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   8233篇
  免费   828篇
  国内免费   3篇
  2024年   9篇
  2023年   68篇
  2022年   149篇
  2021年   311篇
  2020年   168篇
  2019年   245篇
  2018年   201篇
  2017年   212篇
  2016年   332篇
  2015年   556篇
  2014年   630篇
  2013年   628篇
  2012年   818篇
  2011年   749篇
  2010年   430篇
  2009年   366篇
  2008年   476篇
  2007年   449篇
  2006年   378篇
  2005年   356篇
  2004年   316篇
  2003年   220篇
  2002年   209篇
  2001年   97篇
  2000年   100篇
  1999年   78篇
  1998年   28篇
  1997年   35篇
  1996年   26篇
  1995年   27篇
  1994年   28篇
  1993年   26篇
  1992年   34篇
  1991年   36篇
  1990年   29篇
  1989年   20篇
  1988年   18篇
  1986年   13篇
  1985年   12篇
  1984年   19篇
  1983年   9篇
  1982年   13篇
  1980年   14篇
  1979年   14篇
  1977年   10篇
  1976年   8篇
  1975年   9篇
  1974年   12篇
  1973年   8篇
  1970年   8篇
排序方式: 共有9064条查询结果,搜索用时 349 毫秒
51.
Inhibitors of the mammalian target of rapamycin (mTOR) have been proposed to improve vaccine responses, especially in the elderly. Accordingly, testing mTOR inhibitors (such as Sirolimus) and other geroprotective drugs might be considered a key strategy to improve overall health resilience of aged populations. In this respect, Sirolimus (also known as rapamycin) is of great interest, in consideration of the fact that it is extensively used in routine therapy and in clinical studies for the treatment of several diseases. Recently, Sirolimus has been considered in laboratory and clinical studies aimed to find novel protocols for the therapy of hemoglobinopathies (e.g. β-Thalassemia). The objective of the present study was to analyse the activity of CD4+ and CD8+ T cells in β-Thalassemia patients treated with Sirolimus, taking advantages from the availability of cellular samples of the NCT03877809 clinical trial. The approach was to verify IFN-γ releases following stimulation of peripheral blood mononuclear cells (PBMCs) to stimulatory CEF and CEFTA peptide pools, stimulatory for CD4+ and CD8+ T cells, respectively. The main results of the present study are that treatment of β-Thalassemia patients with Sirolimus has a positive impact on the biological activity and number of memory CD4+ and CD8+ T cells releasing IFN-γ following stimulation with antigenic stimuli present in immunological memory. These data are to our knowledge novel and in our opinion of interest, in consideration of the fact that β-Thalassemia patients are considered prone to immune deficiency.  相似文献   
52.
In this study levels of prostaglandin E(2) (PGE(2)), tumour necrosis factor (TNF) and interleukin-1 (IL-1) alpha in medium from monocyte derived macrophages (MdM) infected with Chlamydia trachomatis (L(2)/434/Bu or K biovars). TNF and PGE(2) were found in both cases while IL-1 alpha was not detected. Both TNF and PGE(2) levels were higher in the medium of the MdM infected with K biovars. TNF reached maximum levels 24 h postinfection, and then declined, while PGE(2) levels increased continuously during the infection time up to 96 h post-infection. Addition of dexamethasone inhibited production of TNF and PGE(2). Inhibition of PGE(2) production by indomethacin resulted in increased production of TNF, while addition of PGE(2) caused partial inhibition of TNF production from infected MdM.  相似文献   
53.
Many tropical animals inhabit mosaic landscapes including human-modified habitat. In such landscapes, animals commonly adjust feeding behavior, and may incorporate non-natural foods. These behavioral shifts can influence consumers' nutritional states, with implications for population persistence. However, few studies have addressed the nutritional role of non-natural food. We examined nutritional ecology of wild blue monkeys to understand how dietary habits related to non-natural foods might support population persistence in a mosaic landscape. We documented prevalence and nutritional composition of non-natural foods in monkey diets to assess how habitat use influenced their consumption, and their contribution to nutritional strategies. While most energy and macronutrients came from natural foods, subjects focused non-natural feeding activity on five exotic plants, and averaged about a third of daily calories from non-natural foods. Most non-natural food calories came from non-structural carbohydrates and least from protein. Consumption of non-natural foods related to time in human-modified habitats, which two groups used non-randomly. Non-natural and natural foods were similar in nutrients, and the amount of non-natural food consumed drove variation in nutritional strategy. When more daily calories came from non-natural foods, females consumed a higher ratio of non-protein energy to protein (NPE:P). Females also prioritized protein while allowing NPE:P to vary, increasing NPE while capitalizing on non-natural foods. Overall, these tropical mammals achieved a similar nutrient balance regardless of their intake of non-natural foods. Forest and forest-adjacent areas with non-natural vegetation may provide adequate nutrient access for consumers, and thus contribute to wildlife conservation in mosaic tropical landscapes.  相似文献   
54.
Summary A new method, a restrained Monte Carlo (rMC) calculation, is demonstrated for generating high-resolution structures of DNA oligonucleotides in solution from interproton distance restraints and bounds derived from complete relaxation matrix analysis of two-dimensional nuclear Overhauser effect (NOE) spectral peak intensities. As in the case of restrained molecular dynamics (rMD) refinement of structures, the experimental distance restraints and bounds are incorporated as a pseudo-energy term (or penalty function) into the mathematical expression for the molecular energy. However, the use of generalized helical parameters, rather than Cartesian coordinates, to define DNA conformation increases efficiency by decreasing by an order of magnitude the number of parameters needed to describe a conformation and by simplifying the potential energy profile. The Metropolis Monte Carlo method is employed to simulate an annealing process. The rMC method was applied to experimental 2D NOE data from the octamer duplex d(GTA-TAATG)·d(CATTATAC). Using starting structures from different locations in conformational space (e.g. A-DNA and B-DNA), the rMC calculations readily converged, with a root-mean-square deviation (RMSD) of <0.3 Å between structures generated using different protocols and starting structures. Theoretical 2D NOE peak intensities were calculated for the rMC-generated structures using the complete relaxation matrix program CORMA, enabling a comparison with experimental intensities via residual indices. Simulation of the vicinal proton coupling constants was carried out for the structures generated, enabling a comparison with the experimental deoxyribose ring coupling constants, which were not utilized in the structure determination in the case of the rMC simulations. Agreement with experimental 2D NOE and scalar coupling data was good in all cases. The rMC structures are quite similar to that refined by a traditional restrained MD approach (RMSD<0.5 Å) despite the different force fields used and despite the fact that MD refinement was conducted with additional restraints imposed on the endocyclic torsion angles of deoxyriboses. The computational time required for the rMC and rMD calculations is about the same. A comparison of structural parameters is made and some limitations of both methods are discussed with regard to the average nature of the experimental restraints used in the refinement.Abbreviations MC Monte Carlo - rMC restrained Monte Carlo - MD molecular dynamics - rMD restrained molecular dynamics - DG distance geometry - EM energy minimization - 2D NOE two-dimensional nuclear Overhauser effect - DQF-COSY double-quantum-filtered correlation spectroscopy - RMSD root-mean-square deviation To whom correspondence should be addressed.  相似文献   
55.
56.
57.
The “Framework Messel” formulates regulations, recommendations and instructions for planning and carrying out palaeontological excavations in the Messel pit. The excavation of fossils in the Land Hessen is to some extent controlled by the Monument Protection Law. Therefore, the Framework was developed in close cooperation with the Land Monument Office. Administrative, scientific and organizational conditions for the excavations in the Middle Eocene oil-shales of Messel are listed. Instructional pamphlets explain technical details regarding methods of documentation, excavation and preparation of finds and results.  相似文献   
58.
A receptor for protein import into potato mitochondria   总被引:3,自引:0,他引:3  
Five potential surface receptors for protein import into plant mitochondria were identified by gentle trypsin treatment of intact mitochondria from potato tubers and subsequent preparation of outer mitochondrial membranes. One of them, a 23 kDa protein, was purified to homogeneity and analysed by direct protein sequencing. Copy DNA clones encoding the corresponding polypeptide were isolated with labelled oligonucleotides derived from the amino acid data. The 23 kDa protein shares significant sequence similarity with protein import receptors from fungal mitochondria and contains one of their typical tetratricopeptide motifs. Its integration into the outer membrane is independent of protease accessible surface receptors and not accompanied by proteolytic processing. Monospecific antibodies against the 23 kDa protein significantly reduce import capacity of isolated mitochondria indicating that this component is indeed involved in the recognition or import of precursor proteins. As in fungi, immunological inhibition of protein import with IgGs against a single receptor is incomplete suggesting the existence of other receptors in the outer mitochondrial membrane of plant mitochondria.  相似文献   
59.
The mdr1 gene, first member of the human multidrug-resistance gene family, is a major gene involved in cellular resistance to several drugs used in anticancer chemotherapy. Its product, the drug-excreting P-glycoprotein, shows a bipartite structure formed by two similar adjacent halves. According to one hypothesis, the fusion of two related ancestral genes during evolution could have resulted in this structure. The DNA sequence analysis of the introns located in the region connecting the two halves of the human mdr1 gene revealed a highly conserved poly(CA) · poly (TG) sequence in intron 15 and repeated sequences of the Alu family in introns 14 and 17. These repeated sequences most likely represent molecular fossils of ancient DNA elements which were involved in such a recombination event. Correspondence to: M. Pauly  相似文献   
60.
This study addresses the ability of DNA fragments from various sources to mediate autonomous DNA replication in cultured Drosophila melanogaster cells. We created a series of plasmids containing genomic DNA fragments from the Ultrabithorax gene of Drosophila and tested them for autonomous replication after transfection into Schneider line 2 cells. We found that all plasmids containing Drosophila DNA fragments were able to replicate autonomously, as were plasmids containing random human and Escherichia coli genomic DNA fragments. Most of the plasmids were detectable 18 days after transfection in the absence of selection, suggesting that transfected DNA is maintained in Drosophila cells without rapid loss or degradation. The finding that all plasmids containing Drosophila, human, or bacterial DNA replicate autonomously in Drosophila cells suggests that the signals that direct autonomous replication in Drosophila contain a low degree of sequence specificity. A two-dimensional gel analysis of initiation on one of the plasmids was consistent with many dispersed initiation sites. Low sequence specificity and dispersed initiation sites also characterize autonomous replication in human cells and Senopus eggs and may be general properties of autonomous replication in animal cells.  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