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排序方式: 共有103条查询结果,搜索用时 687 毫秒
71.
本文详细讨论了圆环压缩数学解的求解过程,给出了 Avitzur提出的分流面半径近似计算公式的误差。利用牛顿逼近法计算出了分流面半径的足够精确值,并由此计算绘制出了理论标定曲线,从理论和实际上,都证明是一种更精确的标定曲线。 相似文献
72.
K. W. Hong W. S. Lee B. Y. Rhim Y. W. Shin 《Cellular and molecular life sciences : CMLS》1989,45(4):320-322
Summary Release of a vascular-inhibitory factor from endothelial cells (EC), different from endothelium-derived relaxant factor (EDRF), was identified through use of a two-bath system. This two-bath system precluded the effects of oxygen-free radicals that appear when electrical field stimulation (EFS) is directly imposed on detector muscle. 相似文献
73.
Crystal structure of the intensely sweet protein monellin 总被引:2,自引:0,他引:2
Two unusual proteins, discovered in African berries, possess the interesting property of having a very high specificity for the sweet receptors. These proteins, monellin and thaumatin, are approximately 100,000 times sweeter than sugar on a molar basis and several thousand times sweeter on a weight basis. Neither contains carbohydrates or modified amino acids. Several interesting observations have been made about the two proteins: native conformations are important for the sweet taste, although both proteins are intensely sweet, there are no statistically significant sequence similarities between them; and despite the absence of sequence similarity, antibodies against thaumatin compete for monellin (as well as many other sweet compounds, but not for chemically modified non-sweet monellin) and vice versa. To understand the structural basis of these observations we determined the crystal structure of thaumatin, and report here the structure of monellin at 3 A resolution. Monellin consists of two peptide chains, the A chain of 44 residues and the B chain of 50 residues. We find no similarity between the backbone structure of monellin and that of thaumatin. 相似文献
74.
Group I metabotropic glutamate receptors (consisting of mGluR1 and mGluR5) are G-protein-coupled neurotransmitter receptors that are found in the perisynaptic region of the postsynaptic membrane. These receptors are not activated by single synaptic volleys but rather require bursts of activity. They are implicated in many forms of neural plasticity including hippocampal long-term potentiation and depression, cerebellar long-term depression, associative learning, and cocaine addiction. When activated, group I mGluRs engage two G-protein-dependent signalling mechanisms: stimulation of phospholipase C and activation of an unidentified, mixed-cation excitatory postsynaptic conductance (EPSC), displaying slow activation, in the plasma membrane. Here we report that the mGluR1-evoked slow EPSC is mediated by the TRPC1 cation channel. TRPC1 is expressed in perisynaptic regions of the cerebellar parallel fibre-Purkinje cell synapse and is physically associated with mGluR1. Manipulations that interfere with TRPC1 block the mGluR1-evoked slow EPSC in Purkinje cells; however, fast transmission mediated by AMPA-type glutamate receptors remains unaffected. Furthermore, co-expression of mGluR1 and TRPC1 in a heterologous system reconstituted a mGluR1-evoked conductance that closely resembles the slow EPSC in Purkinje cells. 相似文献
75.
Emison ES McCallion AS Kashuk CS Bush RT Grice E Lin S Portnoy ME Cutler DJ Green ED Chakravarti A 《Nature》2005,434(7035):857-863
The identification of common variants that contribute to the genesis of human inherited disorders remains a significant challenge. Hirschsprung disease (HSCR) is a multifactorial, non-mendelian disorder in which rare high-penetrance coding sequence mutations in the receptor tyrosine kinase RET contribute to risk in combination with mutations at other genes. We have used family-based association studies to identify a disease interval, and integrated this with comparative and functional genomic analysis to prioritize conserved and functional elements within which mutations can be sought. We now show that a common non-coding RET variant within a conserved enhancer-like sequence in intron 1 is significantly associated with HSCR susceptibility and makes a 20-fold greater contribution to risk than rare alleles do. This mutation reduces in vitro enhancer activity markedly, has low penetrance, has different genetic effects in males and females, and explains several features of the complex inheritance pattern of HSCR. Thus, common low-penetrance variants, identified by association studies, can underlie both common and rare diseases. 相似文献
76.
Wallis JW Aerts J Groenen MA Crooijmans RP Layman D Graves TA Scheer DE Kremitzki C Fedele MJ Mudd NK Cardenas M Higginbotham J Carter J McGrane R Gaige T Mead K Walker J Albracht D Davito J Yang SP Leong S Chinwalla A Sekhon M Wylie K Dodgson J Romanov MN Cheng H de Jong PJ Osoegawa K Nefedov M Zhang H McPherson JD Krzywinski M Schein J Hillier L Mardis ER Wilson RK Warren WC 《Nature》2004,432(7018):761-764
Strategies for assembling large, complex genomes have evolved to include a combination of whole-genome shotgun sequencing and hierarchal map-assisted sequencing. Whole-genome maps of all types can aid genome assemblies, generally starting with low-resolution cytogenetic maps and ending with the highest resolution of sequence. Fingerprint clone maps are based upon complete restriction enzyme digests of clones representative of the target genome, and ultimately comprise a near-contiguous path of clones across the genome. Such clone-based maps are used to validate sequence assembly order, supply long-range linking information for assembled sequences, anchor sequences to the genetic map and provide templates for closing gaps. Fingerprint maps are also a critical resource for subsequent functional genomic studies, because they provide a redundant and ordered sampling of the genome with clones. In an accompanying paper we describe the draft genome sequence of the chicken, Gallus gallus, the first species sequenced that is both a model organism and a global food source. Here we present a clone-based physical map of the chicken genome at 20-fold coverage, containing 260 contigs of overlapping clones. This map represents approximately 91% of the chicken genome and enables identification of chicken clones aligned to positions in other sequenced genomes. 相似文献
77.
The MLH1 D132H variant is associated with susceptibility to sporadic colorectal cancer 总被引:18,自引:0,他引:18
Lipkin SM Rozek LS Rennert G Yang W Chen PC Hacia J Hunt N Shin B Fodor S Kokoris M Greenson JK Fearon E Lynch H Collins F Gruber SB 《Nature genetics》2004,36(7):694-699
Most susceptibility to colorectal cancer (CRC) is not accounted for by known risk factors. Because MLH1, MSH2 and MSH6 mutations underlie high-penetrance CRC susceptibility in hereditary nonpolyposis colon cancer (HNPCC), we hypothesized that attenuated alleles might also underlie susceptibility to sporadic CRC. We looked for gene variants associated with HNPCC in Israeli probands with familial CRC unstratified with respect to the microsatellite instability (MSI) phenotype. Association studies identified a new MLH1 variant (415G-->C, resulting in the amino acid substitution D132H) in approximately 1.3% of Israeli individuals with CRC self-described as Jewish, Christian and Muslim. MLH1 415C confers clinically significant susceptibility to CRC. In contrast to classic HNPCC, CRCs associated with MLH1 415C usually do not have the MSI defect, which is important for clinical mutation screening. Structural and functional analyses showed that the normal ATPase function of MLH1 is attenuated, but not eliminated, by the MLH1 415G-->C mutation. The new MLH1 variant confers a high risk of CRC and identifies a previously unrecognized mechanism in microsatellite-stable tumors. These studies suggest that variants of mismatch repair proteins with attenuated function may account for a higher proportion of susceptibility to sporadic microsatellite-stable CRC than previously assumed. 相似文献
78.
一种新型计算机硬盘保护卡 总被引:2,自引:0,他引:2
介绍一种适用于公共机房维护的硬盘保护卡,给出了保护卡中ROM获取BI0S系统控制的方法和启动ROM的构造,并介绍了保护卡对硬盘I/O重新定位和COMS数据保护及保护卡的硬件设计,该卡可防止人为因素和计算机病毒对计算机软件系统的破坏. 相似文献
79.
The pairing of fermions lies at the heart of superconductivity and superfluidity. The stability of these pairs determines the robustness of the superfluid state, and the quest for superconductors with high critical temperature equates to a search for systems with strong pairing mechanisms. Ultracold atomic Fermi gases present a highly controllable model system for studying strongly interacting fermions. Tunable interactions (through Feshbach collisional resonances) and the control of population or mass imbalance among the spin components provide unique opportunities to investigate the stability of pairing-and possibly to search for exotic forms of superfluidity. A major controversy has surrounded the stability of superfluidity against an imbalance between the two spin components when the fermions interact resonantly (that is, at unitarity). Here we present the phase diagram of a spin-polarized Fermi gas of (6)Li atoms at unitarity, experimentally mapping out the superfluid phases versus temperature and density imbalance. Using tomographic techniques, we reveal spatial discontinuities in the spin polarization; this is the signature of a first-order superfluid-to-normal phase transition, and disappears at a tricritical point where the nature of the phase transition changes from first-order to second-order. At zero temperature, there is a quantum phase transition from a fully paired superfluid to a partially polarized normal gas. These observations and the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity. 相似文献
80.
Because plants are sessile, they have developed intricate strategies to adapt to changing environmental variables, including light. Their growth and development, from germination to flowering, is critically influenced by light, particularly at red (660 nm) and far-red (730 nm) wavelengths. Higher plants perceive red and far-red light by means of specific light sensors called phytochromes(A-E). However, very little is known about how light signals are transduced to elicit responses in plants. Here we report that nucleoside diphosphate kinase 2 (NDPK2) is an upstream component in the phytochrome signalling pathway in the plant Arabidopsis thaliana. In animal and human cells, NDPK acts as a tumour suppressor. We show that recombinant NDPK2 in Arabidopsis preferentially binds to the red-light-activated form of phytochrome in vitro and that this interaction increases the activity of recombinant NDPK2. Furthermore, a mutant lacking NDPK2 showed a partial defect in responses to both red and farred light, including cotyledon opening and greening. These results indicate that NDPK2 is a positive signalling component of the phytochrome-mediated light-signal-transduction pathway in Arabidopsis. 相似文献