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   5475篇
  免费   474篇
  国内免费   446篇
  6395篇
  2024年   12篇
  2023年   82篇
  2022年   191篇
  2021年   322篇
  2020年   206篇
  2019年   232篇
  2018年   261篇
  2017年   175篇
  2016年   207篇
  2015年   360篇
  2014年   353篇
  2013年   458篇
  2012年   516篇
  2011年   470篇
  2010年   251篇
  2009年   249篇
  2008年   281篇
  2007年   246篇
  2006年   188篇
  2005年   179篇
  2004年   136篇
  2003年   137篇
  2002年   107篇
  2001年   121篇
  2000年   85篇
  1999年   106篇
  1998年   60篇
  1997年   48篇
  1996年   39篇
  1995年   42篇
  1994年   35篇
  1993年   25篇
  1992年   40篇
  1991年   25篇
  1990年   24篇
  1989年   25篇
  1988年   19篇
  1987年   15篇
  1986年   20篇
  1985年   20篇
  1984年   7篇
  1983年   9篇
  1982年   7篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1976年   1篇
排序方式: 共有6395条查询结果,搜索用时 9 毫秒
81.
? Premise of the study: Microsatellite primers were developed for a heterostylous herb, Hedyotis chrysotricha to investigate the effect of habitat fragmentation on its genetic diversity and population structure. ? Methods and Results: Twelve primer sets were developed and their polymorphisms were tested on 47 individuals from two island populations of H. chrysotricha in Thousand Island Lake of China. The number of alleles per locus ranged from five to 10, with an average of seven alleles. Expected heterozygosity per locus ranged from 0.284 to 0.821 and observed heterozygosity ranged from 0.191 to 0.851. ? Conclusions: We showed that all of the 12 microsatellite markers developed for H. chrysotricha are polymorphic within populations, which should provide a powerful tool for assessing population structure and genetic diversity across fragmented and continuous populations, and for studying the genetic effects of habitat fragmentation on this species.  相似文献   
82.
Thraustochytrium is a unicellular marine protist for the commercial production of very long-chain polyunsaturated fatty acids (VLCPUFAs). Biosynthesis of these VLCPUFAs in the protist is catalysed by a PUFA synthase comprising three subunits, each with multiple catalytic domains. Among these domains, two tandem FabA-like dehydratase domains (DH1 and DH2) in subunit-C together are responsible for introducing double bonds in VLCPUFAs. Domain swapping analysis in yeast showed that the defective phenotype of a Scfas1 mutant could be complemented by expressing an engineered ScFAS1 gene in which the DH domain was replaced by a single DH1 or mutated DH2 of the two. Heterologous expression of the PUFA synthase in E. coli showed that the mutation of DH1 of the two or deletion of DH1 or substitution of DH1 with DH2 resulted in the complete loss of activity in the biosynthesis of VLCPUFAs. Mutation of DH2 of the two or deletion of the DH2 domain produced a small amount of DPA, but not docosahexaenoic acid (DHA). These results indicate that each of the two FabA-like domains of the PUFA synthase possesses distinct function. DH1 domain is essential for the biosynthesis of VLCPUFAs, but DH2 domain is required for the biosynthesis of DHA.  相似文献   
83.
Even though the incidence of pregnancies in the female recipients is lower and also chronic renal disease in male patients is associated with impaired spermatogenesis, the health of the children born to these patients was not studied. In this report, we discuss information on the growth and development of offspring of 248 male and female kidney recipient patients. Physical and routine clinical measurements of the 252 offspring (129 male and 123 female) born to these transplantation patients were made along with the intelligence tests. In some of these children chest X-ray and immune indices were assessed. Among the recipients, 219 males fathered 223 children with an average birth weight of 3,255 ± 374 g and 29 female recipients gave birth to 29 children with an average birth weight of 2,923 ± 551. While most of these children were normal, we noticed a case of soft double toe, a case of short tongue tie, five cases of marginal mental retardation, three cases of proteinuria, six cases of microscopic hematuria, 15 cases of low hemoglobin, and 21 cases with recurrent respiratory tract infections. We conclude that kidney transplantation has no significant impact on the growth, development, health, and intelligence of the offspring born to recipients.  相似文献   
84.
We studied a sample of 75 Chinese, 73 Malay, and 29 Indian healthy neonates taking part in a cohort study to examine potential differences in neonatal brain morphology and white matter microstructure as a function of ethnicity using both structural T2-weighted magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). We first examined the differences in global size and morphology of the brain among the three groups. We then constructed the T2-weighted MRI and DTI atlases and employed voxel-based analysis to investigate ethnic differences in morphological shape of the brain from the T2-weighted MRI, and white matter microstructure measured by fractional anisotropy derived from DTI. Compared with Malay neonates, the brains of Indian neonates’ tended to be more elongated in anterior and posterior axis relative to the superior-inferior axis of the brain even though the total brain volume was similar among the three groups. Although most anatomical regions of the brain were similar among Chinese, Malay, and Indian neonates, there were anatomical variations in the spinal-cerebellar and cortical-striatal-thalamic neural circuits among the three populations. The population-related brain regions highlighted in our study are key anatomical substrates associated with sensorimotor functions.  相似文献   
85.
Jiang D  Ying W  Lu Y  Wan J  Zhai Y  Liu W  Zhu Y  Qiu Z  Qian X  He F 《Proteomics》2003,3(5):724-737
Very little is currently known about mechanisms underlying cancer metastasis. In the present study, metastasis-associated proteomes were separated and identified by comparative proteomic analysis, and the metastasis-related function of candidate protein interleukin-18 (IL-18) was further elucidated. First, a pair of highly and poorly metastatic sublines (termed PLA801D and PLA801C, respectively), originating from the same parental PLA801 cell line, was identified by spontaneous tumorigenicity and metastasis in vivo and characterized by metastatic phenotypes analysis in vitro. Subsequently, a proteomic approach was used to compare the protein expression profiles between PLA801C and PLA801D sublines. Eleven proteins were identified and further verified by one-dimensional Western blotting, Northern blot and/or semiquantitative reverse transciptase polymerase chain reaction analysis. Compared with those in poorly metastatic PLA801C subline, cytokeratin 18, tissue transglutaminase, Rho GDP-dissociation inhibitor 1, tropomyosin, fibroblast type, IL-18 and annexin I were significantly up-regulated, while protein disulfide isomerase, heat shock protein 60, peroxiredoxin 1, chlorine intracellular channel protein 1 (CLI1) and creatine kinase, B chain were significantly down-regulated in the highly metastatic PLA801D subline. Intriguingly, all the identified candidate proteins except for CLI1 have been shown to be somehow associated with distinct aspects of tumor metastasis such as cell growth, motility, invasion, adhesion, apoptosis and tumor immunity, etc. Considering that IL-18 was present in highly metastatic PLA801D but absent in poorly metastatic PLA801C, the association of IL-18 with metastasis was further elucidated by introducing IL-18 sense/IL-18 antisense into PLA801C/PLA801D sublines simultaneously. The results demonstrated that ectopically expressed IL-18 promoted cell motility in vitro and down-regulated E-cadherin expression of PLA801C transfectants, while IL-18 antisense remarkably decreased cell invasion potency in vitro and notably increased E-cadherin expression of PLA801D transfectants, indicating that IL-18 might play a role in metastasis by inhibiting E-cadherin expression.  相似文献   
86.
87.
Hao J  Ma C  Gao C  Qiu J  Wang M  Zhang Y  Cui X  Xu P 《Biotechnology letters》2007,29(1):105-110
Pseudomonas stutzeri SDM oxidized dl-lactic acid (25.5 g l-1) into pyruvic acid (22.6 g l-1) over 24 h. Both NAD+-independent d-lactate dehydrogenase and NAD+-independent l-lactate dehydrogenase were found for the first time in the bioconversion of lactate to pyruvate based on the enzyme activity assay and proteomic analysis. Jianrong Hao and Cuiqing Ma contributed equally to this work  相似文献   
88.
Ovarian high-grade serous carcinomas (HGSCs) and invasive low-grade serous carcinomas (LGSCs) are considered to be distinct entities. In particular, LGSCs are thought to arise from non-invasive serous borderline ovarian tumors (SBOTs) and show poor responsiveness to conventional chemotherapy. The pro-apoptotic effects of CD40 ligand (CD40L) have been demonstrated in HGSC, though the underlying mechanisms are not fully understood. Conversely, the therapeutic potential of the CD40L-CD40 system has yet to be evaluated in LGSC. We now show that CD40 protein is focally expressed on tumor cells in two of five primary LGSCs compared with no expression in eight primary SBOTs. Treatment with CD40L or agonistic CD40 antibody decreased the viability of LGSC-derived MPSC1 and VOA1312 cells, but not SBOT3.1 cells. Small interfering RNA (siRNA) targeting CD40 was used to show that it is required for these reductions in cell viability. CD40L treatment increased cleaved caspase-3 levels in MPSC1 cells though, surprisingly, neither pan-caspase inhibitor nor caspase-3 siRNA reversed or even attenuated CD40L-induced cell death. In addition, CD40-induced cell death was not affected by knockdown of the mitochondrial proteins apoptosis-inducing factor (AIF) and endonuclease G (EndoG). Interestingly, CD40L-induced cell death was blocked by necrostatin-1, an inhibitor of receptor-interacting protein 1 (RIP1), and attenuated by inhibitors of RIP3 (GSK''872) or MLKL (mixed lineage kinase domain-like; necrosulfonamide). Our results indicate that the upregulation of CD40 may be relatively common in LGSC and that CD40 activation induces RIP1-dependent, necroptosis-like cell death in LGSC cells.Epithelial ovarian cancer accounts for approximately 90% of all ovarian malignancies and is the leading cause of gynecological cancer death in developed countries.1, 2 Recently, differences in molecular alterations and clinicopathological features have established a dualistic model dividing ovarian serous carcinomas into high-grade serous carcinoma (HGSC) and low-grade serous carcinoma (LGSC) subtypes. HGSCs are more common and are thought to develop directly from the ovarian surface epithelium or from serous tubal intra-epithelial carcinomas in the fallopian tube. In contrast, LGSCs are rare and are generally considered to develop from benign serous cystadenomas through serous borderline ovarian tumors (SBOT). SBOTs are slow-growing, non-invasive epithelial neoplasms that have a better prognosis compared with other types of ovarian cancer.3, 4, 5 Our previous studies have shown that the inhibition of p53 or treatment of epidermal growth factor or transforming growth factor-β1 increases SBOT cell invasion by inducing epithelial–mesenchymal transition, which suggests a possible mechanism that mediates the progression from SBOT to LGSC.6, 7, 8, 9 However, many of SBOTs recur as LGSCs that display poor responsiveness to conventional chemotherapy and for which survival rates are <50%.1, 3, 10 Thus, the development of novel, targeted therapeutic strategies is likely required to significantly improve patient survival.CD40, a transmembrane glycoprotein belonging to the tumor necrosis factor receptor superfamily, is expressed by a wide range of cell types including immune, endothelial and epithelial cells. Engagement of CD40 with its ligand, CD40L, has been shown to have important roles in a variety of physiological and pathological processes, especially in immunity.11, 12 In addition, CD40 expression has been demonstrated in several types of cancer, including colon, lung, cervical, bladder and prostate cancer.13 However, reported functions of CD40 in tumor cells vary, with both pro-apoptotic and anti-proliferative effects observed depending on the cellular context.14, 15, 16 Alternatively, some studies have shown that CD40 activation may promote the neoplastic transformation and growth of normal cells.17, 18, 19 Expression of CD40 has been demonstrated in ovarian cancer cell lines and tumor samples, but not in normal ovarian tissue, suggesting that CD40 may have an important role in ovarian tumors.20, 21, 22, 23, 24 Indeed, CD40L-CD40 signaling has been shown to induce growth-inhibitory effects in HGSC cells,20, 21, 23, 24, 25 however, the therapeutic potential of CD40 in LGSC and SBOT has not been evaluated.In the present study, we report for the first time elevated CD40 expression in a significant proportion of LGSCs compared with SBOTs. Moreover, CD40 expression is elevated in LGSC-derived MPSC1 and VOA1312 cells compared with SBOT3.1 cells, and CD40 activation induces cell death via CD40 only in LGSC-derived cells. Neither pan-caspase inhibitor nor caspase-3 small interfering RNA (siRNA) has any effect on CD40L-induced MPSC1 cell death. Moreover, CD40L-induced cell death was unaffected by individual or combined knockdown of the mitochondrial proteins apoptosis-inducing factor (AIF) and endonuclease G (EndoG). Interestingly, our results suggest that receptor-interacting protein 1 (RIP1), RIP3 and MLKL are involved in CD40-induced MPSC1 cell death. These results demonstrate that CD40 induces RIP1-dependent, necroptosis-like cell death in LGSC cells.  相似文献   
89.
Ye H  Qiu B  Lin Z  Chen G 《Luminescence》2011,26(5):336-341
The interaction between tamibarotene and bovine serum albumin (BSA) was studied using fluorescence quenching technique and ultraviolet–visible spectrophotometry. The results of experiments showed that tamibarotene could strongly quench the intrinsic fluorescence of BSA by a dynamic quenching mechanism. The apparent binding constant, number of binding site and corresponding thermodynamic parameters at different temperatures were calculated respectively, and the main interaction force between tamibarotene and BSA was proved to be hydrophobic force. Synchronous fluorescence spectra showed that tamibarotene changed the molecular conformation of BSA. When BSA concentration was 1.00 × 10?6 mol L?1, the quenched fluorescence ΔF had a good linear relationship with the concentration of tamibarotene in the range 1.00 × 10?6 to 12.00 × 10?6 mol L?1 with the detection limit of 6.52 × 10?7 mol L?1. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
90.
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