全文获取类型
收费全文 | 57380篇 |
免费 | 4809篇 |
国内免费 | 37篇 |
出版年
2023年 | 268篇 |
2022年 | 601篇 |
2021年 | 1102篇 |
2020年 | 819篇 |
2019年 | 895篇 |
2018年 | 1306篇 |
2017年 | 1090篇 |
2016年 | 1732篇 |
2015年 | 2473篇 |
2014年 | 2474篇 |
2013年 | 3054篇 |
2012年 | 3604篇 |
2011年 | 3326篇 |
2010年 | 2052篇 |
2009年 | 1975篇 |
2008年 | 2415篇 |
2007年 | 2376篇 |
2006年 | 2176篇 |
2005年 | 2462篇 |
2004年 | 2445篇 |
2003年 | 2043篇 |
2002年 | 1652篇 |
2001年 | 1520篇 |
2000年 | 1424篇 |
1999年 | 1293篇 |
1998年 | 590篇 |
1997年 | 552篇 |
1996年 | 614篇 |
1995年 | 470篇 |
1994年 | 476篇 |
1993年 | 448篇 |
1992年 | 949篇 |
1991年 | 851篇 |
1990年 | 785篇 |
1989年 | 779篇 |
1988年 | 773篇 |
1987年 | 706篇 |
1986年 | 666篇 |
1985年 | 645篇 |
1984年 | 606篇 |
1983年 | 453篇 |
1982年 | 344篇 |
1981年 | 345篇 |
1980年 | 328篇 |
1979年 | 446篇 |
1978年 | 368篇 |
1977年 | 309篇 |
1975年 | 307篇 |
1974年 | 307篇 |
1973年 | 307篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
E. Modesto P. D. Lampe M. C. Ribeiro D. C. Spray A. C. Campos de Carvalho 《The Journal of membrane biology》1996,154(3):239-249
Membrane fractions highly enriched in chicken lens MIP (MIP28) were found to form ion channels when incorporated into planar
lipid bilayers. The channels displayed prominent unitary conductances of about 60 and 290 pS in symmetric 150 mm KCl solution and were slightly anion selective. For both depolarizing and hyperpolarizing voltages, voltage sensitivity of
the MIP28-induced conductance could be fit by a Boltzmann relation, symmetric around zero mV, with V
0
= 18.5 mV, n= 4.5 and g
min/g
max= 0.17. Channel properties were not appreciably altered by pH in the range of 5.8 to 7, although channel incorporation was
observed to occur more frequently at lower pH values. Calcium, at millimolar concentrations, decreased the channel mean open
time. Partial proteolysis of MIP28 to yield MIP21 did not appreciably affect single-channel conductance or voltage sensitivity
of the reconstituted channels. MIP28 was not phosphorylated by cAMP dependent protein kinase (PKA). Although unitary conductance
and selectivity of the chicken MIP channel are similar to those reported for the bovine MIP (MIP26), the voltage sensitivity
of MIP28 was higher than that of the bovine homologue, and voltage sensitivity of MIP28 was not modulated by treatments previously
shown to affect MIP26 voltage gating (partial proteolysis and protein phosphorylation by PKA: (Ehring et al., 1990). The existence
of such strikingly different functional properties in highly homologous channel isoforms may provide a useful system for exploration
of the structure-function relations of MIP channels.
Received: 27 March 1996/Revised: 5 August 1996 相似文献
992.
Mar Pérez José M. Valpuesta Miguel Medina Esteban Montejo de Garcini Jesús Avila 《Journal of neurochemistry》1996,67(3):1183-1190
Abstract: Paired helical filaments isolated from the brains of patients with Alzheimer's disease are composed of a major protein component, the microtubule-associated protein termed τ, together with other nonprotein components, including heparan, a glycosaminoglycan, the more extensively sulfated form of which is heparin. As some of these nonprotein components may modulate the assembly of τ into filamentous structures, we have analyzed the ability of the whole τ protein or some of its fragments to self-assemble in the presence of heparin. Different τ fragments, all of them containing some sequences of the tubulin-binding motif, can assemble in vitro into filaments. We have also found formation of polymers with the 18-residue-long peptide corresponding to the third tubulin-binding motif of τ. This suggests that the ability of τ for self-assembly could be localized in a short sequence of amino acids present in the tubulin-binding repeats of the τ molecule. 相似文献
993.
Fokko J. Bosker Tessa Y. C. E. de Winter ré A. Klompmakers Herman G. M. Westenberg 《Journal of neurochemistry》1996,66(6):2546-2555
Abstract: The effects of systemic administration of the serotonin (5-hydroxytryptamine) 5-HT1A receptor agonists flesinoxan and 8-hydroxy-2-(di- n -propylamino)tetralin on extracellular 5-HT were measured using microdialysis probes in both median raphe nucleus and dorsal hippocampus. Both 5-HT1A agonists dose-dependently decreased dialysate 5-HT levels from both brain regions. The effects of flesinoxan in the median raphe (0.3 mg/kg) and dorsal hippocampus (1.0 mg/kg) could be blocked by the 5-HT1A receptor antagonist N -[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N -(2-pyridyl)cyclohexane carboxamide trihydrochloride (WAY 100,635) at a dose of 0.05 mg/kg s.c. The antagonist itself had no effect at this dosage. Local perfusion of flesinoxan for 30 min through the dialysis probe into the median raphe region at concentrations of 20, 100, and 1,000 n M resulted in a significant decrease in dialysate 5-HT content from both regions. The effect of 100 n M flesinoxan could be blocked by coperfusion of 1,000 n M WAY 100,635. The data indicate that flesinoxan is a potent 5-HT1A receptor agonist and also support the notion that somatodendritic 5-HT1A autoreceptors regulate both terminal and somatodendritic 5-HT release. 相似文献
994.
Carlos García-Ferris Asuncin de los Ríos Carmen Ascaso Joaquín Moreno 《Journal of phycology》1996,32(6):953-963
Growth of Euglena gracilis Z Pringsheim under photoheterotrophic conditions in a nitrogen-deprived medium resulted in progressive loss of chloroplastic material until total bleaching of the cells occurred. Biochemical analysis and ultrastructural observation of the first stages of the starvation process demonstrated an early lag phase (from 0 to 9 h) in which cells increased in size, followed by a period of cell division, apparently supported by the mobilization of some chloroplastic proteins such as the photosynthetic CO2-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. The degradation of the enzyme started after 9 h of starvation and was preceded by a transient concentration of this protein in pyrenoidal structures. Protein nitrogen and photosynthetic pigments as well as number of chloroplasts per cell decreased during proliferation through mere distribution among daughter cells. However, after 24 h, when cell division had almost ceased, there was a slow but steady decline of photosynthetic pigments. This was paralleled by observable ultrastructural changes including progressive loss of chloroplast structure and accumulation of paramylon granules and lipid globules in the cytoplasm. These findings reinforce the role of chloroplastic materials as a nitrogen source during starvation of E. gracilis in a carbon-rich medium. The excess of ribulose-1,5-bisphosphate carboxylase/oxygenase acts as a first reservoir that, once exhausted, is superseded by the generalized disassembly of the photosynthetic structures, if the adverse environment persists more than 24 h. 相似文献
995.
Mutational analysis identifies functional domains in the influenza A virus PB2 polymerase subunit. 总被引:4,自引:3,他引:1 下载免费PDF全文
A collection of influenza virus PB2 mutant genes was prepared, including N-terminal deletions, C-terminal deletions, and single-amino-acid insertions. These mutant genes, driven by a T7 promoter, were expressed by transfection into COS-1 cells infected with a vaccinia virus encoding T7 RNA polymerase. Mutant proteins accumulated to levels similar to that of wild-type PB2. Immunofluorescence analyses showed that the C-terminal region of the protein is essential for nuclear transport and that internal sequences affect nuclear localization, confirming previous results (J. Mukaijawa and D. P. Nayak, J. Virol. 65:245-253, 1991). The biological activity of these mutants was tested by determining their capacity to (i) reconstitute RNA polymerase activity in vivo by cotransfection with proteins NP, PB1, and PA and a virion-like RNA encoding the cat gene into vaccinia virus T7-infected COS-1 cells and (ii) complete with the wild-type PB2 activity. In addition, when tested at different temperatures in vivo, two mutant PB2 proteins showed a temperature-sensitive phenotype. The lack of interference shown by some N-terminal deletion mutants and the complete interference obtained with a C-terminal deletion mutant encoding only 124 amino acids indicated that this protein domain is responsible for interaction with another component of the polymerase, probably PB1. To further characterize the mutants, their ability to induce in vitro synthesis of viral cRNA or mRNA was tested by using ApG or beta-globin mRNA as a primer. One of the mutants, 1299, containing an isoleucine insertion at position 299, was able to induce cRNA and mRNA synthesis in ApG-primed reactions but required a higher beta-globin mRNA concentration than wild-type PB2 for detection of in vitro synthesis. This result suggested that mutant I299 has diminished cap-binding activity. 相似文献
996.
The Epstein-Barr virus-encoded nuclear antigen EBNA-5 accumulates in PML-containing bodies. 总被引:8,自引:6,他引:2 下载免费PDF全文
L Szekely K Pokrovskaja W Q Jiang H de The N Ringertz G Klein 《Journal of virology》1996,70(4):2562-2568
EBNA-5 is one of the Epstein-Barr virus (EBV)-encoded nuclear proteins required for immortalization of human B lymphocytes. In the nuclei of EBV-transformed lymphoblastoid cell lines EBNA-5 is preferentially targetted to distinct nuclear foci. Previously we have shown (W.Q. Jiang, L. Szekely, V. Wendel-Hansen, N. Ringertz, G. Klein, and A. Rosen, Exp. Cell Res. 197:314-318, 1991) that the same foci also contained the retinoblastoma (Rb) protein. Using a similar double immunofluorescence technique, we now show that these foci colocalize with nuclear bodies positive for PML, the promyelocytic leukemia-associated protein. Artificial spreading of the chromatin by exposure to the forces of fluid surface tension disrupts this colocalization gradually, suggesting that the bodies consist of at least two subcomponents. Heat shock or metabolic stress induced by high cell density leads to the release of EBNA-5 from the PML-positive nuclear bodies and induces it to translocate to the nucleoli. In addition to their presence in nuclear bodies, both proteins are occasionally present in nuclear aggregates and doughnut-like structures in which PML is concentrated in an outer shell. Nuclear bodies with prominent PML staining are seen in resting B lymphocytes. This staining pattern does not change upon EBV infection. In freshly infected cells EBNA-5 antigens are first distributed throughout the nucleoplasm. After a few days intensely staining foci develop. These foci coincide with PML-positive nuclear bodies. At a later stage and in established lymphoblastoid cell lines EBNA-5 is almost exclusively present in the PML-positive nuclear foci. The colocalization is restricted to EBV-infected human lymphoblasts. The data presented indicate that the distinct EBNA-5 foci are not newly formed structures but the result of translocation of the viral protein to a specialized domain present already in the nuclei of uninfected cells. 相似文献
997.
998.
Cristina de Oliveira Dias 《Hydrobiologia》1996,324(3):253-256
Seven monstrilloid species of the genera Thaumaleus and Monstrilla (Copepoda) are reported from the Brazilian coast. Occurrence, distribution and ecological information is given for: T. longispinosum, T. rigidum, T. quadridens, M. recticulata, M. grandis, M. dubia and M. rugosa. 相似文献
999.
By means of gel electrophoresis of tissue extracts we have studied protein polymorphism inEligmodontia typus. The analysis was performed on specimens from five population samples collected at different sites in Patagonia (Argentina). Mean heterozygosity (\-h) and proportion of polymorphic loci (P) were determined on the basis of 19 loci. Considering all individuals as one sample, \-h gave a value of 0.16 and P was 70%. Although these values are much higher than those reported for most rodent species, they are very similar to those obtained by us for four species of the genusCalomys and forGraomys griseoflavus. There is a striking genetic identity (IN=0.99) among populations from regions with different environmental conditions, indicating that the species possesses a common genic pool. Genetic distance with other species of the Phyllotini was estimated. DN was lower betweenE. typus andCalomys (mean DN=0.88) than betweenE. typus andGraomys griseoflavus (DN=1.01). The high morphological similarity between these last two species, especially regarding those characters related to desert life adaptation, could be assigned, at least in part, to convergent evolution. 相似文献
1000.
Sigma-B, a putative operon encoding alternate sigma factor of Staphylococcus aureus RNA polymerase: molecular cloning and DNA sequencing. 总被引:4,自引:1,他引:3 下载免费PDF全文
We have identified a gene cluster located on the chromosomal SmaI I fragment of a highly methicillin resistant strain of Staphylococcus aureus, consisting of four open reading frames (ORFs), named after the number of deduced amino acid residues, in the sequential order orf333-orf108-orf159-orf256. The gene cluster showed close similarities to the Bacillus subtilis sigB operon both in overall organization and in primary sequences of the gene products. The complete gene cluster (provisionally named sigma-B or sigB) was preceded by an sigmaA-like promoter (PA) and had an internal sigmaB-like promoter sequence (PB) between orf333 and orf108, suggesting a complex regulatory mechanism. The polypeptides encoded by orf333, -108, -159, and -256 showed 62, 67, 71, and 77% homologies, respectively, with the RsbU, RsbV, RsbW, and SigB polypeptides encoded by the B. subtilis sigB operon. A Tn551 insertional mutant, RUSA168 (insert in orf256 of the staphylococcal sigma-B operon), showed drastic reduction in methicillin resistance (decrease in MIC from 1,600 microg ml-1 to 12 to 25 microg ml-1off 相似文献