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   1434篇
  免费   59篇
  国内免费   39篇
  1532篇
  2024年   3篇
  2023年   19篇
  2022年   67篇
  2021年   101篇
  2020年   31篇
  2019年   49篇
  2018年   41篇
  2017年   37篇
  2016年   58篇
  2015年   63篇
  2014年   85篇
  2013年   111篇
  2012年   130篇
  2011年   113篇
  2010年   49篇
  2009年   63篇
  2008年   68篇
  2007年   70篇
  2006年   58篇
  2005年   70篇
  2004年   41篇
  2003年   41篇
  2002年   29篇
  2001年   9篇
  2000年   10篇
  1999年   12篇
  1998年   6篇
  1997年   7篇
  1996年   7篇
  1995年   12篇
  1994年   9篇
  1993年   9篇
  1992年   2篇
  1991年   3篇
  1990年   3篇
  1989年   6篇
  1988年   3篇
  1987年   4篇
  1986年   4篇
  1984年   3篇
  1983年   3篇
  1982年   2篇
  1981年   5篇
  1980年   5篇
  1979年   1篇
  1978年   2篇
  1977年   1篇
  1976年   3篇
  1973年   1篇
  1970年   2篇
排序方式: 共有1532条查询结果,搜索用时 15 毫秒
21.
Reversible phosphorylation of proteins regulates numerous aspects of cell function, and abnormal phosphorylation is causal in many diseases. Pyruvate dehydrogenase complex (PDC) is central to the regulation of glucose homeostasis. PDC exists in a dynamic equilibrium between de-phospho-(active) and phosphorylated (inactive) forms controlled by pyruvate dehydrogenase phosphatases (PDP1,2) and pyruvate dehydrogenase kinases (PDK1-4). In contrast to the reciprocal regulation of the phospho-/de-phospho cycle of PDC and at the level of expression of the isoforms of PDK and PDP regulated by hormones and diet, there is scant evidence for regulatory factors acting in vivo as reciprocal "on-off" switches. Here we show that the putative insulin mediator inositol phosphoglycan P-type (IPG-P) has a sigmoidal inhibitory action on PDK in addition to its known linear stimulation of PDP. Thus, at critical levels of IPG-P, this sigmoidal/linear model markedly enhances the switchover from the inactive to the active form of PDC, a "push-pull" system that, combined with the developmental and hormonal control of IPG-P, indicates their powerful regulatory function. The release of IPGs from cell membranes by insulin is significant in relation to diabetes. The chelation of IPGs with Mn2+ and Zn2+ suggests a role as "catalytic chelators" coordinating the traffic of metal ions in cells. Synthetic inositol hexosamine analogues are shown here to have a similar linear/sigmoidal reciprocal action on PDC exerting push-pull effects, suggesting their potential for treatment of metabolic disorders, including diabetes.  相似文献   
22.
Tomato is one of the most consumed vegetables in the world. Diseases are the number one concern in the development of high-yield and disease-resistant tomato hybrids which is the foremost priority of breeders. Present study was conducted (1) to develop DNA-based markers for genetic confirmation of tomato F1 hybrids, (2) to utilize sequenced characterized amplified region (SCAR) marker linked to the Ph-3 gene for Phytophthora infestans resistance in tomato and (3) to evaluate male and female parental genotypes and their F1 hybrids against late blight (LB) and cucumber mosaic virus (CMV). For molecular studies, 58 previously reported markers including RAPDs (10), SCAR (01), EST-SSR (01) and SSR (46) were applied. The SCAR marker clearly differentiated the LB3 and LB4 from Roma and T-1359 and provided evidence for Ph-3 gene. The SCAR marker was able to confirm the Ph-3 gene in the hybrids Roma × LB4, Roma × LB3, Riogrande × LB2, Riogrande × LB3 and Roma × LB7. Out of several tested primers, SSR-22 proved useful for genetic confirmation of F1 hybrid TMS1 × Money Maker (MM). For LB, tested hybrids/genotypes were ranked as susceptible to highly susceptible with different infection percentage (IP). However, the pace of symptom development was slower in hybrid Rio × LB2, 45% IP after 10 days of inoculation compared with 85% disease in one of the parent genotypes (Riogrande). None of the tested genotypes was found resistant; however, TMS1 responded as tolerant against CMV using mechanical inoculation. Under natural field conditions, TMS1 was found resistant while hybrids TMS1 × Naqeeb and TMS1 × MM were tolerant where as others were found to be susceptible. In conclusion, all tomato hybrids were genetically confirmed using DNA-based markers. SCAR marker was useful for marker-assisted confirmation of the Ph-3 gene in parental lines and hybrids; however, this gene was unable to provide protection against the local population of P. infestans.  相似文献   
23.
A series of unsymmetrically disubstituted urea derivatives 128 has been synthesized and screened for their antiglycation activity in vitro. Compounds 26 (IC50 = 4.26 ± 0.25 μM), 1 (IC50 = 5.8 ± 0.08 μM), 22 (IC50 = 4.26 ± 0.25 μM), 6 (IC50 = 6.4 ± 0.02 μM), 5 (IC50 = 6.6 ± 0.26 μM), 2 (IC50 = 7.02 ± 0.31 μM), 3 (IC50 = 7.14 ± 0.84 μM), 27 (IC50 = 7.27 ± 0.36 μM), 4 (IC50 = 8.16 ± 1.04 μM), 21 (IC50 = 8.4 ± 0.15 μM), 23 (IC50 = 9.0 ± 0.35 μM) and 13 (IC50 = 15.22 ± 6.7 μM) showed an excellent antiglycation activity far better than the standard (rutin, IC50 = 41.9 ± 2.3 μM). This study thus provides a series of potential molecules for further studies of antiglycation agents.  相似文献   
24.
Pheasant reintroduction and conservation efforts have been in place in Pakistan since the 1980 s, yet there is still a scarcity of data on pheasant microbiome and zoonosis. Instead of growing vast numbers of bacteria in the laboratory, to investigate the fecal microbiome, pheasants (green and ring neck pheasant) were analyzed using 16S rRNA metagenomics and using IonS5TMXL sequencing from two flocks more than 10 birds. Operational taxonomic unit (OTU) cluster analysis and phylogenetic tree analysis was performed using Mothur software against the SSUrRNA database of SILVA and the MUSCLE (Version 3.8.31) software. Results of the analysis showed that firmicutes were the most abundant phylum among the top ten phyla, in both pheasant species, followed by other phyla such as actinobacteria and proteobacteria in ring necked pheasant and bacteroidetes in green necked pheasant. Bacillus was the most relatively abundant genus in both pheasants followed by Oceanobacillus and Teribacillus for ring necked pheasant and Lactobacillus for green necked pheasant. Because of their well-known beneficial characteristics, these genus warrants special attention. Bird droppings comprise germs from the urinary system, gut, and reproductive sites, making it difficult to research each anatomical site at the same time. We conclude that metagenomic analysis and classification provides baseline information of the pheasant fecal microbiome that plays a role in disease and health.  相似文献   
25.
It is recognized that genetic factors play a role in the susceptibility to COPD. COPD is characterized by airflow limitation. Chronic inflammation causes small airway disease and parenchymal destruction, leading to the airflow limitation. Polymorphisms in pro-inflammatory cytokine genes may confer a risk for the development of COPD. A case-control association study was performed in Japanese population (88 COPD patients and 61 controls) and Egyptian population (106 patients and 72 controls). Genotype and allele frequencies of the TNFalpha -308 G/A and +489 G/A polymorphisms, the IL1beta -511 C/T, -31 T/C, and +3954 C/T polymorphisms, and a VNTR polymorphism in intron 2 of the IL1RN gene were investigated. In addition, pairwise haplotype frequencies were analyzed. When studied independently, none of the polymorphisms were associated with the development of COPD in both populations. However, in the Egyptian population, the distributions of the haplotype (IL1beta -31 T/C : IL1beta +3954 C/T) were significantly different between the COPD patients and the controls (p(corr)=0.0037). Our findings suggest that this haplotype within the IL1beta gene may be involved in the pathogenesis of COPD and that the genetic factors of COPD susceptibility might be different between different populations.  相似文献   
26.
Second messengers involved in the signal transduction pathway leading to induction of the plasminogen activator inhibitor (PAI-1) have not yet been well characterized. This study focuses on the mechanisms of regulation of PAI-1 expression by reactive oxygen species (ROS) in human endothelial cells. Inhibition of the tumor necrosis factor alpha (TNFalpha?-induced expression of PAI-1 by antioxidant N-acetyl-L-cysteine (NAC) indicated redox-sensitive mechanisms involved in the signaling pathway. Because TNFalpha induces PAI-1 production in endothelial cells, and NAC attenuated this response, we attempted to investigate the possible involvement of ROS in the activation of PAI-1 by TNFalpha. Upregulation of PAI-1 expression in endothelial cells by the stimulation with TNFalpha (50 ng/ml) or H2O2 (10-200 micro M), observed by measurement of the antigen and mRNA levels, was reversed in the presence of NAC (20mM). The stimulatory effect of ROS was detected also at the level of the PAI-1 promoter in endothelial cells transfected with plasmid p800 LUC containing a PAI-1 promoter fragment (+71 to -800). The PAI-1 promoter activity was increased in the presence of ROS, and was suppressed by up to 75% in the presence of antioxidants. On the basis of this study we can conclude that reactive oxygen species play an important role in a cytokine-induced activation of PAI-1 expression, and may act as a signal transduction messenger.  相似文献   
27.
辽宁是东北林蛙Rana dybowskii主要分布地之一,种群数量巨大,其群体遗传多样性有待评估.本研究应用ISSR标记技术对东北林蛙4个种群105个样本进行研究,5个引物共获得44条清晰谱带,4个种群的多态位点率均大于75%,Nei's基因多样性为0.2851,Shannon信息指数为0.4476,显示了较高的遗传多样性.对遗传分化系数、Nei's遗传距离、AMOVA分子变异巢式方差分析和F-统计量等遗传参数的统计结果表明,辽宁东北林蛙种群间已经出现一定程度的遗传分化,分析认为,自然屏障(高山和平原等)以及栖息地片段化是其遗传分化形成的主要因素.  相似文献   
28.
29.
Abouthiouzine is a newly synthesized antithyroid agent with a proposed less adverse effects profile than other currently used drugs. A simple and rapid reversed phase high performance liquid chromatography assay was developed to determine the concentration of abouthiouzine in human plasma. The procedure involved extraction of the drug and propranolol (internal standard) from the plasma using ethylacetate. The extract was evaporated under nitrogen and the residue was constituted with the mobile phase and injected onto micro-Bondapack phenyl column (10 microm, 3.9 mm x 150 mm). The mobile phase consisted of 10 mM potassium dihydrogen phosphate buffer, acetonitrile, and methanol in the ratio of 60:25:15 (v/v/v, pH=3.0), which was delivered at a rate of 1.5 ml/min. Abouthiouzine and the internal standard were monitored using UV detection at 240 nm; the run time was less than 5 min. The detection limit of abouthiouzine is 0.5 microg/ml. The within- and between-day coefficients of variation were less than 7%. Our method has been successfully used to measure abouthiouzine plasma concentrations in a rabbit model following an intravenous administration of the drug.  相似文献   
30.
Root-knot nematodes Meloidogyne incognita (Kofoid and White) Chitwood and Rhizoctonia bataticola (Taub.) Butler, fungus, are very dangerous root damaging pathogens. Present study was planned to establish a chemical control of these root deteriorating pathogens under lab conditions as well as in field. Maximum death rate of nematode juveniles and minimum numbers of nematode eggs hatched were recorded in plates treated with Cadusafos (Rugby® 100G) @12 g/100 ml and Cartap® (4% G) @9g/100 ml. Chemical treatment of Rhizoctonia bataticola with Trifloxystrobin + Tebuconazole (Nativo®) @0.2 g/100 ml and Mancozeb + Matalaxyl (Axiom) @0.25 g/100 ml significantly controlled the mycelial growth in plates. The best treatments tested in laboratory were applied in field as protective and curative treatments. Results proved that chemical control of root-knot nematode and root rot fungi by tested chemicals at recommended time and dose is a significant management technique under field conditions.  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