全文获取类型
收费全文 | 12191篇 |
免费 | 904篇 |
国内免费 | 361篇 |
专业分类
13456篇 |
出版年
2024年 | 26篇 |
2023年 | 102篇 |
2022年 | 289篇 |
2021年 | 387篇 |
2020年 | 289篇 |
2019年 | 351篇 |
2018年 | 403篇 |
2017年 | 317篇 |
2016年 | 423篇 |
2015年 | 679篇 |
2014年 | 790篇 |
2013年 | 860篇 |
2012年 | 1079篇 |
2011年 | 1019篇 |
2010年 | 642篇 |
2009年 | 541篇 |
2008年 | 728篇 |
2007年 | 648篇 |
2006年 | 534篇 |
2005年 | 489篇 |
2004年 | 508篇 |
2003年 | 395篇 |
2002年 | 315篇 |
2001年 | 255篇 |
2000年 | 211篇 |
1999年 | 207篇 |
1998年 | 95篇 |
1997年 | 64篇 |
1996年 | 55篇 |
1995年 | 62篇 |
1994年 | 59篇 |
1993年 | 44篇 |
1992年 | 81篇 |
1991年 | 75篇 |
1990年 | 55篇 |
1989年 | 48篇 |
1988年 | 35篇 |
1987年 | 24篇 |
1986年 | 27篇 |
1985年 | 25篇 |
1984年 | 12篇 |
1983年 | 17篇 |
1982年 | 13篇 |
1980年 | 20篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1977年 | 18篇 |
1975年 | 14篇 |
1974年 | 16篇 |
1970年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Inflammation is known to be an important underlying condition in the development of a variety of diseases. To investigate whether blood lead induces inflammatory reactions in non-occupationally exposed adults and the effects of genetic susceptibility associated with GSTM1 and TNF-alpha gene polymorphisms on this inflammatory response, we measured blood lead levels in 300 healthy university students. Total serum TNF-alpha and IL-6 levels and WBC counts were determined to evaluate the inflammatory response. Allelic loss of GSTM1 and the TNF-alpha-308 G>A polymorphism were determined by PCR and RFLP. Positive relations between blood lead and three inflammation biomarkers were shown in male subjects with blood lead > or =2.51microg/dl (median value) (TNF-alpha, p=0.015; IL-6, p=0.082; and WBC, p=0.044). However, subgroup analysis by genotype showed an effect of blood lead on the three biomarkers only in individuals with the GSTM1 null (TNF-alpha, p=0.020; IL-6, p=0.096; and WBC, p=0.017) or TNF-alpha GG (TNF-alpha, p=0.017; IL-6, p=0.088; and WBC, p=0.095) genotype, and not in individuals with GSTM1 present (all three inflammatory biomarkers, p>0.1) or the TNF-alpha GA or AA (all three biomarkers, p>0.1) genotype. These results suggest that blood lead affects the inflammatory response and that GSTM1 and TNF-alpha gene polymorphisms are genetic factors associated with lead-induced inflammatory response. 相似文献
992.
Senthil-Kumar M Hema R Anand A Kang L Udayakumar M Mysore KS 《The New phytologist》2007,176(4):782-791
Virus-induced gene silencing (VIGS) is a rapid and robust method for determining and studying the function of plant genes or expressed sequence tags (ESTs). However, only a few plant species are amenable to VIGS. There is a need for a systematic study to identify VIGS-efficient plant species and to determine the extent of homology required between the heterologous genes and their endogenous orthologs for silencing. Two approaches were used. First, the extent of phytoene desaturase (PDS) gene silencing was studied in various Solanaceous plant species using Nicotiana benthamiana NbPDS sequences. In the second approach, PDS sequences from a wide range of plant species were used to silence the PDS gene in N. benthamiana. The results showed that tobacco rattle virus (TRV)-mediated VIGS can be performed in a wide range of Solanaceous plant species and that heterologous gene sequences from far-related plant species can be used to silence their respective orthologs in the VIGS-efficient plant N. benthamiana. A correlation was not always found between gene silencing efficiency and percentage homology of the heterologous gene sequence with the endogenous gene sequence. It was concluded that a 21-nucleotide stretch of 100% identity between the heterologous and endogenous gene sequences is not absolutely required for gene silencing. 相似文献
993.
In this study, via a bioactivity-guided fractionation of MeOH extracts of the fruits of Piper nigrum, alkamide (5) and five previously-identified alkamides were isolated. Their structures were elucidated via spectroscopic analysis ((1)H, (13)C NMR and ESI-MS), as follows: retrofractamide A (1), pipercide (2), piperchabamide D (3), pellitorin (4), dehydroretrofractamide C (5) and dehydropipernonaline (6). The IC(50) values determined for the compounds were 24.5 (1), 3.7 (2), 13.5 (3), 40.5 (4), 60 (5) and 90 microM (6), according to the results of an ACAT enzyme assay system using rat liver microsomes. These compounds all inhibited cholesterol esterification in HepG2 cells. 相似文献
994.
995.
Comparison of APRI and Hydrus-2D models to simulate soil water dynamics in a vineyard under alternate partial root zone drip irrigation 总被引:2,自引:0,他引:2
Alternate partial root zone irrigation (APRI) is a new water-saving irrigation technique. It can reduce irrigation water and
transpiration without reduction in crop yield, thus increase water and nutrient use efficiency. Understanding of soil moisture
distribution and dynamic under the alternate partial root zone drip irrigation (APDI) can help to develop the efficient irrigation
schemes. In this paper, a two-dimensional (2D) root water uptake model was proposed based on soil water dynamic and root distribution
of grape vine, and a function of soil evaporation related to soil water content was defined under the APDI. Then the soil
water dynamic model of APDI (APRI-model) was developed based on the 2D root water uptake model and soil evaporation function
combined with average measured soil moisture content at 0–10 cm soil layer. Soil water dynamic in APDI was respectively simulated
by Hydrus-2D model and APRI-model. The simulated soil water contents by two models were compared with the measured value.
The results showed that the values of root-mean-square-error (RMSE) range from 0.01 to 0.022 cm3/cm3 for APRI-model, and from 0.012 to 0.031 cm3/cm3 for Hydrus-2D model. The average relative error between the simulated and measured soil water content is about 10% for APRI-model,
and from 11% to 29% for Hydrus-2D model, indicating that two models perform well in simulating soil moisture dynamic under
the APDI, but the APRI-model is more suitable for modeling the soil water dynamic in the arid region with greater soil evaporation
and uneven root distribution. 相似文献
996.
997.
Characterization of rice tryptophan decarboxylases and their direct involvement in serotonin biosynthesis in transgenic rice 总被引:1,自引:0,他引:1
l-Tryptophan decarboxylase (TDC) and l-tyrosine decarboxylase (TYDC) belong to a family of aromatic l-amino acid decarboxylases and catalyze the conversion of tryptophan and tyrosine into tryptamine and tyramine, respectively.
The rice genome has been shown to contain seven TDC or TYDC-like genes. Three of these genes for which cDNA clones were available were characterized to assign their functions using
heterologous expression in Escherichia coli and rice (Oryza sativa cv. Dongjin). The purified products of two of the genes were expressed in E. coli and exhibited TDC activity, whereas the remaining gene could not be expressed in E. coli. The recombinant TDC protein with the greatest TDC activity showed a K
m of 0.69 mM for tryptophan, and its activity was not inhibited by phenylalanine or tyrosine, indicating a high level of substrate
specificity toward tryptophan. The ectopic expression of the three cDNA clones in rice led to the abundant production of the
products of the encoded enzymes, tyramine and tryptamine. The overproduction of TYDC resulted in stunted growth and a lack
of seed production due to tyramine accumulation, which increased as the plant aged. In contrast, transgenic plants that produced
TDC showed a normal phenotype and contained 25-fold and 11-fold higher serotonin in the leaves and seeds, respectively, than
the wild-type plants. The overproduction of either tyramine or serotonin was not strongly related to the enhanced synthesis
of tyramine or serotonin derivatives, such as feruloyltyramine and feruloylserotonin, which are secondary metabolites that
act as phytoalexins in plants. 相似文献
998.
Lyon CM Klinge DM Liechty KC Gentry FD March TH Kang T Gilliland FD Adamova G Rusinova G Telnov V Belinsky SA 《Radiation research》2007,168(4):409-414
Epigenetic inactivation of genes by promoter hypermethylation, a major mechanism in the initiation and progression of tobacco-induced cancer, has also been associated with lung cancer induced through environmental and occupational exposures. Our previous study of gene methylation in workers from the MAYAK nuclear enterprise identified a significantly higher prevalence for methylation of the p16 gene (CDKN2A) in adenocarcinomas from workers compared to tumors from non-worker controls. The purpose of this investigation was to determine whether genes in addition to p16 are "targeted" for silencing and whether overall gene methylation was more common in radiation-induced adenocarcinoma. A significant increase in the prevalence of methylation of GATA5 was seen in tumors from workers compared to tumors from controls. The prevalence for methylation of PAX5 beta and H-cadherin did not differ in tumors from workers and controls. Evaluating the frequency for methylation of a five-gene panel revealed that 93% of adenocarcinomas from workers compared to 66% of tumors from controls were methylated for at least one gene. Moreover, a twofold increase was seen in the number of tumors methylated for three or more genes for tumors from workers compared to controls. Increased frequency for inactivation of genes by promoter hypermethylation and targeting of tumor suppressor genes such as GATA5 may be factors that contribute to the increased risk for lung cancer associated with radiation exposure. 相似文献
999.
Moon YH Nam SH Kang J Kim YM Lee JH Kang HK Breton V Jun WJ Park KD Kimura A Kim D 《Applied microbiology and biotechnology》2007,77(3):559-567
Two arbutin glucosides were synthesized via the acceptor reaction of a glucansucrase from Leuconostoc mesenteroides B-1299CB with arbutin and sucrose. The glucosides were purified by Bio-gel P-2 column chromatography and high-performance
liquid chromatography, and the structures were elucidated as 4-hydroxyphenyl β-isomaltoside (arbutin-G1), 4-hydroxyphenyl
β-isomaltotrioside (arbutin-G2), according to the results of 1H, 13C, heteronuclear single-quantum coherence, 1H-1H COSY, and heteronuclear multiple-bond correlation analyses. Arbutin glucoside (4-hydroxyphenyl β-isomaltoside) exhibited
slower effects on 1,1-diphenyl-2-picrylhydrazyl radical scavenging and similar effects on tyrosinase inhibition, and increased
inhibitory effect on matrix metalloproteinase-1 production induced by UVB than arbutin.
Young Hwan Moon and Seung Hee Nam contributed equally to this work. 相似文献
1000.
Subnanomolar doses of an unaltered, naturally occurring nucleosomal histone peptide epitope, H4(71-94), when injected s.c. into lupus-prone mice, markedly prolong lifespan by generating CD4+25+ and CD8+ regulatory T cells (Treg) producing TGF-beta. The induced Treg cells suppress nuclear autoantigen-specific Th and B cells and block renal inflammation. Splenic dendritic cells (DC) captured the s.c.-injected H4(71-94) peptide rapidly and expressed a tolerogenic phenotype. The DC of the tolerized animal, especially plasmacytoid DC, produced increased amounts of TGF-beta, but diminished IL-6 on stimulation via the TLR-9 pathway by nucleosome autoantigen and other ligands; and those plasmacytoid DC blocked lupus autoimmune disease by simultaneously inducing autoantigen-specific Treg and suppressing inflammatory Th17 cells that infiltrated the kidneys of untreated lupus mice. Low-dose tolerance with H4(71-94) was effective even though the lupus immune system is spontaneously preprimed to react to the autoepitope. Thus, H4(71-94) peptide tolerance therapy that preferentially targets pathogenic autoimmune cells could spare lupus patients from chronically receiving toxic agents or global immunosuppressants and maintain remission by restoring autoantigen-specific Treg cells. 相似文献