全文获取类型
收费全文 | 1015篇 |
免费 | 0篇 |
专业分类
系统科学 | 13篇 |
教育与普及 | 1篇 |
理论与方法论 | 18篇 |
现状及发展 | 365篇 |
研究方法 | 169篇 |
综合类 | 443篇 |
自然研究 | 6篇 |
出版年
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 11篇 |
2014年 | 11篇 |
2013年 | 10篇 |
2012年 | 58篇 |
2011年 | 72篇 |
2010年 | 26篇 |
2009年 | 7篇 |
2008年 | 38篇 |
2007年 | 52篇 |
2006年 | 54篇 |
2005年 | 50篇 |
2004年 | 27篇 |
2003年 | 28篇 |
2002年 | 40篇 |
2001年 | 32篇 |
2000年 | 41篇 |
1999年 | 34篇 |
1992年 | 31篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 23篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 20篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 11篇 |
1975年 | 8篇 |
1974年 | 11篇 |
1973年 | 12篇 |
1972年 | 14篇 |
1971年 | 15篇 |
1970年 | 14篇 |
1969年 | 12篇 |
1968年 | 9篇 |
1967年 | 11篇 |
1966年 | 8篇 |
1964年 | 6篇 |
1960年 | 4篇 |
1956年 | 6篇 |
1947年 | 4篇 |
排序方式: 共有1015条查询结果,搜索用时 15 毫秒
911.
Stittelaar KJ Neyts J Naesens L van Amerongen G van Lavieren RF Holý A De Clercq E Niesters HG Fries E Maas C Mulder PG van der Zeijst BA Osterhaus AD 《Nature》2006,439(7077):745-748
There is concern that variola virus, the aetiological agent of smallpox, may be used as a biological weapon. For this reason several countries are now stockpiling (vaccinia virus-based) smallpox vaccine. Although the preventive use of smallpox vaccination has been well documented, little is known about its efficacy when used after exposure to the virus. Here we compare the effectiveness of (1) post-exposure smallpox vaccination and (2) antiviral treatment with either cidofovir (also called HPMPC or Vistide) or with a related acyclic nucleoside phosphonate analogue (HPMPO-DAPy) after lethal intratracheal infection of cynomolgus monkeys (Macaca fascicularis) with monkeypox virus (MPXV). MPXV causes a disease similar to human smallpox and this animal model can be used to measure differences in the protective efficacies of classical and new-generation candidate smallpox vaccines. We show that initiation of antiviral treatment 24 h after lethal intratracheal MPXV infection, using either of the antiviral agents and applying various systemic treatment regimens, resulted in significantly reduced mortality and reduced numbers of cutaneous monkeypox lesions. In contrast, when monkeys were vaccinated 24 h after MPXV infection, using a standard human dose of a currently recommended smallpox vaccine (Elstree-RIVM), no significant reduction in mortality was observed. When antiviral therapy was terminated 13 days after infection, all surviving animals had virus-specific serum antibodies and antiviral T lymphocytes. These data show that adequate preparedness for a biological threat involving smallpox should include the possibility of treating exposed individuals with antiviral compounds such as cidofovir or other selective anti-poxvirus drugs. 相似文献
912.
Episodic fresh surface waters in the Eocene Arctic Ocean 总被引:1,自引:0,他引:1
Brinkhuis H Schouten S Collinson ME Sluijs A Sinninghe Damsté JS Dickens GR Huber M Cronin TM Onodera J Takahashi K Bujak JP Stein R van der Burgh J Eldrett JS Harding IC Lotter AF Sangiorgi F van Konijnenburg-van Cittert H de Leeuw JW Matthiessen J Backman J Moran K;Expedition Scientists 《Nature》2006,441(7093):606-609
It has been suggested, on the basis of modern hydrology and fully coupled palaeoclimate simulations, that the warm greenhouse conditions that characterized the early Palaeogene period (55-45 Myr ago) probably induced an intensified hydrological cycle with precipitation exceeding evaporation at high latitudes. Little field evidence, however, has been available to constrain oceanic conditions in the Arctic during this period. Here we analyse Palaeogene sediments obtained during the Arctic Coring Expedition, showing that large quantities of the free-floating fern Azolla grew and reproduced in the Arctic Ocean by the onset of the middle Eocene epoch (approximately 50 Myr ago). The Azolla and accompanying abundant freshwater organic and siliceous microfossils indicate an episodic freshening of Arctic surface waters during an approximately 800,000-year interval. The abundant remains of Azolla that characterize basal middle Eocene marine deposits of all Nordic seas probably represent transported assemblages resulting from freshwater spills from the Arctic Ocean that reached as far south as the North Sea. The termination of the Azolla phase in the Arctic coincides with a local sea surface temperature rise from approximately 10 degrees C to 13 degrees C, pointing to simultaneous increases in salt and heat supply owing to the influx of waters from adjacent oceans. We suggest that onset and termination of the Azolla phase depended on the degree of oceanic exchange between Arctic Ocean and adjacent seas. 相似文献
913.
914.
Mokken scale analysis uses an automated bottom-up stepwise item selection procedure that suffers from two problems. First, when selected during the procedure items satisfy the scaling conditions but they may fail to do so after the scale has been completed. Second, the procedure is approximate and thus may not produce the optimal item partitioning. This study investigates a variation on Mokken’s item selection procedure, which alleviates the first problem, and proposes a genetic algorithm, which alleviates both problems. The genetic algorithm is an approximation to checking all possible partitionings. A simulation study shows that the genetic algorithm leads to better scaling results than the other two procedures. 相似文献
915.
van Veen HW 《Nature》2010,467(7318):926-927
916.
Andresen GB Ashkezari MD Baquero-Ruiz M Bertsche W Bowe PD Butler E Cesar CL Chapman S Charlton M Deller A Eriksson S Fajans J Friesen T Fujiwara MC Gill DR Gutierrez A Hangst JS Hardy WN Hayden ME Humphries AJ Hydomako R Jenkins MJ Jonsell S Jørgensen LV Kurchaninov L Madsen N Menary S Nolan P Olchanski K Olin A Povilus A Pusa P Robicheaux F Sarid E el Nasr SS Silveira DM So C Storey JW Thompson RI van der Werf DP Wurtele JS Yamazaki Y 《Nature》2010,468(7324):673-676
Antimatter was first predicted in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 10(14) for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 10(7) antiprotons and 7?×?10(8) positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4?±?1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen. 相似文献
917.
918.
δ-Protocadherins constitute a group of cadherins characterized by several conserved motifs in their cytoplasmic domains. We
present a phylogenetic analysis that further divides this group into δ1-protocadherins (comprising protocadherin-1, −7, −9
and −11 or -X/Y) and δ2-protocadherins (comprising protocadherin-8, −10, −17, −18 and −19). The δ-protocadherin genes, which
are located on different chromosomes in man and mouse, have a similar gene structure. They are expressed as multiple splice
forms, differing mostly in their cytoplasmic domains. Some δ-protocadherins were reported to mediate weak cell-cell adhesion
in vitro and cell sorting in vivo. In addition, individual δ-protocadherins might play important roles in signaling pathways,
as they bind to proteins such as TAF1/Set, protein phosphatase-1α and the Frizzled 7 receptor. The spatiotemporally restricted
expression of δ-protocadherins in different tissues and species and the results of their functional analysis, mainly in Xenopus, suggest that they play multiple, tightly regulated roles in vertebrate development.
Received 18 July 2005; received after revision 26 August 2005; accepted 2 September 2005 相似文献
919.
Chfr is required for tumor suppression and Aurora A regulation 总被引:7,自引:0,他引:7
Yu X Minter-Dykhouse K Malureanu L Zhao WM Zhang D Merkle CJ Ward IM Saya H Fang G van Deursen J Chen J 《Nature genetics》2005,37(4):401-406
Tumorigenesis is a consequence of loss of tumor suppressors and activation of oncogenes. Expression of the mitotic checkpoint protein Chfr is lost in 20-50% of primary tumors and tumor cell lines. To explore whether downregulation of Chfr contributes directly to tumorigenesis, we generated Chfr knockout mice. Chfr-deficient mice are cancer-prone, develop spontaneous tumors and have increased skin tumor incidence after treatment with dimethylbenz(a)anthracene. Chfr deficiency leads to chromosomal instability in embryonic fibroblasts and regulates the mitotic kinase Aurora A, which is frequently upregulated in a variety of tumors. Chfr physically interacts with Aurora A and ubiquitinates Aurora A both in vitro and in vivo. Collectively, our data suggest that Chfr is a tumor suppressor and ensures chromosomal stability by controlling the expression levels of key mitotic proteins such as Aurora A. 相似文献
920.
Genetics,cytokines and human infectious disease: lessons from weakly pathogenic mycobacteria and salmonellae 总被引:22,自引:0,他引:22
Ottenhoff TH Verreck FA Lichtenauer-Kaligis EG Hoeve MA Sanal O van Dissel JT 《Nature genetics》2002,32(1):97-105
Host genetic factors are important in determining the outcome of infections caused by intracellular pathogens, including mycobacteria and salmonellae, but until now have been poorly characterized. Recently, some individuals with severe infections due to otherwise weakly pathogenic mycobacteria (non-tuberculous mycobacteria or Mycobacterium bovis bacille Calmette-Guérin) or Salmonella species have been shown to be unable to produce or respond to interferon-gamma. This inability results from mutations in any of five genes encoding essential proteins of the type 1 cytokine cascade: interleukin-12p40, interleukin-12R beta 1, interferon-gamma R1, interferon-gamma R2 or STAT1. Ten syndromes have thus far been identified. Recent insights in genetically controlled host defense and susceptibility to mycobacterial disease are discussed. 相似文献