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   951篇
  免费   44篇
林业   24篇
农学   15篇
基础科学   4篇
  77篇
综合类   238篇
农作物   18篇
水产渔业   75篇
畜牧兽医   492篇
园艺   9篇
植物保护   43篇
  2020年   9篇
  2019年   9篇
  2018年   7篇
  2017年   14篇
  2016年   14篇
  2015年   13篇
  2014年   18篇
  2013年   79篇
  2012年   32篇
  2011年   49篇
  2010年   38篇
  2009年   28篇
  2008年   48篇
  2007年   34篇
  2006年   46篇
  2005年   39篇
  2004年   26篇
  2003年   29篇
  2002年   34篇
  2001年   19篇
  2000年   24篇
  1999年   29篇
  1998年   14篇
  1997年   9篇
  1996年   7篇
  1995年   9篇
  1994年   7篇
  1993年   7篇
  1992年   23篇
  1991年   20篇
  1990年   19篇
  1989年   15篇
  1988年   9篇
  1987年   14篇
  1986年   11篇
  1985年   10篇
  1984年   12篇
  1983年   8篇
  1979年   11篇
  1978年   9篇
  1977年   7篇
  1976年   8篇
  1974年   10篇
  1973年   8篇
  1972年   8篇
  1971年   6篇
  1970年   6篇
  1969年   9篇
  1967年   6篇
  1966年   7篇
排序方式: 共有995条查询结果,搜索用时 15 毫秒
81.
Understanding cancer pathogenesis requires knowledge of not only the specific contributory genetic mutations but also the cellular framework in which they arise and function. Here we explore the clonal evolution of a form of childhood precursor-B cell acute lymphoblastic leukemia that is characterized by a chromosomal translocation generating a TEL-AML1 fusion gene. We identify a cell compartment in leukemic children that can propagate leukemia when transplanted in mice. By studying a monochorionic twin pair, one preleukemic and one with frank leukemia, we establish the lineal relationship between these "cancer-propagating" cells and the preleukemic cell in which the TEL-AML1 fusion first arises or has functional impact. Analysis of TEL-AML1-transduced cord blood cells suggests that TEL-AML1 functions as a first-hit mutation by endowing this preleukemic cell with altered self-renewal and survival properties.  相似文献   
82.
An RNA processing activity that debranches RNA lariats   总被引:68,自引:0,他引:68  
The excised introns of pre-messenger RNA's (pre-mRNA's) and intron-containing splicing intermediates are in a lariat configuration in which the 5' end of the intron is covalently joined by a 2',5'-phosphodiester bond to a specific adenosine residue near the 3' end of the intron. A 2',5'-phosphodiesterase activity in HeLa cell extracts has been detected that debranches RNA lariats, converting them to linear RNA molecules by specific cleavage of the 2',5'-phosphodiester bond. This lariat debranching activity is distinct from previously reported 2',5'-phosphodiesterases with regard to its biochemical and substrate requirements as well as its stringent cleavage specificity. The debranching activity is observed only if the RNA lariats generated during in vitro processing are deproteinized and added back to the extract. These results suggest that during the normal in vitro splicing reaction the 2',5'-phosphodiester bond of RNA lariats is protected from cleavage by the lariat debranching activity.  相似文献   
83.
DM Kolb  R Ullmann  T Will 《Science (New York, N.Y.)》1997,275(5303):1097-1099
The use of scanning tunneling microscopy in an electrochemical environment as a tool for the nanoscale modification of gold electrodes was demonstrated. Small copper clusters, typically two to four atomic layers in height, were precisely positioned on a gold(111) electrode by a process in which copper was first deposited onto the tip of the scanning tunneling microscope, which then acted as a reservoir from which copper could be transferred to the surface during an appropriate approach of the tip to the surface. Tip approach and position were controlled externally by a microprocessor unit, allowing the fabrication of various patterns, cluster arrays, and "conducting wires" in a very flexible and convenient manner.  相似文献   
84.
85.
86.
Group Research     
  相似文献   
87.
Coordinated observations of the earth's bow shock were made as Vela 3A and Explorer 33 passed within 6 earth radii of each other. Elapsed time measurements of shock motion give directly determined velocities in the range 1 to 10 kilometers per second and establish the existence of two regions, one of large amplitude magnetic "shock" oscillations and another of smaller, sunward, upstream oscillations. Each region is as thick as 1 earth radius, or more.  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