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   166175篇
  免费   9471篇
  国内免费   87篇
  2021年   1564篇
  2020年   928篇
  2019年   1123篇
  2018年   2326篇
  2017年   2286篇
  2016年   4774篇
  2015年   9393篇
  2014年   9392篇
  2013年   9908篇
  2012年   10969篇
  2011年   8312篇
  2010年   6040篇
  2009年   5656篇
  2008年   5891篇
  2007年   5861篇
  2006年   5708篇
  2005年   11379篇
  2004年   9979篇
  2003年   8019篇
  2002年   5432篇
  2001年   1947篇
  2000年   951篇
  1999年   2421篇
  1998年   1544篇
  1997年   988篇
  1996年   852篇
  1992年   2480篇
  1991年   2529篇
  1990年   2536篇
  1989年   2392篇
  1988年   2373篇
  1987年   2157篇
  1986年   1942篇
  1985年   2056篇
  1984年   1609篇
  1983年   1235篇
  1982年   963篇
  1981年   907篇
  1979年   1371篇
  1978年   1084篇
  1977年   888篇
  1976年   897篇
  1975年   1078篇
  1974年   1237篇
  1973年   1163篇
  1972年   1068篇
  1971年   1030篇
  1970年   902篇
  1969年   909篇
  1967年   819篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
73.
We present a time‐calibrated phylogeny of the charismatic green lacewings (Neuroptera: Chrysopidae). Previous phylogenetic studies on the family using DNA sequences have suffered from sparse taxon sampling and/or limited amounts of data. Here we combine all available previously published DNA sequence data and add to it new DNA sequences generated for this study. We analysed these data in a supermatrix using Bayesian and maximum likelihood methods and provide a phylogenetic hypothesis for the family that recovers strong support for the monophyly of all subfamilies and resolves relationships among a large proportion of chrysopine genera. Chrysopinae tribes Leucochrysini and Belonopterygini were recovered as monophyletic sister clades, while the species‐rich tribe Chrysopini was rendered paraphyletic by Ankylopterygini. Relationships among the subfamilies were resolved, although with relatively low statistical support, and the topology varied based on the method of analysis. Greatest support was found for Apochrysinae as sister to Nothochrysinae and Chrysopinae, which is in contrast to traditional concepts that place Nothochrysinae as sister to the rest of the family. Divergence estimates suggest that the stem groups to the various subfamilies diverged during the Triassic‐Jurassic, and that stem groups of the chrysopine tribes diverged during the Cretaceous.  相似文献   
74.
Following treatment of hen erythrocyte nuclei with dimethyl 3,3'-dithiobispropionimidate, dimers between histones H1a, H1b, and H5 were extracted with 5% perchloric acid. They resolved electrophoretically into four sub-bands and these were identified by non-reducing/reducing gel electrophoresis. The H5-H5 homodimer species was purified by gel electrophoresis and was treated sequentially with BrCN and dithiothreitol. The pattern of resulting fragments indicates that cross-links were mainly formed between the COOH-terminal portions and at a significantly lower frequency between the COOH-terminal and the NH2-terminal portions.  相似文献   
75.
Enzyme activity capable of converting the glycine-extended substance P precursor, substance P-Gly12, into substance P was purified from human cerebrospinal fluid. The conversion reaction was monitored by radioimmunoassay measurement of substance P formation. The chemical identity of the product was verified by reversed-phase HPLC. The enzyme reaction was stimulated by Cu(II) ion and ascorbic acid and inhibited by the presence of diethyldithiocarbamate. By HPLC molecular sieving, the major enzyme activity appeared as a protein of 26,000 molecular weight.  相似文献   
76.
77.
In has been shown that cortisol immobilized on polyvinylpyrrolidone (PVP-GC) affects cyclic AMP production stimulated by adenosine and isoproterenol in rat thymocytes. This effect of PVP-GC is specific for cortisol: antiglucocorticoid progesterone (at a concentration of 10(-5) M) inhibited completely the action of PVP-GC on the intracellular cAMP level. It is suggested that cortisol effect on cAMP production is one of the mechanisms of glucocorticoid hormone action in target cells.  相似文献   
78.
79.
80.
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