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Gabrielli P Barbante C Plane JM Varga A Hong S Cozzi G Gaspari V Planchon FA Cairns W Ferrari C Crutzen P Cescon P Boutron CF 《Nature》2004,432(7020):1011-1014
An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65 million years ago. It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate or the nanophase material of the vaporized impactor. But the fate of platinum-group elements (such as iridium) that regularly enter the atmosphere via ablating meteoroids remains largely unknown. Here we report a record of iridium and platinum fluxes on a climatic-cycle timescale, back to 128,000 years ago, from a Greenland ice core. We find that unexpectedly constant fallout of extraterrestrial matter to Greenland occurred during the Holocene, whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust-laden atmosphere of the last glacial age. We suggest that nanometre-sized meteoric smoke particles, formed from the recondensation of ablated meteoroids in the atmosphere at altitudes >70 kilometres, are transported into the winter polar vortices by the mesospheric meridional circulation and are preferentially deposited in the polar ice caps. This implies an average global fallout of 14 +/- 5 kilotons per year of meteoric smoke during the Holocene. 相似文献
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Nucleosome mobilization catalysed by the yeast SWI/SNF complex. 总被引:18,自引:0,他引:18
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This paper presents systematically a method for image compression/decompression viawavelet transform.It consists of filter,quantization and Huffman coding etc..Different methodshave been compared.Finally,some suggestions for further studies are proposed.In fact,the paper isa summary of our recent research. 相似文献
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The origin of human cancers 总被引:32,自引:0,他引:32
The limited evidence available suggests that most human cancers are not caused by conventional mutagens but are more likely to be the result of genetic transpositions. Although the molecular biology of transposition is starting to be understood, the external factors that influence its frequency have not yet been studied in any detail. 相似文献
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Mutation selection and the natural history of cancer. 总被引:76,自引:0,他引:76
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James R. Ketudat Cairns Asim Esen 《Cellular and molecular life sciences : CMLS》2010,67(20):3389-3405
β-Glucosidases (3.2.1.21) are found in all domains of living organisms, where they play essential roles in the removal of
nonreducing terminal glucosyl residues from saccharides and glycosides. β-Glucosidases function in glycolipid and exogenous
glycoside metabolism in animals, defense, cell wall lignification, cell wall β-glucan turnover, phytohormone activation, and
release of aromatic compounds in plants, and biomass conversion in microorganisms. These functions lead to many agricultural
and industrial applications. β-Glucosidases have been classified into glycoside hydrolase (GH) families GH1, GH3, GH5, GH9,
and GH30, based on their amino acid sequences, while other β-glucosidases remain to be classified. The GH1, GH5, and GH30
β-glucosidases fall in GH Clan A, which consists of proteins with (β/α)8-barrel structures. In contrast, the active site of GH3 enzymes comprises two domains, while GH9 enzymes have (α/α)6 barrel structures. The mechanism by which GH1 enzymes recognize and hydrolyze substrates with different specificities remains
an area of intense study. 相似文献