全文获取类型
收费全文 | 1134篇 |
免费 | 75篇 |
国内免费 | 64篇 |
专业分类
1273篇 |
出版年
2024年 | 1篇 |
2023年 | 21篇 |
2022年 | 37篇 |
2021年 | 80篇 |
2020年 | 43篇 |
2019年 | 58篇 |
2018年 | 48篇 |
2017年 | 41篇 |
2016年 | 61篇 |
2015年 | 104篇 |
2014年 | 73篇 |
2013年 | 114篇 |
2012年 | 121篇 |
2011年 | 95篇 |
2010年 | 65篇 |
2009年 | 35篇 |
2008年 | 43篇 |
2007年 | 24篇 |
2006年 | 29篇 |
2005年 | 19篇 |
2004年 | 17篇 |
2003年 | 5篇 |
2002年 | 17篇 |
2001年 | 15篇 |
2000年 | 12篇 |
1999年 | 15篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
排序方式: 共有1273条查询结果,搜索用时 15 毫秒
61.
62.
Self incompatibility (SI) in Phalaris coerulescens is gametophytically determined by two unlinked multi allelic loci (S and Z). Neither the S nor Z genes have yet been cloned. As part of a map-based cloning strategy, high-resolution maps of the S and Z regions were generated from distorted segregating populations using RFLP probes from wheat, barley, oat, and Phalaris. The S locus was delimited to 0.26 cM with two boundary markers (Xwg811 and Xpsr168) and cosegregated with Xbm2 and Xbcd762. Xbcd266 was the closest marker linked to Z (0.9 cM). A high level of colinearity in the S and Z regions was found in both self-incompatible and -compatible species. The S locus was localized to the subcentromere region of chromosome 1 and the Z locus to the long arm end of chromosome 2. Several rice BAC clones orthologous to the S and Z locus regions were identified. This opens the possibility of using the rice genome sequence data to generate more closely linked markers and identify SI candidate genes. These results add further support to the conservation of gene order in the S and Z regions of the grass genomes. 相似文献
63.
Liangqia Guo Zenghong Xie Xintong Bian Xucong Lin Weilin Zhang Xiaohua Liu And Guonan Chen 《Luminescence》2005,20(3):129-134
A flow-injection chemiluminescent method for the determination of oxytetracycline was developed. The method is based on an enhancement by oxytetracycline of the chemiluminescence light emission of tris(2,2'-bipyridine) ruthenium (II), generated by the continuous oxidation of tris(2,2'-bipyridine) ruthenium (II) by cerium (IV) sulphate in sulphuric acid. Under the optimum conditions, the calibration curve was linear over the range 1.0 x 10(-7)-1.0 x 10(-5) g/mL for oxytetracycline with the linear equation: DeltaINT = 148.77 x C + 0.6637 (R2 = 0.9994). The detection limit was 4.52 x 10(-8) g/mL. The proposed method was also successfully used to determine oxytetracycline in pharmaceutical formulations. The mean recovery of determination of oxytetracycline was 92.73%. A mechanism for the chemiluminescence enhancement by oxytetracycline of tris(2,2'-bipyridine)-ruthenium (II) and cerium (IV) sulphate system is also proposed. 相似文献
64.
65.
群体感应是细菌根据细胞密度变化调控基因表达的一种调节机制。铜绿假单胞菌中QS系统由lasI和rhlI合成的信号分子3OC12-HSL和C4-HSL以及各自的受体蛋白LasR、RhlR组成,它们以级联方式调控多个基因表达。【目的】研究细菌群体感应(QS)对聚羟基脂肪酸酯合成的调控。【方法】利用铜绿假单胞菌PAO1及其QS突变株为材料通过气相色谱、荧光定量PCR在生理和分子水平上研究QS对聚羟基脂肪酸酯合成的调控。【结果】QS信号分子合成抑制剂阿奇霉素处理铜绿假单胞菌PAO1和QS突变株导致胞内PHA积累量显著减少;铜绿假单胞菌PAO1中C4-HSL合成酶基因rhlI缺失突变株PAO210胞内PHA积累量与野生型无差别;而3OC12-HSL合成酶基因lasI缺失突变株PAO55、3OC12-HSL受体合成酶基因lasR缺失突变株PAO56以及lasI/lasR双缺失突变株PAO57胞内PHA含量与野生型相比明显减少;lasI和lasR的突变株体内PHA合成酶基因phaC1的表达量显著降低,信号分子3OC12-HSL回补实验使phaC1的表达量可恢复到野生株水平,但只可部分恢复lasI缺失导致的胞内PHA合成。【结论】由此推测,铜绿假单胞菌群体感应系统中lasI/lasR系统参与胞内聚羟基脂肪酸酯合成的调控。 相似文献
66.
Introduction
Hypodontia, hypohidrosis, sparse hair and characteristic faces are the main characters of X-linked hypohidrotic ectodermal dysplasia (XLHED) which is caused by genetic ectodysplasin A (EDA) deficiency. Heterozygous female carriers tend to have mild to moderate XLHED phenotype, even though 30% of them present no obvious symptom.Methods
A large Chinese XLHED family was reported and the entire coding region and exon–intron boundaries of EDA gene were sequenced. To elucidate the mechanism for carriers’ tempered phenotype, we analyzed the methylation level on four sites of the promoter of EDA by the pyrosequencing system.Results
A known frameshift mutation (c.573–574 insT) was found in this pedigree. Combined with the pedigrees we reported before, 120 samples comprised of 23 carrier females from 11 families and 97 healthy females were analyzed for the methylation state of EDA promoter. Within 95% confidence interval (CI), 18 (78.26%) carriers were hypermethylated at these 4 sites.Conclusion
Chinese XLHED carriers often have a hypermethylated EDA promoter. 相似文献67.
Pei Jian-Ming; Yu Xiao-Chun; Bian Jin-Song; Wong Tak-Ming 《American journal of physiology. Cell physiology》1999,277(3):C492
To study the effects of -opioid receptor stimulation onintracellular Ca2+ concentration([Ca2+]i)homeostasis during extracellular acidosis, we determined the effects of-opioid receptor stimulation on[Ca2+]iresponses during extracellular acidosis in isolated single ratventricular myocytes, by a spectrofluorometric method. U-50488H (10-30 µM), a selective -opioid receptor agonist, dosedependently decreased the electrically induced[Ca2+]itransient, which results from the influx ofCa2+ and the subsequentmobilization of Ca2+ from thesarcoplasmic reticulum (SR). U-50488H (30 µM) also increased theresting[Ca2+]iand inhibited the[Ca2+]itransient induced by caffeine, which mobilizesCa2+ from the SR, indicating thatthe effects of the -opioid receptor agonist involved mobilization ofCa2+ from its intracellular poolinto the cytoplasm. The Ca2+responses to 30 µM U-50488H were abolished by 5 µMnor-binaltorphimine, a selective -opioid receptorantagonist, indicating that the event was mediated by the -opioidreceptor. The effects of the agonist on[Ca2+]iand the electrically induced[Ca2+]itransient were significantly attenuated when the extracellular pH(pHe) was loweredto 6.8, which itself reduced intracellular pH(pHi) and increased[Ca2+]i.The inhibitory effects of U-50488H were restored during extracellular acidosis in the presence of 10 µM ethylisopropyl amiloride, a potentNa+/H+exchange blocker, or 0.2 mM Ni2+,a putativeNa+/Ca2+exchange blocker. The observations indicate that acidosismay antagonize the effects of -opioid receptor stimulation viaNa+/H+andNa+/Ca2+exchanges. When glucose at 50 mM, known to activate theNa+/H+exchange, was added, both the resting[Ca2+]iand pHi increased. Interestingly,the effects of U-50488H on [Ca2+]iand the electrically induced[Ca2+]itransient during superfusion with glucose were significantly attenuated; this mimicked the responses during extracellular acidosis. When a high-Ca2+ (3 mM) solutionwas superfused, the resting[Ca2+]iincreased; the increase was abolished by 0.2 mMNi2+, but thepHi remained unchanged. Like theresponses to superfusion with high-concentration glucose andextracellular acidosis, the responses of the[Ca2+]iand electrically induced[Ca2+]itransients to 30 µM U-50488H were also significantly attenuated. Results from the present study demonstrated for the first time thatextracellular acidosis antagonizes the effects of -opioid receptorstimulation on the mobilization ofCa2+ from SR. Activation of bothNa+/H+andNa+/Ca2+exchanges, leading to an elevation of[Ca2+]i,may be responsible for the antagonistic action of extracellular acidosis against -opioid receptor stimulation. 相似文献
68.
Genistein is an isoflavone and phytoestrogen that is a potent inhibitor of cell proliferation and angiogenesis. This study was designed to investigate the binding of genistein to human serum albumin (HSA) under physiological conditions with drug concentrations in the range of 6.7 × 10−6 to 2.0 × 10−5 mol L−1 and HSA concentration at 1.5 × 10−6 mol L−1. Fluorescence quenching methods in combination with Fourier transform infrared (FT-IR) spectroscopy and circular dichroism (CD) spectroscopy was used to determine the binding mode, the binding constant and the protein structure changes in the presence of genistein in aqueous solution. Changes in the CD spectra and FT-IR spectra were observed upon ligand binding, and the degree of tryptophan fluorescence quenching change did significantly in the complexes. These data have proved the change in protein secondary structure accompanying ligand binding. The change in tryptophan fluorescence intensity was used to determine the binding constants. The thermodynamic parameters, the enthalpy change (ΔH) and the entropy change (ΔS) were calculated to be −22.24 kJ mol−1and 19.60 J mol−1 K−1 according to the van’t Hoff equation, which indicated that hydrophobic and electrostatic interactions play the main role in the binding of genistein to HSA. 相似文献
69.
Kasumov T Martini WZ Reszko AE Bian F Pierce BA David F Roe CR Brunengraber H 《Analytical biochemistry》2002,305(1):90-96
We developed gas chromatography-mass spectrometry assays for the concentration and mass isotopomer distribution of propionyl-CoA, methylmalonyl-CoA, and succinyl-CoA in tissues. The assays involves perchloric acid extraction of the tissue, spiking the extract with [(2)H(5)]propionyl-CoA and [(2)H(4)]succinyl-CoA internal standards, and isolation of short-chain acyl-CoA fraction on an oligonucleotide purification cartridge. Propionyl-CoA is reacted with sarcosine and the formed N-propionylsarcosine is assayed as its pentafluorobenzyl derivative. Methylmalonyl-CoA and succinyl-CoA are hydrolyzed and the corresponding acids assayed as tert-butyl dimethylsilyl derivatives. The assay was applied to a study of [U-(13)C(3)]propionate metabolism in perfused rat livers. While propionyl-CoA is only M3 labeled, succinyl-CoA is M3, M2, and M1 labeled because of isotopic exchanges in the citric acid cycle. Methylmalonyl-CoA is M3 and M2 labeled, reflecting reversal of S-methylmalonyl-CoA mutase. Thus, our assays allow measuring the turnover of the coenzyme A derivatives involved in anaplerosis of the citric acid cycle via precursors of propionyl-CoA, i.e., propionate, odd-chain fatty acids, isoleucine, threonine, and valine. 相似文献
70.
A key commonality of most age-related neurodegenerative diseases is the accumulation of aggregation-prone proteins in the brain. Except for the prionoses, the initiation and propagation of these proteopathies in vivo remains poorly understood. In a previous study, we found that the deposition of the amyloidogenic peptide Abeta can be induced by injection of dilute extracts of Alzheimeric neocortex into the brains of Tg2576 transgenic mice overexpressing the human beta-amyloid precursor protein. The present study was undertaken to assess the pathology after long-term (12 months) incubation, and to clarify the distinctive anatomical distribution of seeded Abeta-immunoreactivity. All mice were injected at 3 months of age; 5 months later, as expected, Abeta deposits were concentrated mostly in the injected hemisphere. After 12 months, abundant, transgene-derived Abeta deposits were present bilaterally in the forebrain, but plaque load was still clearly greater in the extract-injected hemisphere. There was also evidence of tau hyperphosphorylation in axons of the corpus callosum that had been injured by the injection, most prominently in transgenic mice, but also, to a lesser degree, in non-transgenic mice. Five months following injection of AD-extract, an isolated cluster of Abeta-immunoreactive microglia was sometimes evident in the ipsilateral entorhinal cortex; the strong innervation of the hippocampus by entorhinal cortical neurons suggests the possible spread of seeded pathology from the injection site via neuronal transport mechanisms. Finally, using India Ink to map the local dispersion of injectate, we found that Abeta induction is especially potent in places where the injectate is sequestered. The AD-seeding model can illuminate the emergence and spread of cerebral beta-amyloidosis and tau hyperphosphorylation, and thus could enhance our understanding of AD and its pathogenic commonalties with other cerebral proteopathies. 相似文献