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   310篇
  完全免费   23篇
  2018年   3篇
  2017年   6篇
  2016年   9篇
  2015年   3篇
  2014年   8篇
  2013年   7篇
  2012年   17篇
  2011年   13篇
  2010年   7篇
  2009年   2篇
  2008年   8篇
  2007年   14篇
  2006年   7篇
  2005年   14篇
  2004年   8篇
  2003年   12篇
  2002年   14篇
  2001年   14篇
  2000年   14篇
  1999年   14篇
  1998年   4篇
  1997年   4篇
  1996年   10篇
  1995年   6篇
  1994年   5篇
  1992年   14篇
  1991年   9篇
  1990年   7篇
  1989年   8篇
  1988年   4篇
  1987年   9篇
  1986年   7篇
  1985年   4篇
  1984年   5篇
  1983年   4篇
  1982年   2篇
  1981年   2篇
  1980年   2篇
  1979年   1篇
  1978年   7篇
  1977年   2篇
  1976年   1篇
  1975年   3篇
  1974年   2篇
  1973年   2篇
  1971年   2篇
  1969年   1篇
  1968年   1篇
  1967年   2篇
  1966年   2篇
  1964年   2篇
  1956年   1篇
  1955年   1篇
  1954年   1篇
  1910年   1篇
  1909年   1篇
排序方式: 共有333条查询结果,搜索用时 62 毫秒
1.
Imaging of photo-oxidative stress responses in leaves   总被引:24,自引:0,他引:24  
High resolution digital imaging was used to identify sites of photo-oxidative stress responses in Arabidopsis leaves non-invasively, and to demonstrate the potential of using a suite of imaging techniques for the study of oxidative metabolism in planta. Tissue-specific photoinhibition of photosynthesis in individual chloroplasts in leaves was imaged by chlorophyll fluorescence microscopy. Singlet oxygen production was assessed by imaging the quenching of the fluorescence of dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole (DanePy) that results from its reaction with singlet oxygen. Superoxide and hydrogen peroxide accumulation were visualized by the reduction of nitroblue tetrazolium (NBT) to formazan deposits and by polymerization with 3,3'-diaminobenzidine (DAB), respectively. Stress-induced expression of a gene involved with antioxidant metabolism was imaged from the bioluminescence from leaves of an Arabidopsis APX2-LUC transformant, which co-expresses an ascorbate peroxidase (APX2) with firefly luciferase. Singlet oxygen and superoxide production were found to be primarily located in mesophyll tissues whereas hydrogen peroxide accumulation and APX2 gene expression were primarily localized in the vascular tissues.  相似文献
2.
RNA interference can be considered as an antisense mechanism of action that utilizes a double-stranded RNase to promote hydrolysis of the target RNA. We have performed a comparative study of optimized antisense oligonucleotides designed to work by an RNA interference mechanism to oligonucleotides designed to work by an RNase H-dependent mechanism in human cells. The potency, maximal effectiveness, duration of action, and sequence specificity of optimized RNase H-dependent oligonucleotides and small interfering RNA (siRNA) oligonucleotide duplexes were evaluated and found to be comparable. Effects of base mismatches on activity were determined to be position-dependent for both siRNA oligonucleotides and RNase H-dependent oligonucleotides. In addition, we determined that the activity of both siRNA oligonucleotides and RNase H-dependent oligonucleotides is affected by the secondary structure of the target mRNA. To determine whether positions on target RNA identified as being susceptible for RNase H-mediated degradation would be coincident with siRNA target sites, we evaluated the effectiveness of siRNAs designed to bind the same position on the target mRNA as RNase H-dependent oligonucleotides. Examination of 80 siRNA oligonucleotide duplexes designed to bind to RNA from four distinct human genes revealed that, in general, activity correlated with the activity to RNase H-dependent oligonucleotides designed to the same site, although some exceptions were noted. The one major difference between the two strategies is that RNase H-dependent oligonucleotides were determined to be active when directed against targets in the pre-mRNA, whereas siRNAs were not. These results demonstrate that siRNA oligonucleotide- and RNase H-dependent antisense strategies are both valid strategies for evaluating function of genes in cell-based assays.  相似文献
3.
Interaction of interleukin 2 (IL2) with its high affinity membrane receptor complex (IL2R) is sufficient to induce proliferation of T lymphocytes. However, the biochemical mechanisms by which IL2 induces this process remain unresolved. The IL2R complex consists of at least two distinct polypeptides that bind IL2, a 75-kDa intermediate affinity subunit (IL2R beta) and a 55-kDa low affinity subunit (IL2R alpha). As indicated by Western blotting with anti-phosphotyrosine-specific antibodies and confirmed by phosphoamino acid analysis, we now demonstrate that interaction of the T cell growth factor interleukin 2 (IL2) with its high affinity receptor on IL2-sensitive human peripheral blood lymphoblasts induces tyrosine phosphorylation of proteins of 92, 80, 78, 70-75, and 57 kDa. IL2 induced tyrosine phosphorylation in YT 2C2 cells which express only the 75-kDa intermediate affinity IL2 binding molecule (IL2R beta) but not in cells which either express only the 55-kDa low affinity IL2 receptor molecule (IL2R alpha) or no IL2-binding sites. Therefore, IL2R beta, in the absence of IL2R alpha, appears sufficient to transduce the transmembrane signal leading to tyrosine phosphorylation. Two different antibodies reactive with phosphotyrosine specifically immunoprecipitated IL2R beta cross-linked to radiolabeled IL2. These findings suggest that IL2R beta is a substrate for the tyrosine kinase which is activated by IL2 binding to its receptor. Thus, like several other growth factor receptors, activation of the IL2R results in an increase in tyrosine phosphorylation with the receptor itself serving as one substrate.  相似文献
4.
The structure of human lactoferrin has been refined crystallographically at 2.8 A (1 A = 0.1 nm) resolution using restrained least squares methods. The starting model was derived from a 3.2 A map phased by multiple isomorphous replacement with solvent flattening. Rebuilding during refinement made extensive use of these experimental phases, in combination with phases calculated from the partial model. The present model, which includes 681 of the 691 amino acid residues, two Fe3+, and two CO3(2-), gives an R factor of 0.206 for 17,266 observed reflections between 10 and 2.8 A resolution, with a root-mean-square deviation from standard bond lengths of 0.03 A. As a result of the refinement, two single-residue insertions and one 13-residue deletion have been made in the amino acid sequence, and details of the secondary structure and tertiary interactions have been clarified. The two lobes of the molecule, representing the N-terminal and C-terminal halves, have very similar folding, with a root-mean-square deviation, after superposition, of 1.32 A for 285 out of 330 C alpha atoms; the only major differences being in surface loops. Each lobe is subdivided into two dissimilar alpha/beta domains, one based on a six-stranded mixed beta-sheet, the other on a five-stranded mixed beta-sheet, with the iron site in the interdomain cleft. The two iron sites appear identical at the present resolution. Each iron atom is coordinated to four protein ligands, 2 Tyr, 1 Asp, 1 His, and the specific Co3(2-), which appears to bind to iron in a bidentate mode. The anion occupies a pocket between the iron and two positively charged groups on the protein, an arginine side-chain and the N terminus of helix 5, and may serve to neutralize this positive charge prior to iron binding. A large internal cavity, beyond the Arg side-chain, may account for the binding of larger anions as substitutes for CO3(2-). Residues on the other side of the iron site, near the interdomain crossover strands could provide secondary anion binding sites, and may explain the greater acid-stability of iron binding by lactoferrin, compared with serum transferrin. Interdomain and interlobe interactions, the roles of charged side-chains, heavy-atom binding sites, and the construction of the metal site in relation to the binding of different metals are also discussed.  相似文献
5.
Pol I transcription: which comes first, the end or the beginning?   总被引:18,自引:0,他引:18  
S M Baker  T Platt 《Cell》1986,47(6):839-840
6.
METAL TOLERANCE   总被引:18,自引:0,他引:18  
7.
8.
Isolation of ouabain-resistant human diploid fibroblasts   总被引:17,自引:0,他引:17  
R Mankovitz  M Buchwald  R M Baker 《Cell》1974,3(3):221-226
9.
Two human rhinovirus serotypes complexed with two- and five-domain soluble fragments of the cellular receptor, intercellular adhesion molecule-1, have been investigated by X-ray crystallographic analyses of the individual components and by cryo-electron microscopy of the complexes. The three-dimensional image reconstructions provide a molecular envelope within which the crystal structures of the viruses and the receptor fragments can be positioned with accuracy. The N-terminal domain of the receptor binds to the rhinovirus 'canyon' surrounding the icosahedral 5-fold axes. Fitting of molecular models into the image reconstruction density identified the residues on the virus that interact with those on the receptor surface, demonstrating complementarity of the electrostatic patterns for the tip of the N-terminal receptor domain and the floor of the canyon. The complexes seen in the image reconstructions probably represent the first stage of a multistep binding process. A mechanism is proposed for the subsequent viral uncoating process.  相似文献
10.
Measurement of mass flow of water in the stems of herbaceous plants   总被引:15,自引:2,他引:13  
Abstract. Heat balance methods of stem flow measurement offer the opportunity to measure directly the mass flow rate of water in plants. We have tested one such approach; the constant power heat balance method of Sakuratani (1981). The results supported his statement of an approximate accuracy of 10% when measuring the transpiration rate of herbaceous plants. The response to sudden changes in stem flow rate is not instantaneous, but investigation of the time constant shows that it decreases as stem flow increases, to the extent that, at flow rates typical of daytime conditions the system is capable of accurately tracking changes in stem flow within 5 min or less.
We describe a new gauge design that is relatively rugged, simple to use with an appropriate digital datalogger and suitable for field use over prolonged periods of time. It does not injure or penetrate the stem, is amenable to continuous and direct recording of the mass flow rate of water in the stem and requires no calibration. A further refinement, which should improve both the accuracy and the dynamic response of the system, is proposed.  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