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   7990篇
  免费   757篇
  国内免费   2篇
  2021年   110篇
  2020年   69篇
  2019年   100篇
  2018年   114篇
  2017年   98篇
  2016年   168篇
  2015年   240篇
  2014年   269篇
  2013年   355篇
  2012年   453篇
  2011年   442篇
  2010年   274篇
  2009年   255篇
  2008年   363篇
  2007年   350篇
  2006年   369篇
  2005年   364篇
  2004年   339篇
  2003年   336篇
  2002年   291篇
  2001年   145篇
  2000年   138篇
  1999年   138篇
  1998年   109篇
  1997年   76篇
  1996年   97篇
  1995年   86篇
  1994年   89篇
  1993年   101篇
  1992年   119篇
  1991年   119篇
  1990年   120篇
  1989年   100篇
  1988年   62篇
  1987年   82篇
  1986年   83篇
  1985年   107篇
  1984年   93篇
  1983年   70篇
  1982年   85篇
  1981年   75篇
  1980年   95篇
  1979年   72篇
  1978年   66篇
  1977年   63篇
  1976年   49篇
  1975年   76篇
  1974年   58篇
  1973年   68篇
  1972年   43篇
排序方式: 共有8749条查询结果,搜索用时 31 毫秒
1.
2.
The distribution of grooming and touching behaviours was recorded in a group of captive ring-tailed lemurs. Grooming was found to be performed chiefly by older, higher ranking animals; touching (i.e., “reach out and touch” behaviour) was directed primarily by younger, low ranking animals to older, high ranking individuals. It is suggested that such touching is a submissive gesture in this species.  相似文献   
3.
Yeast DNA ligase is radioactively labelled in vitro by incubating a crude cell extract with [α-32P]ATP. The product of this reaction is the stable covalent ligase-AMP adduct, which can be characterized by its reactivity with either pyrophosphate or nicked DNA and visualized by gel electrophoresis and autoradiography. The Saccharomyces cerevisiae DNA ligase was identified as an 89 kDa polypeptide by exploiting the fact that transformants with multiple copies of the plasmid-encoded DNA ligase (CDC9) gene overproduce the enzyme by two orders of magnitude. A similar strategy has been used to identify the Schizosaccharomyces pombe DNA ligase as an 87 kDa polypeptide. Both values agree well with the coding capacities of the respective cloned gene sequences. When the S. cerevisiae ligase is greatly overproduced with respect to wild-type levels, a second polypeptide of 78.5 kDa is also labelled and has the same properties as the 89 kDa adduct. We suggest that this polypeptide is generated by proteolysis.  相似文献   
4.
5.
6.
Myoglobin (Mb) is the classic vertebrate oxygen-binding protein present in aerobic striated muscles. It functions principally in oxygen delivery and provides muscle with its characteristic red colour. Members of the Antarctic icefish family (Channichthyidae) are widely thought to be extraordinary for lacking cardiac Mb expression, a fact that has been attributed to their low metabolic rate and unusual evolutionary history. Here, we report that cardiac Mb deficit, associated with pale heart colour, has evolved repeatedly during teleost evolution. This trait affects both gill- and air-breathing species from temperate to tropical habitats across a full range of salinities. Cardiac Mb deficit results from total pseudogenization in three-spined stickleback and is associated with a massive reduction in mRNA level in two species that evidently retain functional Mb. The results suggest that near or complete absence of Mb-assisted oxygen delivery to heart muscle is a common facet of teleost biodiversity, even affecting lineages with notable oxygen demands. We suggest that Mb deficit may affect how different teleost species deal with increased tissue oxygen demands arising under climate change.  相似文献   
7.
8.
From the time of its introduction, in 1973, through November 1983, fine needle aspiration biopsy ( FNAB ) of the lung was used in the diagnostic evaluation of 1,015 patients at the Duke University Medical Center and Durham Veterans Administration Medical Center. A diagnosis of a primary lung cancer was established in 49.4% of the cases and malignant neoplasm metastatic to the lung in 14.1%; a benign diagnosis was made in 31.1% and an inconclusive one in 5.4%. In 123 patients in whom lung tissue was also available, cancer was correctly diagnosed by FNAB in 83.8%, not diagnosed by FNAB in 14.6% and incorrectly diagnosed in 1.6%. In 37 patients without cancer, FNAB detected an infectious organism or a specific morphologic type of inflammation (abscess and granuloma).  相似文献   
9.
Abstract— 2-Hydroxy-, 2-chloro-, 2- and Cmethyl-GABA are linear competitive inhibitors of GABA uptake in rat brain slices. These analogues are thus potential substrates for the GABA transport system and possible'false transmitters'. 2-Hydroxy-GABA is the most potent inhibitor of GABA uptake yet described. No specific inhibitor of GABA uptake was revealed amongst the drugs tested.  相似文献   
10.
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