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   924篇
  免费   41篇
林业   24篇
农学   15篇
基础科学   4篇
  77篇
综合类   223篇
农作物   18篇
水产渔业   73篇
畜牧兽医   479篇
园艺   9篇
植物保护   43篇
  2020年   12篇
  2019年   12篇
  2018年   9篇
  2017年   11篇
  2016年   14篇
  2015年   11篇
  2014年   14篇
  2013年   53篇
  2012年   31篇
  2011年   48篇
  2010年   38篇
  2009年   29篇
  2008年   47篇
  2007年   35篇
  2006年   47篇
  2005年   40篇
  2004年   27篇
  2003年   28篇
  2002年   33篇
  2001年   19篇
  2000年   24篇
  1999年   26篇
  1998年   13篇
  1997年   9篇
  1996年   8篇
  1995年   9篇
  1994年   7篇
  1993年   7篇
  1992年   23篇
  1991年   21篇
  1990年   22篇
  1989年   16篇
  1988年   7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   10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965条查询结果,搜索用时 20 毫秒
61.
62.
63.
Dark matter     
Green S 《Science (New York, N.Y.)》1992,255(5046):858-859
  相似文献   
64.
65.
The design and catalytic properties of a simplified ribonuclease P RNA   总被引:21,自引:0,他引:21  
Ribonuclease P (RNase P) RNA is the catalytic moiety of the ribonucleoprotein enzyme that removes precursor sequences from the 5' ends of pre-transfer RNAs in eubacteria. Phylogenetic variation according to recently proposed secondary structure models was used to identify structural elements of the RNase P RNA that are dispensable for catalysis. A simplified RNase P RNA that consists only of evolutionarily conserved features was designed, synthesized, and characterized. Although the simplified RNA (Min 1 RNA) is only 263 nucleotides in length, in contrast to the 354 to 417 nucleotides of naturally occurring RNase P RNAs, its specificity of pre-tRNA cleavage is identical to that of the native enzymes. Moreover, the catalytic efficiencies of the Min 1 RNA and the native RNA enzymes are similar. These results focus the search for the catalytic elements of RNase P RNAs to their conserved structure.  相似文献   
66.
An acceptor stem G3.U70 base pair is a major determinant of the identity of an alanine transfer RNA. Hairpin helices and RNA duplexes consisting of complementary single strands are aminoacylated with alanine if they contain G3.U70. Chemical synthesis of RNA duplexes enabled the introduction of base analogs that tested the role of specific functional groups in the major and minor grooves of the RNA helix. The results of these experiments indicate that an unpaired guanine 2-amino group at a specific position in the minor groove of an RNA helix marks a molecule for aminoacylation with alanine.  相似文献   
67.
Lymphocyte culture lines can be established by using buffy coat cells from normal or diseased individuals. Established lymphocytoblasts from a normal donor were used in the one-way mixed leukocyte culture test and were found to stimulate the peripheral lymphocytes of that donor. These lymphocytoblasts were also capable of stimulating allogeneic lymphocyte populations.  相似文献   
68.
Acetabularia chloroplast DNA: electron microscopic visualization   总被引:2,自引:0,他引:2  
Isolated chloroplasts of Acetabularia mediterranea, lysed by osmotic shock in a protein monolayer, released large amounts of supercoiled DNA. In some cases, the DNA radiated in large loops from centers of organization. The amount of DNA per chloroplast may be similar to that of a bacterium.  相似文献   
69.
Energized configurations of heart mitochondria in situ   总被引:3,自引:0,他引:3  
Changes in the configurational state of the cristal membranes of rat heart mitochondria within the living cell can be induced by imposing energizing conditions. An exact correlation has been established between the configurational states of the cristal membrane and the energy states of the mitochondrion. The configurational changes observed in mitochondria in situ are comparable to those established for beef heart mitochondria in vitro and are consistent with the postulate of the conformational basis of energy transductions in membrane systems. The formation of paracrystalline arrays is one of the noteworthy features of configurational changes of mitochondria in situ.  相似文献   
70.
Phagocytosis of bacteria by rabbit alveolar macrophages is inhibited quantitatively by cigarette smoke in vitro. This phagocytoxic effect was abolished by addition of 0.2 to 0.4 micromole of glutathione or cysteine per milliliter of cigarette smoke. Serum protein was required to obtain both the toxic effect of the smoke and the protective action of the sulfhydryl compounds. The protective role of the sulfhydryl agents suggests an oxidant action of the cigarette smoke on these pulmonary cells.  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