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   137683篇
  免费   10611篇
  国内免费   9100篇
  157394篇
  2024年   274篇
  2023年   1646篇
  2022年   3644篇
  2021年   6255篇
  2020年   4330篇
  2019年   5424篇
  2018年   5342篇
  2017年   4106篇
  2016年   5792篇
  2015年   8171篇
  2014年   9461篇
  2013年   10444篇
  2012年   12403篇
  2011年   11229篇
  2010年   7032篇
  2009年   6237篇
  2008年   7235篇
  2007年   6590篇
  2006年   5883篇
  2005年   4668篇
  2004年   4121篇
  2003年   3780篇
  2002年   3251篇
  2001年   2400篇
  2000年   2211篇
  1999年   2098篇
  1998年   1338篇
  1997年   1213篇
  1996年   1146篇
  1995年   1023篇
  1994年   916篇
  1993年   760篇
  1992年   1038篇
  1991年   800篇
  1990年   628篇
  1989年   587篇
  1988年   472篇
  1987年   479篇
  1986年   386篇
  1985年   377篇
  1984年   274篇
  1983年   240篇
  1982年   183篇
  1981年   149篇
  1980年   114篇
  1979年   139篇
  1978年   99篇
  1977年   96篇
  1974年   112篇
  1973年   94篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
A cDNA library was prepared from poly(A) mRNA extracted from 9-day hamster-yolk-sac erythroid cells. Two clones containing inserts coding for embryonic beta-like z or y globin-chains were isolated. Their identity was confirmed by (a) translation of hybrid selected mRNAs and (b) nucleotide sequence analysis of the inserts and comparison to the embryonic beta-like globin genes of Balb/c mouse. Availability of sequences for embryonic and adult globin cDNAs will aid in investigations of the molecular mechanisms of the globin switch in hamster YSEC.  相似文献   
22.
23.
A Melanesian variant of the genetic disease α-thalassemia has recently been shown to be due to a single-nucleotide polymorphism located between the adult α-globin genes and their enhancers. The finding that this mutation creates a novel promoter provides support for a mechanism of gene regulation by facilitated chromatin looping.  相似文献   
24.
25.
26.
Unpublished portions of the synthesis of a phosphinate-phosphonate diether analog of CDPdiacylglycerol are reported. The liponucleotide analog was found to be a very powerful inhibitor of platelet PI synthetase; kinetic data suggest a competitive inhibition mechanism. The structural specificity of CDPdiacylglycerol for liponucleotide-mediated biosynthetic reactions is discussed.  相似文献   
27.
28.
An iso-random Bi Bi mechanism has been proposed for adenylate kinase. In this mechanism, one of the enzyme forms can bind the substrates MgATP and AMP, whereas the other form can bind the products MgADP and ADP. In a catalytic cycle, the conformational changes of the free enzyme and the ternary complexes are the rate-limiting steps. The AP(5)A inhibition equations derived from this mechanism show theoretically that AP(5)A acts as a competitive inhibitor for the forward reaction and a mixed noncompetitive inhibitor for the backward reaction.  相似文献   
29.
How Can the Eco‐efficiency of a Region be Measured and Monitored?   总被引:2,自引:0,他引:2  
The concept of eco-efficiency is commonly referred to as a business link to sustainable development. In this article, ecoefficiency is examined at a regional level as an approach to promoting the competitiveness of economic activities in the Finnish Kymenlaakso region and mitigating their harmful impacts on the environment. The aim is to develop appropriate indicators for monitoring changes in the eco-efficiency of the region. A starting point is to produce indicators for the environmental and economic dimensions of regional development and use them for measuring regional eco-efficiency. The environmental impact indicators are based on a life-cycle assessment method, producing different types of environmental impact indicators: pressure indicators (e.g., emissions of CO2), impact category indicators (e.g., CO2 equivalents in the case of climate change), and a total impact indicator (aggregating different impact category indicator results into a single value). Environmental impact indicators based on direct material input, total material input, and total material requirement of the Kymenlaakso region are also assessed. The economic indicators used are the gross domestic product, the value added, and the output of the main economic sectors of Kymenlaakso. In the eco-efficiency assessment, the economic and environmental impact indicators are monitored in the same graph. In a few cases eco-efficiency ratios can also be calculated (the economic indicators are divided by the environmental indicators). Output (= value added + intermediate consumption) is used as an economic indicator related to the environmental impact indicators, which also cover the upstream processes of the region's activities. In the article, we also discuss the strengths and weaknesses of using the different environmental impact indicators.  相似文献   
30.
Two analogs of human beta-endorphin (beta-EP) which contain cystine bridges, [Cys15-Cys26,Phe27,Gly31]-beta-EP (I) and [Cys16-Cys26,Phe27,Gly31]-beta-EP (II), were synthesized by the solid-phase method. Peptides I and II were shown to contain 2-2.5 times the opiate receptor binding activity of beta-endorphin. We also synthesized two analogs with reduced alkylated cysteine residues and these peptides, [Arg9,19,24,28,29 Cys(Cam)11,26,Phe27,Gly31] and [Arg9,19,24,28,29,Cys-(Cam)12,26,Phe27,Gly31], were shown to have approximately the same opiate receptor activity as beta-endorphin.  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