全文获取类型
收费全文 | 320篇 |
免费 | 1篇 |
国内免费 | 8篇 |
专业分类
系统科学 | 1篇 |
丛书文集 | 1篇 |
教育与普及 | 1篇 |
理论与方法论 | 1篇 |
现状及发展 | 48篇 |
研究方法 | 49篇 |
综合类 | 202篇 |
自然研究 | 26篇 |
出版年
2021年 | 1篇 |
2019年 | 1篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 2篇 |
2014年 | 1篇 |
2013年 | 7篇 |
2012年 | 22篇 |
2011年 | 48篇 |
2010年 | 3篇 |
2009年 | 1篇 |
2008年 | 28篇 |
2007年 | 34篇 |
2006年 | 21篇 |
2005年 | 17篇 |
2004年 | 22篇 |
2003年 | 17篇 |
2002年 | 26篇 |
2001年 | 14篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1996年 | 2篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1969年 | 1篇 |
1968年 | 4篇 |
1967年 | 2篇 |
1965年 | 3篇 |
1960年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有329条查询结果,搜索用时 1 毫秒
231.
Processing of primary microRNAs by the Microprocessor complex 总被引:4,自引:0,他引:4
232.
Glazebrook K Abraham RG McCarthy PJ Savaglio S Chen HW Crampton D Murowinski R Jørgensen I Roth K Hook I Marzke RO Carlberg RG 《Nature》2004,430(6996):181-184
Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr (refs 4-7), but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe. 相似文献
233.
Most biological catalysts are made of protein; however, eight classes of natural ribozymes have been discovered that catalyse fundamental biochemical reactions. The central functions of ribozymes in modern organisms support the hypothesis that life passed through an 'RNA world' before the emergence of proteins and DNA. We have identified a new class of ribozymes that cleaves the messenger RNA of the glmS gene in Gram-positive bacteria. The ribozyme is activated by glucosamine-6-phosphate (GlcN6P), which is the metabolic product of the GlmS enzyme. Additional data indicate that the ribozyme serves as a metabolite-responsive genetic switch that represses the glmS gene in response to rising GlcN6P concentrations. These findings demonstrate that ribozyme switches may have functioned as metabolite sensors in primitive organisms, and further suggest that modern cells retain some of these ancient genetic control systems. 相似文献
234.
Aligianis IA Johnson CA Gissen P Chen D Hampshire D Hoffmann K Maina EN Morgan NV Tee L Morton J Ainsworth JR Horn D Rosser E Cole TR Stolte-Dijkstra I Fieggen K Clayton-Smith J Mégarbané A Shield JP Newbury-Ecob R Dobyns WB Graham JM Kjaer KW Warburg M Bond J Trembath RC Harris LW Takai Y Mundlos S Tannahill D Woods CG Maher ER 《Nature genetics》2005,37(3):221-223
Warburg Micro syndrome (WARBM1) is a severe autosomal recessive disorder characterized by developmental abnormalities of the eye and central nervous system and by microgenitalia. We identified homozygous inactivating mutations in RAB3GAP, encoding RAB3 GTPase activating protein, a key regulator of the Rab3 pathway implicated in exocytic release of neurotransmitters and hormones, in 12 families with Micro syndrome. We hypothesize that the underlying pathogenesis of Micro syndrome is a failure of exocytic release of ocular and neurodevelopmental trophic factors. 相似文献
235.
An expression profile for diagnosis of lymph node metastases from primary head and neck squamous cell carcinomas 总被引:15,自引:0,他引:15
Roepman P Wessels LF Kettelarij N Kemmeren P Miles AJ Lijnzaad P Tilanus MG Koole R Hordijk GJ van der Vliet PC Reinders MJ Slootweg PJ Holstege FC 《Nature genetics》2005,37(2):182-186
Metastasis is the process by which cancers spread to distinct sites in the body. It is the principal cause of death in individuals suffering from cancer. For some types of cancer, early detection of metastasis at lymph nodes close to the site of the primary tumor is pivotal for appropriate treatment. Because it can be difficult to detect lymph node metastases reliably, many individuals currently receive inappropriate treatment. We show here that DNA microarray gene-expression profiling can detect lymph node metastases for primary head and neck squamous cell carcinomas that arise in the oral cavity and oropharynx. The predictor, established with an 82-tumor training set, outperforms current clinical diagnosis when independently validated. The 102 predictor genes offer unique insights into the processes underlying metastasis. The results show that the metastatic state can be deciphered from the primary tumor gene-expression pattern and that treatment can be substantially improved. 相似文献
236.
全球纳米技术研究文献计量分析 总被引:1,自引:0,他引:1
本研究采用文本挖掘技术,从在全球公开的纳米科学和技术研究文献数据库(SCI/SSCI 数据
库)中提取技术情报。在SCI/SSCI数据库中利用纳米科学和技术领域的300多个术语进行相关文献的检
索;利用摘要共现词汇的自相关图来揭示的技术核心领域;通过词汇间的互相关图来揭示因共现而发生
关系的词汇的技术核心领域;采用因子矩阵进一步量化研究机构之间、国家之间的关系强度,并对图中
揭示的合作出版网络给予确证。 相似文献
库)中提取技术情报。在SCI/SSCI数据库中利用纳米科学和技术领域的300多个术语进行相关文献的检
索;利用摘要共现词汇的自相关图来揭示的技术核心领域;通过词汇间的互相关图来揭示因共现而发生
关系的词汇的技术核心领域;采用因子矩阵进一步量化研究机构之间、国家之间的关系强度,并对图中
揭示的合作出版网络给予确证。 相似文献
237.
Zusammenfassung Das aus Bienengift vonApis mellifera gewonnene, oberflächenaktive, kationische Polypeptid Melittin bildet mit Hautzellenextrakt, nicht aber mit Serumalbumin, einen stöchiometrischen Komplex, der vermutlich die Transportform des Giftes im Körper darstellt.
This work was sponsored by the Glenn B. and Gertrude Warren Foundation, Inc., 1361 Myron Street, Schenectedy (New York 12309, USA). 相似文献
This work was sponsored by the Glenn B. and Gertrude Warren Foundation, Inc., 1361 Myron Street, Schenectedy (New York 12309, USA). 相似文献
238.
Raxworthy CJ Martinez-Meyer E Horning N Nussbaum RA Schneider GE Ortega-Huerta MA Townsend Peterson A 《Nature》2003,426(6968):837-841
Despite the importance of tropical biodiversity, informative species distributional data are seldom available for biogeographical study or setting conservation priorities. Modelling ecological niche distributions of species offers a potential solution; however, the utility of old locality data from museums, and of more recent remotely sensed satellite data, remains poorly explored, especially for rapidly changing tropical landscapes. Using 29 modern data sets of environmental land coverage and 621 chameleon occurrence localities from Madagascar (historical and recent), here we demonstrate a significant ability of our niche models in predicting species distribution. At 11 recently inventoried sites, highest predictive success (85.1%) was obtained for models based only on modern occurrence data (74.7% and 82.8% predictive success, respectively, for pre-1978 and all data combined). Notably, these models also identified three intersecting areas of over-prediction that recently yielded seven chameleon species new to science. We conclude that ecological niche modelling using recent locality records and readily available environmental coverage data provides informative biogeographical data for poorly known tropical landscapes, and offers innovative potential for the discovery of unknown distributional areas and unknown species. 相似文献
239.
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
240.
Caudy AA Ketting RF Hammond SM Denli AM Bathoorn AM Tops BB Silva JM Myers MM Hannon GJ Plasterk RH 《Nature》2003,425(6956):411-414
RNA interference (RNAi) regulates gene expression by the cleavage of messenger RNA, by mRNA degradation and by preventing protein synthesis. These effects are mediated by a ribonucleoprotein complex known as RISC (RNA-induced silencing complex). We have previously identified four Drosophila components (short interfering RNAs, Argonaute 2 (ref. 2), VIG and FXR) of a RISC enzyme that degrades specific mRNAs in response to a double-stranded-RNA trigger. Here we show that Tudor-SN (tudor staphylococcal nuclease)--a protein containing five staphylococcal/micrococcal nuclease domains and a tudor domain--is a component of the RISC enzyme in Caenorhabditis elegans, Drosophila and mammals. Although Tudor-SN contains non-canonical active-site sequences, we show that purified Tudor-SN exhibits nuclease activity similar to that of other staphylococcal nucleases. Notably, both purified Tudor-SN and RISC are inhibited by a specific competitive inhibitor of micrococcal nuclease. Tudor-SN is the first RISC subunit to be identified that contains a recognizable nuclease domain, and could therefore contribute to the RNA degradation observed in RNAi. 相似文献