全文获取类型
收费全文 | 1384篇 |
免费 | 73篇 |
出版年
2022年 | 16篇 |
2021年 | 26篇 |
2020年 | 25篇 |
2019年 | 26篇 |
2018年 | 40篇 |
2017年 | 54篇 |
2016年 | 55篇 |
2015年 | 62篇 |
2014年 | 64篇 |
2013年 | 73篇 |
2012年 | 79篇 |
2011年 | 81篇 |
2010年 | 53篇 |
2009年 | 44篇 |
2008年 | 69篇 |
2007年 | 69篇 |
2006年 | 55篇 |
2005年 | 48篇 |
2004年 | 47篇 |
2003年 | 36篇 |
2002年 | 37篇 |
2001年 | 17篇 |
2000年 | 23篇 |
1999年 | 11篇 |
1992年 | 12篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 18篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 16篇 |
1975年 | 13篇 |
1974年 | 18篇 |
1973年 | 14篇 |
1972年 | 7篇 |
1971年 | 10篇 |
1970年 | 10篇 |
1968年 | 6篇 |
1967年 | 8篇 |
1966年 | 5篇 |
排序方式: 共有1457条查询结果,搜索用时 15 毫秒
81.
Several bacterial species carry in their genomes a so-called "mutagenesis" gene cluster encoding ImuB which is similar to Y-family DNA polymerases, and DnaE2 related to the catalytic subunit DnaE of Pol III. Y-family DNA polymerases are known to be involved in stationary-phase mutagenesis and DnaE2 homologues characterized so far have expressed a mutator phenotype. In this study, we raised a question about the involvement of ImuB and DnaE2 in stationary-phase mutagenesis. Here, we show that Pseudomonas putida ImuB and DnaE2 have antagonistic effects on stationary-phase mutagenesis. ImuB facilitated accumulation of stationary-phase mutants up to two-fold. In contrast to that, DnaE2 had no significant effect on emergence of 1-bp deletion mutants and moreover, it acted as an anti-mutator in accumulation of base substitution mutants in starving bacteria. Similar antagonistic effects of DnaE2 and ImuB on mutagenesis appeared also in UV-mutagenesis study. This data distinguishes the DnaE2 of P. putida from its homologues studied in other organisms. 相似文献
82.
Amaral Joo Henrique Fernandes Melack John Michael Barbosa Pedro Maia Borges Alberto V. Kasper Daniele Corts Alicia Cortes Zhou Wencai MacIntyre Sally Forsberg Bruce Rider 《Ecosystems》2022,25(4):911-930
Ecosystems - Extensive floodplains and numerous lakes in the Amazon basin are well suited to examine the role of floodable lands within the context of the sources and processing of carbon within... 相似文献
83.
Natlia Erdens Maron Freitas Emily Ferreira Santos Leonardo Maia Leony ngelo Antnio Oliveira Silva Ramona Tavares Daltro Larissa de Carvalho Medrado Vasconcelos Gabriela Agra Duarte Cristiane Oliveira da Mota Edimilson Domingos Silva Paola Alejandra Fiorani Celedon Nilson Ivo Tonin Zanchin Fred Luciano Neves Santos 《PLoS neglected tropical diseases》2022,16(3)
BackgroundEnzyme-linked immunosorbent assays (ELISA) are generally the chosen test for Chagas disease (CD) diagnosis; however, its performance depends on the antigen preparation adsorbed to the solid phase, which may lead to false-positive results and cross-reactions. The use of chimeric recombinant antigens can overcome this limitation. Four chimeric antigens from Trypanosoma cruzi (IBMP-8.1, IBMP-8.2, IBMP-8.3 and IBMP-8.4) were developed and evaluated in phase I, II and III studies using indirect ELISA as diagnostic platform. However, peroxidase-labeled secondary anti-human IgG antibody, which is employed in indirect ELISAs, limits its use for the detection of species-specific and class-specific antibodies. To overcome this limitation, peroxidase-labeled antigens can be utilized, diagnosing both acute or chronic infection, in a species and immunoglobulin class-independent manner, through the use of a double-antigen sandwich ELISA (DAgS-ELISA). We aimed to evaluate and validate the diagnostic performance of the chimeric antigens IBMP-8.1, IBMP-8.2, IBMP-8.3 and IBMP-8.4 in the DAgS-ELISA platform.Methodology/Principal findingsDAgS-ELISA was optimized by checkerboard titration. In phase I study, 207 positive and 205 negative samples were evaluated. Cross-reactivity to other infections was also assessed using 68 samples. The selected conditions for the tests utilized 25 ng of antigen per well and the conjugate diluted at 1:2,000 for all molecules. In the phase I study, the areas under the curve of IBMP-8.1, IBMP-8.2, IBMP-8.3 and IBMP-8.4 were 98.7%, 99.5%, 98.6% and 98.8%, respectively. Among the positive samples, IBMP-8.1 antigen classified 53 (25.6%) as false negative, IBMP-8.2, 27 (13%), IBMP-8.3, 24 (11.6%) and IBMP-8.4, 43 (20.8%), giving sensitivities of 74.4%, 87%, 88.4% and 79.2%, respectively. The only antigen that did not reach 100% specificity was IBMP-8.3, with 96.6%. IBMP-8.3 was also the only molecule to show cross-reactivity with HTLV.Conclusions/SignificanceDAgS-ELISA is a promising tool for immunodiagnosis, and despite the high AUC values, the performance of this assay was different from the values obtained by our group when using these antigens in the indirect ELISA, for this reason, improvements are being considered to increase the sensitivity of the DAgS-ELISA. 相似文献
84.
Thais Maia Torres Priscila Silva de Almeida-Monteiro Renata Vieira do Nascimento Vanessa Alves Pereira Yasmim Maia Ferreira Jessica Sales Lobato Romulo Roberto Ribeiro Pinheiro Yara Silvino Sales Assis Rubens Montenegro Carminda Sandra Brito Salmito-Vanderley 《Animal Reproduction》2022,19(1)
The action of substances with non-permeable cryoprotectant potential, besides glucose, has not yet been studied for the species Prochilodus brevis. The objective of this work was to evaluate the action of four non-permeable cryoprotectants on this species sperm cryopreservation. Five pools were cryopreserved in a solution of 5% glucose and 10% dimethyl sulfoxide (Me2SO) associated or not (control) with cryoprotectants egg yolk (5, 10 or 12%), soy lecithin (2.5, 7.5 or 10%), sucrose (5, 10 or 20%) and lactose (5, 8 or 15%). After thawing, samples were evaluated for sperm kinetics (total motility, motility duration, velocities, and wobble - WOB), morphology and membrane and DNA integrity. The treatments containing egg yolk improved significantly (P<0.05) results when compared the control for the membrane integrity parameter. When compared to other treatments, egg yolk, at any concentration, presented higher results (P<0.05) for membrane integrity, total motility, curvilinear velocity (VCL) and average path velocity (VAP) parameters. Egg yolk also showed the best results for WOB, but it did not differ from 5% and 8% lactose and 5% and 20% sucrose. Soy lecithin had the lowest percentages of morphologically normal sperm (P<0.05), while the other treatments did not differ from each other. There was no difference regarding DNA integrity data. Thus, 5% egg yolk is indicated as a non-permeable cryoprotectant for P. brevis, in association with 5% glucose and 10% Me2SO. 相似文献
85.
86.
Borecký J Maia IG Costa AD Jezek P Chaimovich H de Andrade PB Vercesi AE Arruda P 《FEBS letters》2001,505(2):240-244
The Arabidopsis thaliana uncoupling protein (UCP) gene was expressed in Escherichia coli and isolated protein reconstituted into liposomes. Linoleic acid-induced H+ fluxes were sensitive to purine nucleotide inhibition with an apparent K(i) (in mM) of 0.8 (GDP), 0.85 (ATP), 0.98 (GTP), and 1.41 (ADP); the inhibition was pH-dependent. Kinetics of AtPUMP1-mediated H+ fluxes were determined for lauric, myristic, palmitic, oleic, linoleic, and linolenic acids. Properties of recombinant AtPUMP1 indicate that it represents a plant counterpart of animal UCP2 or UCP3. This work brings the functional and genetic approaches together for the first time, providing strong support that AtPUMP1 is truly an UCP. 相似文献
87.
da Cunha EF de Castro Ramalho T Bicca de Alencastro R Maia ER 《Journal of biomolecular structure & dynamics》2004,22(2):119-130
There are major differences between the structures of human dihydrofolate reductase (hDHFR) and Mycobacterium tuberculosis dihydrofolate reductase (mtDHFR). These differences may allow us to design more selective mtDHFR inhibitors. In this paper we study the reactions of six different compounds derived from 5-deazapteridine with human and bacterial enzymes. Results suggest that the addition of hydrophobic groups to the aminophenyl ring would increase mtDHFR-inhibitor affinity and selectivity. 相似文献
88.
ApoE and clusterin cooperatively suppress Abeta levels and deposition: evidence that ApoE regulates extracellular Abeta metabolism in vivo 总被引:9,自引:0,他引:9
DeMattos RB Cirrito JR Parsadanian M May PC O'Dell MA Taylor JW Harmony JA Aronow BJ Bales KR Paul SM Holtzman DM 《Neuron》2004,41(2):193-202
Apolipoprotein E (apoE) and clusterin can influence structure, toxicity, and accumulation of the amyloid-beta (Abeta) peptide in brain. Both molecules may also be involved in Abeta metabolism prior to its deposition. To assess this possibility, we compared PDAPP transgenic mice that develop age-dependent Abeta accumulation in the absence of apoE or clusterin as well as in the absence of both proteins. apoE(-/-) and clusterin(-/-) mice accumulated similar Abeta levels but much less fibrillar Abeta. In contrast, apoE(-/-)/clusterin(-/-) mice had both earlier onset and markedly increased Abeta and amyloid deposition. Both apoE(-/-) and apoE(-/-)/clusterin(-/-) mice had elevated CSF and brain interstitial fluid Abeta, as well as significant differences in the elimination half-life of interstitial fluid Abeta measured by in vivo microdialysis. These findings demonstrate additive effects of apoE and clusterin on influencing Abeta deposition and that apoE plays an important role in regulating extracellular CNS Abeta metabolism independent of Abeta synthesis. 相似文献
89.
Aedes aegypti L. is the major vector of dengue fever, an endemic disease in Brazil. In an effort to find effective and affordable ways to control this mosquito, the larvicidal activities of essential oils from nine plants widely found in the Northeast of Brazil were analyzed by measurement of their LC50. The essential oils were extracted by steam distillation and their chemical composition determined by GL-chromatography coupled to mass spectroscopy. The essential oils from Cymbopogon citratus and Lippia sidoides, reported in the literature to have larvicidal properties against A. aegypti, were used for activity comparison. The results show that Ocimum americanum and Ocimum gratissimum have LC50 of 67 ppm and 60 ppm respectively, compared to 63 ppm for L. sidoides and 69 ppm for C. citratus. These results suggest a potential utilization of the essential oil of these two Ocimum species for the control of A. aegypti. 相似文献
90.
Maternal behavior of Koltushi High- and Low-avoidance (KHA and KLA) rats strains was assessed over the prewealing period (days 6-21). Ten litters of each strain were observed during the light phase of the light/dark cycle. In a series of experiments, rat pups were taken from the maternal nest and placed into the opposite corner of the cage. The following parameters of the maternal behavior were recorded: the latency of the first contact with the pups, pup licking, latency of carrying/retrieval of the first pup back to the nest, time of returning to the nest of the whole litter, and mother's spontaneous behavior (grooming and locomotion time) over the course of 10 min of observation. KLA mothers stayed with their pups and took care of them more frequently than KHA mothers during the light phase of the circadian cycle. Time of self-grooming was significantly higher in KHA rats. The highest levels of self-grooming of mothers was registered on the first day of testing. The latency of the first coming to pups after their removal from the nest was lower in KHA rats, but they needed more time to returned all pups to the nest. The experimental evidence suggests that the KHA but not KLA rats with the active coping style and higher stress reactivity display disorders in maternal behavior in a novel situation. 相似文献