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排序方式: 共有894条查询结果,搜索用时 15 毫秒
81.
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84.
Howard LO 《Science (New York, N.Y.)》1915,41(1052):312-315
85.
Genetic structure of human populations 总被引:1,自引:0,他引:1
Rosenberg NA Pritchard JK Weber JL Cann HM Kidd KK Zhivotovsky LA Feldman MW 《Science (New York, N.Y.)》2002,298(5602):2381-2385
We studied human population structure using genotypes at 377 autosomal microsatellite loci in 1056 individuals from 52 populations. Within-population differences among individuals account for 93 to 95% of genetic variation; differences among major groups constitute only 3 to 5%. Nevertheless, without using prior information about the origins of individuals, we identified six main genetic clusters, five of which correspond to major geographic regions, and subclusters that often correspond to individual populations. General agreement of genetic and predefined populations suggests that self-reported ancestry can facilitate assessments of epidemiological risks but does not obviate the need to use genetic information in genetic association studies. 相似文献
86.
Einsle O Tezcan FA Andrade SL Schmid B Yoshida M Howard JB Rees DC 《Science (New York, N.Y.)》2002,297(5587):1696-1700
A high-resolution crystallographic analysis of the nitrogenase MoFe-protein reveals a previously unrecognized ligand coordinated to six iron atoms in the center of the catalytically essential FeMo-cofactor. The electron density for this ligand is masked in structures with resolutions lower than 1.55 angstroms, owing to Fourier series termination ripples from the surrounding iron and sulfur atoms in the cofactor. The central atom completes an approximate tetrahedral coordination for the six iron atoms, instead of the trigonal coordination proposed on the basis of lower resolution structures. The crystallographic refinement at 1.16 angstrom resolution is consistent with this newly detected component being a light element, most plausibly nitrogen. The presence of a nitrogen atom in the cofactor would have important implications for the mechanism of dinitrogen reduction by nitrogenase. 相似文献
87.
Disseminated infection with Histoplasma capsulatum was diagnosed in a 7-yr-old female Bengal tiger (Panthera tigris). Clinical signs were nonspecific with the exception of brief periods of tachypnea for 5 days prior to death. H. capsulatum organisms were found in the lungs, tracheobronchial lymph nodes, and liver. Diagnosis was confirmed by tracheal wash, urine H. capsulatum enzyme immunoassay, and necropsy results. This report represents the first published account of disseminated histoplasmosis in a tiger. 相似文献
88.
Use of organic carbon and loss-on-ignition to estimate soil organic matter in different soil types and horizons 总被引:2,自引:0,他引:2
Summary Loss-on-ignition (LOI) and the organic C content have been used to estimate soil organic matter. Organic matter is often estimated from organic C by applying a factor of 1.724. Several authors have examined the relationship between LOI, used as an estimate of organic matter, and C by simple linear regressions. In the present study, this approach was examined in relation to two sets of data. LOI overestimates organic matter in soils with significant proportions of clay minerals because of bound water, and correcting for bound water gives some LOI: C ratios of less than 1. It is concluded that differences in the nature of the organic matter in different soils and horizons make the simple regression approach unsuitable. More attention needs to be paid to studies of the nature of the organic matter. 相似文献
89.
Zhai Y Pierre D Si R Deng W Ferrin P Nilekar AU Peng G Herron JA Bell DC Saltsburg H Mavrikakis M Flytzani-Stephanopoulos M 《Science (New York, N.Y.)》2010,329(5999):1633-1636
We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H(2)O + CO → H(2) + CO(2)) used for producing H(2). The alkali ion-associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-O(x)(OH)(y) species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria. 相似文献
90.
Howard CJ Brooke GP Werling D Sopp P Hope JC Parsons KR Collins RA 《Veterinary immunology and immunopathology》1999,72(1-2):119-124
Dendritic cells are professional antigen presenting cells derived from the bone marrow and distributed throughout body tissues where they are located in sites that are suitable for antigen uptake. They are central to the induction of immune responses in naive animals and thus have become targets in strategies that are aimed at modulating resistance to infection. Studies in cattle have shown that the dendritic cells are phenotypically heterogeneous and that the different phenotypes have different biological properties. The molecular basis for this variation has begun to be investigated and has led to the identification of a member of the SIRPalpha family of signal regulatory proteins (MyD1) on a subset of dendritic cells in afferent lymph. Uptake of antigen by cattle dendritic cells is by a number of mechanisms that can involve endocytosis via clathrin coated pits or via caveolae as well as macropinocytosis. 相似文献