Local-Global Long Short-Term Memory
Semantic object parsing is a fundamental task for understanding objects in
detail in computer vision community, where incorporating multi-level contextual
information is critical for achieving such fine-grained pixel-level
recognition. Prior methods often leverage the contextual information through
post-processing predicted confidence maps. In this work, we propose a novel
deep Local-Global Long Short-Term Memory (LG-LSTM) architecture to seamlessly
incorporate short-distance and long-distance spatial dependencies into the
feature learning over all pixel positions. In each LG-LSTM layer, local
guidance from neighboring positions and global guidance from the whole image
are imposed on each position to better exploit complex local and global
contextual information. Individual LSTMs for distinct spatial dimensions are
also utilized to intrinsically capture various spatial layouts of semantic
parts in the images, yielding distinct hidden and memory cells of each position
for each dimension. In our parsing approach, several LG-LSTM layers are stacked
and appended to the intermediate convolutional layers to directly enhance
visual features, allowing network parameters to be learned in an end-to-end
way. The long chains of sequential computation by stacked LG-LSTM layers also
enable each pixel to sense a much larger region for inference benefiting from
the memorization of previous dependencies in all positions along all
dimensions. Comprehensive evaluations on three public datasets well demonstrate
the significant superiority of our LG-LSTM over other state-of-the-art methods.Comment: 10 page
Type-I superconductivity in AlRe
While the pure elements tend to exhibit Type-I rather than Type-II
superconductivity, nearly all compound superconductors are Type-II, with only a
few known exceptions. We report single crystal growth and physical
characterization of the rhenium aluminide AlRe, which we conclude is a
Type-I superconductor based on magnetization, ac-susceptibility, and
specific-heat measurements. This detection of superconductivity, despite the
strong similarity of AlRe to a family of W and Mo aluminides that do not
superconduct, suggests that these aluminides are an ideal testbed for
identifying the relative importance of valence electron count and inversion
symmetry in determining whether a material will superconduct.Comment: 9 pages, 7 figures, CIF file as ancillar
Evaluating the performance of empirical models of total electron density and whistler-mode wave amplitude in the Earth’s inner magnetosphere
Empirical models have been previously developed using the large dataset of satellite observations to obtain the global distributions of total electron density and whistler-mode wave power, which are important in modeling radiation belt dynamics. In this paper, we apply the empirical models to construct the total electron density and the wave amplitudes of chorus and hiss, and compare them with the observations along Van Allen Probes orbits to evaluate the model performance. The empirical models are constructed using the Hp30 and SME (or SML) indices. The total electron density model provides an overall high correlation coefficient with observations, while large deviations are found in the dynamic regions near the plasmapause or in the plumes. The chorus wave model generally agrees with observations when the plasma trough region is correctly modeled and for modest wave amplitudes of 10–100 pT. The model overestimates the wave amplitude when the chorus is not observed or weak, and underestimates the wave amplitude when a large-amplitude chorus is observed. Similarly, the hiss wave model has good performance inside the plasmasphere when modest wave amplitudes are observed. However, when the modeled plasmapause location does not agree with the observation, the model misidentifies the chorus and hiss waves compared to observations, and large modeling errors occur. In addition, strong (>200 pT) hiss waves are observed in the plumes, which are difficult to capture using the empirical model due to their transient nature and relatively poor sampling statistics. We also evaluate four metrics for different empirical models parameterized by different indices. Among the tested models, the empirical model considering a plasmapause and controlled by Hp* (the maximum Hp30 during the previous 24 h) and SME* (the maximum SME during the previous 3 h) or Hp* and SML has the best performance with low errors and high correlation coefficients. Our study indicates that the empirical models are applicable for predicting density and whistler-mode waves with modest power, but large errors could occur, especially near the highly-dynamic plasmapause or in the plumes
Dirac Fermions in Antiferromagnetic FeSn Kagome Lattices with Combined Space Inversion and Time Reversal Symmetry
Symmetry principles play a critical role in formulating the fundamental laws
of nature, with a large number of symmetry-protected topological states
identified in recent studies of quantum materials. As compelling examples,
massless Dirac fermions are jointly protected by the space inversion symmetry
and time reversal symmetry supplemented by additional crystalline
symmetry, while evolving into Weyl fermions when either or is broken.
Here, based on first-principles calculations, we reveal that massless Dirac
fermions are present in a layered FeSn crystal containing antiferromagnetically
coupled ferromagnetic Fe kagome layers, where each of the and
symmetries is individually broken but the combined symmetry is preserved.
These stable Dirac fermions protected by the combined symmetry with
additional non-symmorphic symmetry can be transformed to either
massless/massive Weyl or massive Dirac fermions by breaking the or
symmetry. Our angle-resolved photoemission spectroscopy
experiments indeed observed the Dirac states in the bulk and two-dimensional
Weyl-like states at the surface. The present study substantially enriches our
fundamental understanding of the intricate connections between symmetries and
topologies of matter, especially with the spin degree of freedom playing a
vital role.Comment: 6 pages, 4 figure
Photoemission Evidence of a Novel Charge Order in Kagome Metal FeGe
A charge order has been discovered to emerge deep into the antiferromagnetic
phase of the kagome metal FeGe. To study its origin, the evolution of the
low-lying electronic structure across the charge order phase transition is
investigated with angle-resolved photoemission spectroscopy. We do not find
signatures of nesting between Fermi surface sections or van-Hove singularities
in zero-frequency joint density of states, and there are no obvious energy gaps
at the Fermi level, which exclude the nesting mechanism for the charge order
formation in FeGe. However, two obvious changes in the band structure have been
detected, i.e., one electron-like band around the K point and another one
around the A point move upward in energy position when the charge order forms.
These features can be well reproduced by our density-functional theory
calculations, where the charge order is primarily driven by magnetic energy
saving via large dimerizations of a quarter of Ge1-sites (in the kagome plane)
along the c-axis. Our results provide strong support for this novel charge
order formation mechanism in FeGe, in contrast to the conventional nesting
mechanism.Comment: 6 pages, 4 figure
Deep learning model of hiss waves in the plasmasphere and plumes and their effects on radiation belt electrons
Hiss waves play an important role in removing energetic electrons from Earth’s radiation belts by precipitating them into the upper atmosphere. Compared to plasmaspheric hiss that has been studied extensively, the evolution and effects of plume hiss are less understood due to the challenge of obtaining their global observations at high cadence. In this study, we use a neural network approach to model the global evolution of both the total electron density and the hiss wave amplitudes in the plasmasphere and plume. After describing the model development, we apply the model to a storm event that occurred on 14 May 2019 and find that the hiss wave amplitude first increased at dawn and then shifted towards dusk, where it was further excited within a narrow region of high density, namely, a plasmaspheric plume. During the recovery phase of the storm, the plume rotated and wrapped around Earth, while the hiss wave amplitude decayed quickly over the nightside. Moreover, we simulated the overall energetic electron evolution during this storm event, and the simulated flux decay rate agrees well with the observations. By separating the modeled plasmaspheric and plume hiss waves, we quantified the effect of plume hiss on energetic electron dynamics. Our simulation demonstrates that, under relatively quiet geomagnetic conditions, the region with plume hiss can vary from L = 4 to 6 and can account for up to an 80% decrease in electron fluxes at hundreds of keV at L > 4 over 3 days. This study highlights the importance of including the dynamic hiss distribution in future simulations of radiation belt electron dynamics