503 research outputs found
Mitochondrial DNA replication in sea urchin oocytes
Mitochondrial DNA (mtDNA) replicative intermediates from Strongylocentrotus purpuratus oocytes were isolated by ethidium bromide-CsCl density gradient centrifugation and examined by electron microscopy after formamide spreading. In some experiments, the mtDNA was radioactively labeled by exposing isolated oocytes to [3H]thymidine. Oocyte mtDNA replication appears to follow the displacement loop model outlined in mouse L cells. There are differences in detail. The frequency of D-loop DNA is much lower in oocytes, suggesting that the relative holding time at the D-loop stage is shorter. Duplex synthesis on the displaced strand occurs early and with multiple initiations. The frequency of totally duplex replicative forms, or Cairns' forms, is the highest reported for mtDNA. The differences may be related to the fact that oocyte mtDNA replication occurs in the absence of cell division and need not be coordinated with a cell cycle. Molecules with expanded D loops banded in the intermediate region between the lower and upper bands in an ethidium bromide-CsCl gradient, supporting the notion that displacement replication proceeds on a closed circular template which is subject to nicking-closing cycles. In mature sea urchin eggs, replicative forms are absent and virtually all the mtDNA is stored as clean circular duplexes. Some novel structural variants of superhelical circular DNA (molecules with denaturation loops and double branch-migrated replicative forms) are reported
The ballistic to diffusive crossover in a weakly-interacting Fermi gas
Charge and energy are expected to diffuse in interacting systems of fermions
at finite temperatures, even in the absence of disorder, with the interactions
inducing a crossover from the coherent and ballistic streaming of
quasi-particles at early times, to incoherent diffusive behavior at late times.
The relevant crossover timescales and the transport coefficients are both
controlled by the strength of interactions. In this work we develop a numerical
method to simulate such systems at high temperatures, applicable in a wide
range of interaction strengths, by adapting Dissipation-assisted Operator
Evolution (DAOE) to fermions. Our fermion DAOE, which approximates the exact
dynamics by systematically discarding information from high -point
functions, is tailored to capture non-interacting dynamics exactly, thus
providing a good starting point for the weakly interacting problem. Applying
our method to a microscopic model of weakly interacting fermions, we
numerically demonstrate that the crossover from ballistic to diffusive
transport happens at a time and that the diffusion
constant similarly scales as , where is the
interaction strength. We substantiate this scaling with a Fermi's golden rule
calculation in the operator spreading picture, interpreting as the
fermion-fermion scattering time and lifetime of the single-particle Green's
function
Statistical mechanics of dimers on quasiperiodic tilings
We study classical dimers on two-dimensional quasiperiodic Ammann-Beenker
(AB) tilings. Despite the lack of periodicity we prove that each infinite
tiling admits 'perfect matchings' in which every vertex is touched by one
dimer. We introduce an auxiliary 'AB' tiling obtained from the AB tiling by
deleting all 8-fold coordinated vertices. The AB tiling is again
two-dimensional, infinite, and quasiperiodic. The AB tiling has a single
connected component, which admits perfect matchings. We find that in all
perfect matchings, dimers on the AB tiling lie along disjoint
one-dimensional loops and ladders, separated by 'membranes', sets of edges
where dimers are absent. As a result, the dimer partition function of the
AB tiling factorizes into the product of dimer partition functions along
these structures. We compute the partition function and free energy per edge on
the AB tiling using an analytic transfer matrix approach. Returning to the
AB tiling, we find that membranes in the AB tiling become
'pseudomembranes', sets of edges which collectively host at most one dimer.
This leads to a remarkable discrete scale-invariance in the matching problem.
The structure suggests that the AB tiling should exhibit highly inhomogenous
and slowly decaying connected dimer correlations. Using Monte Carlo
simulations, we find evidence supporting this supposition in the form of
connected dimer correlations consistent with power law behaviour. Within the
set of perfect matchings we find quasiperiodic analogues to the staggered and
columnar phases observed in periodic systems.Comment: 33 pages, 26 figure
Hamiltonian cycles on Ammann-Beenker tilings
We provide a simple algorithm for constructing Hamiltonian graph cycles (visiting every vertex exactly once) on a set of arbitrarily large finite subgraphs of aperiodic two-dimensional Ammann-Beenker (AB) tilings. Using this result, and the discrete scale symmetry of AB tilings, we find exact solutions to a range of other problems which lie in the complexity class NP-complete for general graphs. These include the equal-weight traveling salesperson problem, providing, for example, the most efficient route a scanning tunneling microscope tip could take to image the atoms of physical quasicrystals with AB symmetries; the longest path problem, whose solution demonstrates that collections of flexible molecules of any length can adsorb onto AB quasicrystal surfaces at density one, with possible applications to catalysis; and the three-coloring problem, giving ground states for th
Landau polaritons in highly non-parabolic 2D gases in the ultra-strong coupling regime
We probe ultra-strong light matter coupling between metallic terahertz
metasurfaces and Landau-level transitions in high mobility 2D electron and hole
gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron
cyclotron resonances in InSb quantum wells, both within highly non-parabolic
bands, and compare our results to well known parabolic AlGaAs/GaAs quantum well
(QW) systems. Tuning the coupling strength of the system by two methods,
lithographically and by optical pumping, we observe a novel behavior clearly
deviating from the standard Hopfield model previously verified in cavity
quantum electrodynamics: an opening of a lower polaritonic gap
HECTD2 Is Associated with Susceptibility to Mouse and Human Prion Disease
Prion diseases are fatal transmissible neurodegenerative disorders, which include Scrapie, Bovine Spongiform Encephalopathy (BSE), Creutzfeldt-Jakob Disease (CJD), and kuru. They are characterised by a prolonged clinically silent incubation period, variation in which is determined by many factors, including genetic background. We have used a heterogeneous stock of mice to identify Hectd2, an E3 ubiquitin ligase, as a quantitative trait gene for prion disease incubation time in mice. Further, we report an association between HECTD2 haplotypes and susceptibility to the acquired human prion diseases, vCJD and kuru. We report a genotype-associated differential expression of Hectd2 mRNA in mouse brains and human lymphocytes and a significant up-regulation of transcript in mice at the terminal stage of prion disease. Although the substrate of HECTD2 is unknown, these data highlight the importance of proteosome-directed protein degradation in neurodegeneration. This is the first demonstration of a mouse quantitative trait gene that also influences susceptibility to human prion diseases. Characterisation of such genes is key to understanding human risk and the molecular basis of incubation periods
9th Biennial Judge Joe Lee Bankruptcy Institute
Materials from the 9th Biennial Judge Joe Lee Bankruptcy Institute held December 1999
Evidence and Ideology in Macroeconomics: The Case of Investment Cycles
The paper reports the principal findings of a long term research project on the description and explanation of business cycles. The research strongly confirmed the older view that business cycles have large systematic components that take the form of investment cycles. These quasi-periodic movements can be represented as low order, stochastic, dynamic processes with complex eigenvalues. Specifically, there is a fixed investment cycle of about 8 years and an inventory cycle of about 4 years. Maximum entropy spectral analysis was employed for the description of the cycles and continuous time econometrics for the explanatory models. The central explanatory mechanism is the second order accelerator, which incorporates adjustment costs both in relation to the capital stock and the rate of investment. By means of parametric resonance it was possible to show, both theoretically and empirically how cycles aggregate from the micro to the macro level. The same mathematical tool was also used to explain the international convergence of cycles. I argue that the theory of investment cycles was abandoned for ideological, not for evidential reasons. Methodological issues are also discussed
Meta-analysis of genome-wide association studies from the CHARGE consortium identifies common variants associated with carotid intima media thickness and plaque
Carotid intima media thickness (cIMT) and plaque determined by ultrasonography are established measures of subclinical atherosclerosis that each predicts future cardiovascular disease events. We conducted a meta-analysis of genome-wide association data in 31,211 participants of European ancestry from nine large studies in the setting of the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium. We then sought additional evidence to support our findings among 11,273 individuals using data from seven additional studies. In the combined meta-analysis, we identified three genomic regions associated with common carotid intima media thickness and two different regions associated with the presence of carotid plaque (P < 5 × 10 -8). The associated SNPs mapped in or near genes related to cellular signaling, lipid metabolism and blood pressure homeostasis, and two of the regions were associated with coronary artery disease (P < 0.006) in the Coronary Artery Disease Genome-Wide Replication and Meta-Analysis (CARDIoGRAM) consortium. Our findings may provide new insight into pathways leading to subclinical atherosclerosis and subsequent cardiovascular events
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