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   866854篇
  免费   79811篇
  国内免费   189篇
  946854篇
  2016年   9388篇
  2015年   13176篇
  2014年   15508篇
  2013年   22166篇
  2012年   24471篇
  2011年   24935篇
  2010年   16616篇
  2009年   15599篇
  2008年   22128篇
  2007年   23003篇
  2006年   21391篇
  2005年   20850篇
  2004年   20399篇
  2003年   20028篇
  2002年   19271篇
  2001年   36305篇
  2000年   36861篇
  1999年   29480篇
  1998年   10784篇
  1997年   11551篇
  1996年   11017篇
  1995年   10483篇
  1994年   10391篇
  1993年   10349篇
  1992年   25498篇
  1991年   25181篇
  1990年   24402篇
  1989年   23960篇
  1988年   22149篇
  1987年   21442篇
  1986年   20135篇
  1985年   20264篇
  1984年   17016篇
  1983年   14942篇
  1982年   11606篇
  1981年   10612篇
  1980年   10098篇
  1979年   16788篇
  1978年   13184篇
  1977年   12151篇
  1976年   11515篇
  1975年   12701篇
  1974年   13195篇
  1973年   13022篇
  1972年   12125篇
  1971年   10793篇
  1970年   9328篇
  1969年   8867篇
  1968年   8055篇
  1967年   7083篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
There are too many kinds of organisms to be able to study and manage each, yet the loss of a single species can sometimes unravel an ecosystem. Such `fusewire species'– critical in the same sense that an electrical fuse can cut out a whole circuit – would be a rewarding focus for research and management effort. However, this approach can only be effective if these `fusewires' represent but a small proportion of the number of species in the system.  

Aim


To demonstrate methods for measuring what proportion of the species in a system are critical to ecosystem function.  

Methods


The prevalence of fusewire species was measured in manipulative experiments on an aquatic microcosm.  

Results


No single genus deletion caused changes in key characteristics of the system.  

Main conclusions


Comparison of these results with other published studies shows that the proportion of critical fusewire species varies amongst different ecosystems. The oxidation pond microcosms were shown to contain no single species indispensable to system function. They appear to be ill-suited to a management strategy which focuses on priority eukaryote species. However, a single study provides no evidence that this result is general or even typical of other kinds of ecosystems; it is presented here as an empirical model. Other methods of investigation are available; they are less experimentally rigorous but more practical. These could provide important guidance in planning an approach to management in a particular ecosystem.  相似文献   
2.
Mitogen-activated protein kinase (MAPK) pathways are activated by a plethora of stimuli. The literature is filled with papers describing the activation of different MAPKs by almost any stimulus or insult imaginable to cells. In this review, we use signal transduction wiring diagrams to illustrate putative upstream regulators for the MAPK kinase kinases, MEKK1, 2, and 3. Targeted gene disruption of MEKK1, 2, or 3 defined phenotypes for each MEKK associated with loss of specific MAPK regulation. Genetic analysis of MEKK function clearly defines specific components of the wiring diagram that require MEKK1, 2, or 3 for physiological responses. We propose that signal transduction network wiring diagrams are valuable tools for hypothesis building and filtering physiologically relevant phenotypic responses from less connected protein relations in the regulation of MAPK pathways.  相似文献   
3.
4.
5.
6.
7.
8.
9.
Structural maintenance of chromosomes (SMC) proteins play fundamental roles in higher-order chromosome dynamics from bacteria to humans. It has been proposed that the Bacillus subtilis SMC (BsSMC) homodimer is composed of two anti-parallel coiled-coil arms, each having an ATP-binding domain at its distal end. It remains totally unknown, however, how the two-armed structure supports ATP-dependent actions of BsSMC. By constructing a number of mutant derivatives including 'single-armed' BsSMC, we show here that the central hinge domain provides a structural flexibility that allows opening and closing of the two arms. This unique structure brings about bimodal regulation of the SMC ATPase cycle. Closing the arm can trigger ATP hydrolysis by allowing an end-end interaction within a dimer (intramolecular mode). When bound to DNA, ATP promotes a dimer-dimer interaction, which in turn activates their DNA-dependent ATPase activity (intermolecular mode). Our results reveal a novel mechanism of ATPase regulation and provide mechanistic insights into how eukaryotic SMC protein complexes could mediate diverse chromosomal functions, such as chromosome condensation and sister chromatid cohesion.  相似文献   
10.
Methods of synthesis of coproporphyrin and uroporphyrin by using bacteria of the genus Arthrobacter are proposed. Metal complexes of coproporphyrin and uroporphyrin with Pt, Pd, and Zn were synthesized. Their structures were identified by spectrophotometry, IR spectrometry, 1H-NMR, mass spectrometry, and HPLC. Data showing the possibility to use coproporphyrin III-metal complexes as luminophores for fluorescence detection of tumors. The current and prospective uses of metal complexes of water-soluble natural porphyrins in advanced immunofluorescence assays are discussed.  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