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   1230262篇
  免费   99308篇
  国内免费   1346篇
  2021年   17648篇
  2020年   12547篇
  2019年   16100篇
  2018年   17470篇
  2017年   16270篇
  2016年   27808篇
  2015年   41950篇
  2014年   49904篇
  2013年   76120篇
  2012年   32105篇
  2011年   19918篇
  2010年   42496篇
  2009年   44206篇
  2008年   19490篇
  2007年   17225篇
  2006年   22930篇
  2005年   24331篇
  2004年   23480篇
  2003年   21234篇
  2002年   19475篇
  2001年   26057篇
  2000年   23345篇
  1999年   25421篇
  1998年   24477篇
  1997年   24164篇
  1996年   23816篇
  1995年   21964篇
  1994年   21776篇
  1993年   20843篇
  1992年   22551篇
  1991年   21255篇
  1990年   19887篇
  1989年   21089篇
  1988年   19335篇
  1987年   18286篇
  1986年   17386篇
  1985年   19535篇
  1984年   20136篇
  1983年   17867篇
  1982年   19135篇
  1981年   18530篇
  1980年   17345篇
  1979年   16046篇
  1978年   15922篇
  1977年   15347篇
  1976年   14620篇
  1975年   13995篇
  1974年   14394篇
  1973年   14796篇
  1972年   12507篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
The measles virus (MV) accessory proteins V and C play important roles in MV replication and pathogenesis. Infection with recombinant MV lacking either V or C causes more cell death than infection with the parental vaccine-equivalent virus (MVvac), and C-deficient virus grows poorly relative to the parental virus. Here, we show that a major effector of the C phenotype is the RNA-dependent protein kinase PKR. Using human HeLa cells stably deficient in PKR as a result of RNA interference-mediated knockdown (PKRkd cells), we demonstrated that a reduction in PKR partially rescued the growth defect of C knockout (Cko) virus but had no effect on the growth of either wild-type (WT) or V knockout (Vko) virus. Increased growth of the Cko virus in PKRkd cells correlated with increased viral protein expression, while defective growth and decreased protein expression in PKR-sufficient cells correlated with increased phosphorylation of PKR and the α subunit of eukaryotic initiation factor 2. Furthermore, infection with WT, Vko, or especially Cko virus caused significantly less apoptosis in PKRkd cells than in PKR-sufficient cells. Although apoptosis induced by Cko virus infection in PKR-sufficient cells was blocked by a caspase antagonist, the growth of Cko virus was not restored to the WT level by treatment with this pharmacologic inhibitor. Taken together, these results indicate that PKR plays an important antiviral role during MV infection but that the virus growth restriction by PKR is not dependent upon the induction of apoptosis. Furthermore, the results establish that a principal function of the MV C protein is to antagonize the proapoptotic and antiviral activities of PKR.  相似文献   
82.
Summary The results of a recent quantitative analysis of the Teorell membrane oscillator are utilized to explore its role as an excitability analogue. Special attention is paid to its role as a mechano-electric transducer. A membrane of exceptionally well-defined pore structure has been used in this study. The analogue properties arise from nonlinear coupling between water and salt fluxes. When the membrane is simultaneously subjected to controlled gradients of hydrostatic pressure, electrical potential and concentration, bi-stable stationary states can be produced. These arise from the opposing effects of pressure and electro-osmosis on the volume flow. Transitions between these states show hysteresis. The factors governing such transitions are analogous to certain types of stimuli encountered in the natural excitation process. The membrane system also shows oscillatory behavior when the hydrostatic pressure gradient is allowed to vary under constant current conditions. This property is related to the bi-stable stationary state phenomena and is compared to the regenerative behavior found in biologically excitable tissues. Particular emphasis is placed upon analogies between the membrane oscillator and certain natural tissues. The importance of the nonlinear nature of the force-flux coupling in the analogue is stressed, and its possible relevance to biological excitability indicated. Some consideration is also given to the role of electro-osmotic flux coupling in biological tissues.  相似文献   
83.
84.
The literature relating to chemical, biochemical and biological aspects of the steroidal glycoalkaloid, α-tomatine, is reviewed. The alkaloid, which can be used as a starting compound for the synthesis of steroidal hormones, is toxic to a wide range of living organisms. The significance of tomatine to plants which elaborate it is discussed and some possible uses of the compound are mentioned.  相似文献   
85.
86.
87.
Censuses of buffalo and wildebeest in the Serengeti-Mara region have been carried out by various workers at intervals since 1958. The methods of these censuses, which normally employ a total count, are described and the errors inherent in such techniques are analysed in detail. After the appropriate corrections had been made the results showed that both populations have increased considerably over the years 1961–1971.  相似文献   
88.
89.
90.
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