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   4283篇
  免费   330篇
  国内免费   4篇
  2021年   49篇
  2020年   29篇
  2019年   46篇
  2018年   47篇
  2017年   55篇
  2016年   102篇
  2015年   155篇
  2014年   193篇
  2013年   266篇
  2012年   317篇
  2011年   303篇
  2010年   176篇
  2009年   182篇
  2008年   266篇
  2007年   284篇
  2006年   250篇
  2005年   258篇
  2004年   246篇
  2003年   239篇
  2002年   218篇
  2001年   37篇
  2000年   30篇
  1999年   53篇
  1998年   62篇
  1997年   38篇
  1996年   41篇
  1995年   42篇
  1994年   32篇
  1993年   33篇
  1992年   43篇
  1991年   30篇
  1990年   32篇
  1989年   21篇
  1988年   25篇
  1987年   26篇
  1986年   19篇
  1985年   24篇
  1984年   38篇
  1983年   17篇
  1982年   34篇
  1981年   17篇
  1980年   10篇
  1979年   15篇
  1978年   17篇
  1977年   11篇
  1976年   12篇
  1975年   17篇
  1974年   11篇
  1973年   10篇
  1969年   10篇
排序方式: 共有4617条查询结果,搜索用时 312 毫秒
101.
Antonie van Leeuwenhoek - Strain M2T was isolated from the beach of Cuxhaven, Wadden Sea, Germany, in course of a program to attain new producers of bioactive natural products. Strain M2T produces...  相似文献   
102.
103.
Hydrobiologia - Aphanius fasciatus is a small fish occurring in Mediterranean brackish environments. In Cyprus it is known from three localities separated by long stretches of coast. The genetic...  相似文献   
104.
Malaria parasites are fast replicating unicellular organisms and require substantial amounts of folate for DNA synthesis. Despite the central role of this critical co‐factor for parasite survival, only little is known about intraparasitic folate trafficking in Plasmodium. Here, we report on the expression, subcellular localisation and function of the parasite's folate transporter 2 (FT2) during life cycle progression in the murine malaria parasite Plasmodium berghei. Using live fluorescence microscopy of genetically engineered parasites, we demonstrate that FT2 localises to the apicoplast. In invasive P. berghei stages, a fraction of FT2 is also observed at the apical end. Upon genetic disruption of FT2, blood and liver infection, gametocyte production and mosquito colonisation remain unaltered. But in the Anopheles vector, FT2‐deficient parasites develop inflated oocysts with unusual pulp formation consisting of numerous single‐membrane vesicles, which ultimately fuse to form large cavities. Ultrastructural analysis suggests that this defect reflects aberrant sporoblast formation caused by abnormal vesicular traffic. Complete sporogony in FT2‐deficient oocysts is very rare, and mutant sporozoites fail to establish hepatocyte infection, resulting in a complete block of parasite transmission. Our findings reveal a previously unrecognised organellar folate transporter that exerts critical roles for pathogen maturation in the arthropod vector.  相似文献   
105.
The extensive periodic vertical movements of up to 14 h and 209 m observed in this study for an individual goosefish, Lophius americanus, challenges previous assumptions about the benthic and highly sedentary behavior of the species as well as of other lophiids. Researchers should consider conducting similar data storage tagging studies with other benthic fishes to test assumptions of sedentary behavior.  相似文献   
106.
Abstract

The genus Crassocephalum in Asia, introduced there from Africa, was examined by extensive field work, herbarium studies, analyses of pollen and seed fertility, chromosome counts and ITS and trnL‐F sequencing. We found that Crassocephalum in Asia comprises two species and their interspecific hybrid. The two species are C. crepidioides (Benth.) S. Moore and C. rubens (Juss. ex Jacq.) S. Moore, of which the latter is a new record for Asia (north Thailand). The hybrid between these two species in north Thailand originated from a cross between C. crepidioides (2n = 40) as female and C. rubens (2n = 40) as male parent.  相似文献   
107.
Abstract

A procedure was developed for the chemical synthesis of P1,P2-dinucleoside-5′-diphosphates (N1(5′)pp(5′)N2) on a nanomolar scale Reaction conditions for activating purine-5′-monophosphates (pA, pG, and pm7G) by 1,1′-carbonyldiimidazole were studied and optimized in respect to solvents and amount of activating reagent used. Various dinucleoside-5′-diphosphates were synthesized in 62-98% yield by incubating activated and non-activated purine-5′-monophosphates. Two unexpected by-products were formed by competition reactions: the imidazolidate of the non-activated nucleotide and the corresponding symmetrically substituted dinucleoside-5′-diphosphate. A mechanism is proposed to explain the observed side reactions.  相似文献   
108.
Nucleoside analogues are chemical means to investigate hydrogen bonds, base stacking, and solvation as the three predominant forces that are responsible for the stability of secondary structure of nucleic acids. To obtain deeper insight into the contributions of these interactions to RNA stability apart from the ones exerted by the predominant nucleosides we decided to synthesize some novel nucleic acid analogues where the nucleobases are replaced by fluoroindoles. Fluorinated indoles can be compared to fluorinated benzimidazoles to determine the role of nitrogen in five membered ring system. The synthesis of fluoroindole ribonucleosides is described here.  相似文献   
109.
The design and generation of DNA constructs is among the necessary but generally tedious tasks for molecular biologists and, typically, the cloning strategy is restricted by available restriction sites. However, increasingly sophisticated experiments require increasingly complex DNA constructs, with an intricacy that exceeds what is achievable using standard cloning procedures. Many transgenes such as inducible gene cassettes or recombination elements consist of multiple components that often require precise in-frame fusions. Here, we present an efficient protocol that facilitates the generation of these complex constructs. The golden GATEway cloning approach presented here combines two established cloning methods, namely golden Gate cloning and Multisite GatewayTM cloning. This allows efficient and seamless assembly as well as reuse of predefined DNA elements. The golden Gate cloning procedure follows clear and simple design rules and allows the assembly of multiple fragments with different sizes into one open reading frame. The final product can be directly integrated into the widely used Multisite GatewayTM cloning system, granting more flexibility when using a transgene in the context of multiple species. This adaptable and streamlined cloning procedure overcomes restrictions of “classical construct generation” and allows focusing on construct design.  相似文献   
110.
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