Showing 251 - 260 of 733 Items
Mercury in the snow and firn at Summit Station, Central Greenland, and implications for the study of past atmospheric mercury levels
Date: 2008-06-30
Creator: X. Faïn, C. P. Ferrari, A. Dommergue, M. Albert, M., Battle, L. Arnaud, J. M. Barnola, W. Cairns, C. Barbante, C. Boutron
Access: Open access
- Gaseous Elemental Mercury (Hg° or GEM) was investigated at Summit Station, Greenland, in the interstitial air extracted from the perennial snowpack (firn) at depths ranging from the surface to 30 m, during summer 2005 and spring 2006. Photolytic production and destruction of Hg° were observed close to the snow surface during summer 2005 and spring 2006, and we observed dark oxidation of GEM up to 270 cm depth in June 2006. Photochemical transformation of gaseous elemental mercury resulted in diel variations in the concentrations of this gas in the near-surface interstitial air, but destruction of Hg° was predominant in June, and production was the main process in July. This seasonal evolution of the chemical mechanisms involving gaseous elemental mercury produces a signal that propagates downward through the firn air, but is unobservably small below 15 m in depth. As a consequence, multi-annual averaged records of GEM concentration should be well preserved in deep firn air at depths below 15 m, and available for the reconstruction of the past atmospheric history of GEM over the last decades.
Localization of DNA sequences promoting RNA polymerase I activity in Drosophila
Date: 1983-01-01
Creator: B. D. Kohorn, P. M.M. Rae
Access: Open access
Resonance in the menstrual cycle: A new model of the LH surge
Date: 2003-01-01
Creator: Mary Lou Zeeman, W. Weckesser, D. Gokhman
Access: Open access
- In vertebrates, ovulation is triggered by a surge of LH from the pituitary. The precise mechanism by which rising oestradiol concentrations initiate the LH surge in the human menstrual cycle remains a fundamental open question of reproductive biology. It is well known that sampling of serum LH on a time scale of minutes reveals pulsatile release from the pituitary in response to pulses of gonadotrophin releasing hormone from the hypothalamus. The LH pulse frequency and amplitude vary considerably over the cycle, with the highest frequency and amplitude at the midcycle surge. Here a new mathematical model is presented of the pituitary as a damped oscillator (pulse generator) driven by the hypothalamus. The model LH surge is consistent with LH data on the time scales of both minutes and days. The model is used to explain the surprising pulse frequency characteristics required to treat human infertility disorders such as Kallmann's syndrome, and new experimental predictions are made.
Extinction in nonautonomous competitive Lotka-Volterra Systems
Date: 1996-01-01
Creator: Francisco Montes De Oca, Mary Lou Zeeman
Access: Open access
- It is well known that for the two species autonomous competitive Lotka-Volterra model with no fixed point in the open positive quadrant, one of the species is driven to extinction, whilst the other population stabilises at its own carrying capacity. In this paper we prove a generalisation of this result to nonautonomous systems of arbitrary finite dimension. That is, for the n species nonautonomous competitive Lotka-Volterra model, we exhibit simple algebraic criteria on the parameters which guarantee that all but one of the species is driven to extinction. The restriction of the system to the remaining axis is a nonautonomous logistic equation, which has a unique solution u(t) that is strictly positive and bounded for all time; see Coleman (Math. Biosci. 45 (1979), 159-173) and Ahmad (Proc. Amer. Math. Soc. 117 (1993), 199-205). We prove in addition that all solutions of the n-dimensional system with strictly positive initial conditions are asymptotic to u(t). © 1996 American Mathematical Society.
Ocean primary production derived from satellite data: An evaluation with atmospheric oxygen measurements
Date: 1999-06-01
Creator: Yves Balkanski, Patrick Monfray, Mark Battle, Martin Heimann
Access: Open access
- Recently, very precise measurements have detected the seasonal variability in the atmospheric O2/N2 ratio at several sites in the northern and southern hemispheres. In this paper, we derive marine primary productivity (PP) from satellite ocean color data. To infer air-sea oxygen fluxes, a simple one-dimensional diagnostic model of ocean biology has been developed that depends on only two parameters: a time delay between organic production and oxidation (set to 2 weeks) and an export scale length (50 m). This model gives a global net community production of 4.3 mol C m-2 yr-1 in the euphotic zone and 3.2 mol C m-2 yr-1 in the mixed layer. This last value corresponds to a global f ratio (net community production (NCP)/PP) at the base of the mixed layer of 0.37. The air-sea fluxes derived from this model are then used at the base of a three-dimensional atmospheric model to compare the atmospheric seasonal cycle of O2/N2 at five sites: Cape Grim (40.6S, 144.6E), Baring Head (41.3S, 174.8E), Mauna Loa (19.5N,154.8W), La Jolla (32.9N, 117.3W), and Barrow (71.3N, 156.6W). The agreement between model and observations is very encouraging. We infer from the agreement that the seasonal variations in O2/N2 are largely controlled by the photosynthesis rate but also by the remineralization linked to the deepening and shoaling of the mixed layer. Lateral ventilation to high latitudes may also be an important factor controlling the amplitude of the seasonal cycle.
Identification of the determinants for the specific recognition of single-strand telomeric DNA by Cdc13
Date: 2006-01-24
Creator: Aimee M. Eldridge, Wayne A. Halsey, Deborah S. Wuttke
Access: Open access
- The single-strand overhang present at telomeres plays a critical role in mediating both the capping and telomerase regulation functions of telomeres. The telomere end-binding proteins, Cdc13 in Saccharomyces cerevisiae, Pot1 in higher eukaryotes, and TEBP in the ciliated protozoan Oxytricha nova, exhibit sequence-specific binding to their respective single-strand overhangs. S. cerevisiae telomeres are composed of a heterogeneous mixture of GT-rich telomeric sequence, unlike in higher eukaryotes which have a simple repeat that is maintained with high fidelity. In yeast, the telomeric overhang is recognized by the essential protein Cdc13, which coordinates end-capping and telomerase activities at the telomere. The Cdc13 DNA-binding domain (Cdc13-DBD) binds these telomere sequences with high affinity (3 pM) and sequence specificity. To better understand the basis for this remarkable recognition, we have investigated the binding of the Cdc13-DBD to a series of altered DNA substrates. Although an 11-mer of GT-rich sequence is required for full binding affinity, only three of these 11 bases are recognized with high specificity. This specificity differs from that observed in the other known telomere end-binding proteins, but is well suited to the specific role of Cdc13 at yeast telomeres. These studies expand our understanding of telomere recognition by the Cdc13-DBD and of the unique molecular recognition properties of ssDNA binding. © 2006 American Chemical Society.
A record of atmospheric halocarbons during the twentieth century from polar firn air
Date: 1999-06-24
Creator: James H. Butler, Mark Battle, Michael L. Bender, Stephen A. Montzka, Andrew D., Clarke, Eric S. Saltzman, Cara M. Sucher, Jeffrey P. Severinghaus, James W. Elkins
Access: Open access
- Measurements of trace gases in air trapped in polar firn (unconsolidated snow) demonstrate that natural sources of chlorofluorocarbons, halons, persistent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are minimal or non-existent. Atmospheric concentrations of these gases, reconstructed back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric lifetimes. The measurements confirm the predominance of human activity in the atmospheric budget of organic chlorine, and allow the estimation of atmospheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to that at present, while the burden of methyl bromide was probably over half of today's value.
Disruption of topoisomerase II perturbs pairing in Drosophila cell culture
Date: 2007-09-01
Creator: Benjamin R. Williams, Jack R. Bateman, Natasha D. Novikov, C. Ting Wu
Access: Open access
- Homolog pairing refers to the alignment and physical apposition of homologous chromosomal segments. Although commonly observed during meiosis, homolog pairing also occurs in nonmeiotic cells of several organisms, including humans and Drosophila. The mechanism underlying nonmeiotic pairing, however, remains largely unknown. Here, we explore the use of established Drosophila cell lines for the analysis of pairing in somatic cells. Using fluorescent in situ hybridization (FISH), we assayed pairing at nine regions scattered throughout the genome of Kc167 cells, observing high levels of homolog pairing at all six euchromatic regions assayed and variably lower levels in regions in or near centromeric heterochromatin. We have also observed extensive pairing in six additional cell lines representing different tissues of origin, different ploidies, and two different species, demonstrating homolog pairing in cell culture to be impervious to cell type or culture history. Furthermore, by sorting Kc167 cells into G1, S, and G2 subpopulations, we show that even progression through these stages of the cell cycle does not significantly change pairing levels. Finally, our data indicate that disrupting Drosophila topoisomerase II (Top2) gene function with RNAi and chemical inhibitors perturbs homolog pairing, suggesting Top2 to be a gene important for pairing. Copyright © 2007 by the Genetics Society of America.
A 350-year atmospheric history for carbonyl sulfide inferred from Antarctic firn air and air trapped in ice
Date: 2004-11-27
Creator: Stephen A. Montzka, M. Aydin, M. Battle, J. H. Butler, E. S., Saltzman, B. D. Hall, A. D. Clarke, D. Mondeel, J. W. Elkins
Access: Open access
- Carbonyl sulfide (COS) and other trace gases were measured in firn air collected near South Pole (89.98°S) and from air trapped in ice at Siple Dome, Antarctica (81.65°S). The results, when considered with ambient air data and previous ice core measurements, provide further evidence that atmospheric mixing ratios of COS over Antarctica between 1650 and 1850 A.D. were substantially lower than those observed today. Specifically, the results suggest annual mean COS mixing ratios between 300 and 400 pmol mol-1 (ppt) during 1650-1850 A.D. and increases throughout most of the twentieth century. Measurements of COS in modern air and in the upper layers of the firn at South Pole indicate ambient, annual mean mixing ratios between 480 and 490 ppt with substantial seasonal variations. Peak mixing ratios are observed during austral summer in ambient air at South Pole and Cape Grim, Tasmania (40.41°S). Provided COS is not produced or destroyed in firn, these results also suggest that atmospheric COS mixing ratios have decreased 60-90 ppt (10-16%) since the 1980s in high latitudes of the Southern Hemisphere. The history derived for atmospheric mixing ratios of COS in the Southern Hemisphere since 1850 is closely related to historical anthropogenic sulfur emissions. The fraction of anthropogenic sulfur emissions released as COS (directly or indirectly) needed to explain the secular changes in atmospheric COS over this period is 0.3-0.6%. Copyright 2004 by the American Geophysical Union.
One-loop SYM-supergravity relation for five-point amplitudes
Date: 2011-11-21
Creator: Stephen G. Naculich, Howard J. Schnitzer
Access: Open access
- We derive a linear relation between the one-loop five-point amplitude of N = 8 supergravity and the one-loop five-point subleading-color amplitudes of N = 4 supersymmetric Yang-Mills theory. © 2011 SISSA.