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Search Results: 1 - 10 of 475087 matches for " Mark A. Hallen "
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Anastral spindle assembly and γ-tubulin in Drosophila oocytes
Sharyn A Endow, Mark A Hallen
BMC Cell Biology , 2011, DOI: 10.1186/1471-2121-12-1
Abstract: We show, for the first time, using a bright GFP fusion protein and live imaging, that the Drosophila maternally-expressed γTub37C is present at low levels in oocyte meiosis I spindles. Despite this, we find that formation of bipolar meiosis I spindles does not require functional γTub37C, extending previous findings by others. Fluorescence photobleaching assays show rapid recovery of γTub37C in the meiosis I spindle, similar to the cytoplasm, indicating weak binding by γTub37C to spindles, and fits of a new, potentially more accurate model for fluorescence recovery yield kinetic parameters consistent with transient, diffusional binding.The FRAP results, together with its mutant effects late in meiosis I, indicate that γTub37C may perform a role subsequent to metaphase I, rather than nucleating microtubules for meiosis I spindle formation. Weak binding to the meiosis I spindle could stabilize pre-existing microtubules or position γ-tubulin for function during meiosis II spindle assembly, which follows rapidly upon oocyte activation and completion of the meiosis I division.Anastral spindles assemble without centrosomes by a pathway that is still not fully understood. In particular, the mechanism by which microtubule nucleation occurs has not been well defined. Although chromatin has been shown to play an essential role [1], the involvement of the microtubule nucleator, γ-tubulin, is still an open question. γ-Tubulin localizes to centrosomes and other microtubule organizing centers in mitosis and is essential for nucleating microtubules in organisms as diverse as yeast, Drosophila, Xenopus, humans, and higher plants [2-5]. γ-Tubulin is also found on spindle microtubules, where it has been proposed to nucleate microtubules for spindle maintenance by functioning in a chromatin-mediated nucleation pathway that augments the dominant pathway of nucleation by centrosomes [6,7].γ-Tubulin is present in cells as a large ring complex, γTuRC, comprising 12-13 γ-tubulin molecules a
Altered Nucleotide-Microtubule Coupling and Increased Mechanical Output by a Kinesin Mutant
Hong-Lei Liu, Mark A. Hallen, Sharyn A. Endow
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0047148
Abstract: Kinesin motors hydrolyze ATP to produce force and do work in the cell – how the motors do this is not fully understood, but is thought to depend on the coupling of ATP hydrolysis to microtubule binding by the motor. Transmittal of conformational changes from the microtubule- to the nucleotide-binding site has been proposed to involve the central β-sheet, which could undergo large structural changes important for force production. We show here that mutation of an invariant residue in loop L7 of the central β-sheet of the Drosophila kinesin-14 Ncd motor alters both nucleotide and microtubule binding, although the mutated residue is not present in either site. Mutants show weak-ADP/tight-microtubule binding, instead of tight-ADP/weak-microtubule binding like wild type – they hydrolyze ATP faster than wild type, move faster in motility assays, and assemble long spindles with greatly elongated poles, which are also produced by simulations of assembly with tighter microtubule binding and faster sliding. The mutated residue acts like a mechanochemical coupling element – it transmits changes between the microtubule-binding and active sites, and can switch the state of the motor, increasing mechanical output by the motor. One possibility, based on our findings, is that movements by the residue and the loop that contains it could bend or distort the central β-sheet, mediating free energy changes that lead to force production.
Comportamiento Mecánico de Grietas No Coplanares en Tubos Aplicando el Método de los Elementos Finitos/[title] [title language=en]Mechanical Behavior of Non-Coplanar Cracks in Pipes Applying the Finite Elements Method
Morales,A.; González,J. L.; Hallen,J. M.;
Información tecnológica , 2004, DOI: 10.4067/S0718-07642004000600005
Abstract: planar-type cracks formed at different depths within tube walls, which are generally caused by hydrogen-induced cracking were analyzed. modeling was carried out on the interaction of pressurized cracks contained within the tube wall with internal pressure prior to their coalescence, varying the radius of the defects. the finite elements method was applied under non-linear conditions of the material following the isotropic hardening law and considering the properties of the material (api 5l-x52). the results suggested the evolution of stress fields and deformations in the crack tips area as a function of the pressure of the defect. defects of less than 38.1 mm resist a pressure of 70 to 124 mpa, and those larger than 63.5 mm severely affect the mechanical integrity. the function of the critical pressure supported by symmetrical cracks is of the potential type, and this pressure produces the interaction which plastifies the region among the cracks, prior to stepping
Comportamiento Mecánico de Grietas No Coplanares en Tubos Aplicando el Método de los Elementos Finitos/[title] [title language=en]Mechanical Behavior of Non-Coplanar Cracks in Pipes Applying the Finite Elements Method
A. Morales,J. L. González,J. M. Hallen
Información Tecnológica , 2004,
Abstract: Se analizan grietas del tipo planar formadas a diferentes profundidades del espesor del tubo y que generalmente son causadas por el agrietamiento inducido por hidrógeno. Se modeló la interacción de grietas presurizadas contenidas en la pared del tubo con presión interna previas a su coalescencia, variando el radio de los defectos. Se aplicó el método de los elementos finitos en condiciones no lineales del material bajo la ley de endurecimiento isotrópico y considerando las propiedades del material (API 5L-X52). Los resultados indican la evolución de los campos de esfuerzos y deformaciones en el área de puntas de grieta en función de la presión del defecto. Defectos menores a 38.1 mm resisten una presión de 70 hasta 124 MPa, y mayores de 63.5 mm afectan severamente la integridad mecánica. La función de la presión critica que soportan grietas simétricas es de tipo potencial y esta presión produce la interacción que plastifica la región entre las grietas, previamente al escalonamiento Planar-type cracks formed at different depths within tube walls, which are generally caused by hydrogen-induced cracking were analyzed. Modeling was carried out on the interaction of pressurized cracks contained within the tube wall with internal pressure prior to their coalescence, varying the radius of the defects. The finite elements method was applied under non-linear conditions of the material following the isotropic hardening law and considering the properties of the material (API 5L-X52). The results suggested the evolution of stress fields and deformations in the crack tips area as a function of the pressure of the defect. Defects of less than 38.1 mm resist a pressure of 70 to 124 MPa, and those larger than 63.5 mm severely affect the mechanical integrity. The function of the critical pressure supported by symmetrical cracks is of the potential type, and this pressure produces the interaction which plastifies the region among the cracks, prior to stepping
“Ethnophilosophy” Redefined?
B Hallen
Thought and Practice: A Journal of the Philosophical Association of Kenya , 2010,
Abstract: The meaning of the term “ethnophilosophy” has evolved in both a significant and controversial variety of ways since it was first introduced by Paulin Hountondji in 1970. It was first challenged by the Kenyan philosopher, H. Odera Oruka, as based upon Hountondji’s unfair appreciation of Africa’s indigenous cultural heritage. Barry Hallen and J. Olubi Sodipo, using a form of analytic philosophy as foundational, thereafter argued that Yoruba ordinary language discourse also served to undermine Hountondji’s critique. The later work of the Ghanaian philosopher, Kwame Gyekye, and the Kenyan D. A. Masolo have further legitimized the epistemological status of elements of African culture that once would have been labeled as of no genuine philosophical significance because they were ‘ethnophilosophical’ in character. The end result of this debate seems to be that both the form and content of philosophy in culture generally must be relativized. The most significant consequence of this would be that African and non-Western philosophy generally would finally be culturally liberated from the oppressive influence, indeed dominance, of what has conventionally come to be known as ‘mainstream’ (Western) philosophy.
Computation of Steady-State Probability Distributions in Stochastic Models of Cellular Networks
Mark Hallen,Bochong Li,Yu Tanouchi,Cheemeng Tan,Mike West,Lingchong You
PLOS Computational Biology , 2011, DOI: 10.1371/journal.pcbi.1002209
Abstract: Cellular processes are “noisy”. In each cell, concentrations of molecules are subject to random fluctuations due to the small numbers of these molecules and to environmental perturbations. While noise varies with time, it is often measured at steady state, for example by flow cytometry. When interrogating aspects of a cellular network by such steady-state measurements of network components, a key need is to develop efficient methods to simulate and compute these distributions. We describe innovations in stochastic modeling coupled with approaches to this computational challenge: first, an approach to modeling intrinsic noise via solution of the chemical master equation, and second, a convolution technique to account for contributions of extrinsic noise. We show how these techniques can be combined in a streamlined procedure for evaluation of different sources of variability in a biochemical network. Evaluation and illustrations are given in analysis of two well-characterized synthetic gene circuits, as well as a signaling network underlying the mammalian cell cycle entry.
Modelado del crecimiento de picaduras en tuberías enterradas que transportan hidrocarburos utilizando técnicas estadísticas
Velázquez, J. C.,Caleyo, F.,Valor, A.,Hallen, J. M.
Revista de Metalurgia , 2011,
Abstract: New deterministic and stochastic predictive models are proposed for external pitting corrosion in underground pipelines. The deterministic model takes into consideration the local chemical and physical properties of the soil as well as the pipeline coating to predict the time dependence of pitting depth and rate in a range of soils. This model, based on results from a field study, was used to conduct Monte Carlo simulations that established the probability distribution of pitting depth and growth rate in the studied soils and their evolution over the life of the pipeline. In the last stage of the study, an empirical Markov chain-based stochastic model was developed for predicting the evolution of pitting corrosion depth and rate distributions from the observed properties of the soil. Esta investigación, resume los resultados de un estudio, enfocado en contribuir a resolver las necesidades de predicción de crecimiento de picadura en ductos que transportan hidrocarburos. Primero, se describe el desarrollo de un modelo determinista, considerando las propiedades físicas y químicas del suelo y el tipo de recubrimiento que protege la tubería. Este modelo fue utilizado para realizar simulaciones de Monte Carlo, logrando establecer las distribuciones de probabilidad de profundidad de picadura y su velocidad de crecimiento en suelos. Finalmente, se detalla el desarrollo de un modelo estocástico basado en cadenas de Markov. Este modelo tiene como objetivo, predecir la evolución en el tiempo de las distribuciones de profundidad de picadura.
Degradación de la tenacidad al impacto durante el envejecimiento acelerado de soldadura en acero microaleado
Vargas-Arista, B.,Hallen, J. M.,Albiter, A.,ángeles-Chávez, C.
Revista de Metalurgia , 2008,
Abstract: The effect of accelerated aging on the toughness and fracture of the longitudinal weld metal of an API5L-X52 linepipe steel was evaluated by Charpy V-notch impact test, fracture analysis and transmission electron microscopy. Aging was performed at 250 °C for 100 to 1000 h. The impact results indicated a significant reduction in the fracture energy and impact toughness as a function of aging time, which were achieved by the scanning electron microscope fractographs that showed a decrease in the vol fraction of microvoids by Charpy ductile failure with the aging time, which favored the brittle fracture by transgranular cleavage. The minimum vol fraction of microvoids was reached at 500 h due to the peak-aged. The microstructural analysis indicated the precipitation of transgranular iron nanocarbides in the aged specimens, which was related to the deterioration of toughness and change in the ductile to brittle behavior. Se evaluó, mediante el ensayo de impacto Charpy, fractografía y microscopia electrónica de transmisión el efecto del envejecimiento acelerado sobre la tenacidad y fractura de la soldadura en tubería de acero API5L-X52. El envejecimiento se realizó a 250 °C por 1.000 h, con control cada 100 h. Los resultados de impacto indicaron una disminución en la energía de fractura y tenacidad al impacto en función del tiempo del envejecimiento, los cuales se evidenciaron mediante fractografía, por la reducción en la fracción volumétrica de microhuecos por fractura dúctil con el tiempo, favoreciendo la fractura frágil por clivaje transgranular. Sin embargo, a 500 h, se observó la fracción volumétrica mínima debido al pico del envejecimiento. El análisis microestructural evidenció la precipitación de nanocarburos de hierro transgranulares en las muestras envejecidas, la cual se relaciono con la pérdida de tenacidad y cambio en el comportamiento dúctil a frágil, confirmado por fractografía.
Gender and Anxiety: A Comparison of Student Anxiety Levels in Face-to-Face and Video Conferencing Courses  [PDF]
Jodi McKnight, Mark A. McKnight
Creative Education (CE) , 2012, DOI: 10.4236/ce.2012.31015
Abstract: This research focuses on the role of gender in face-to-face instruction and video conferencing instruction on students’ levels of anxiety. This is due, in part, to the fact that gender and anxiety levels of students enrolled in remote video conferencing learning environments has received little attention in either psychological or educational research. A difference in gender as it relates to education is an important focus of research. This is due to the increasing learning opportunities for female students (online in particular). Explored later, further research should investigate various demographics and delivery options for courses.
Forest Fragmentation and Its Potential Implications in the Brazilian Amazon between 2001 and 2010  [PDF]
Izaya Numata, Mark A. Cochrane
Open Journal of Forestry (OJF) , 2012, DOI: 10.4236/ojf.2012.24033
Abstract: In recent decades, human development pressures have results in conversions of vast tracts of Amazonian tropical rain forests to agriculture and other human land uses. In addition to the loss of large forest cover, remaining forests are also fragmented into smaller habitats. Fragmented forests suffer several biological and ecological changes due to edge effects that can exacerbate regional forest degradation. The Brazilian Amazon has had greatly contrasting land cover dynamics in the past decade with the highest historical rates of deforestation (2001-2005) followed by the lowest rates of forest loss in decades, since 2006. Currently, the basin-wide status and implications of forest fragmentation on remnant forests is not well known. We performed a regional forest fragmentation analysis for seven states of the Brazilian Amazon between 2001 and 2010 using a recent deforestation data. During this period, the number of forest fragments (>2 ha) doubled, nearly 125,000 fragments were formed by human activities with more than 50% being smaller than 10 ha. Over the decade, forest edges increased by an average of 36,335 km/year. However, the rate was much greater from 2001-2005 (50,046 km/year) then 2006-2010 (25,365 km/year) when deforestation rates dropped drastically. In 2010, 55% of basin-wide forest edges were < 10 years old due to the creation of large number of small fragments where intensive biological and ecological degradation is ongoing. Over the past decade protected areas have been expanded dramatically over the Brazilian Amazon and, as of 2010, 51% of remaining forests across the basin are within protected areas and only 1.5% of protected areas has been deforested. Conversely, intensive forest cover conversion has been occurred in unprotected forests. While 17% of Amazonian forests are within 1 km of forest edges in 2010, the proportion increases to 34% in unprotected areas varying between 14% and 95% among the studied states. Our results indicate that the Brazilian Amazon now largely consists of two contrasting forest conditions: protected areas with vast undisturbed forests and unprotected forests that are highly fragmented and disturbed landscapes.
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