全文获取类型
收费全文 | 1857篇 |
免费 | 178篇 |
专业分类
2035篇 |
出版年
2023年 | 19篇 |
2022年 | 27篇 |
2021年 | 52篇 |
2020年 | 47篇 |
2019年 | 56篇 |
2018年 | 45篇 |
2017年 | 43篇 |
2016年 | 80篇 |
2015年 | 141篇 |
2014年 | 121篇 |
2013年 | 143篇 |
2012年 | 186篇 |
2011年 | 163篇 |
2010年 | 93篇 |
2009年 | 79篇 |
2008年 | 97篇 |
2007年 | 88篇 |
2006年 | 73篇 |
2005年 | 48篇 |
2004年 | 59篇 |
2003年 | 38篇 |
2002年 | 45篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 24篇 |
1998年 | 15篇 |
1997年 | 9篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1993年 | 9篇 |
1992年 | 15篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 6篇 |
1988年 | 10篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有2035条查询结果,搜索用时 15 毫秒
11.
Gene flow in genetically modified wheat 总被引:1,自引:0,他引:1
Understanding gene flow in genetically modified (GM) crops is critical to answering questions regarding risk-assessment and the coexistence of GM and non-GM crops. In two field experiments, we tested whether rates of cross-pollination differed between GM and non-GM lines of the predominantly self-pollinating wheat Triticum aestivum. In the first experiment, outcrossing was studied within the field by planting "phytometers" of one line into stands of another line. In the second experiment, outcrossing was studied over distances of 0.5-2.5 m from a central patch of pollen donors to adjacent patches of pollen recipients. Cross-pollination and outcrossing was detected when offspring of a pollen recipient without a particular transgene contained this transgene in heterozygous condition. The GM lines had been produced from the varieties Bobwhite or Frisal and contained Pm3b or chitinase/glucanase transgenes, respectively, in homozygous condition. These transgenes increase plant resistance against pathogenic fungi. Although the overall outcrossing rate in the first experiment was only 3.4%, Bobwhite GM lines containing the Pm3b transgene were six times more likely than non-GM control lines to produce outcrossed offspring. There was additional variation in outcrossing rate among the four GM-lines, presumably due to the different transgene insertion events. Among the pollen donors, the Frisal GM line expressing a chitinase transgene caused more outcrossing than the GM line expressing both a chitinase and a glucanase transgene. In the second experiment, outcrossing after cross-pollination declined from 0.7-0.03% over the test distances of 0.5-2.5 m. Our results suggest that pollen-mediated gene flow between GM and non-GM wheat might only be a concern if it occurs within fields, e.g. due to seed contamination. Methodologically our study demonstrates that outcrossing rates between transgenic and other lines within crops can be assessed using a phytometer approach and that gene-flow distances can be efficiently estimated with population-level PCR analyses. 相似文献
12.
Gene therapy is considered a feasible approach for the treatment and prevention of HIV/AIDS. Targeting both viral genes and host dependency factors can interfere with the viral lifecycle and prevent viral replication. A number of approaches have been taken to target these genes, including ribozymes, aptamers, and RNAi based therapies. A number of these therapies are now beginning to make their way into clinical trials and providing proof of principle that gene therapy is a safe and realistic option for treating HIV. Here, we focus on those therapies that have progressed along the pipeline to preclinical and clinical testing. 相似文献
13.
14.
15.
Hao Wu Lei Sun Fabian Blombach Stan J.J. Brouns Ambrosius P. L. Snijders Kristina Lorenzen Robert H. H. van den Heuvel Albert J. R. Heck Sheng Fu Xuemei Li Xuejun C. Zhang Zihe Rao John van der Oost 《Proteins》2010,78(3):705-713
The HflX‐family is a widely distributed but poorly characterized family of translation factor‐related guanosine triphosphatases (GTPases) that interact with the large ribosomal subunit. This study describes the crystal structure of HflX from Sulfolobus solfataricus solved to 2.0‐Å resolution in apo‐ and GDP‐bound forms. The enzyme displays a two‐domain architecture with a novel “HflX domain” at the N‐terminus, and a classical G‐domain at the C‐terminus. The HflX domain is composed of a four‐stranded parallel β‐sheet flanked by two α‐helices on either side, and an anti‐parallel coiled coil of two long α‐helices that lead to the G‐domain. The cleft between the two domains accommodates the nucleotide binding site as well as the switch II region, which mediates interactions between the two domains. Conformational changes of the switch regions are therefore anticipated to reposition the HflX‐domain upon GTP‐binding. Slow GTPase activity has been confirmed, with an HflX domain deletion mutant exhibiting a 24‐fold enhanced turnover rate, suggesting a regulatory role for the HflX domain. The conserved positively charged surface patches of the HflX‐domain may mediate interaction with the large ribosomal subunit. The present study provides a structural basis to uncover the functional role of this GTPases family whose function is largely unknown. Proteins 2010. © 2009 Wiley‐Liss, Inc. 相似文献
16.
17.
We examined the dynamics of fish species and how they relate to species assemblage coherence in the heavily exploited Georges
Bank fish community. Coherence is defined as reduced temporal variability of total assemblage biomass. We assumed that a higher
degree of compensation hence coherence occurs within competitively coupled species assemblages; therefore, fisheries may directly
alter the dynamics of certain targeted species sizes but assemblage structure will be relatively more stable owing to compensatory
interactions. Species-sizes were grouped, based on negative covariance coupling in biomass time series from survey data. Assemblages
representing benthic feeders were clearly identified by this method; furthermore, the most heavily exploited species-sizes
were decoupled from other species-sizes suggesting that fisheries have diminished their potential to compensate or to be compensated
for by competitive interactions. Biomass of species-sizes within known trophic guilds strongly compensated other guild-member
biomass fluctuations if the diet of guild members was more specialized. This is an indication that more competitive conditions
(more specialization) foster greater compensatory responses between competitors biomass fluctuations. 相似文献
18.
Leendertz FH Lankester F Guislain P Néel C Drori O Dupain J Speede S Reed P Wolfe N Loul S Mpoudi-Ngole E Peeters M Boesch C Pauli G Ellerbrok H Leroy EM 《American journal of primatology》2006,68(9):928-933
During the period of December 2004 to January 2005, Bacillus anthracis killed three wild chimpanzees (Pan troglodytes troglodytes) and one gorilla (Gorilla gorilla gorilla) in a tropical forest in Cameroon. While this is the second anthrax outbreak in wild chimpanzees, this is the first case of anthrax in gorillas ever reported. The number of great apes in Central Africa is dramatically declining and the populations are seriously threatened by diseases, mainly Ebola. Nevertheless, a considerable number of deaths cannot be attributed to Ebola virus and remained unexplained. Our results show that diseases other than Ebola may also threaten wild great apes, and indicate that the role of anthrax in great ape mortality may have been underestimated. These results suggest that risk identification, assessment, and management for the survival of the last great apes should be performed with an open mind, since various pathogens with distinct characteristics in epidemiology and pathogenicity may impact the populations. An animal mortality monitoring network covering the entire African tropical forest, with the dual aims of preventing both great ape extinction and human disease outbreaks, will create necessary baseline data for such risk assessments and management plans. 相似文献
19.
Sakine Simsekyilmaz Fabian Schreiber Stefan Weinandy Felix Gremse Tolga Taha S?nmez Elisa A. Liehn 《Journal of visualized experiments : JoVE》2013,(75)
Despite the considerable progress made in the stent development in the last decades, cardiovascular diseases remain the main cause of death in western countries. Beside the benefits offered by the development of different drug-eluting stents, the coronary revascularization bears also the life-threatening risks of in-stent thrombosis and restenosis. Research on new therapeutic strategies is impaired by the lack of appropriate methods to study stent implantation and restenosis processes. Here, we describe a rapid and accessible procedure of stent implantation in mouse carotid artery, which offers the possibility to study in a convenient way the molecular mechanisms of vessel remodeling and the effects of different drug coatings. 相似文献
20.
Radioimmunoimaging of Liver Metastases with PET Using a 64Cu-Labeled CEA Antibody in Transgenic Mice
Stefanie Nittka Marcel A. Krueger John E. Shively Hanne Boll Marc A. Brockmann Fabian Doyon Bernd J. Pichler Michael Neumaier 《PloS one》2014,9(9)