全文获取类型
收费全文 | 63907篇 |
免费 | 5445篇 |
国内免费 | 141篇 |
专业分类
69493篇 |
出版年
2023年 | 230篇 |
2022年 | 701篇 |
2021年 | 1153篇 |
2020年 | 646篇 |
2019年 | 854篇 |
2018年 | 1256篇 |
2017年 | 1004篇 |
2016年 | 1764篇 |
2015年 | 2949篇 |
2014年 | 3221篇 |
2013年 | 3809篇 |
2012年 | 4929篇 |
2011年 | 4692篇 |
2010年 | 2951篇 |
2009年 | 2616篇 |
2008年 | 3739篇 |
2007年 | 3464篇 |
2006年 | 3205篇 |
2005年 | 2887篇 |
2004年 | 2774篇 |
2003年 | 2492篇 |
2002年 | 2129篇 |
2001年 | 1861篇 |
2000年 | 1737篇 |
1999年 | 1390篇 |
1998年 | 601篇 |
1997年 | 525篇 |
1996年 | 466篇 |
1995年 | 448篇 |
1994年 | 349篇 |
1993年 | 356篇 |
1992年 | 760篇 |
1991年 | 609篇 |
1990年 | 569篇 |
1989年 | 582篇 |
1988年 | 481篇 |
1987年 | 472篇 |
1986年 | 403篇 |
1985年 | 419篇 |
1984年 | 331篇 |
1983年 | 284篇 |
1982年 | 240篇 |
1981年 | 211篇 |
1980年 | 205篇 |
1979年 | 292篇 |
1978年 | 258篇 |
1977年 | 223篇 |
1976年 | 217篇 |
1974年 | 238篇 |
1973年 | 192篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Sauquet H Ho SY Gandolfo MA Jordan GJ Wilf P Cantrill DJ Bayly MJ Bromham L Brown GK Carpenter RJ Lee DM Murphy DJ Sniderman JM Udovicic F 《Systematic biology》2012,61(2):289-313
Although temporal calibration is widely recognized as critical for obtaining accurate divergence-time estimates using molecular dating methods, few studies have evaluated the variation resulting from different calibration strategies. Depending on the information available, researchers have often used primary calibrations from the fossil record or secondary calibrations from previous molecular dating studies. In analyses of flowering plants, primary calibration data can be obtained from macro- and mesofossils (e.g., leaves, flowers, and fruits) or microfossils (e.g., pollen). Fossil data can vary substantially in accuracy and precision, presenting a difficult choice when selecting appropriate calibrations. Here, we test the impact of eight plausible calibration scenarios for Nothofagus (Nothofagaceae, Fagales), a plant genus with a particularly rich and well-studied fossil record. To do so, we reviewed the phylogenetic placement and geochronology of 38 fossil taxa of Nothofagus and other Fagales, and we identified minimum age constraints for up to 18 nodes of the phylogeny of Fagales. Molecular dating analyses were conducted for each scenario using maximum likelihood (RAxML + r8s) and Bayesian (BEAST) approaches on sequence data from six regions of the chloroplast and nuclear genomes. Using either ingroup or outgroup constraints, or both, led to similar age estimates, except near strongly influential calibration nodes. Using "early but risky" fossil constraints in addition to "safe but late" constraints, or using assumptions of vicariance instead of fossil constraints, led to older age estimates. In contrast, using secondary calibration points yielded drastically younger age estimates. This empirical study highlights the critical influence of calibration on molecular dating analyses. Even in a best-case situation, with many thoroughly vetted fossils available, substantial uncertainties can remain in the estimates of divergence times. For example, our estimates for the crown group age of Nothofagus varied from 13 to 113 Ma across our full range of calibration scenarios. We suggest that increased background research should be made at all stages of the calibration process to reduce errors wherever possible, from verifying the geochronological data on the fossils to critical reassessment of their phylogenetic position. 相似文献
972.
Sequence polymorphisms of Der f 1, Der p 1, Der f 2 and Der p 2 from Korean house dust mite isolates
Jeong KY Lee IY Yong TS Lee JH Kim EJ Lee JS Hong CS Park JW 《Experimental & applied acarology》2012,58(1):35-42
Amino acid sequence variations have possible influences on the allergenicity of allergens and may be important factors in allergen standardization. This study was undertaken to investigate the sequence polymorphisms of group 1 and 2 allergens from Korean isolates of the house dust mites Dermatophagoides farinae and D. pteronyssinus. cDNA sequences encoding group 1 and 2 allergens were amplified by RT-PCR and compared the deduced amino acid sequences. Der f 1.0101, which appeared in 64.0 % of the 50 sequences analyzed, was found to be predominant. Among the Der p 1 sequences, Der p 1.0102 and 1.0105 were predominant (58 %). Among the Der f 2 sequences, Der f 2.0102 (40.7 %) and a new variant with Gly at position 42 (27.8 %) were predominant. The deduced amino acid sequences of 60 Der p 2 clones were examined, and 28 variants with 1-5 amino acid substitutions were found. Interestingly, all of the Der p 2 sequences had Thr instead of Lys at position 49. Two variants (Leu40, Thr49, and Asn114 (26.6 %); Val40, Thr49, and Asn114 (20.0 %)) were found to be the most predominant forms of Der p 2. Der p 1 has a high rate of sporadic substitutions and the group 2 allergens show a more regular pattern with orderly associations of amino acid substitutions. Der f 1 and Der p 2 from Korean mite isolates have unique amino acid sequence polymorphisms. These findings provide important data for house dust mite allergen standardization. 相似文献
973.
Growing evidence from human and animal studies has shown adverse consequences of maternal usage of antidepressants in their newborn babies. To study the effects of early antidepressant exposure on motor function later in life, we treated neonatal rat pups with fluoxetine (FLX), a selective serotonin reuptake inhibitor (SSRI)-type antidepressant, from the day of birth to postnatal day 4 and examined motor performance during adolescence. FLX-treated rats had reduced locomotor activities in an open field and poorer motor performance on an accelerating rotarod compared to the control group of saline-treated animals. Nevertheless, the poorer motor performance largely improved after repetitive practices. To elucidate the structural alterations in the motor system, we examined the structure of neurons in motor-related brain regions. The shape, density, and soma size of cerebellar Purkinje cells were comparable in the two groups, however, density of dendritic spine in medial spiny neurons of striatum and Layer 5 pyramidal neurons in the primary motor cortex (M1) were reduced in FLX-rats. Furthermore, the basilar dendrites in M1 Layer 5 neurons had reduced dendritic complexity than those of the control animals. The impaired dendritic structure in striatal and cortical neurons in FLX-treated rats might account for their poorer motor performances. Together, the structure and function of the motor system are affected by early FLX exposure, the long-term effects of early exposure to SSRI-type antidepressants should be concerned. 相似文献
974.
Yujeong Cho Jeehye Maeng Jungmin Ryu Hyekyoung Shin Miyoung Kim Goo Taeg Oh Moo-Yeol Lee Kyunglim Lee 《Transgenic research》2012,21(6):1245-1254
Hypertension is a well-established etiological factor for atherogenesis. We previously showed that transgenic mice overexpressing translationally controlled tumor protein (TCTP) develop systemic arterial hypertension. In this study we explored the cardiovascular effects of TCTP overexpression and possibly of the resultant hypertension on the severity of atherosclerosis in apolipoprotein E-deficient mice. Through multiple mating of TCTP-overexpressing transgenic mice (TCTP-TG) with apolipoprotein E knock-out mice (ApoE KO), we generated non-transgenic (nTG), TCTP-TG, nTG/ApoE KO and TCTP-TG/ApoE KO mice with similar genetic background. Male mice, 7-week old, were fed a lipid-enriched Western diet for 16?weeks, and blood pressure and body weight change were monitored every 2?weeks. Plasma lipid profiles and atherosclerotic lesions in aorta were quantified at the end of study. We found that blood pressure levels of TCTP-TG and TCTP-TG/ApoE KO, were similarly elevated while nTG and nTG/ApoE KO mice were normotensive. TCTP overexpression in ApoE KO mice led to significant exacerbation of atherosclerotic lesions. Feeding Western diet resulted in increases in total cholesterol, triglyceride (TG) and low density lipoprotein, and decreased high density lipoprotein (HDL) in ApoE KO mice. No significant differences were found in plasma lipid profiles of nTG/ApoE KO and TCTP-TG/ApoE KO. This study suggests that overexpression of TCTP, which induces hypertension, also accelerates the development of atherosclerotic lesion caused by high-fat and high-cholesterol diet without significantly altering plasma lipid profiles. We conclude that TCTP-induced hypertension could increase the severity of atherosclerotic lesion and suggest that inhibition of TCTP or its signaling pathways may be a potential approach to the therapy of both diseases, hypertension and atherosclerosis. 相似文献
975.
Hye Jung Choi Dong Wan Kim Young Whan Choi Young Geun Lee Yong-Ill Lee Yong Kee Jeong Woo Hong Joo 《Biotechnology and Bioprocess Engineering》2012,17(3):576-583
Streptomyces sp. strain BCNU 1001 was isolated from forest soil samples. Cultural, morphological, and physiological characteristics as well as 16S rDNA analysis revealed that the isolate, BCNU 1001, belonged to the genus Streptomyces. The antimicrobial activity of the ethyl acetate extract was confirmed using the broth microdilution technique. The minimum inhibitory concentration (MIC) of the BCNU 1001 ethyl acetate extract was 0.25 mg/mL for Bacillus subtilis, Escherichia coli, and Pseudomonas aeruginosa, and 0.125 mg/mL for Micrococcus luteus, Staphylococcus aureus, and Pseudomonas fluorescens. The MIC of the BCNU 1001 ethyl acetate extract for Aspergillus niger, Candida albicans, and Saccharomyces cerevisiae was 0.5, 0.125, and 0.25 mg/mL, respectively. BCNU 1001 was also active against dermatophytic fungi such as Trichophyton mentagrophytes and T. rubrum. Furthermore, BCNU 1001 was also found to be effective against Methicillin-resistant Staphylococcus aureus (MRSA), and its ethyl acetate extract showed MIC = 0.5 mg/mL against MRSA. The most abundant antimicrobial compound was identified as a 2-hydroxybenzyl alcohol through analysis utilizing a nuclear magnetic resonance spectroscopy. This compound was seen to be very effective against some kinds of bacteria and fungi. 相似文献
976.
Err-Cheng Chan Peter Pear Ueng Li Fu Chen 《Applied microbiology and biotechnology》1989,31(5-6):524-528
Summary The Escherichia coli xylose isomerase gene was transformed into Schizosaccharomyces pombe for direct d-xylose utilization. In order to understand d-xylose metabolism and determine the limiting factors on d-xylose utilization by the transformed yeast, d-xylose transport, xylose isomerization, and xylulose phosphorylation were investigated. The results indicated that low activity of xylose isomerization in the cloned yeast was the limiting step for d-xylose fermentation. An in vitro study showed that yeast proteases decreased xylose isomerase activity. Xylitol, a by-product of d-xylose fermentation, had no effect on the activity of xylose isomerase. 相似文献
977.
978.
Results from recent studies support the hypothesis that cancer stem cells (CSCs) are responsible for tumor initiation and formation. Here, we applied a proteome profiling approach to investigate the mechanisms of CSCs and to identify potential biomarkers in the prostate cancer cell line DU145. Using MACS, the DU145 prostate cancer cell line was isolated into CD44+ or CD44− cells. In sphere culture, CD44+ cells possessed stem cell characteristics and highly expressed genes known to be important in stem cell maintenance. In addition, they showed strong tumorigenic potential in the clonogenic assay and soft agar colony formation assay. We then analyzed and identified proteins that were differentially expressed between CD44+ and CD44− using two-dimensional gel electrophoresis and LC-MS/MS. Cofilin and Annexin A5, which are associated with proliferation or metastasis in cancer, were found to be positively correlated with CD44 expression. These results provide information that will be important to the development of new cancer diagnostic tools and understanding the mechanisms of CSCs although a more detailed study is necessary to investigate the roles of Cofilin and Annexin A5 in CSCs. 相似文献
979.
Kim JS Heo P Yang TJ Lee KS Jin YS Kim SK Shin D Kweon DH 《Biochemical and biophysical research communications》2011,(1):105-110
In a phenomenon called persistence, small numbers of bacterial cells survive even after exposure to antibiotics. Recently, bactericidal antibiotics have been demonstrated to kill bacteria by increasing the levels of hydroxyl radicals inside cells. In the present study, we report a direct correlation between intracellular hydroxyl radical formation and bacterial persistence. By conducting flow cytometric analysis in a three-dimensional space, we resolved distinct bacterial populations in terms of intracellular hydroxyl radical levels, morphology and viability. We determined that, upon antibiotic treatment, a small sub-population of Escherichia coli survivors do not overproduce hydroxyl radicals and maintain normal morphology, whereas most bacterial cells were killed by accumulating hydroxyl radicals and displayed filamentous morphology. Our results suggest that bacterial persisters can be formed once they have transient defects in mediating reactions involved in the hydroxyl radical formation pathway. Thus, it is highly probable that persisters do not share a common mechanism but each persister cell respond to antibiotics in different ways, while they all commonly show lowered hydroxyl radical formation and enhanced tolerance to antibiotics. 相似文献
980.
Nam Jin Noh Yowhan Son Rae Hyun Kim Kyung Won Seo Jin Woo Koo In Hyeop Park Young Jin Lee Kyeong Hak Lee Yeong Mo Son 《Journal of Plant Biology》2007,50(4):461-466
Above- and belowground biomass and nitrogen (N) and phosphorus (P) distribution within threeQuercus acutissima stands were investigated in central Korea. The average age (year) and diameter at breast height (DBH, cm) were 10.8 and 7.9
for Stand 1, 38.2 and 17.1 for Stand 2, and 44.0 and 20.7 for Stand 3, respectively. Fifteen trees were destructively harvested
for dimension analysis of component biomass (stem wood, stem bark, foliage, branches, and roots) plus N and P concentrations.
Total biomass (t ha-1) was 88.7 for Stand 1, 154.9 for Stand 2, and 278.1 for Stand 3 while N and P contents in all tree components (kg ha-1) were 483.3 and 52.2, 697.1 and 55.0, and 1113.9 and 83.7. Nitrogen concentrations were highest in the foliage, followed
by the stem bark, branches or roots, and stem wood. In contrast, P concentrations were greatest in the roots, then foliage,
branches, stem bark, and stem wood. In general, N and P concentrations in these components significantly decreased with tree
age and DBH, while N and P contents significantly increased with age and size. These relationships were stronger for size
than for age. Our current data could be utilized to estimate N and P budgets for silvicultural practices, including fertilization,
thinning, and harvesting. 相似文献