全文获取类型
收费全文 | 38387篇 |
免费 | 2482篇 |
国内免费 | 15篇 |
专业分类
40884篇 |
出版年
2023年 | 224篇 |
2022年 | 520篇 |
2021年 | 907篇 |
2020年 | 518篇 |
2019年 | 673篇 |
2018年 | 939篇 |
2017年 | 784篇 |
2016年 | 1315篇 |
2015年 | 1992篇 |
2014年 | 2137篇 |
2013年 | 2928篇 |
2012年 | 3349篇 |
2011年 | 3191篇 |
2010年 | 1949篇 |
2009年 | 1680篇 |
2008年 | 2383篇 |
2007年 | 2314篇 |
2006年 | 2054篇 |
2005年 | 1851篇 |
2004年 | 1739篇 |
2003年 | 1677篇 |
2002年 | 1514篇 |
2001年 | 304篇 |
2000年 | 208篇 |
1999年 | 296篇 |
1998年 | 378篇 |
1997年 | 253篇 |
1996年 | 251篇 |
1995年 | 230篇 |
1994年 | 224篇 |
1993年 | 219篇 |
1992年 | 135篇 |
1991年 | 144篇 |
1990年 | 147篇 |
1989年 | 109篇 |
1988年 | 100篇 |
1987年 | 90篇 |
1986年 | 70篇 |
1985年 | 95篇 |
1984年 | 95篇 |
1983年 | 71篇 |
1982年 | 83篇 |
1981年 | 80篇 |
1980年 | 79篇 |
1979年 | 72篇 |
1978年 | 39篇 |
1977年 | 50篇 |
1976年 | 36篇 |
1975年 | 38篇 |
1973年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
DePaoli HC Brito MS Quiapim AC Teixeira SP Goldman GH Dornelas MC Goldman MH 《The New phytologist》2011,190(4):882-895
A cDNA encoding a small lysine-rich protein of unknown function was identified in a tobacco (Nicotiana tabacum) stigma/style suppression subtractive hybridization cDNA library. After its characterization, the corresponding gene was designated stigma/style cell cycle inhibitor 1 (SCI1). Fluorescence microscopy with an SCI1-GFP protein fusion demonstrated its nuclear localization, which was confined to the interchromatic region. Real-time RT-PCR and in situ hybridization experiments showed that SCI1 is stigma/style-specific and developmentally regulated. SCI1 RNAi knockdown and overexpression plants had stigmas/styles with remarkably enlarged and reduced areas, respectively, which was attributable to differences in cell numbers. These results indicate that SCI1 is a tissue-specific negative cell cycle regulator. The differences in cell division had an effect on the timing of the differentiation of the stigmatic papillar cells, suggesting that their differentiation is coupled to stigma cell divisions. This is consistent with a role for SCI1 in triggering differentiation through cell proliferation control. Our results revealed that SCI1 is a novel tissue-specific gene that controls cell proliferation/differentiation, probably as a component of a developmental signal transduction pathway. 相似文献
993.
Lefrançois LH Pujol C Bodier CC Teixeira-Gomez AP Drobecq H Rosso ML Raze D Dias AA Hugot JP Chacon O Barletta RG Locht C Vidal Pessolani MC Biet F 《Microbes and infection / Institut Pasteur》2011,13(6):585-594
Mycobacterium avium subsp. paratuberculosis (Map) causes a chronic enteric disease in ruminants, called paratuberculosis or Johne's disease. The current model proposes that after ingestion by the host, Map crosses the intestinal barrier via internalization by the M cells. Experimental observations suggest, however, that Map may also transcytose the intestinal wall via the enterocytes, but the mechanisms involved in this process remain poorly understood. Cytoadherence assays performed on epithelial cells with Map revealed that the addition of laminin to the cell culture increases adhesion. A Map protein was isolated by heparin-Sepharose chromatography and identified as a laminin-binding protein like. The gene encoding this protein named Lbp/Hlp was identified in the Map genome sequence at locus MAP3024 (annotated Hup B). The deduced Map Lbp/Hlp amino acid sequence reveals 80% identity with that reported for other mycobacteria. The C-terminal domain involved in adhesion is mainly composed of arginine and lysine residues modified by methylation. In vitro tests demonstrated that recombinant Lbp/Hlp binds laminin, heparin, collagen and epithelial cells. Interestingly, we found that this adhesin corresponds to the antigen described as the target of pANCA and serum antibodies of patients with Crohn's disease. 相似文献
994.
de Freitas VL da Silva SC Sartori AM Bezerra RC Westphalen EV Molina TD Teixeira AR Ibrahim KY Shikanai-Yasuda MA 《PLoS neglected tropical diseases》2011,5(8):e1277
Background
Reactivation of chronic Chagas disease, which occurs in approximately 20% of patients coinfected with HIV/Trypanosoma cruzi (T. cruzi), is commonly characterized by severe meningoencephalitis and myocarditis. The use of quantitative molecular tests to monitor Chagas disease reactivation was analyzed.Methodology
Polymerase chain reaction (PCR) of kDNA sequences, competitive (C-) PCR and real-time quantitative (q) PCR were compared with blood cultures and xenodiagnosis in samples from 91 patients (57 patients with chronic Chagas disease and 34 with HIV/T. cruzi coinfection), of whom 5 had reactivation of Chagas disease and 29 did not.Principal Findings
qRT-PCR showed significant differences between groups; the highest parasitemia was observed in patients infected with HIV/T. cruzi with Chagas disease reactivation (median 1428.90 T. cruzi/mL), followed by patients with HIV/T. cruzi infection without reactivation (median 1.57 T. cruzi/mL) and patients with Chagas disease without HIV (median 0.00 T. cruzi/mL). Spearman''s correlation coefficient showed that xenodiagnosis was correlated with blood culture, C-PCR and qRT-PCR. A stronger Spearman correlation index was found between C-PCR and qRT-PCR, the number of parasites and the HIV viral load, expressed as the number of CD4+ cells or the CD4+/CD8+ ratio.Conclusions
qRT-PCR distinguished the groups of HIV/T. cruzi coinfected patients with and without reactivation. Therefore, this new method of qRT-PCR is proposed as a tool for prospective studies to analyze the importance of parasitemia (persistent and/or increased) as a criterion for recommending pre-emptive therapy in patients with chronic Chagas disease with HIV infection or immunosuppression. As seen in this study, an increase in HIV viral load and decreases in the number of CD4+ cells/mm3 and the CD4+/CD8+ ratio were identified as cofactors for increased parasitemia that can be used to target the introduction of early, pre-emptive therapy. 相似文献995.
Munawar A Trusch M Georgieva D Spencer P Frochaux V Harder S Arni RK Duhalov D Genov N Schlüter H Betzel C 《Molecular bioSystems》2011,7(12):3298-3307
Snake venom peptidomes are valuable sources of pharmacologically active compounds. We analyzed the peptidic fractions (peptides with molecular masses < 10,000 Da) of venoms of Vipera ammodytes meridionalis (Viperinae), the most toxic snake in Europe, and Bothrops jararacussu (Crotalinae), an extremely poisonous snake of South America. Liquid chromatography/mass spectrometry (LC/MS), direct infusion electrospray mass spectrometry (ESI-MS) and matrix-assisted desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) were applied to characterize the peptides of both snake venoms. 32 bradykinin-potentiating peptides (BPPs) were identified in the Crotalinae venom and their sequences determined. 3 metalloproteinase inhibitors, 10 BPPs and a Kunitz-type inhibitor were observed in the Viperinae venom peptidome. Variability in the C-terminus of homologous BPPs was observed, which can influence the pharmacological effects. The data obtained so far show a subfamily specificity of the venom peptidome in the Viperidae family: BPPs are the major peptide component of the Crotalinae venom peptidome lacking Kunitz-type inhibitors (with one exception) while the Viperinae venom, in addition to BPPs, can contain peptides of the bovine pancreatic trypsin inhibitor family. We found indications for a post-translational phosphorylation of serine residues in Bothrops jararacussu venom BPP (S[combining low line]QGLPPGPPIP), which could be a regulatory mechanism in their interactions with ACE, and might influence the hypotensive effect. Homology between venom BPPs from Viperidae snakes and venom natriuretic peptide precursors from Elapidae snakes suggests a structural similarity between the respective peptides from the peptidomes of both snake families. The results demonstrate that the venoms of both snakes are rich sources of peptides influencing important physiological systems such as blood pressure regulation and hemostasis. The data can be used for pharmacological and medical applications. 相似文献
996.
Ramirez-Llodra E Tyler PA Baker MC Bergstad OA Clark MR Escobar E Levin LA Menot L Rowden AA Smith CR Van Dover CL 《PloS one》2011,6(8):e22588
The deep sea, the largest ecosystem on Earth and one of the least studied, harbours high biodiversity and provides a wealth of resources. Although humans have used the oceans for millennia, technological developments now allow exploitation of fisheries resources, hydrocarbons and minerals below 2000 m depth. The remoteness of the deep seafloor has promoted the disposal of residues and litter. Ocean acidification and climate change now bring a new dimension of global effects. Thus the challenges facing the deep sea are large and accelerating, providing a new imperative for the science community, industry and national and international organizations to work together to develop successful exploitation management and conservation of the deep-sea ecosystem. This paper provides scientific expert judgement and a semi-quantitative analysis of past, present and future impacts of human-related activities on global deep-sea habitats within three categories: disposal, exploitation and climate change. The analysis is the result of a Census of Marine Life--SYNDEEP workshop (September 2008). A detailed review of known impacts and their effects is provided. The analysis shows how, in recent decades, the most significant anthropogenic activities that affect the deep sea have evolved from mainly disposal (past) to exploitation (present). We predict that from now and into the future, increases in atmospheric CO(2) and facets and consequences of climate change will have the most impact on deep-sea habitats and their fauna. Synergies between different anthropogenic pressures and associated effects are discussed, indicating that most synergies are related to increased atmospheric CO(2) and climate change effects. We identify deep-sea ecosystems we believe are at higher risk from human impacts in the near future: benthic communities on sedimentary upper slopes, cold-water corals, canyon benthic communities and seamount pelagic and benthic communities. We finalise this review with a short discussion on protection and management methods. 相似文献
997.
Kasperavičiūtė D Catarino CB Chinthapalli K Clayton LM Thom M Martinian L Cohen H Adalat S Bockenhauer D Pope SA Lench N Koltzenburg M Duncan JS Hammond P Hennekam RC Land JM Sisodiya SM 《PloS one》2011,6(8):e23182
Background
Patients with epilepsy often suffer from other important conditions. The existence of such co-morbidities is frequently not recognized and their relationship with epilepsy usually remains unexplained.Methodology/Principal Findings
We describe three patients with common, sporadic, non-syndromic epilepsies in whom large genomic microdeletions were found during a study of genetic susceptibility to epilepsy. We performed detailed gene-driven clinical investigations in each patient. Disruption of the function of genes in the deleted regions can explain co-morbidities in these patients.Conclusions/Significance
Co-morbidities in patients with epilepsy can be part of a genomic abnormality even in the absence of (known) congenital malformations or intellectual disabilities. Gene-driven phenotype examination can also reveal clinically significant unsuspected condition. 相似文献998.
Testa MP Alvarado O Wournell A Lee J Guilford FT Henriksen SH Phillips TR 《Journal of visualized experiments : JoVE》2011,(53):e2841
An often-suggested mechanism of virus induced neuronal damage is oxidative stress. Astrocytes have an important role in controlling oxidative stress of the Central Nervous System (CNS). Astrocytes help maintain a homeostatic environment for neurons as well as protecting neurons from Reactive Oxygen Species (ROS). CM-H2DCFDA is a cell-permeable indicator for the presence of ROS. CM-H(2)DCFDA enters the cell as a non-fluorescent compound, and becomes fluorescent after cellular esterases remove the acetate groups, and the compound is oxidized. The number of cells, measured by flow cytometry, that are found to be green fluorescing is an indication of the number of cells that are in an oxidative state. CM-H(2)DCFDA is susceptible to oxidation by a large number of different ROS. This lack of specificity, regarding which ROS can oxidize CM-H(2)DCFDA, makes this compound a valuable regent for use in the early stages of a pathogenesis investigation, as this assay can be used to screen for an oxidative cellular environment regardless of which oxygen radical or combination of ROS are responsible for the cellular conditions. Once it has been established that ROS are present by oxidation of CM-H(2)DCFDA, then additional experiments can be performed to determine which ROS or combination of ROSs are involved in the particular pathogenesis process. The results of this study demonstrate that with the addition of hydrogen peroxide an increase in CM-H(2)DCFDA fluorescence was detected relative to the saline controls, indicating that this assay is a valuable test for detecting an oxidative environment within G355-5 cells, a feline astrocyte cell line. 相似文献
999.
Medicago truncatula mtpt4 mutants reveal a role for nitrogen in the regulation of arbuscule degeneration in arbuscular mycorrhizal symbiosis 总被引:1,自引:0,他引:1
Javot H Penmetsa RV Breuillin F Bhattarai KK Noar RD Gomez SK Zhang Q Cook DR Harrison MJ 《The Plant journal : for cell and molecular biology》2011,68(6):954-965
Plants acquire essential mineral nutrients such as phosphorus (P) and nitrogen (N) directly from the soil, but the majority of the vascular plants also gain access to these mineral nutrients through endosymbiotic associations with arbuscular mycorrhizal (AM) fungi. In AM symbiosis, the fungi deliver P and N to the root through branched hyphae called arbuscules. Previously we identified MtPT4, a Medicago truncatula phosphate transporter located in the periarbuscular membrane that is essential for symbiotic phosphate transport and for maintenance of the symbiosis. In mtpt4 mutants arbuscule degeneration occurs prematurely and symbiosis fails. Here, we show that premature arbuscule degeneration occurs in mtpt4 mutants even when the fungus has access to carbon from a nurse plant. Thus, carbon limitation is unlikely to be the primary cause of fungal death. Surprisingly, premature arbuscule degeneration is suppressed if mtpt4 mutants are deprived of nitrogen. In mtpt4 mutants with a low N status, arbuscule lifespan does not differ from that of the wild type, colonization of the mtpt4 root system occurs as in the wild type and the fungus completes its life cycle. Sulphur is another essential macronutrient delivered to the plant by the AM fungus; however, suppression of premature arbuscule degeneration does not occur in sulphur-deprived mtpt4 plants. The mtpt4 arbuscule phenotype is strongly correlated with shoot N levels. Analyses of an mtpt4-2 sunn-1 double mutant indicates that SUNN, required for N-mediated autoregulation of nodulation, is not involved. Together, the data reveal an unexpected role for N in the regulation of arbuscule lifespan in AM symbiosis. 相似文献
1000.
Methicillin-resistant Staphylococcus aureus (MRSA) is a significant pathogen in both human and veterinary medicine. The importance of companion animals as reservoirs of human infections is currently unknown. The companion animals of 49 MRSA-infected outpatients (cases) were screened for MRSA carriage, and their bacterial isolates were compared with those of the infected patients using Pulsed-Field Gel Electrophoresis (PFGE). Rates of MRSA among the companion animals of MRSA-infected patients were compared to rates of MRSA among companion animals of pet guardians attending a “veterinary wellness clinic” (controls). MRSA was isolated from at least one companion animal in 4/49 (8.2%) households of MRSA-infected outpatients vs. none of the pets of the 50 uninfected human controls. Using PFGE, patient-pets MRSA isolates were identical for three pairs and discordant for one pair (suggested MRSA inter-specie transmission p-value = 0.1175). These results suggest that companion animals of MRSA-infected patients can be culture-positive for MRSA, representing a potential source of infection or re-infection for humans. Further studies are required to better understand the epidemiology of MRSA human-animal inter-specie transmission. 相似文献