Showing 2811 - 2820 of 5708 Items
Assembly of the precursor and processed light-harvesting chlorophyll a/b protein of Lemna into the light-harvesting complex II of barley etiochloroplasts.
Date: 1986-01-01
Creator: P. R. Chitnis, E. Harel, B. D. Kohorn, E. M. Tobin, J. P., Thornber
Access: Open access
- When the in vitro synthesized precursor of a light-harvesting chlorophyll a/b binding protein (LHCP) from Lemna gibba is imported into barley etiochloroplasts, it is processed to a single form. Both the processed form and the precursor are found in the thylakoid membranes, assembled into the light-harvesting complex of photosystem II. Neither form can be detected in the stromal fraction. The relative amounts of precursor and processed forms observed in the thylakoids are dependent on the developmental stage of the plastids used for uptake. The precursor as well as the processed form can also be detected in thylakoids of greening maize plastids used in similar uptake experiments. This detection of a precursor in the thylakoids, which has not been previously reported, could be a result of using rapidly developing plastids and/or using an heterologous system. Our results demonstrate that the extent of processing of LHCP precursor is not a prerequisite for its inclusion in the complex. They are also consistent with the possibility that the processing step can occur after insertion of the protein into the thylakoid membrane.
Animal-to-animal variability in the phasing of the crustacean cardiac motor pattern: An experimental and computational analysis
Date: 2013-01-01
Creator: Alex H. Williams, Molly A. Kwiatkowski, Adam L. Mortimer, Eve Marder, Mary Lou, Zeeman, Patsy S. Dickinson
Access: Open access
- The cardiac ganglion (CG) of Homarus americanus is a central pattern generator that consists of two oscillatory groups of neurons: "small cells" (SCs) and "large cells" (LCs). We have shown that SCs and LCs begin their bursts nearly simultaneously but end their bursts at variable phases. This variability contrasts with many other central pattern generator systems in which phase is well maintained. To determine both the consequences of this variability and how CG phasing is controlled, we modeled the CG as a pair of Morris-Lecar oscillators coupled by electrical and excitatory synapses and constructed a database of 15,000 simulated networks using random parameter sets. These simulations, like our experimental results, displayed variable phase relationships, with the bursts beginning together but ending at variable phases. The model suggests that the variable phasing of the pattern has important implications for the functional role of the excitatory synapses. In networks in which the two oscillators had similar duty cycles, the excitatory coupling functioned to increase cycle frequency. In networks with disparate duty cycles, it functioned to decrease network frequency. Overall, we suggest that the phasing of the CG may vary without compromising appropriate motor output and that this variability may critically determine how the network behaves in response to manipulations. © 2013 the American Physiological Society.
Third-generation effects on fermion mass predictions in supersymmetric grand unified theories
Date: 1993-01-01
Creator: Stephen G. Naculich
Access: Open access
- Relations among fermion masses and mixing angles at the scale of grand unification are modified at lower energies by renormalization group running induced by gauge and Yukawa couplings. In super-symmetric theories, the b quark and lepton Yukawa couplings, as well as the t quark coupling, may cause significant running if tan, the ratio of Higgs field expectation values, is large. We present approximate analytic expressions for the scaling factors for fermion masses and CKM matrix elements induced by all three third generation Yukawa couplings. We then determine how running caused by the third generation of fermions affects the predictions arising from three possible forms for the Yukawa coupling matrices at the GUT scale: the Georgi-Jarlskog, Giudice, and Fritzsch textures. © 1993 The American Physical Society.
Quantum kinks: Solitons at strong coupling
Date: 1992-01-01
Creator: Stephen G. Naculich
Access: Open access
- We examine solitons in theories with heavy fermions. These "quantum" solitons differ dramatically from semiclassical (perturbative) solitons because fermion loop effects are important when the Yukawa coupling is strong. We focus on kinks in a (1 + 1)-dimensional 4 theory coupled to fermions; a large-N expansion is employed to treat the Yukawa coupling g nonperturbatively. A local expression for the fermion vacuum energy is derived using the WKB approximation for the Dirac eigenvalues. We find that fermion loop corrections increase the energy of the kink and (for large g) decrease its size. For large g, the energy of the quantum kink is proportional to g, and its size scales as 1g, unlike the classical kink; we argue that these features are generic to quantum solitons in theories with strong Yukawa couplings. We also discuss the possible instability of fermions to solitons. © 1992 The American Physical Society.
Digital Authoritarianism in China and Russia: A Comparative Study
Date: 2020-01-01
Creator: Laura H.C. Howells
Access: Open access
- Digital authoritarianism is on the rise around the world and threatens the data privacy and rights of both domestic and international Internet users. However, scholarship on digital authoritarianism remains limited in scope and case study selection. This study contributes a new, more comprehensive analytical framework for the study of Internet governance and applies it to the case studies of China and Russia. Special attention is paid to the still understudied Russian Internet governance model. After thorough literature review and novel data collection and analysis, this paper identifies relative centralization of network infrastructure and the extent and pace of change in governance as the most notable differences between the two models. These points of divergence may be explained by two theories; the varieties of authoritarianism hypothesis posits that different political systems face persistent and unique constraints to governance of the digital realm. The development trajectory theory argues that each country’s technological development path foreshadows the systems’ capacity for and extent of governance. This study is among the first to distinguish between Internet governance strategies of authoritarian regimes.
Phononic rogue waves
Date: 2018-09-13
Creator: E. G. Charalampidis, J. Lee, P. G. Kevrekidis, C. Chong
Access: Open access
- We present a theoretical study of extreme events occurring in phononic lattices. In particular, we focus on the formation of rogue or freak waves, which are characterized by their localization in both spatial and temporal domains. We consider two examples. The first one is the prototypical nonlinear mass-spring system in the form of a homogeneous Fermi-Pasta-Ulam-Tsingou (FPUT) lattice with a polynomial potential. By deriving an approximation based on the nonlinear Schrödinger (NLS) equation, we are able to initialize the FPUT model using a suitably transformed Peregrine soliton solution of the NLS equation, obtaining dynamics that resembles a rogue wave on the FPUT lattice. We also show that Gaussian initial data can lead to dynamics featuring a rogue wave for sufficiently wide Gaussians. The second example is a diatomic granular crystal exhibiting rogue-wave-like dynamics, which we also obtain through an NLS reduction and numerical simulations. The granular crystal (a chain of particles that interact elastically) is a widely studied system that lends itself to experimental studies. This study serves to illustrate the potential of such dynamical lattices towards the experimental observation of acoustic rogue waves.
The Impact of Armed Conflict on Maternal Health in Colombia
Date: 2020-01-01
Creator: Madeleine Squibb
Access: Open access
- This study combines data from the 2010 Demographic and Health Survey and the Conflict Analysis Resource Center (CERAC) to examine the impact of conflict on maternal health service utilization and outcomes in Colombia. The primary results indicate a significant, negative relationship between conflict level and antenatal and postnatal care utilization. Conflict is insignificant in determining the use of professional assistance at delivery. Although rural women are, overall, less likely to access maternal health services, further analysis along rural-urban lines reveals that the negative effect of violence on prenatal and postnatal care is stronger among urban women. Secondary estimation of the occurrence of complications during or after delivery employs a Two-Stage Residuals Inclusion model to address potential endogeneity in service use. Estimated results show that conflict levels are insignificant, but that Indigenous women and women in lower wealth quintiles are significantly more likely to experience complications, even after controlling for service use. The conclusions of this paper suggest that Colombia’s universal healthcare system has been successful in reducing economic barriers to prenatal care and professional delivery, but that significant wealth-related inequalities remain in maternal health outcomes. Additionally, Indigenous and women with lower levels of education are less likely to access services and more likely to experience complications. The primary contribution of this paper is the inclusion of a conflict measure. The significant, negative impact on prenatal and postnatal care utilization, especially for urban women, warrants further study to better inform policy to increase service use and reduce maternal mortality and morbidity.
A genomewide survey argues that every zygotic gene product is dispensable for the initiation of somatic homolog pairing in Drosophila
Date: 2008-11-01
Creator: Jack R. Bateman, C. Ting Wu
Access: Open access
- Studies from diverse organisms show that distinct interchromosomal interactions are associated with many developmental events. Despite recent advances in uncovering such phenomena, our understanding of how interchromosomal interactions are initiated and regulated is incomplete. During the maternal-to-zygotic transition (MZT) of Drosophila embryogenesis, stable interchromosomal contacts form between maternal and paternal homologous chromosomes, a phenomenon known as somatic homolog pairing. To better understand the events that initiate pairing, we performed a genomewide assessment of the zygotic contribution to this process. Specifically, we took advantage of the segregational properties of compound chromosomes to generate embryos lacking entire chromosome arms and, thus, all zygotic gene products derived from those arms. Using DNA fluorescence in situ hybridization (FISH) to assess the initiation of pairing at five separate loci, this approach allowed us to survey the entire zygotic genome using just a handful of crosses. Remarkably, we found no defect in pairing in embryos lacking any chromosome arm, indicating that no zygotic gene product is essential for pairing to initiate. From these data, we conclude that the initiation of pairing can occur independently of zygotic control and may therefore be part of the developmental program encoded by the maternal genome. Copyright © 2008 by the Genetics Society of America.
Responses of central pattern generators in the American lobster STNS to multiple members of a novel neuropeptide family
Date: 2020-01-01
Creator: Benjamin Harley Wong
Access: Open access
- Neuropeptides are important modulators of neural activity, allowing neural networks, such as the central pattern generators (CPGs) that control rhythmic movements, to alter their output and thus generate behavioral flexibility. Isoforms of a neuropeptide family vary in physical structure, allowing potentially distinct functional neuromodulatory effects on CPG systems. While some familial neuropeptide isoforms can differentially affect a system, others in the same family may elicit indistinguishable effects. Here, we examined the effects elicited by members of a novel family of six peptide hormone isoforms (GSEFLamides: I-, M-, AL-, AM-, AV-, and VM-GSEFLamide) on the pyloric filter and gastric mill CPGs in the stomatogastric nervous system (STNS) of the American lobster, Homarus americanus. Recent unpublished work from the Dickinson lab found that five of the six GSEFLamides elicited similar increases in contraction amplitude when perfused through the isolated lobster heart, while one (AVGSEFLamide) had virtually no effect. Using extracellular recordings, we found the pattern of GSEFLamide effects on the STNS gastric mill to be similar to the pattern observed in the lobster cardiac system; the gastric mill circuit was fairly consistently activated by all isoforms except AVGSEFLamide. The intrinsically active pyloric pattern was also significantly enhanced by three out of five peptide isoforms, and nearly significantly enhanced by two more, but was likewise non-responsive to AVGSEFLamide. While the reason AVGSEFLamide had no effect on either pattern is unknown, the similar phenomenon noted in the isolated whole heart potentially indicates that this isoform lacks any function in the lobster.
That’s DOPE: the delayed-onset, prolonged excitation response of a primary auditory interneuron in Gryllus bimaculatus This record is embargoed.
- Embargo End Date: 2025-05-13
Date: 2020-01-01
Creator: Samuel G. Brill-Weil
Access: Embargoed