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   27329篇
  免费   2348篇
  国内免费   2312篇
  31989篇
  2024年   45篇
  2023年   234篇
  2022年   500篇
  2021年   787篇
  2020年   678篇
  2019年   808篇
  2018年   835篇
  2017年   724篇
  2016年   973篇
  2015年   1610篇
  2014年   1784篇
  2013年   2015篇
  2012年   2546篇
  2011年   2214篇
  2010年   1569篇
  2009年   1370篇
  2008年   1749篇
  2007年   1588篇
  2006年   1488篇
  2005年   1356篇
  2004年   1286篇
  2003年   1148篇
  2002年   959篇
  2001年   593篇
  2000年   498篇
  1999年   465篇
  1998年   311篇
  1997年   236篇
  1996年   216篇
  1995年   188篇
  1994年   163篇
  1993年   117篇
  1992年   149篇
  1991年   119篇
  1990年   93篇
  1989年   95篇
  1988年   60篇
  1987年   74篇
  1986年   59篇
  1985年   35篇
  1984年   21篇
  1983年   38篇
  1982年   16篇
  1981年   20篇
  1979年   17篇
  1978年   14篇
  1977年   14篇
  1976年   15篇
  1972年   12篇
  1970年   14篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Plasmonics - The funneling profile of enhanced light transmission through a subwavelength slit in a perfect electric conductor is studied with finite-difference time-domain simulation. From the...  相似文献   
62.

Background

Several case-control studies have been performed to examine the association of genetic variants in lysyl oxidase (LOX) with keratoconus. However, the results remained inconclusive and great heterogeneity might exist across populations.

Method

A comprehensive literature search for studies that published up to June 25, 2015 was performed. Summary odds ratios (OR) and 95% confidence intervals (CI) of each single nucleotide polymorphism (SNP) were estimated with fixed effects model when I 2<50% in the test for heterogeneity or random effects model when I 2>50%. Publication bias was evaluated using funnel plots and Egger’s test.

Results

A total of four studies including 1,467 keratoconus cases and 4,490 controls were involved in this meta-analysis. SNPs rs2956540 and rs10519694 showed significant association with keratoconus, with ORs of 0.71 (95% CI: 0.63–0.80, P = 1.43E-08) and 0.77 (95% CI: 0.61–0.97, P = 0.026), respectively. In contrast, our study lacked sufficient evidences to support the association of rs1800449/rs2288393 with keratoconus across populations.

Conclusion

This meta-analysis suggested that two LOX variants, rs2956540 and rs10519694, may affect individual susceptibility to keratoconus, while distinct heterogeneity existed within this locus. Larger-scale and multi-ethnic genetic studies on keratoconus are required to further validate the results.  相似文献   
63.
Expressed sequence tags (ESTs) provide a valuable tool that can be used to identify genes in secondary metabolite biosynthesis. Ginseng (Panax ginseng C.A Meyer) is a medicinal plant that accumulates ginsenosides in roots. We sequenced 11,636 ESTs from five ginseng libraries in order to create a gene resource for biosynthesis of ginsenosides, which are thought to be the major active component in roots. Only 59% of the ginseng ESTs exhibited significant homology to previously known polypeptide sequences. Stress- and pathogen-response proteins were most abundant in 4-year-old ginseng roots. ESTs involved in ginsenoside biosynthesis were identified by a keyword search of BLASTX results and a domain search of ginseng ESTs. We identified 4 oxidosqualene cyclase candidates involved in the cyclization reaction of 2,3-oxidosqualene, 9 nine cytochrome P450 and 12 glycosyltransferse candidates, which may be involved in modification of the triterpene backbone.Abbreviations cDNA Complementary DNA - ESTs Expressed sequence tagsCommunicated by I.S. Chung  相似文献   
64.
65.
66.
报道间斑寇蛛首次在南方大批量人工养殖成功所获得的行为生态学资料:养殖方法、捕食与食谱、活动规律、蜕皮与体色的关系;温度和湿度对间斑寇蛛活动能力、成活率、蜕皮率的影响;温度和食物对其若蛛生长发育的交互影响作用。间斑寇蛛生长发育的适宜温区是20~30℃,RH55%~75%;最适合温度为25℃,RH65%,苍蝇为较理想的食物。结合野外进行了种群密度的调查、初步分析了其种群分布与栖境生物群落的关系。  相似文献   
67.
As studies on vehicular ad hoc networks have been conducted actively in recent years, convenient and reliable services can be provided to vehicles through traffic information, surrounding information, and file sharing. To provide services for multiple requests, road side units (RSUs) should receive requests from vehicles and provide a scheduling scheme for data transfer according to priority. In this paper, we propose a new scheduling scheme by which multiple RSUs are connected through wired networks and data is transferred through the collaboration of RSUs. The proposed scheme transfers safety and non-safety data by employing a collaborative strategy of multiple RSUs as well as reducing the deadline miss ratio and average response time. When safety data is generated, data is transferred from the previous RSU in advance, while priority is assigned considering the deadline and reception rate. Since non-safety data is an on-demand data processed by user requests, the proposed scheme provides a method that reduces the deadline miss ratio upon loads generated in RSUs. To prove the superiority of the proposed scheme, we perform a performance evaluation in which the number and velocities of vehicles were changed. It is shown through the performance evaluation that the proposed scheme has better deadline miss ratios and faster response time than the existing schemes.  相似文献   
68.
Glycoproteins are involved in many important molecular recognition processes including invasion, adhesion, differentiation, and development. To identify the glycoproteins of Toxoplasma gondii, a proteomic analysis was undertaken. T. gondii proteins were prepared and fractioned using lectin affinity chromatography. The proteins in each fraction were then separated using SDS-PAGE and identified by tryptic in gel digestion followed by tandem mass spectrometry. Utilizing these methods 132 proteins were identified. Among the identified proteins were 17 surface proteins, 9 microneme proteins, 15 rhoptry proteins, 11 heat shock proteins (HSP), and 32 hypothetical proteins. Several proteins had 1–5 transmembrane domains (TMD) with some being as large as 608.3 kDa. Both lectin-fluorescence labeling and lectin blotting were employed to confirm the presence of carbohydrates on the surface or cytoplasm of T. gondii parasites. PCR demonstrated that selected hypothetical proteins were expressed in T. gondii tachyzoites. This data provides a large-scale analysis of the T. gondii glycoproteome. Studies of the function of glycosylation of these proteins may help elucidate mechanism(s) involved in invasion improving drug therapy as well as identify glycoproteins that may prove to be useful as vaccine candidates.  相似文献   
69.
HIV-1 gp41 prehairpin fusion intermediate (PFI) composed of three N-terminal heptad repeats (NHR) plays a crucial role in viral fusion and entry and represents an attractive target for anti-HIV therapeutics (e.g., enfuvirtide) and vaccines. In present study, we constructed and expressed two recombinant gp41 PFI mimetics, designated N46Fd and N46FdFc. N46Fd consists of N46 (residues 536-581) in gp41 NHR and foldon (Fd), a trimerization motif. N46FdFc is composed of N46Fd fused with human IgG Fc fragment as an immunoenhancer. We immunized mice with N46 peptide, N46Fd and N46FdFc, respectively, and found that only N46FdFc elicited neutralizing antibody response in mice against infection by HIV-1 strains IIIB (clade B, X4), 92US657 (clade B, R5), and 94UG103 (clade A, X4R5). Anti-N46FdFc antibodies inhibited PIE7 binding to PFI, blocked gp41 six-helix bundle formation, and suppressed HIV-1 mediated cell-cell fusion. These findings provide an important clue for developing recombinant gp41 PFI mimetics-based HIV vaccines.  相似文献   
70.
Vascular smooth muscle cell (VSMC) proliferation is a hallmark of neointimal hyperplasia (NIH) in atherosclerosis and restenosis post-balloon angioplasty and stent insertion. Although numerous cytotoxic and cytostatic therapeutics have been developed to reduce NIH, it is improbable that a multifactorial disease can be successfully treated by focusing on a preconceived hypothesis. We, therefore, aimed to identify key molecules involved in NIH via a hypothesis-free approach. We analyzed four datasets (GSE28829, GSE43292, GSE100927, and GSE120521), evaluated differentially expressed genes (DEGs) in wire-injured femoral arteries of mice, and determined their association with VSMC proliferation in vitro. Moreover, we performed RNA sequencing on platelet-derived growth factor (PDGF)-stimulated human VSMCs (hVSMCs) post-phosphoenolpyruvate carboxykinase 2 (PCK2) knockdown and investigated pathways associated with PCK2. Finally, we assessed NIH formation in Pck2 knockout (KO) mice by wire injury and identified PCK2 expression in human femoral artery atheroma. Among six DEGs, only PCK2 and RGS1 showed identical expression patterns between wire-injured femoral arteries of mice and gene expression datasets. PDGF-induced VSMC proliferation was attenuated when hVSMCs were transfected with PCK2 siRNA. RNA sequencing of PCK2 siRNA-treated hVSMCs revealed the involvement of the Akt-FoxO-PCK2 pathway in VSMC proliferation via Akt2, Akt3, FoxO1, and FoxO3. Additionally, NIH was attenuated in the wire-injured femoral artery of Pck2-KO mice and PCK2 was expressed in human femoral atheroma. PCK2 regulates VSMC proliferation in response to vascular injury via the Akt-FoxO-PCK2 pathway. Targeting PCK2, a downstream signaling mediator of VSMC proliferation, may be a novel therapeutic approach to modulate VSMC proliferation in atherosclerosis.  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