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   19470篇
  免费   1038篇
  国内免费   13篇
  2023年   164篇
  2022年   222篇
  2021年   560篇
  2020年   437篇
  2019年   462篇
  2018年   672篇
  2017年   629篇
  2016年   817篇
  2015年   949篇
  2014年   1195篇
  2013年   1624篇
  2012年   1733篇
  2011年   1542篇
  2010年   894篇
  2009年   767篇
  2008年   902篇
  2007年   866篇
  2006年   747篇
  2005年   674篇
  2004年   531篇
  2003年   439篇
  2002年   407篇
  2001年   335篇
  2000年   308篇
  1999年   275篇
  1998年   114篇
  1997年   83篇
  1996年   84篇
  1995年   87篇
  1994年   74篇
  1993年   69篇
  1992年   181篇
  1991年   174篇
  1990年   140篇
  1989年   103篇
  1988年   142篇
  1987年   111篇
  1986年   96篇
  1985年   90篇
  1984年   93篇
  1983年   56篇
  1982年   56篇
  1981年   56篇
  1980年   49篇
  1979年   67篇
  1978年   49篇
  1977年   56篇
  1976年   43篇
  1975年   39篇
  1974年   42篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
Horsegram [Macrotyloma uniflorum (Lam.) Verdc.] commonly known as kulthi or Madras gram is an important drought tolerant legume crop used as food and fodder in India and across the globe. Horsegram is tolerant to many biotic and abiotic stresses and considered a potential future food legume. Despite being a multiutility crop, insufficient genomic information is available in this species, which is otherwise required for genetic improvement. Hence, in the present work we used next-generation sequencing (NGS) technology for genome-wide development and characterization of novel simple sequence repeat (SSR) markers in horsegram. In all, 2458 SSR primer pairs were designed from NGS data and 117 SSRs were characterized in 48 diverse lines of horsegram. Cross-transferability of these markers was also checked in nine related legume species. The polymorphic SSRs revealed high diversity measures such as mean values of expected heterozygosity (He; 0.54), observed heterozygosity (Ho; 0.64), and polymorphism information content (PIC; 0.46). Analysis of molecular variance (AMOVA) revealed high degree of genetic variance within the populations. Dendrogram based on Jaccard’s similarity coefficient and principal component analysis (PCA) revealed two groups in the analyzed accessions. This observation was further confirmed by Bayesian genetic STRUCTURE analysis. The SSR markers developed herein can be used in diverse genetic analysis including association mapping in this crop and also in related legume crops with limited marker resources. Hence, this new SSR dataset can be useful for molecular breeding research in this underutilized pulse crop. In addition, genetic diversity estimates of analyzed germplasm can be important for devising future breeding programmes in horsegram.  相似文献   
2.
Angiotensinogen (AGT) belongs to the serpin superfamily. It acts as the unique substrate of all angiotensin peptides, which generates a spectrum of angiotensin peptides in the renin-angiotensin system and regulates hypertension. This serpin belongs to the multiple member group V2 of the intron encoded vertebrate serpin classification. Despite huge advancements in the understanding of angiotensinogen based on biochemical properties and its roles in the RAS, phylogenetic history of AGT remains forgotten. To date, there is no comprehensive study illustrating the phylogenetic history of AGT. Herein, we investigated phylogenetic traits of AGT gene across vertebrates. Gene structures of AGT gene from selected ray-finned fishes varied in exon I and II with insertions of two novel introns in the core domain for ray-finned fishes at the position 77c and 233c. We that found AGT loci is conserved from lampreys to human and estimated to be older than 500 MY. By comparing AGT protein in 57 vertebrate genomes, we illustrated that the reactive center loop (RCL) of AGT protein became from inhibitory (in lampreys, GTEAKAETVVGIMPI†SMPPT) to non-inhibitory (in human, EREPTESTQQLNKPE†VLEVT) during period of 500 MY. We identified 690 AGT variants by analysis of 1092 human genomes with top three variation classes belongs to SNPs (89.7%), somatic SNVs (5.2%) and deletion (2.9%). There are 32 key residues out of 121 missense variants, which are deleterious for AGT protein, computed by combination of SIFT and PolyPhen V2 methods. These results may have clinical implications for understanding hypertension.  相似文献   
3.
The small size and cryptic nature of thrips pests often make their monitoring and the identification process difficult. With the aim to obtain accurate identification of various pest species in the order Thysanoptera, a pilot study was conducted on an invasive thrips species Scirtothrips dorsalis Hood in Florida. The specific objective of this pilot study was to assess if the thrips specimens processed to be observed under scanning electron microscopy (SEM) can be further utilized for DNA based assays. Larvae and adults of S. dorsalis were subjected to traditional morphological identification using high resolution SEM prior to their DNA extraction. Sequence results of both mtCO1 and ITS rDNA of individual larva and adult S. dorsalis were found to be in agreement with the taxonomic identification conducted using SEM and each result confirmed the other technique. Our results suggest that steps involved during specimen preparation and observation under SEM does not impact DNA analysis of the sample. The two techniques together could be used for the correct identification of various thrips species using the same specimen. The proof of concept was further tested on three other important thrips species Frankliniella occidentalis (Pergande), F. schultzei (Trybom) and Thrips palmi Karny, which confirmed the robustness of the technique. The technique has broader significance for different areas under entomological sciences especially medical, forensic and taxonomic studies where the samples put under a high beam of ions for investigation are often required for DNA analysis.  相似文献   
4.
Novel synthetic mimics of antimicrobial peptides have been developed to exhibit structural properties and antimicrobial activity similar to those of natural antimicrobial peptides (AMPs) of the innate immune system. These molecules have a number of potential advantages over conventional antibiotics, including reduced bacterial resistance, cost-effective preparation, and customizable designs. In this study, we investigate a family of nylon-3 polymer-based antimicrobials. By combining vesicle dye leakage, bacterial permeation, and bactericidal assays with small-angle X-ray scattering (SAXS), we find that these polymers are capable of two interdependent mechanisms of action: permeation of bacterial membranes and binding to intracellular targets such as DNA, with the latter necessarily dependent on the former. We systemically examine polymer-induced membrane deformation modes across a range of lipid compositions that mimic both bacteria and mammalian cell membranes. The results show that the polymers' ability to generate negative Gaussian curvature (NGC), a topological requirement for membrane permeation and cellular entry, in model Escherichia coli membranes correlates with their ability to permeate membranes without complete membrane disruption and kill E. coli cells. Our findings suggest that these polymers operate with a concentration-dependent mechanism of action: at low concentrations permeation and DNA binding occur without membrane disruption, while at high concentrations complete disruption of the membrane occurs. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.  相似文献   
5.

Background

Coronary Artery Disease (CAD) is clearly a multifactorial disease that develops from childhood and ultimately leads to death. Several reports revealed having a First Degree Relatives (FDRS) with premature CAD is a significant autonomous risk factor for CAD development. C - reactive protein (CRP) is a member of the pentraxin family and is the most widely studied proinflammatory biomarker. IL-18 is a pleiotrophic and proinflammatory cytokine which is produced mainly by macrophages and plays an important role in the inflammatory cascade.

Methods and Results

Hs-CRP levels were estimated by ELISA and Genotyping of IL-18 gene variant located on promoter -137 (G/C) by Allele specific PCR in blood samples of 300 CAD patients and 300 controls and 100 FDRS. Promoter Binding sites and Protein interacting partners were identified by Alibaba 2.1 and Genemania online tools respectively. Hs-CRP levels were significantly high in CAD patients followed by FDRS when compared to controls. In IL-18 -137 (G/C) polymorphism homozygous GG is significantly associated with occurrence of CAD and Hs-CRP levels were significantly higher in GG genotype subjects when compared to GC and CC. IL-18 was found to be interacting with 100 protein interactants.

Conclusion

Our results indicate that Hs-CRP levels and IL-18-137(G/C) polymorphism may help to identify risk of future events of CAD in asymptomatic healthy FDRS.  相似文献   
6.

Background

Sterol glycosyltransferases (SGTs) are ubiquitous but one of the most diverse group of enzymes of glycosyltransferases family. Members of this family modulate physical and chemical properties of secondary plant products important for various physiological processes. The role of SGTs has been demonstrated in the biosynthesis of pharmaceutically important molecules of medicinal plants like Withania somnifera.

Results

Analysis suggested conserved behaviour and high similarity in active sites of WsSGTs with other plant GTs. Substrate specificity of WsSGTs were analysed through docking performance of WsSGTs with different substrates (sterols and withanolides). Best docking results of WsSGTL1 in the form of stable enzyme-substrate complex having lowest binding energies were obtained with brassicasterol, transandrosteron and WsSGTL4 with solasodine, stigmasterol and 24-methylene cholesterol.

Conclusion

This study reveals topological characters and conserved nature of two SGTs from W. somnifera (WsSGTs) i.e. WsSGTL1 and WsSGTL4. However, besides being ubiquitous in nature and with broad substrate specificity, difference between WsSGTL1 and WsSGTL4 is briefly described by difference in stability (binding energy) of enzyme-substrate complexes through comparative docking.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0563-7) contains supplementary material, which is available to authorized users.  相似文献   
7.
8.
9.
Protein aggregation leads to several burdensome human maladies, but a molecular level understanding of how human proteome has tackled the threat of aggregation is currently lacking. In this work, we survey the human proteome for incidence of aggregation prone regions (APRs), by using sequences of experimentally validated amyloid‐fibril forming peptides and via computational predictions. While approximately 30 human proteins are currently known to be amyloidogenic, we found that 260 proteins (~1% of human proteome) contain at least one experimentally validated amyloid‐fibril forming segment. Computer predictions suggest that more than 80% of the human proteins contain at least one potential APR and approximately two‐thirds (65%) contain two or more APRs; spanning 3–5% of their sequences. Sequence randomizations show that this apparently high incidence of APRs has been actually significantly reduced by unique amino acid composition and sequence patterning of human proteins. The human proteome has utilized a wide repertoire of sequence‐structural optimization strategies, most of them already known, to minimize deleterious consequences due to the presence of APRs while simultaneously taking advantage of their order promoting properties. This survey also found that APRs tend to be located near the active and ligand binding sites in human proteins, but not near the post translational modification sites. The APRs in human proteins are also preferentially found at heterotypic interfaces rather than homotypic ones. Interestingly, this survey reveals that APRs play multiple, often opposing, roles in the human protein sequence‐structure‐function relationships. Insights gained from this work have several interesting implications towards novel drug discovery and development. Proteins 2017; 85:1099–1118. © 2017 Wiley Periodicals, Inc.  相似文献   
10.

Purpose

The main aim of the study is to assess the environmental and economic impacts of the lodging sector located in the Himalayan region of Nepal, from a life cycle perspective. The assessment should support decision making in technology and material selection for minimal environmental and economic burden in future construction projects.

Methods

The study consists of the life cycle assessment and life cycle costing of lodging in three building types: traditional, semi-modern and modern. The life cycle stages under analysis include raw material acquisition, manufacturing, construction, use, maintenance and material replacement. The study includes a sensitivity analysis focusing on the lifespan of buildings, occupancy rate and discount and inflation rates. The functional unit was formulated as the ‘Lodging of one additional guest per night’, and the time horizon is 50 years of building lifespan. Both primary and secondary data were used in the life cycle inventory.

Results and discussion

The modern building has the highest global warming potential (kg CO2-eq) as well as higher costs over 50 years of building lifespan. The results show that the use stage is responsible for the largest share of environmental impacts and costs, which are related to energy use for different household activities. The use of commercial materials in the modern building, which have to be transported mostly from the capital in the buildings, makes the higher GWP in the construction and replacement stages. Furthermore, a breakdown of the building components shows that the roof and wall of the building are the largest contributors to the production-related environmental impact.

Conclusions

The findings suggest that the main improvement opportunities in the lodging sector lie in the reduction of impacts on the use stage and in the choice of materials for wall and roof.
  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