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   5524篇
  免费   373篇
  国内免费   3篇
  2021年   59篇
  2020年   27篇
  2019年   54篇
  2018年   79篇
  2017年   47篇
  2016年   91篇
  2015年   150篇
  2014年   189篇
  2013年   350篇
  2012年   267篇
  2011年   342篇
  2010年   200篇
  2009年   188篇
  2008年   328篇
  2007年   305篇
  2006年   291篇
  2005年   276篇
  2004年   313篇
  2003年   285篇
  2002年   280篇
  2001年   158篇
  2000年   171篇
  1999年   155篇
  1998年   69篇
  1997年   66篇
  1996年   47篇
  1995年   38篇
  1994年   50篇
  1993年   54篇
  1992年   97篇
  1991年   86篇
  1990年   81篇
  1989年   86篇
  1988年   86篇
  1987年   58篇
  1986年   51篇
  1985年   51篇
  1984年   43篇
  1983年   37篇
  1982年   25篇
  1981年   34篇
  1980年   24篇
  1979年   27篇
  1978年   25篇
  1977年   20篇
  1976年   30篇
  1975年   17篇
  1974年   11篇
  1973年   16篇
  1971年   12篇
排序方式: 共有5900条查询结果,搜索用时 15 毫秒
21.
Dialyzable factors (DF) were prepared from ribosomal fractions of several organisms including rough mutants of Salmonella typhimurium LT2, salmonella species of different serogroups, other enteric bacteria and gram-positive organisms, and tested for their immunogenicity against S. typhimurium infection in mice. All of them conferred local resistance on mice challenged intramuscularly with S. typhimurium LT2 in the early stage of immunization before the establishment of delayed-type hypersensitivity (DTH) to salmonella antigens. Although DFs of enteric bacteria including rough mutants of S. typhimurium induced DTH to salmonella antigens, only DF of a two-heptose mutant of S. typhimurium LT2 afforded significant mouse protection but others only prolonged the mean time to death. DF of Listeria monocytogenes induced the cross-reacting immunity which afforded the low level of mouse protection as well as an increase in mean time to death without inducing DTH. Passive transfer of anti-O antibody did not enhance the mouse protection provided by each DF. Resistance conferred by DF of S. typhimurium LT2 consisted of two phases: (i) nonspecific macrophage activation resulting in reduction of organisms at the infected site, which became active in the early stage of immunization and (ii) salmonella-specific immunity capable of preventing systemic infection, which became active in the late stage of immunization.  相似文献   
22.
23.
Human calcium/calmodulin-dependent protein kinase IV (CAMKIV) is a member of Ser/Thr kinase family, and is associated with different types of cancer and neurodegenerative diseases. Vanillin is a natural compound, a primary component of the extract of the vanilla bean which possesses varieties of pharmacological features including anti-oxidant, anti-inflammatory, anti-bacterial and anti-tumor. Here, we have investigated the binding mechanism and affinity of vanillin to the CAMKIV which is being considered as a potential drug target for cancer and neurodegenerative diseases. We found that vanillin binds strongly to the active site cavity of CAMKIV and stabilized by a large number of non-covalent interactions. We explored the utility of vanillin as anti-cancer agent and found that it inhibits the proliferation of human hepatocyte carcinoma (HepG2) and neuroblastoma (SH-SY5Y) cells in a dose-dependent manner. Furthermore, vanillin treatment resulted into the significant reduction in the mitochondrial membrane depolarization and ROS production that eventually leads to apoptosis in HepG2 and SH-SY5Y cancer cells. These findings may offer a novel therapeutic approach by targeting the CAMKIV using natural product and its derivative with a minimal side effect.  相似文献   
24.
25.
26.
We investigated the basic technology of cell culture conditions for production of useful substances such as cytokines, and related proteins produced by Namalwa cells. Namalwa cells (Klein, 1972), human B lymphoblastoid cells, were used for large scale production of alpha-interferon (Klein, 1979). Namalwa KJM-1, a subline of Namalwa cells, adapted to serum- and albumin-free medium, can grow at a high density above 1 × 107 cells/ml in suspension mode by the use of a perfusion culture system, Biofermenter?, containing a cone-type cell-sedimentation column as cell separator (Sato, 1983).Several kinds of cytokine cDNA can be introduced and expressed in Namalwa KJM-1 cells (Miyaji, 1990a,b,c). Some of these were produced in large quantities by use of a gene amplification method with dhfr (Miyaji, 1990c), even though the Namalwa KJM-1 cells contained endogenous dhfr genes. For stable production of the target protein, Namalwa KJM-1 cells are very useful host cells, because they have no effective endogenous protease activity in the conditioned medium.Using Biofermenter with micro-silicone fibers and a dialysis system, the specific productivity of the target proteins was not depressed at a high cell density.  相似文献   
27.
Abstract

The antitumor mechanism of action of 2′-C-cyano-2′-deoxy-1-β-d-arabinofuranosylcytosine (CNDAC) has been examined. CNDAC was designed as a potentially DNA-self-strand-breaking nucleoside. It had potent antitumor effects against various solid tumors in vitro as well as in vivo. Using a chain-extension method with Vent (exo?) DNA polymerase and a short primer/template system, we found that 5′-triphosphate of CNDAC (CNDACTP) was incorporated into the primer at a site opposite a guanine residue in the template. After further chain-extension reaction of the primer containing CNDAC at the 3′-terminus, chain elongation was not observed. Therefore, CNDACTP appeared to act as a chain-terminator. Analyses of the structure of the 3′-terminus in the primer revealed 2′-C-cyano-2′,3′-didehydro-2′,3′-dideoxycytidine (ddCNC) together with CNDAC and 2′-C-cyano-2′-deoxy-1-β-d-ribofuranosylcytosine (CNDC). The existence of ddCNC in the 3′-end of the primer would be due to the self-strand-break by the nucleotide incorporated next to CNDAC. We also found that CNDAC was epimerized to CNDC in near-neutral to alkaline media. Therefore, CNDC found in the primer was epimerized after incorporation of CNDACTP into the primer. We also described the metabolism of CNDAC.  相似文献   
28.
The NSm nonstructural protein of Rift Valley fever virus (family Bunyaviridae, genus Phlebovirus) has an antiapoptotic function and affects viral pathogenesis. We found that NSm integrates into the mitochondrial outer membrane and that the protein''s N terminus is exposed to the cytoplasm. The C-terminal region of NSm, which contains a basic amino acid cluster and a putative transmembrane domain, targeted the protein to the mitochondrial outer membrane and exerted antiapoptotic function.  相似文献   
29.
Abstract

On treatment with bisulfite at neutral pH, 5-trifluoromethyl-2′-deoxyuridine (CF3dUR) underwent rapid substitution of the fluorine atoms by bisulfite to give first the monosulfonate and then the disulfonate derivatives. It was shown that the monosulfonate product has reactivity to bisulfite with a potency half that of CF3dUR. These findings demonstrate the stepwise nature of the fluorine release from CF3dUR and constitute evidence that 5-exo-methylene type intermediates are involved in the nucleophile-mediated release of the fluorine from CF3dUR.  相似文献   
30.
Numerous radionuclides were released from the Fukushima Daiichi Nuclear Power Station (F1-NPS) in Japan following the magnitude 9.0 earthquake and tsunami on March 11, 2011. Local residents have been eager to calculate their individual radiation exposure. Thus, absorbed dose rates in the indoor and outdoor air at evacuation sites in the Fukushima Prefecture were measured using a gamma-ray measuring devices, and individual radiation exposure was calculated by assessing the radiation dose reduction efficiency (defined as the ratio of absorbed dose rate in the indoor air to the absorbed dose rate in the outdoor air) of wood, aluminum, and reinforced concrete buildings. Between March 2011 and July 2011, dose reduction efficiencies of wood, aluminum, and reinforced concrete buildings were 0.55±0.04, 0.15±0.02, and 0.19±0.04, respectively. The reduction efficiency of wood structures was 1.4 times higher than that reported by the International Atomic Energy Agency. The efficiency of reinforced concrete was similar to previously reported values, whereas that of aluminum structures has not been previously reported. Dose reduction efficiency increased in proportion to the distance from F1-NPS at 8 of the 18 evacuation sites. Time variations did not reflect dose reduction efficiencies at evacuation sites although absorbed dose rates in the outdoor air decreased. These data suggest that dose reduction efficiency depends on structure types, levels of contamination, and evacuee behaviors at evacuation sites.  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