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   746篇
  免费   37篇
  2022年   9篇
  2021年   17篇
  2020年   7篇
  2019年   8篇
  2018年   20篇
  2017年   13篇
  2016年   23篇
  2015年   30篇
  2014年   41篇
  2013年   30篇
  2012年   57篇
  2011年   66篇
  2010年   38篇
  2009年   39篇
  2008年   41篇
  2007年   49篇
  2006年   37篇
  2005年   32篇
  2004年   41篇
  2003年   33篇
  2002年   28篇
  2001年   9篇
  2000年   4篇
  1999年   4篇
  1998年   3篇
  1996年   5篇
  1995年   3篇
  1994年   2篇
  1993年   3篇
  1992年   5篇
  1991年   3篇
  1990年   12篇
  1989年   5篇
  1988年   9篇
  1987年   10篇
  1986年   9篇
  1985年   3篇
  1984年   7篇
  1979年   3篇
  1978年   2篇
  1975年   2篇
  1973年   1篇
  1972年   5篇
  1971年   2篇
  1970年   1篇
  1969年   3篇
  1965年   1篇
  1946年   1篇
  1945年   2篇
  1944年   1篇
排序方式: 共有783条查询结果,搜索用时 31 毫秒
701.
Mechanism of testicular toxicity induced by dietary cadmium (Cd) has been less investigated than that following acute Cd injection. In the present study we characterized testicular injury in a small rodent, the bank vole, exposed subchronically to dietary Cd in a quantity of 0.9 mol/g, and determined the importance of some factors (Cd accumulation, metallothionein (MT), oxidative stress, and zinc (Zn)) in the injury. Dietary Cd induced moderate histopathological changes (hemorrhage in interstitium, necrosis and apoptosis in seminiferous tubule epithelium) in young (1 month old) bank voles fed, for 6 weeks, Fe-adequate (1.1–1.4 mol/g) and Fe-enriched (4.5–4.8 mol/g) diets. In contrast, adult (5 months old) bank voles appeared to be resistant to the toxic effects of dietary Cd, despite the fact that testicular Cd contents were higher and MT levels lower than those in the young animals. The Cd-induced histopathological changes and apoptosis were accompanied by increased testicular lipid peroxidation, decreased testicular Zn concentration and elevated levels of hepatic and renal MT and Zn. Supplemental dietary Zn (1.7–1.8 mol/g) prevented the Cd-induced testicular Zn depletion and injury. The data indicate that dietary Cd produces testicular lesions indirectly, through decreasing testicular Zn, which seems to be due to the sequestration of this element by the Cd-induced hepatic and renal MT.  相似文献   
702.
Here we report the purification and biochemical characterization of recombinant Arabidopsis thaliana DNA ligase I. We show that this ligase requires ATP as a source for adenylation. The calculated K m [ATP] for ligation is 3 M. This enzyme is able to ligate nicks in oligo(dT)/poly(dA) and oligo(rA)/poly(dT) substrates, but not in oligo(dT)/poly(rA) substrates. Double-stranded DNAs with cohesive or blunt ends are also good substrates for the ligase. These biochemical features of the purified enzyme show the characteristics typical of a type I DNA ligase. Furthermore, this DNA ligase is able to perform the reverse reaction (relaxation of supercoiled DNA) in an AMP-dependent and PPi-stimulated manner.  相似文献   
703.
Enhanced expression of transforming growth factor-β1(TGF-β1) demonstrated in human colonic mucosa of patients with ulcerative colitis (UC), indicates its possible significance in the pathogenesis of this disease. The aim of this study was to evaluate plasma TGF-β1concentration in patients with different degrees of colonic mucosal injury, as a possible indicator of ulcerative colitis activity. TGF-β1concentration was measured with an enzyme immunoassay (EIA) in plasma of 45 patients with endoscopically confirmed UC. Values observed in UC patients (40.5±15.9 ng/ml) were significantly higher than in healthy people (18.3±11.6 ng/ml) and higher than in patients with irritable colon syndrome (ICS), (20.5±13.6 ng/ml). The highest plasma TGF-β1(58.6±112.1 ng/ml) was in patients with the severe UC course. TGF-β1level analysed in all UC patients revealed significant positive correlation with scored degree of mucosal injury (r=0.396;P<0.01). Among other possible laboratory markers of the disease activity, only C-reactive protein concentration demonstrated significant correlation. Enhanced production of TGF-β1can be related to inflammation activity. Measurement of plasma TGF-β1may be considered as a biomarker of the disease activity.  相似文献   
704.
The effects of auxins and cytokinins at the range of concentrations 0.0001–100 µM on Acutodesmus obliquus (Chlorophyceae) cultures were studied. Microalga exhibited sensitivity to cytokinins in the following order: 0.01 µM tZ?>?0.1 µM Kin?>?1 µM DPU, whereas the hierarchy of auxin activity was: 0.01 µM IAA?>?0.1 µM IBA?>?0.1 µM PAA. Cytokinins possessed higher stimulating properties on the cell number, whereas auxins increased the size of cells. Differences in the metabolite profiles of the cultures treated with phytohormones were observed. Auxins and cytokinins had a positive effect on the photosynthetic apparatus enhancing the level of chlorophylls, carotenes, and xanthophylls. In comparison with auxins, cytokinins more effectively delayed oxidative damage by increasing the level of non-enzymatic antioxidants (ascorbate, glutathione) and the activity of enzymes scavenging reactive oxidative species (catalase, glutathione reductase, ascorbate peroxidase). On the other hand, auxins stimulated superoxide dismutase activity and provoked hydrogen peroxide generation, which may be involved in cell enlargement. All phytohormones reduced the content of abscisic acid and controlled the level of endogenous auxin and cytokinins suggesting complex interactions. Different dynamics of A. obliquus responses to auxins and cytokinins clearly demonstrated their diverse roles in algal growth and metabolism.  相似文献   
705.
706.
Soils collected in the proximity of a steel plant in Krakow were analyzed for their pH, the content of total organic carbon, and the total contents of metals and metalloids. Applying the Commission of European Communities Bureau of Reference (BCR) extraction of selected metals: Cd, Fe, Pb, and Zn, their forms of binding in soils have been determined. The results of the soil analyses of the samples taken in 1996 and 2016 were compared, and then the soil contamination changes were assessed using the GI, contamination factor (CF), and PLI indicators. A rehabilitation of the degraded soils was tested using the communal sewage sludge, which was added into the soil at four various ratios. The sewage sludge applied meets the environmental standards as a material for rehabilitating soils for non-agricultural needs but cannot be used for upgrading strongly contaminated soils. The reason is that the sludge addition increases the soil As, Ca, Cr, Mn, and Zn quantities as well as the easily mobilized fractions of the sludge/soil mixtures that contain Cd, Cr, Mg, Mn, Pb, and Tl. The addition of the communal sludge into soils increases the value of the environmental risk in the case of Pb, Mn, Mg, Fe, Cr, Zn, and Ca, but within the same risk classification ranges.  相似文献   
707.
Usher syndrome is rare genetic disorder impairing two human senses, hearing and vision, with the characteristic late onset of vision loss. This syndrome is divided into three types. In all cases, the vision loss is postlingual, while loss of hearing is usually prelingual. The vestibular functions may also be disturbed in Usher type 1 and sometimes in type 3. Vestibular areflexia is helpful in making a proper diagnosis of the syndrome, but, often, the syndrome is misdiagnosed as a nonsyndromic hearing loss. Here, we present a Polish family with hearing loss, which was clinically classified as nonsyndromic. After excluding mutations in the DFNB1 locus, we implemented the next-generation sequencing method and revealed that hearing loss was syndromic and mutations in the USH2A gene indicate Usher syndrome. This research highlights the importance of molecular analysis in establishing a clinical diagnosis of congenital hearing loss.  相似文献   
708.
Bcl-2/adenovirus E1B-19 kDa-interacting protein 3 (BNIP3) is an important mediator of cell survival and a member of the Bcl-2 family of proteins that regulate programmed cell death and autophagy. We have previously established a link between the expression of oncogenic HRas and up-regulation of BNIP3 and the control of autophagy in cancer cells. However, in view of varied expression of BNIP3 in different tumor types and emerging uncertainties as to the role of epigenetic silencing, oncogenic regulation and the role of BNIP3 in cancer are still poorly understood.  相似文献   
709.
This work demonstrates that self‐disproportionation of enantiomers via achiral chromatography can be recommended as inexpensive and general method for optical purification of enantiomerically enriched compounds. In particular, the advantage of this approach over conventional recrystallization is that it can be used for both crystalline as well as liquid compounds. Chirality, 25:365‐368, 2013. © 2013 Wiley Periodicals, Inc.  相似文献   
710.
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