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   19415篇
  免费   1488篇
  国内免费   1093篇
  21996篇
  2024年   53篇
  2023年   226篇
  2022年   578篇
  2021年   905篇
  2020年   601篇
  2019年   738篇
  2018年   826篇
  2017年   549篇
  2016年   845篇
  2015年   1154篇
  2014年   1310篇
  2013年   1468篇
  2012年   1767篇
  2011年   1556篇
  2010年   966篇
  2009年   850篇
  2008年   1069篇
  2007年   935篇
  2006年   822篇
  2005年   732篇
  2004年   584篇
  2003年   548篇
  2002年   472篇
  2001年   359篇
  2000年   298篇
  1999年   290篇
  1998年   185篇
  1997年   139篇
  1996年   107篇
  1995年   112篇
  1994年   80篇
  1993年   70篇
  1992年   96篇
  1991年   96篇
  1990年   87篇
  1989年   68篇
  1988年   57篇
  1987年   54篇
  1986年   40篇
  1985年   44篇
  1984年   24篇
  1983年   24篇
  1982年   21篇
  1981年   13篇
  1979年   20篇
  1978年   16篇
  1977年   19篇
  1976年   15篇
  1975年   15篇
  1972年   15篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
Alien species are brought into countries world wide on a massive scale for agricultural production, ex situ conservation, landscape aesthetics, gardens, and ecosystem restoration. Unfortunately, some of these species have escaped and adversely impacted on regional as well as global biodiversity conservation and agricultural production. To reduce such risks, it is necessary to implement specific and effective measures. Since various government departments and institutions are involved in the management of alien species, it is difficult to prevent native and agroecosystems from being invaded by invited species. We propose the establishment of a supervision and inspection continuum over intentional species introduction, similar to that which exists in some countries over unintentional species introductions. Namely, a justification of the necessity to import, a risk assessment, assurances as to provision of an adequate containment facility assessment, and a damage-limitation protocol should that need to be invoked. These requirements should be satisfied before an alien species is knowingly imported, and the necessary follow-up supervision is important post- importation.  相似文献   
33.
34.
35.
The turnover of tumor necrosis factor (TNF) mRNA in permanently transfected macrophages of the RAW 264.7 cell line was studied directly (by Northern blot analysis using a probe specific for TNF) and indirectly (through studies of the turnover of various reporter mRNAs, either containing or lacking the TNF 3' untranslated region (UTR)). The TNF mRNA was found to be very unstable in RAW 264.7 cells. Instability appeared to result from two distinguishable nucleolytic processes. The major degradative process involved was not specific for the TNF 3' UTR of reporter mRNAs, and was inhibited by actinomycin D pretreatment. It appeared to be expressed constitutively, in that cell activation by lipopolysaccharide (LPS) did not modify message stability. When cells were treated with actinomycin D, a minor nucleolytic activity was 'uncovered'. This minor activity was noted to increase with time following LPS activation. It also exhibited specificity, in that reporter mRNAs bearing the 3' UTR of TNF were more susceptible to degradation in the presence of actinomycin D than were constructs lacking the 3' UTR of TNF. Thus, TNF mRNA turnover appears complex, and depends upon at least two separable degradative pathways. The TNF 3' UTR apparently contributes only modestly to the instability of this mRNA under normal conditions.  相似文献   
36.
Diao Y  Guo X  Li Y  Sun K  Lu L  Jiang L  Fu X  Zhu H  Sun H  Wang H  Wu Z 《Cell Stem Cell》2012,11(2):231-241
In mouse skeletal muscles, Pax7 uniquely marks muscle satellite cells and plays some important yet unknown functions at the perinatal stage. To elucidate its in vivo functions, we initiated a yeast two-hybrid screening to look for Pax7-interacting proteins and identified a previously uncharacterized Pax7- and Pax3-binding protein (Pax3/7BP). Pax3/7BP is a ubiquitously expressed nuclear protein, enriched in Pax7+ muscle precursor cells (MPCs), and serves as an indispensable adaptor for Pax7 to recruit the histone 3 lysine 4 (H3K4) methyltransferase (HMT) complex by bridging Pax7 and Wdr5. Knockdown of Pax3/7BP abolished the Pax3/7-associated H3K4 HMT activity and inhibited the proliferation of Pax7+ MPCs from young mice both in culture and in vivo. Id3 and Cdc20 were direct target genes of Pax7 and Pax3/7BP involved in the proliferation of Pax7+ MPCs. Collectively, our work establishes Pax3/7BP as an essential adaptor linking Pax3/7 with the H3K4 HMT to regulate the proliferation of MPCs.  相似文献   
37.
This study was performed to examine the use of NaOCl as an alternative antimicrobial compound in winemaking because of the potential health problems that may arise as a result of the use of SO2. For this, the blank (non-treated), control (SO2-added), and sample (NaOCl-treated) wines were made, and microbial and chemical changes including sensory characteristics were analyzed during the fermentation periods. Treatment of grapes with NaOCl decreased the initial contaminating microbial population in grape must, resulting in higher growth of yeast and lactic acid bacteria. After 200 days of fermentation, the chemical analysis of sample wine revealed that it had higher ethanol content, redness (a*), and concentrations of fruity ester compounds and lower total acidity than the control. In the sensory analyses, the sample wine obtained a higher overall acceptability score (5.70) than the control (4.26). This result reveals that NaOCl can be used as an alternative to SO2 in winemaking for inhibiting the growth of contaminating microorganisms.  相似文献   
38.
To determine whether genetic heterogeneity exists in patients with Graves'' disease (GD), the cytotoxic T-lymphocyte associated 4 (CTLA-4) gene, which is implicated a susceptibility gene for GD by considerable genetic and immunological evidence, was used for association analysis in a Chinese Han cohort recruited from various geographic regions. Our association study for the SNPs in the CTLA4 gene in 2640 GD patients and 2204 control subjects confirmed that CTLA4 is the susceptibility gene for GD in the Chinese Han population. Moreover, the logistic regression analysis in the combined Chinese Han cohort revealed that SNP rs231779 (allele frequencies p = 2.81×10−9, OR = 1.35, and genotype distributions p = 2.75×10−9, OR = 1.42) is likely the susceptibility variant for GD. Interestingly, the logistic regression analysis revealed that SNP rs35219727 may be the susceptibility variant to GD in the Shandong population; however, SNP, rs231779 in the CTLA4 gene probably independently confers GD susceptibility in the Xuzhou and southern China populations. These data suggest that the susceptibility variants of the CTLA4 gene varied between the different geographic populations with GD.  相似文献   
39.
40.
Members of the Pumilio (Pum) family of RNA-binding proteins act as translational repressors and are required for germ cell development and asymmetric division. We identified the chicken Pum1 and Pum2 genes and analyzed their expression patterns in various tissues. Comparative sequence analysis of the Pum1 and Pum2 proteins from the drosophila, chicken, mouse, and human revealed a high degree of evolutionary conservation in terms of the levels of homology of the peptide sequences and the structure of Pumilio homology domain (PUM-HD), C-terminal RNA-binding domain, with similar spacing between the adjacent Pum eight tandem repeats. In addition, phylogenetic patterns of pumilio family showed that Pum 1 and 2 of chicken are more closely related to those of mouse and human than other species and Pum1 is more conserved than Pum2. Using real-time RT-PCR, the expression levels of the Pum1 and Pum2 genes were found to be highest in hatched female gonads, and high-level expression of Pum2 was detected in 12-day and hatched gonads among the various chicken embryonic tissues tested. In adult tissues, the expression levels of Pum1 and Pum2 were expressed at higher levels in the testis and muscle than in any other tissue. The characteristics of the tissue-specific expression of Pum genes suggest that Pum1 and Pum2 have effects crucially in particular stage during development of chicken gonads depending on sexual maturation.  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