首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   345篇
  免费   0篇
  国内免费   1篇
系统科学   5篇
理论与方法论   2篇
现状及发展   49篇
研究方法   37篇
综合类   241篇
自然研究   12篇
  2019年   2篇
  2017年   1篇
  2016年   2篇
  2015年   2篇
  2013年   1篇
  2012年   16篇
  2011年   24篇
  2010年   4篇
  2009年   2篇
  2008年   11篇
  2007年   20篇
  2006年   13篇
  2005年   14篇
  2004年   10篇
  2003年   4篇
  2002年   8篇
  2001年   7篇
  2000年   13篇
  1999年   2篇
  1997年   1篇
  1996年   1篇
  1994年   1篇
  1992年   11篇
  1991年   5篇
  1990年   12篇
  1989年   8篇
  1988年   4篇
  1987年   8篇
  1986年   12篇
  1985年   9篇
  1984年   3篇
  1983年   5篇
  1982年   6篇
  1981年   4篇
  1980年   1篇
  1979年   7篇
  1978年   3篇
  1977年   9篇
  1976年   3篇
  1975年   6篇
  1974年   10篇
  1973年   5篇
  1972年   7篇
  1971年   11篇
  1970年   7篇
  1969年   3篇
  1968年   9篇
  1967年   8篇
  1966年   4篇
  1965年   6篇
排序方式: 共有346条查询结果,搜索用时 38 毫秒
91.
92.
Summary Two new aromatic derivatives of 5, 12-dihydroxy-6, 7-epoxy-resiniferonol were isolated from the leaves and twigs ofThymelea hirsuta L. (fam. Thymelaeaceae). These compounds were assigned the structures 12-O-cinnamoyl-5-hydroxy-6, 7-epoxyresiniferonol-9, 13, 14-ortho benzoate 1 and 12-O-heptadecenoyl-5-hydroxy-6, 7,-epoxy-resiniferonol-9, 13, 14-ortho benzoate 2. Both compounds induced erythema of mouse ears in a dose of 0.1 g per ear.Acknowledgments. S. Ismail and M. El-Missiry are grateful to the Royal Society for a travel and support grant.  相似文献   
93.
Laros JG  Evans WD  Klebesadel RW  Olson RA  Spalding RE 《Nature》1977,267(5607):131-132
We present here temporal and 0.2-2 MeV spectral data from two gamma bursts observed on 12 June and 16 August 1976, by detectors on the Solrad 11A and 11B satellites. The 12 June burst showed evidence for structure on time scales down to approximately 10 ms throughout its lifetime, whereas the 16 August burst varied only with characteristic times longer than a few tenths of a second. A search for both slow and fast spectral variability gave negative results. Accurate absolute burst times are, however, not yet available, but since both bursts were also observed by at least one Vela satellite, positions are calculable and will be reported.  相似文献   
94.
Cloning and sequence analysis of cDNA for bovine carboxypeptidase E   总被引:10,自引:0,他引:10  
L D Fricker  C J Evans  F S Esch  E Herbert 《Nature》1986,323(6087):461-464
Carboxypeptidase E (enkephalin convertase) was first identified as the carboxypeptidase B-like enzyme involved in the biosynthesis of enkephalin in bovine adrenal chromaffin granules. A similar enzyme is present in many brain regions and in purified secretory granules from rat pituitary and rat insulinoma. Within the secretory granules, carboxypeptidase E (CPE) activity is found in both a soluble and a membrane-bound form, which differ slightly in relative molecular mass (Mr). Here, to investigate whether the CPE activities in the various tissues are produced from a single gene, purified CPE was partially sequenced and oligonucleotide probes were used to isolate a clone encoding CPE from a bovine pituitary complementary DNA library. This cDNA hybridizes to bovine pituitary poly(A)+ RNAs of approximately 3.3, 2.6 and 2.1 kilobases (kb), with the 3.3-kb messenger RNA the predominant species. The predicted amino-acid sequence of the cDNA clone contains the partially determined sequences of CPE, several pairs of basic amino acids and displays some homology with both carboxypeptidases A and B. Restriction analysis of bovine genomic DNA suggests only one gene for CPE. This is consistent with a broad role for CPE in the biosynthesis of many neuropeptides.  相似文献   
95.
Many biological processes are coupled to ATP hydrolysis. We describe here a class of closely related ATP-binding proteins, from several bacterial species, which are associated with a variety of cellular functions including membrane transport, cell division, nodulation in Rhizobium and haemolysin export. These proteins comprise a family of structurally and functionally related subunits which share a common evolutionary origin, bind ATP and probably serve to couple ATP hydrolysis to each of these biological processes. This finding suggests a specific role for ATP in cell division, nodulation during nitrogen fixation and protein export, and allows us to assign a probable function to one of the protein components from each of these systems.  相似文献   
96.
97.
It is important that migration is measured accurately, for example to inform population estimates and projections. However, current sources of information make it difficult to produce robust estimates of emigration from Great Britain. Several other countries, including Ireland, make use of household surveys in their estimates of migration. To investigate the feasibility of obtaining information on emigration from those resident in Great Britain, three questions were included in the Omnibus Survey for two months. Respondents were asked whether they had plans to emigrate or whether anyone in their family had recently emigrated or intended to shortly. This article reports on the results of this pilot, providing evidence on response levels, comparison with the International Passenger Survey and feedback from interviewers on the questions asked.  相似文献   
98.
Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titan's nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titan's history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150-200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.  相似文献   
99.
Identification of the familial cylindromatosis tumour-suppressor gene   总被引:25,自引:0,他引:25  
Familial cylindromatosis is an autosomal dominant genetic predisposition to multiple tumours of the skin appendages. The susceptibility gene (CYLD) has previously been localized to chromosome 16q and has the genetic attributes of a tumour-suppressor gene (recessive oncogene). Here we have identified CYLD by detecting germline mutations in 21 cylindromatosis families and somatic mutations in 1 sporadic and 5 familial cylindromas. All mutations predict truncation or absence of the encoded protein. CYLD encodes three cytoskeletal-associated-protein-glycine-conserved (CAP-GLY) domains, which are found in proteins that coordinate the attachment of organelles to microtubules. CYLD also has sequence homology to the catalytic domain of ubiquitin carboxy-terminal hydrolases (UCH).  相似文献   
100.
Thompson PM  Giedd JN  Woods RP  MacDonald D  Evans AC  Toga AW 《Nature》2000,404(6774):190-193
The dynamic nature of growth and degenerative disease processes requires the design of sensitive strategies to detect, track and quantify structural change in the brain in its full spatial and temporal complexity. Although volumes of brain substructures are known to change during development, detailed maps of these dynamic growth processes have been unavailable. Here we report the creation of spatially complex, four-dimensional quantitative maps of growth patterns in the developing human brain, detected using a tensor mapping strategy with greater spatial detail and sensitivity than previously obtainable. By repeatedly scanning children (aged 3-15 years) across time spans of up to four years, a rostro-caudal wave of growth was detected at the corpus callosum, a fibre system that relays information between brain hemispheres. Peak growth rates, in fibres innervating association and language cortices, were attenuated after puberty, and contrasted sharply with a severe, spatially localized loss of subcortical grey matter. Conversely, at ages 3-6 years, the fastest growth rates occurred in frontal networks that regulate the planning of new actions. Local rates, profiles, and principal directions of growth were visualized in each individual child.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号