全文获取类型
收费全文 | 4323篇 |
免费 | 340篇 |
国内免费 | 1篇 |
出版年
2023年 | 7篇 |
2022年 | 9篇 |
2021年 | 67篇 |
2020年 | 43篇 |
2019年 | 51篇 |
2018年 | 83篇 |
2017年 | 61篇 |
2016年 | 124篇 |
2015年 | 218篇 |
2014年 | 227篇 |
2013年 | 321篇 |
2012年 | 396篇 |
2011年 | 378篇 |
2010年 | 194篇 |
2009年 | 173篇 |
2008年 | 275篇 |
2007年 | 282篇 |
2006年 | 238篇 |
2005年 | 255篇 |
2004年 | 217篇 |
2003年 | 201篇 |
2002年 | 187篇 |
2001年 | 48篇 |
2000年 | 38篇 |
1999年 | 51篇 |
1998年 | 44篇 |
1997年 | 49篇 |
1996年 | 40篇 |
1995年 | 34篇 |
1994年 | 32篇 |
1993年 | 25篇 |
1992年 | 34篇 |
1991年 | 27篇 |
1990年 | 19篇 |
1989年 | 20篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 25篇 |
1985年 | 11篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1969年 | 4篇 |
1965年 | 4篇 |
1927年 | 4篇 |
1925年 | 4篇 |
排序方式: 共有4664条查询结果,搜索用时 78 毫秒
41.
42.
ΦX174 lysis protein E-mediated lysis of Escherichia coli is characterized by a protein E-specific fusion of the inner and outer membrane and formation of a transmembrane tunnel structure. In order to understand the fusion process, the topology of protein E within the envelope complex of E. coli was investigated. Proteinase K protection studies showed that, during the time course of protein E-mediated lysis process, more of the fusion protein E-FXa-streptavidin gradually became accessible to the protease at the cell surface. These observations postulate a conformational change in protein E during induction of the lysis process by movement of the C-terminal end of the protein throughout the envelope complex from the inner side to the outer side spanning the entire pore and fusing the inner and outer membranes at distinct areas. The initiation mechanism for such a conformational change could be the cis–trans isomerization of proline residues within α-helical membrane-spanning segments. Conversion of proline 21, presumed to be in the membrane-embedded α-helix of protein E, to alanine, glycine, serine and valine, respectively, resulted in lysis-negative E mutant proteins. Proteinase K accessibility studies using streptavidin as a reporter fused to the P21G mutant protein showed that the C-terminal part of the fusion protein is not translocated to the outer side of the membrane, suggesting that this proline residue is essential for the correct folding of protein E within the cell wall complex of E. coli . Oligomerization of protein P21G-StrpA was not disturbed. 相似文献
43.
44.
Liedtke Christa; Polsakiewicz Monika; Hartmann Ingrid; Peters Petra; Volkmann Dieter; Scherer Gnther F.E. 《Journal of experimental botany》1997,48(6):1215-1221
The vacuolar membrane, the tonoplast, is a proteinrich membranehitherto only few proteins in it have been identified. As anapproach for the identification of tonoplast proteins by monoclonalantibodies (MABs), purified tonoplast from cress roots (Lepidiumsativum L.) were used for immunization and plasma membranesas a control membrane to test the absence of antigen. The MABTOP 35 identified a glycoprotein of about 35 kDa in purifiedtonoplast of cress roots. Triton X-114 phase separation showedthat it was a hydrophobic integral membrane protein. In immunocytochemistrythe MAB TOP 35 strongly labelled the vacuolar membrane. Theabsence of cell wall or plasma membrane labelling by TOP 35indicates a distinct biosynthetic pathway of this protein tothe vacuolar membrane in plants. Key words: Immnocytochemistry, Lepidium sativum, monoclonal antibody, secretion, vacuole 相似文献
45.
Reovirus variants selected during persistent infections of L cells contain mutations in the viral S1 and S4 genes and are altered in viral disassembly. 总被引:3,自引:3,他引:0 下载免费PDF全文
J D Wetzel G J Wilson G S Baer L R Dunnigan J P Wright D S Tang T S Dermody 《Journal of virology》1997,71(2):1362-1369
Reoviruses isolated from persistently infected cultures (PI viruses) can grow in the presence of ammonium chloride, a weak base that blocks acid-dependent proteolysis of viral outer-capsid proteins during viral entry into cells. We used reassortant viruses isolated from crosses of wild-type (wt) reovirus strain, type 1 Lang, and three independent PI viruses, L/C, PI 2A1, and PI 3-1, to identify viral genes that segregate with the capacity of PI viruses to grow in cells treated with ammonium chloride. Growth of reassortant viruses in ammonium chloride-treated cells segregated with the S1 gene of L/C and the S4 gene of PI 2A1 and PI 3-1. The S1 gene encodes viral attachment protein sigma1, and the S4 gene encodes outer-capsid protein sigma3. To identify mutations in sigma3 selected during persistent reovirus infection, we determined the S4 gene nucleotide sequences of L/C, PI 2A1, PI 3-1, and four additional PI viruses. The deduced amino acid sequences of sigma3 protein of six of these PI viruses contained a tyrosine-to-histidine substitution at residue 354. To determine whether mutations selected during persistent infection alter cleavage of the viral outer capsid, the fate of viral structural proteins was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after treatment of virions of wt and PI viruses with chymotrypsin in vitro. Proteolysis of PI virus outer-capsid proteins sigma3 and mu1C occurred with faster kinetics than proteolysis of wt virus outer-capsid proteins. These results demonstrate that mutations in either the S1 or S4 gene alter acid-dependent disassembly of the reovirus outer capsid and suggest that increased efficiency of proteolysis of viral outer-capsid proteins is important for maintenance of persistent reovirus infections of cultured cells. 相似文献
46.
Mutations in type 3 reovirus that determine binding to sialic acid are contained in the fibrous tail domain of viral attachment protein sigma1. 总被引:1,自引:1,他引:0 下载免费PDF全文
The reovirus attachment protein, sigma1, determines numerous aspects of reovirus-induced disease, including viral virulence, pathways of spread, and tropism for certain types of cells in the central nervous system. The sigma1 protein projects from the virion surface and consists of two distinct morphologic domains, a virion-distal globular domain known as the head and an elongated fibrous domain, termed the tail, which is anchored into the virion capsid. To better understand structure-function relationships of sigma1 protein, we conducted experiments to identify sequences in sigma1 important for viral binding to sialic acid, a component of the receptor for type 3 reovirus. Three serotype 3 reovirus strains incapable of binding sialylated receptors were adapted to growth in murine erythroleukemia (MEL) cells, in which sialic acid is essential for reovirus infectivity. MEL-adapted (MA) mutant viruses isolated by serial passage in MEL cells acquired the capacity to bind sialic acid-containing receptors and demonstrated a dependence on sialic acid for infection of MEL cells. Analysis of reassortant viruses isolated from crosses of an MA mutant virus and a reovirus strain that does not bind sialic acid indicated that the sigma1 protein is solely responsible for efficient growth of MA mutant viruses in MEL cells. The deduced sigma1 amino acid sequences of the MA mutant viruses revealed that each strain contains a substitution within a short region of sequence in the sigma1 tail predicted to form beta-sheet. These studies identify specific sequences that determine the capacity of reovirus to bind sialylated receptors and suggest a location for a sialic acid-binding domain. Furthermore, the results support a model in which type 3 sigma1 protein contains discrete receptor binding domains, one in the head and another in the tail that binds sialic acid. 相似文献
47.
Many theories of human stereovision are based on feature matching and
the related correspondence problem. In this paper, we present
psychophysical experiments indicating that localized image features
such as Laplacian zerocrossings, intensity extrema, or centroids are
not necessary for binocular depth perception. Smooth one-dimensional
intensity profiles were combined into stereograms with
mirror-symmetric half-images such that these localized image features
were either absent or did not carry stereo information. In a
discrimination task, subjects were asked to distinguish between
stereograms differing only by an exchange of these half-images (ortho-
vs. pseudoscopic stereograms). In a depth ordering task, subjects had
to judge which of the two versions appeared in front. Subjects are
able to solve both tasks even in the absence of the mentioned image
features. The performance is compared to various possible stereo
mechanisms. We conclude that localized image features and the
correspondences between them are not necessary to perceive
stereoscopic depth. One mechanism accounting for our data is
correlation or mean square difference.
Received: 8 February 1994 / Accepted in revised form: 15
September
1994 相似文献
48.
Three endocyclic sulfoximides of the 1-aryl- and 1-alkyl-3-oxo-benzo[d]-isothia (IV)-azole 1-oxide type (1-substituent = 2′-carboxyphenyl, 2′-carbethoxyphenyl, and octyl, respectively) were found to be well resolved on a chiral phase derived from bovine serum albumin (BSA). Selectivities (α) of 1.74, 1.12, and 1.44, respectively, were obtained. The retention behaviour of 1-octyl-3-oxo-benzo[d]isothia(IV)-azole 1-oxide was further investigated in some detail as a function of the mobile phase composition and the elution order was established from optically active material obtained from the enantiopure sulfoxide precursor. An enantiomeric excess of 85.4% was obtained in the cyclocondensation reaction of the octyl-substituted sulfoxide precursor with hydrazoic acid to the corresponding endocyclic sulfoximide. © 1995 Wiley-Liss, Inc. 相似文献
49.
It is generally believed that loop regions in globular proteins, and particularly hypervariable loops in immunoglobulins, can accommodate a wide variety of sequence changes without jeopardizing protein structure or stability. We show here, however, that novel sequences introduced within complementarity determining regions (CDRs) 1 and 3 of the immunoglobulin variable domain REI VL can significantly diminish the stability of the native state of this protein. Besides their implications for the general role of loops in the stability of globular proteins, these results suggest previously unrecognized stability constraints on the variability of CDRs that may impact efforts to engineer new and improved activities into antibodies. 相似文献
50.
Summary Fibroblasts of a patient with Bloom syndrome (GM-1492) were cultured in the presence of either mitomycin C, ethylmethanesulfonate, or 4-nitroquinoline-1-oxide, (4-NQ1-O) and sister chromatid exchange was determined. The mutagens enhanced the sister chromatid exchange rate to different degrees, 4-NQ1-O being the most potent substance. Bloom corrective factor, which is present in normal cell-conditioned culture medium, reduced the spontaneously increased SCE in Bloom syndrome cells by about 20 SCE per metaphase but failed to reduce the additional mutagen-induced SCE increase. These findings indicate that only spontaneously, but not mutagen-indeuced, SCE in Bloom syndrome fibroblasts can be decreased by the Bloom corrective factor. 相似文献