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排序方式: 共有196条查询结果,搜索用时 10 毫秒
41.
Fynbo JP Watson D Thöne CC Sollerman J Bloom JS Davis TM Hjorth J Jakobsson P Jørgensen UG Graham JF Fruchter AS Bersier D Kewley L Cassan A Cerón JM Foley S Gorosabel J Hinse TC Horne KD Jensen BL Klose S Kocevski D Marquette JB Perley D Ramirez-Ruiz E Stritzinger MD Vreeswijk PM Wijers RA Woller KG Xu D Zub M 《Nature》2006,444(7122):1047-1049
It is now accepted that long-duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard 'collapsar' model predicts that a broad-lined and luminous type Ic core-collapse supernova accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we report that GRB 060505 (ref. 10) and GRB 060614 (ref. 11) were not accompanied by supernova emission down to limits hundreds of times fainter than the archetypal supernova SN 1998bw that accompanied GRB 980425, and fainter than any type Ic supernova ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in actively star-forming regions. The absence of a supernova to such deep limits is qualitatively different from all previous nearby long-duration GRBs and suggests a new phenomenological type of massive stellar death. 相似文献
42.
Ashley Graham Kennedy 《Studies in history and philosophy of science》2012,43(2):326-332
In this paper, I examine two idealized models in astrophysics, with the aim of showing that the idealizations in these models play an important explanatory role. I argue, against many representationalists, that it is because of the idealizations in these models, rather than in spite of them, that the models turn out to have explanatory power. In many cases, this claim can be extended to the use of idealized models in the sciences more generally, and thus it gives important insight into the nature of model explanation. 相似文献
43.
Graham Rees 《Annals of science》2013,70(5):549-571
Francis Bacon's reflections on atomism have generally been misunderstood because they have never been systematically studied in relation to the speculative chemical philosophy which he developed in the interval between about 1592 and his death in 1626. This philosophy, in many respects unknown to historians until quite recently, was the only body of positive science which Bacon ever accepted. The speculative philosophy was, on the whole, chemical and non-mechanical, and consequently not consistent with atomist doctrines. In fact, Bacon never at any time accepted the principal atomist tenets. It is therefore necessary to explain why he was interested in atomism at all. Of the several reasons for his interest one (hitherto unrecognised) is especially important—namely, his belief that the speculative and atomic philosophies had affinities. In particular, he believed that of all the ancient philosophies atomism alone approached the ideals associated with the key notion of ‘subtlety’—ideals represented in their highest form (as he imagined) in the speculative philosophy. 相似文献
44.
Serum retinol binding protein 4 contributes to insulin resistance in obesity and type 2 diabetes 总被引:3,自引:0,他引:3
Yang Q Graham TE Mody N Preitner F Peroni OD Zabolotny JM Kotani K Quadro L Kahn BB 《Nature》2005,436(7049):356-362
In obesity and type 2 diabetes, expression of the GLUT4 glucose transporter is decreased selectively in adipocytes. Adipose-specific Glut4 (also known as Slc2a4) knockout (adipose-Glut4(-/-)) mice show insulin resistance secondarily in muscle and liver. Here we show, using DNA arrays, that expression of retinol binding protein-4 (RBP4) is elevated in adipose tissue of adipose-Glut4(-/-) mice. We show that serum RBP4 levels are elevated in insulin-resistant mice and humans with obesity and type 2 diabetes. RBP4 levels are normalized by rosiglitazone, an insulin-sensitizing drug. Transgenic overexpression of human RBP4 or injection of recombinant RBP4 in normal mice causes insulin resistance. Conversely, genetic deletion of Rbp4 enhances insulin sensitivity. Fenretinide, a synthetic retinoid that increases urinary excretion of RBP4, normalizes serum RBP4 levels and improves insulin resistance and glucose intolerance in mice with obesity induced by a high-fat diet. Increasing serum RBP4 induces hepatic expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and impairs insulin signalling in muscle. Thus, RBP4 is an adipocyte-derived 'signal' that may contribute to the pathogenesis of type 2 diabetes. Lowering RBP4 could be a new strategy for treating type 2 diabetes. 相似文献
45.
Functional circuits in the adult neocortex adjust to novel sensory experience, but the underlying synaptic mechanisms remain unknown. Growth and retraction of dendritic spines with synapse formation and elimination could change brain circuits. In the apical tufts of layer 5B (L5B) pyramidal neurons in the mouse barrel cortex, a subset of dendritic spines appear and disappear over days, whereas most spines are persistent for months. Under baseline conditions, new spines are mostly transient and rarely survive for more than a week. Transient spines tend to be small, whereas persistent spines are usually large. Because most excitatory synapses in the cortex occur on spines, and because synapse size and the number of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors are proportional to spine volume, the excitation of pyramidal neurons is probably driven through synapses on persistent spines. Here we test whether the generation and loss of persistent spines are enhanced by novel sensory experience. We repeatedly imaged dendritic spines for one month after trimming alternate whiskers, a paradigm that induces adaptive functional changes in neocortical circuits. Whisker trimming stabilized new spines and destabilized previously persistent spines. New-persistent spines always formed synapses. They were preferentially added on L5B neurons with complex apical tufts rather than simple tufts. Our data indicate that novel sensory experience drives the stabilization of new spines on subclasses of cortical neurons. These synaptic changes probably underlie experience-dependent remodelling of specific neocortical circuits. 相似文献
46.
Muzny DM Scherer SE Kaul R Wang J Yu J Sudbrak R Buhay CJ Chen R Cree A Ding Y Dugan-Rocha S Gill R Gunaratne P Harris RA Hawes AC Hernandez J Hodgson AV Hume J Jackson A Khan ZM Kovar-Smith C Lewis LR Lozado RJ Metzker ML Milosavljevic A Miner GR Morgan MB Nazareth LV Scott G Sodergren E Song XZ Steffen D Wei S Wheeler DA Wright MW Worley KC Yuan Y Zhang Z Adams CQ Ansari-Lari MA Ayele M Brown MJ Chen G Chen Z Clendenning J Clerc-Blankenburg KP Chen R Chen Z Davis C Delgado O Dinh HH Dong W 《Nature》2006,440(7088):1194-1198
After the completion of a draft human genome sequence, the International Human Genome Sequencing Consortium has proceeded to finish and annotate each of the 24 chromosomes comprising the human genome. Here we describe the sequencing and analysis of human chromosome 3, one of the largest human chromosomes. Chromosome 3 comprises just four contigs, one of which currently represents the longest unbroken stretch of finished DNA sequence known so far. The chromosome is remarkable in having the lowest rate of segmental duplication in the genome. It also includes a chemokine receptor gene cluster as well as numerous loci involved in multiple human cancers such as the gene encoding FHIT, which contains the most common constitutive fragile site in the genome, FRA3B. Using genomic sequence from chimpanzee and rhesus macaque, we were able to characterize the breakpoints defining a large pericentric inversion that occurred some time after the split of Homininae from Ponginae, and propose an evolutionary history of the inversion. 相似文献
47.
Molecular characterization of Ph1 as a major chromosome pairing locus in polyploid wheat 总被引:1,自引:0,他引:1
Griffiths S Sharp R Foote TN Bertin I Wanous M Reader S Colas I Moore G 《Nature》2006,439(7077):749-752
The foundation of western civilization owes much to the high fertility of bread wheat, which results from the stability of its polyploid genome. Despite possessing multiple sets of related chromosomes, hexaploid (bread) and tetraploid (pasta) wheat both behave as diploids at meiosis. Correct pairing of homologous chromosomes is controlled by the Ph1 locus. In wheat hybrids, Ph1 prevents pairing between related chromosomes. Lack of Ph1 activity in diploid relatives of wheat suggests that Ph1 arose on polyploidization. Absence of phenotypic variation, apart from dosage effects, and the failure of ethylmethane sulphonate treatment to yield mutants, indicates that Ph1 has a complex structure. Here we have localized Ph1 to a 2.5-megabase interstitial region of wheat chromosome 5B containing a structure consisting of a segment of subtelomeric heterochromatin that inserted into a cluster of cdc2-related genes after polyploidization. The correlation of the presence of this structure with Ph1 activity in related species, and the involvement of heterochromatin with Ph1 (ref. 6) and cdc2 genes with meiosis, makes the structure a good candidate for the Ph1 locus. 相似文献
48.
49.
Dendritic spines of pyramidal neurons in the cerebral cortex undergo activity-dependent structural remodelling that has been proposed to be a cellular basis of learning and memory. How structural remodelling supports synaptic plasticity, such as long-term potentiation, and whether such plasticity is input-specific at the level of the individual spine has remained unknown. We investigated the structural basis of long-term potentiation using two-photon photolysis of caged glutamate at single spines of hippocampal CA1 pyramidal neurons. Here we show that repetitive quantum-like photorelease (uncaging) of glutamate induces a rapid and selective enlargement of stimulated spines that is transient in large mushroom spines but persistent in small spines. Spine enlargement is associated with an increase in AMPA-receptor-mediated currents at the stimulated synapse and is dependent on NMDA receptors, calmodulin and actin polymerization. Long-lasting spine enlargement also requires Ca2+/calmodulin-dependent protein kinase II. Our results thus indicate that spines individually follow Hebb's postulate for learning. They further suggest that small spines are preferential sites for long-term potentiation induction, whereas large spines might represent physical traces of long-term memory. 相似文献
50.
Estimates of the effect of increasing atmospheric CO2 concentrations on future global plant production rely on the physiological response of individual plants or plant communities when exposed to high CO2 (refs 1-6). Plant populations may adapt to the changing atmosphere, however, such that the evolved plant communities of the next century are likely to be genetically different from contemporary communities. The properties of these future communities are unknown, introducing a bias of unknown sign and magnitude into projections of global carbon pool dynamics. Here we report a long-term selection experiment to investigate the phenotypic consequences of selection for growth at elevated CO2 concentrations. After about 1,000 generations, selection lines of the unicellular green alga Chlamydomonas failed to evolve specific adaptation to a CO2 concentration of 1,050 parts per million. Some lines, however, evolved a syndrome involving high rates of photosynthesis and respiration, combined with higher chlorophyll content and reduced cell size. These lines also grew poorly at ambient concentrations of CO2. We tentatively attribute this outcome to the accumulation of conditionally neutral mutations in genes affecting the carbon concentration mechanism. 相似文献