全文获取类型
收费全文 | 8934篇 |
免费 | 678篇 |
国内免费 | 676篇 |
出版年
2024年 | 11篇 |
2023年 | 109篇 |
2022年 | 248篇 |
2021年 | 503篇 |
2020年 | 303篇 |
2019年 | 377篇 |
2018年 | 385篇 |
2017年 | 305篇 |
2016年 | 375篇 |
2015年 | 585篇 |
2014年 | 703篇 |
2013年 | 717篇 |
2012年 | 851篇 |
2011年 | 732篇 |
2010年 | 438篇 |
2009年 | 409篇 |
2008年 | 466篇 |
2007年 | 383篇 |
2006年 | 295篇 |
2005年 | 247篇 |
2004年 | 191篇 |
2003年 | 187篇 |
2002年 | 161篇 |
2001年 | 143篇 |
2000年 | 129篇 |
1999年 | 110篇 |
1998年 | 73篇 |
1997年 | 84篇 |
1996年 | 81篇 |
1995年 | 58篇 |
1994年 | 48篇 |
1993年 | 45篇 |
1992年 | 63篇 |
1991年 | 49篇 |
1990年 | 49篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 26篇 |
1986年 | 24篇 |
1985年 | 36篇 |
1984年 | 33篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1978年 | 11篇 |
1974年 | 11篇 |
1973年 | 7篇 |
1971年 | 9篇 |
1967年 | 7篇 |
1966年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
The staphylococcal nuclease, encoded by the nuc1 gene, is an important virulence factor of Staphylococcus aureus. However, the physiological role of the nuclease has not been fully characterized. The current study observed that biofilm
development could be prevented in staphylococcal nuclease-producing strains of S. aureus; however, when the nuc1 gene was knocked out, the ability to form a biofilm significantly increased. Scanning electron and confocal scanning laser
microscopy were used to evaluate the role of the nuc1 gene in biofilm formation. Moreover, the nuc1 gene product, staphylococcal nuclease, and recombinant NUC1 protein were found to have a visible effect on other biofilm-forming
bacteria, such as Pseudomonas aeruginosa, Actinobacillus pleuropneumoniae, and Haemophilus parasuis. The current study showed a direct relationship between staphylococcal nuclease production and the prevention of biofilm
development. The findings from this study underscore the important role of staphylococcal nuclease activity to prevent biofilm
formation in S. aureus. They also provided evidence for the biological role of staphylococcal nucleases in other organisms. 相似文献
972.
Allelopathy of diterpenoids extracted from plants of the genus Robdosia on three common species of soil ciliates, Colpoda inflata, Colpoda cucullus and Euplotes muscicola, was studied by acute toxicity test, sub-lethal effect test and morphological observation. Acute toxicity test showed that there was remarkable toxicity of the diterpenoids on the individuals of the three soil ciliate species, and there was close correlation between toxicity and concentration of the diterpenoids. 12 h-LC50 values of the diterpenoids on the individuals of C. inflata, C. cucullus and E. muscicola were 161.40 mg L?1, 94.80 mg L?1 and 83.70 mg L?1 respectively, and 24 h-LC50 values were 114.90 mg L?1, 92.30 mg L?1and 65.80 mg L?1 separately. Sub-lethal effect test of soil ciliates suggested that there existed significant inhibition of the diterpenoids on population growth of the three ciliates with dose-dependant relationships, population density and growth rate of the test group was obviously lower than that of the control group. Morphological observation indicated that diterpenoids affected the body shapes of the three ciliates and made them shorter and thicker, and the higher the concentration of diterpenoids, the greater the affection. The results are of great significance for understanding the functions of ciliates and their relationships to other organisms, and for the application of allelopathy in biological pest control in the soil ecosystems. 相似文献
973.
Protein splicing is a precise post-translational process mediated by inteins. Inteins are intervening proteins that cleave
themselves from a precursor protein while joining the flanking sequences. Here we report the 15N, 13C, and 1H chemical shift assignments of the intein from DNA polymerase II of Pyrococcus abyssi (Pab PolII intein), which has been recombinantly overexpressed and isotopically labeled. The NMR assignments of Pab PolII intein are essential for solution structure determination and protein dynamics study. 相似文献
974.
Dufau S Duñabeitia JA Moret-Tatay C McGonigal A Peeters D Alario FX Balota DA Brysbaert M Carreiras M Ferrand L Ktori M Perea M Rastle K Sasburg O Yap MJ Ziegler JC Grainger J 《PloS one》2011,6(9):e24974
Investigating human cognitive faculties such as language, attention, and memory most often relies on testing small and homogeneous groups of volunteers coming to research facilities where they are asked to participate in behavioral experiments. We show that this limitation and sampling bias can be overcome by using smartphone technology to collect data in cognitive science experiments from thousands of subjects from all over the world. This mass coordinated use of smartphones creates a novel and powerful scientific "instrument" that yields the data necessary to test universal theories of cognition. This increase in power represents a potential revolution in cognitive science. 相似文献
975.
976.
Identification of miRs-143 and -145 that is associated with bone metastasis of prostate cancer and involved in the regulation of EMT 总被引:1,自引:0,他引:1
Peng X Guo W Liu T Wang X Tu X Xiong D Chen S Lai Y Du H Chen G Liu G Tang Y Huang S Zou X 《PloS one》2011,6(5):e20341
The principal problem arising from prostate cancer (PCa) is its propensity to metastasize to bone. MicroRNAs (miRNAs) play a crucial role in many tumor metastases. The importance of miRNAs in bone metastasis of PCa has not been elucidated to date. We investigated whether the expression of certain miRNAs was associated with bone metastasis of PCa. We examined the miRNA expression profiles of 6 primary and 7 bone metastatic PCa samples by miRNA microarray analysis. The expression of 5 miRNAs significantly decreased in bone metastasis compared with primary PCa, including miRs-508-5p, -145, -143, -33a and -100. We further examined other samples of 16 primary PCa and 13 bone metastases using real-time PCR analysis. The expressions of miRs-143 and -145 were verified to down-regulate significantly in metastasis samples. By investigating relationship of the levels of miRs-143 and -145 with clinicopathological features of PCa patients, we found down-regulations of miRs-143 and -145 were negatively correlated to bone metastasis, the Gleason score and level of free PSA in primary PCa. Over-expression miR-143 and -145 by retrovirus transfection reduced the ability of migration and invasion in vitro, and tumor development and bone invasion in vivo of PC-3 cells, a human PCa cell line originated from a bone metastatic PCa specimen. Their upregulation also increased E-cadherin expression and reduced fibronectin expression of PC-3 cells which revealed a less invasive morphologic phenotype. These findings indicate that miRs-143 and -145 are associated with bone metastasis of PCa and suggest that they may play important roles in the bone metastasis and be involved in the regulation of EMT Both of them may also be clinically used as novel biomarkers in discriminating different stages of human PCa and predicting bone metastasis. 相似文献
977.
978.
von Oheimb PV Albrecht C Riedel F Du L Yang J Aldridge DC Bössneck U Zhang H Wilke T 《PloS one》2011,6(10):e26307
Background
The Tibetan Plateau is not only the highest and largest plateau on earth; it is also home to numerous freshwater lakes potentially harbouring endemic faunal elements. As it remains largely unknown whether these lakes have continuously existed during the Last Glacial Maximum (LGM), questions arise as to whether taxa have been able to exist on the plateau since before the latest Pleistocene, from where and how often the plateau was colonized, and by which mechanisms organisms conquered remote high altitude lentic freshwater systems. In this study, species of the plateau-wide distributed freshwater gastropod genus Radix are used to answer these biogeographical questions.Methodology/Principal Findings
Based on a broad spatial sampling of Radix spp. on the Tibetan Plateau, and phylogenetic analyses of mtDNA sequence data, three probably endemic and one widespread major Radix clade could be identified on the plateau. Two of the endemic clades show a remarkably high genetic diversity, indicating a relatively great phylogenetic age. Phylogeographical analyses of individuals belonging to the most widely distributed clade indicate that intra-plateau distribution cannot be explained by drainage-related dispersal alone.Conclusions/Significance
Our study reveals that Radix spp. persisted throughout the LGM on the Tibetan Plateau. Therefore, we assume the continuous existence of suitable water bodies during that time. The extant Radix diversity on the plateau might have been caused by multiple colonization events combined with a relatively long intra-plateau evolution. At least one colonization event has a Palaearctic origin. In contrast to freshwater fishes, passive dispersal, probably by water birds, might be an important mechanism for conquering remote areas on the plateau. Patterns found in Radix spp. are shared with some terrestrial plateau taxa, indicating that Radix may be a suitable model taxon for inferring general patterns of biotic origin, dispersal and survival on the Tibetan Plateau. 相似文献979.
Background and Objectives
Tumor necrosis factor-α (TNF-α) plays a very important role in the development and progress of cancer. Some TNF-α polymorphisms have been confirmed to increase cancer risks; however, the association between TNF-α-238 polymorphism and cancers remains controversial and ambiguous. The aim of this study is to explore a more precise estimation of its relationship with cancer using meta-analysis.Methods
Electronic searches of several databases were conducted for all publications on the association between this variant and cancer through March 2011. Odds ratios (OR) with 95% confidence intervals (95% CI) were used to access the strength of this association in the random-effect model.Results
Thirty four studies with 34,679 cancer patients and 41,186 healthy controls were included. This meta-analysis showed no significant association between TNF-α-238 polymorphism and cancers (AA+GA vs GG: OR = 1.09, 95%CI = 0.88–1.34). In Caucasian and Asian subgroups, OR values (95% CI) were 1.14 (0.91–1.43) and 0.97 (0.58–1.61), respectively. In the subgroups of cancer type, no significant association was detected. The sensitivity analysis further strengthened the validity of these negative associations. No publication bias was observed in this study.Conclusions
No significant association was found between the TNF-α-238 polymorphism and the risk for cancer. 相似文献980.
The complete mitochondrial DNA genome of the Sichuan taimen (Hucho bleekeri) was determined by the long and accurate polymerase chain reaction (LA-PCR) and primer walking sequence method. The entire mitochondrial genome of this species is 16,997 bp in length, making it the longest among the completely sequenced Salmonidae mitochondrial genomes. It consists of two ribosomal RNA (rRNA) genes, 13 protein-coding genes, 22 transfer RNA (tRNA) genes, and one control region (CR). The gene arrangement, nucleotide composition, and codon usage pattern of the mitochondrial genome are similar to those of other teleosts. A T-type mononucleotide microsatellite and an 82 bp tandem repeat were identified in the control region, which were almost identical among the three H. bleekeri individuals examined. Both phylogenetic analyses based on 12 concatenated protein-coding genes of the heavy strand and on just the control region show that H. bleekeri is a basal species in Salmoninae. In addition, Salmo, Salvelinus and Oncorhynchus all represent monophyletic groups, respectively. All freshwater species occupied basal phylogenetic positions, and also possessed various tandem repeats in their mitochondrial control regions. These results support established phylogenetic relationships among genera in Salmonidae based on morphological and molecular analyses, and are consistent with the hypothesis that Salmonidae evolved from freshwater species. 相似文献