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71.
A large number of high-accuracy vector measurements of the Earth's magnetic field have recently become available from the satellite Oersted, complementing previous vector data from the satellite Magsat, which operated in 1979/80. These data can be used to infer the morphology of the magnetic field at the surface of the fluid core, approximately 2,900 km below the Earth's surface. Here I apply a new methodology to these data to calculate maps of the magnetic field at the core surface which show intense flux spots in equatorial regions. The intensity of these features is unusually large--some have intensities comparable to high-latitude flux patches near the poles, previously identified as the major component of the dynamo field. The tendency for pairing of some of these spots to the north and south of the geographical equator suggests they might be associated with the tops of equatorially symmetric columnar structures in the fluid, or their antisymmetric equivalents. The drift of the equatorial features may represent material flow or could represent wave motion; discrimination of these two effects based on future data could provide new information on the strength of the hidden toroidal magnetic field of the Earth. 相似文献
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It has been a long-standing goal to detect the effects of quantum mechanics on a macroscopic mechanical oscillator. Position measurements of an oscillator are ultimately limited by quantum mechanics, where 'zero-point motion' fluctuations in the quantum ground state combine with the uncertainty relation to yield a lower limit on the measured average displacement. Development of a position transducer, integrated with a mechanical resonator, that can approach this limit could have important applications in the detection of very weak forces, for example in magnetic resonance force microscopy and a variety of other precision experiments. One implementation that might allow near quantum-limited sensitivity is to use a single electron transistor (SET) as a displacement sensor: the exquisite charge sensitivity of the SET at cryogenic temperatures is exploited to measure motion by capacitively coupling it to the mechanical resonator. Here we present the experimental realization of such a device, yielding an unequalled displacement sensitivity of 2 x 10(-15) m x Hz(-1/2) for a 116-MHz mechanical oscillator at a temperature of 30 mK-a sensitivity roughly a factor of 100 larger than the quantum limit for this oscillator. 相似文献
74.
The direction of frequency-modulated (FM) sweeps is an important temporal cue in animal and human communication. FM direction-selective neurons are found in the primary auditory cortex (A1), but their topography and the mechanisms underlying their selectivity remain largely unknown. Here we report that in the rat A1, direction selectivity is topographically ordered in parallel with characteristic frequency (CF): low CF neurons preferred upward sweeps, whereas high CF neurons preferred downward sweeps. The asymmetry of 'inhibitory sidebands', suppressive regions flanking the tonal receptive field (TRF) of the spike response, also co-varied with CF. In vivo whole-cell recordings showed that the direction selectivity already present in the synaptic inputs was enhanced by cortical synaptic inhibition, which suppressed the synaptic excitation of the non-preferred direction more than that of the preferred. The excitatory and inhibitory synaptic TRFs had identical spectral tuning, but with inhibition delayed relative to excitation. The spectral asymmetry of the synaptic TRFs co-varied with CF, as had direction selectivity and sideband asymmetry, and thus suggested a synaptic mechanism for the shaping of FM direction selectivity and its topographic ordering. 相似文献
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76.
Kamath RS Fraser AG Dong Y Poulin G Durbin R Gotta M Kanapin A Le Bot N Moreno S Sohrmann M Welchman DP Zipperlen P Ahringer J 《Nature》2003,421(6920):231-237
77.
Singh N Ebrahimi FA Gimelbrant AA Ensminger AW Tackett MR Qi P Gribnau J Chess A 《Nature genetics》2003,33(3):339-341
Random monoallelic expression and asynchronous replication define an unusual class of autosomal mammalian genes. We show that every cell has randomly chosen either the maternal or paternal copy of each given autosome pair, such that alleles of these genes scattered across the chosen chromosome replicate earlier than the alleles on the homologous chromosome. Thus, chromosome-pair non-equivalence, rather than being limited to X-chromosome inactivation, is a fundamental property of mouse chromosomes. 相似文献
78.
Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells 总被引:27,自引:0,他引:27
Nauli SM Alenghat FJ Luo Y Williams E Vassilev P Li X Elia AE Lu W Brown EM Quinn SJ Ingber DE Zhou J 《Nature genetics》2003,33(2):129-137
Several proteins implicated in the pathogenesis of polycystic kidney disease (PKD) localize to cilia. Furthermore, cilia are malformed in mice with PKD with mutations in TgN737Rpw (encoding polaris). It is not known, however, whether ciliary dysfunction occurs or is relevant to cyst formation in PKD. Here, we show that polycystin-1 (PC1) and polycystin-2 (PC2), proteins respectively encoded by Pkd1 and Pkd2, mouse orthologs of genes mutated in human autosomal dominant PKD, co-distribute in the primary cilia of kidney epithelium. Cells isolated from transgenic mice that lack functional PC1 formed cilia but did not increase Ca(2+) influx in response to physiological fluid flow. Blocking antibodies directed against PC2 similarly abolished the flow response in wild-type cells as did inhibitors of the ryanodine receptor, whereas inhibitors of G-proteins, phospholipase C and InsP(3) receptors had no effect. These data suggest that PC1 and PC2 contribute to fluid-flow sensation by the primary cilium in renal epithelium and that they both function in the same mechanotransduction pathway. Loss or dysfunction of PC1 or PC2 may therefore lead to PKD owing to the inability of cells to sense mechanical cues that normally regulate tissue morphogenesis. 相似文献
79.
Localized mutations in the gene encoding the cytoskeletal protein filamin A cause diverse malformations in humans 总被引:18,自引:0,他引:18
Robertson SP Twigg SR Sutherland-Smith AJ Biancalana V Gorlin RJ Horn D Kenwrick SJ Kim CA Morava E Newbury-Ecob R Orstavik KH Quarrell OW Schwartz CE Shears DJ Suri M Kendrick-Jones J Wilkie AO;OPD-spectrum Disorders Clinical Collaborative Group 《Nature genetics》2003,33(4):487-491
Remodeling of the cytoskeleton is central to the modulation of cell shape and migration. Filamin A, encoded by the gene FLNA, is a widely expressed protein that regulates re-organization of the actin cytoskeleton by interacting with integrins, transmembrane receptor complexes and second messengers. We identified localized mutations in FLNA that conserve the reading frame and lead to a broad range of congenital malformations, affecting craniofacial structures, skeleton, brain, viscera and urogenital tract, in four X-linked human disorders: otopalatodigital syndrome types 1 (OPD1; OMIM 311300) and 2 (OPD2; OMIM 304120), frontometaphyseal dysplasia (FMD; OMIM 305620) and Melnick-Needles syndrome (MNS; OMIM 309350). Several mutations are recurrent, and all are clustered into four regions of the gene: the actin-binding domain and rod domain repeats 3, 10 and 14/15. Our findings contrast with previous observations that loss of function of FLNA is embryonic lethal in males but manifests in females as a localized neuronal migration disorder, called periventricular nodular heterotopia (PVNH; refs. 3-6). The patterns of mutation, X-chromosome inactivation and phenotypic manifestations in the newly described mutations indicate that they have gain-of-function effects, implicating filamin A in signaling pathways that mediate organogenesis in multiple systems during embryonic development. 相似文献
80.
Wnt proteins are lipid-modified and can act as stem cell growth factors 总被引:93,自引:0,他引:93
Willert K Brown JD Danenberg E Duncan AW Weissman IL Reya T Yates JR Nusse R 《Nature》2003,423(6938):448-452
Wnt signalling is involved in numerous events in animal development, including the proliferation of stem cells and the specification of the neural crest. Wnt proteins are potentially important reagents in expanding specific cell types, but in contrast to other developmental signalling molecules such as hedgehog proteins and the bone morphogenetic proteins, Wnt proteins have never been isolated in an active form. Although Wnt proteins are secreted from cells, secretion is usually inefficient and previous attempts to characterize Wnt proteins have been hampered by their high degree of insolubility. Here we have isolated active Wnt molecules, including the product of the mouse Wnt3a gene. By mass spectrometry, we found the proteins to be palmitoylated on a conserved cysteine. Enzymatic removal of the palmitate or site-directed and natural mutations of the modified cysteine result in loss of activity, and indicate that the lipid is important for signalling. The purified Wnt3a protein induces self-renewal of haematopoietic stem cells, signifying its potential use in tissue engineering. 相似文献