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   38712篇
  免费   16438篇
  国内免费   906篇
  2024年   32篇
  2023年   155篇
  2022年   356篇
  2021年   910篇
  2020年   2489篇
  2019年   4093篇
  2018年   4235篇
  2017年   4405篇
  2016年   4523篇
  2015年   4702篇
  2014年   4555篇
  2013年   5037篇
  2012年   2916篇
  2011年   2518篇
  2010年   3688篇
  2009年   2278篇
  2008年   1384篇
  2007年   868篇
  2006年   779篇
  2005年   822篇
  2004年   763篇
  2003年   723篇
  2002年   626篇
  2001年   511篇
  2000年   415篇
  1999年   375篇
  1998年   155篇
  1997年   94篇
  1996年   91篇
  1995年   104篇
  1994年   84篇
  1993年   67篇
  1992年   116篇
  1991年   101篇
  1990年   71篇
  1989年   69篇
  1988年   73篇
  1987年   56篇
  1986年   56篇
  1985年   68篇
  1984年   52篇
  1983年   41篇
  1982年   46篇
  1981年   39篇
  1979年   37篇
  1978年   30篇
  1975年   29篇
  1974年   30篇
  1973年   44篇
  1968年   28篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Aromatic L-amino acid decarboxylase (AADC) is responsible for the conversion of L-3,4-dihydroxyphenylalanine (L-DOPA) and L-5-hydroxytryptophan to dopamine and serotonin, respectively, which are important neurotransmitters. We characterized genomic clones derived from the rat AADC locus by Southern blot and nucleotide sequencing analyses to explore the exonal organization of the gene. Our results suggest that the rat AADC gene is relatively large, containing at least 12 exons and spanning at least 40 kb in the rat genome. In this study, nine exons corresponding to 71% of the published cDNA sequence were identified, the smallest of which was as short as 20 base pairs (bp). In the Drosophila dopa decarboxylase (DDC) gene, the sequences homologous to these nine exons are all present in the fourth exon. This implies that either multiple intron sequences have been added to the vertebrate AADC gene or alternatively, deleted from the invertebrate gene after the divergence of vertebrates and invertebrates during evolution.  相似文献   
112.
113.
A series of new chiral thiosemicarbazones derived from homochiral amines in both enantiomeric forms were synthesized and evaluated for their in vitro antiproliferative activity against A549 (human alveolar adenocarcinoma), MCF‐7 (human breast adenocarcinoma), HeLa (human cervical adenocarcinoma), and HGC‐27 (human stomach carcinoma) cell lines. Some of compounds showed inhibitory activities on the growth of cancer cell lines. Especially, compound 17b exhibited the most potent activity (IC50 4.6 μM) against HGC‐27 as compared with the reference compound, sindaxel (IC50 10.3 μM), and could be used as a lead compound to search new chiral thiosemicarbazone derivatives as antiproliferative agents. Chirality 27:177–188, 2015. © 2014 Wiley Periodicals, Inc.  相似文献   
114.
Although Ficus (Moraceae) is a keystone plant genus in the tropics, providing resources to many frugivorous vertebrates, its population genetic structure, which is an important determinant of its long‐term survival, has rarely been investigated. We examined the population genetic structure of two dioecious fig species (Ficus hispida and Ficus exasperata) in the Indian Western Ghats using co‐dominant nuclear microsatellite markers. We found high levels of microsatellite genetic diversity in both species. The regression slopes between genetic relationship coefficients (fij) and spatial distances were significantly negative in both species indicating that, on average, individuals in close spatial proximity were more likely to be related than individuals further apart. Mean parent–offspring distance (σ) calculated using these slopes was about 200 m in both species. This should be contrasted with the very long pollen dispersal distances documented for monoecious Ficus species. Nevertheless, overall population genetic diversity remained large suggesting immigrant gene flow. Further studies will be required to analyze broader scale patterns.  相似文献   
115.
116.
Space use including territoriality and spatial arrangement within a population can reveal important information on the nature, dynamics, and evolutionary maintenance of alternative strategies in color polymorphic species. Despite the prevalence of color polymorphic species as model systems in evolutionary biology, the interaction between space use and genetic structuring of morphs within populations has rarely been examined. Here, we assess the spatial and genetic structure of male throat color morphs within a population of the tawny dragon lizard, Ctenophorus decresii. Male color morphs do not differ in morphology but differ in aggressive and antipredator behaviors as well as androgen levels. Despite these behavioral and endocrine differences, we find that color morphs do not differ in territory size, with their spatial arrangement being essentially random with respect to each other. There were no differences in genetic diversity or relatedness between morphs; however, there was significant, albeit weak, genetic differentiation between morphs, which was unrelated to geographic distance between individuals. Our results indicate potential weak barriers to gene flow between some morphs, potentially due to nonrandom pre‐ or postcopulatory mate choice or postzygotic genetic incompatibilities. However, space use, spatial structure, and nonrandom mating do not appear to be primary mechanisms maintaining color polymorphism in this system, highlighting the complexity and variation in alternative strategies associated with color polymorphism.  相似文献   
117.
118.
119.
A fast and accurate assay procedure for DNA-RNA hybrids is described in which exhaustive digestion of unhybridized DNA with S1 nuclease is followed by binding of hybrids to filter discs of DEAE-cellulose. The digested DNA can be efficiently washed from the filters so that background levels of 0.1-0.2% of input tracer DNA can be achieved, in contrast to the much higher (approximately 1-5%) backgrounds obtained using TCA precipitation procedures. Short duplexes, as small as 36 nucleotides in length, which are inefficiently bound to hydroxyapatite, are quantitatively bound to the DEAE-cellulose filters.  相似文献   
120.
In this study, the first reported isolates of the genera Snowella and Woronichinia were characterized by 16S rRNA gene sequencing and morphological analysis. Phylogenetic studies and sequences for these genera were not available previously. By botanical criteria, the five isolated strains were identified as Snowella litoralis (Häyrén) Komárek et Hindák Snowella rosea (Snow) Elenkin and Woronichinia naegeliana (Unger) Elenkin. This study underlines the identification of freshly isolated cultures, since the Snowella strains lost the colony structure and were not identifiable after extended laboratory cultivation. In the 16S rRNA gene analysis, the Snowella strains formed a monophyletic cluster, which was most closely related to the Woronichinia strain. Thus, our results show that the morphology of the genera Snowella and Woronichinia was in congruence with their phylogeny, and their phylogeny seems to support the traditional botanical classification of these genera. Furthermore, the genera Snowella and Woronichinia occurred commonly and might occasionally be the most abundant cyanobacterial taxa in mainly oligotrophic and mesotrophic Finnish lakes. Woronichinia occurred frequently and also formed blooms in eutrophic Czech reservoirs.  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