全文获取类型
收费全文 | 2966篇 |
免费 | 378篇 |
出版年
2021年 | 28篇 |
2019年 | 32篇 |
2018年 | 33篇 |
2017年 | 36篇 |
2016年 | 53篇 |
2015年 | 96篇 |
2014年 | 85篇 |
2013年 | 149篇 |
2012年 | 138篇 |
2011年 | 138篇 |
2010年 | 78篇 |
2009年 | 78篇 |
2008年 | 106篇 |
2007年 | 131篇 |
2006年 | 121篇 |
2005年 | 132篇 |
2004年 | 115篇 |
2003年 | 85篇 |
2002年 | 80篇 |
2001年 | 79篇 |
2000年 | 104篇 |
1999年 | 84篇 |
1998年 | 53篇 |
1997年 | 57篇 |
1996年 | 38篇 |
1995年 | 36篇 |
1994年 | 37篇 |
1993年 | 29篇 |
1992年 | 75篇 |
1991年 | 49篇 |
1990年 | 65篇 |
1989年 | 60篇 |
1988年 | 56篇 |
1987年 | 53篇 |
1986年 | 43篇 |
1985年 | 47篇 |
1984年 | 31篇 |
1983年 | 37篇 |
1982年 | 34篇 |
1981年 | 29篇 |
1980年 | 36篇 |
1979年 | 40篇 |
1978年 | 30篇 |
1977年 | 30篇 |
1976年 | 33篇 |
1975年 | 26篇 |
1974年 | 31篇 |
1973年 | 27篇 |
1972年 | 28篇 |
1970年 | 25篇 |
排序方式: 共有3344条查询结果,搜索用时 15 毫秒
81.
Functional role of the glycan cluster of the human immunodeficiency virus type 1 transmembrane glycoprotein (gp41) ectodomain. 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
E Fenouillet I Jones B Powell D Schmitt M P Kieny J C Gluckman 《Journal of virology》1993,67(1):150-160
To examine the role of the glycans of human immunodeficiency virus type 1 transmembrane glycoprotein gp41, conserved glycosylation sites within the env sequence (Asn-621, Asn-630, and Asn-642) were mutated to Gln. The mutated and control wild-type env genes were introduced into recombinant vaccinia virus and used to infect BHK-21 or CD4+ CEM cells. Mutated gp41 appeared as a 35-kDa band in a Western blot (immunoblot), and it comigrated with the deglycosylated form of wild-type gp41. Proteolytic cleavage of the recombinant wild-type and mutant forms of the gp160 envelope glycoprotein precursor was analyzed by pulse-chase experiments and enzyme-linked immunosorbent assay: gp160 synthesis was similar whether cells were infected with control or mutated env-expressing recombinant vaccinia virus, but about 10-fold less cleaved gp120 and gp41 was produced by the mutated construct than the control construct. The rates of gp120-gp41 cleavage at each of the two potential sites appeared to be comparable in the two constructs. By using a panel of antibodies specific for gp41 and gp120 epitopes, it was shown that the overall immunoreactivities of control and mutated gp41 proteins were similar but that reactivity to epitopes at the C and N termini of gp120, as present on gp160 produced by the mutated construct, was enhanced. This was no longer observed for cleaved gp120 in supernatants. Both gp120 proteins, from control and mutated env, were expressed on the cell surface under a cleaved form and could bind to membrane CD4, as determined by quantitative immunofluorescence assay. In contrast, and despite sufficient expression of env products at the cell membrane, gp41 produced by the mutated construct was unable to induce membrane fusion. Therefore, while contradictory results reported in the literature suggest that gp41 individual glycosylation sites are dispensable for the bioactivity and conformation of env products, it appears that such is not the case when the whole gp41 glycan cluster is removed. 相似文献
82.
Summary In asexual reproduction of the water mold,Saprolegnia ferax, four distinct and sequentially produced spores are involved in dispersal, two of which are motile and two of which are nonmotile. Composition of cell surface glycoproteins may be important in dispersal strategies for each of these stages. Binding patterns of fluorescently labelled lectins were investigated to identify differences in glycoproteins of asexually produced dispersal stages. The pattern of lectin binding to zoospores was diverse. FITC-Con A bound to surfaces of zoospores and membranes of the water expulsion vacuole system, indicating the prescence of mannosyl and glucosyl residues. In zoospores incubated for more than 30 min in FITC-WGA and FITC-GS II. which bind N-acetyl glucosamine, fluorescence was sometimes localized in peripheral, intracellular patches. In shorter incubations, secondary zoospores bound these lectins along the groove region where K-bodies were located. Surfaces of cystospores typically bound FITC-WGA, but not FITC-GS II. FITC-GS II, however, bound to empty cystospore walls, probably because reactive sugars were available at the inner surface of the wall. Germ tubes emerging from cystospores bound labelled WGA and GS II, but not Con A. The same lectin binding pattern was found along discharge papilla of primary cystospores, indicating that modifications in cystospore walls associated with direct germination and zoospore discharge were similar. Thus, glycoproteins involved in early establishment of the hyphal system differ from those forming the cell surface of cystospores. Differences in the binding pattern of lectins to zoospores and cystospores highlight differences between cell surface carbohydrates of motile and nonmotile asexual stages.Abbreviations BPA
lectin fromBauhinia purpurea
- C1
primary cystospore
- C2
secondary cystospore
- Con A
concanavalin A, lectin fromCanavalia ensiformis
- DBA
lectin fromDolichos biflorus
- DIC
Nomarski differential interference contrast optics
- DS
dilute salts
- FITC
fluorescein isothiocyanate
- FUC
fucose
- Gal
galactose
- GalNAc
N-acetyl galactosamine
- Glc
glucose
- GlcNAc
N-acetyl glucosamine
- GS I
Griffonia simplicifolia lectin I
- GS II
G. simplicifolia lectin II
- Man
mannose
- MPA
lectin fromMaclura pomifera
- PC
phase contrast optics
- PNA
lectin fromArachis hypogaea
- SBA
soybean agglutinin, lectin fromGlycine max
- UEA-1
lectin fromUlex europaeus
- WGA
wheat germ agglutinin fromTriticum vulgare
- WV
water expulsion vacuole 相似文献
83.
F. Schroeder J. R. Jefferson D. Powell S. Incerpi J. K. Woodford S. M. Colles S. Myers-Payne T. Emge T. Hubbell D. Moncecchi D. R. Prows C. E. Heyliger 《Molecular and cellular biochemistry》1993,123(1-2):73-83
Fatty acid-binding proteins (FABP) are abundant cytosolic proteins whose level is responsive to nutritional, endocrine, and a variety of pathological states. Although FABPs have been investigatedin vitro for several decades, little is known of their physiological function. Liver L-FABP binds both fatty acids and cholesterol. Competitive binding analysis and molecular modeling studies of L-FABP indicate the presence of two ligand binding pockets that accomodate one fatty acid each. One fatty acid binding site is identical to the cholesterol binding site. To test whether these observations obtainedin vitro were physiologically relevant, the cDNA encoding L-FABP was transfected into L-cells, a cell line with very low endogenous FABP and sterol carrier proteins. Uptake of both ligands did not differ between control cells and low expression clones. In contrast, both fatty acid uptake and cholesterol uptake were stimulated in the high expression cells. In high expression cells, uptake of fluorescent cis-parinaric acid was enhanced more than that of trans-parinaric acid. This is consistent with the preferential binding of cis-fatty acids to L-FABP but in contrast to the preferential binding of trans-parinaric acid to the L-cell plasma membrane fatty acid transporter (PMFABP). These data show that the level of cytosolic fatty acids in intact cells can regulate both the extent and specificity of fatty acid uptake. Last, sphingomyelinase treatment of L-cells released cholesterol from the plasma membrane to the cytoplasm and stimulated microsomal acyl-CoA: cholesteryl acyl transferase (ACAT). This process was accelerated in high expression cells. These observations show for the first time in intact cells that L-FABP, a protein most prevalent in liver and intestine where much fat absorption takes place, may have a role in fatty acid and cholesterol absorption.Abbreviations FABP
fatty acid-binding protein
- L-FABP
liver fatty acid-binding protein
- I-FABP
intestinal fatty acid-binding protein
- H-FABP
heart fatty acid-binding protein
- A-FABP
adipocyte fatty acid-binding protein
- PMFABP
plasma membrane fatty acid-binding protein
- SCP-2
sterol carrier protein-2
- Dehydroergosterol (DHE)
d-5,7,9(11),22-ergostatetraene-3b-ol
- cis-parinaric
acid-9Z, 11E, 13E, 15Z-octatetraenoic acid
- trans
parinaric acid, 9E, 11E, 13E, 14E-octatetraenoic acid
- BSA
bovine serum albumin
- KRH
Krebs-Ringer-Henseleit buffer 相似文献
84.
Alycia C. R. Lackey Alyssa C. Murray Nadia A. Mirza Thomas H. Q. Powell 《Journal of evolutionary biology》2023,36(6):882-892
The pace of divergence and likelihood of speciation often depends on how and when different types of reproductive barriers evolve. Questions remain about how reproductive isolation evolves after initial divergence. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 generations) sympatric populations, adapted to different host fruits (hawthorn and apple). We found that flies from both populations were more likely to mate within than between populations. Thus, sexual isolation may play an important role in reducing gene flow allowed by early-acting ecological barriers. We also tested how warmer temperatures predicted under climate change could alter sexual isolation and found that sexual isolation was markedly asymmetric under warmer temperatures – apple males and hawthorn females mated randomly while apple females and hawthorn males mated more within populations than between. Our findings provide a window into the early speciation process and the role of sexual isolation after initial ecological divergence, in addition to examining how environmental conditions could shape the likelihood of further divergence. 相似文献
85.
Neil C. Talbot Caird E. Rexroad Jr. Vernon G. Pursel Anne M. Powell Neil D. Nel 《In vitro cellular & developmental biology. Animal》1993,29(7):543-554
Summary Pig epiblast cells that had been separated from other early embryonic cells were cultured in vitro. A three-step dissection
protocol was used to isolate the epiblast from trophectoderm and primitive endoderm before culturing. Blastocysts collected
at 7 to 8 days postestrus were immunodissected to obtain the inner cell mass (ICM) and destroy trophectodermal cells. The
ICM was cultured for 2 to 3 days on STO feeder cells. The epiblast was then physically dissected free of associated primitive
endoderm. Epiblast-derived cells, grown on STO feeders, produced colonies of small cells resembling mouse embryonic stem cells.
This primary cell morphology changed as the colonies grew and evolved into three distinct colony types (endodermlike, neural
rosette, or complex). Cell cultures derived from these three colony types spontaneously differentiated into numerous specialized
cell types in STO co-culture. These included fibroblasts, endodermlike cells, neuronlike cells, pigmented cells, adipogenic
cells, contracting muscle cells, dome-forming epithelium, ciliated epithelium, tubule-forming epithelium, and a round amoeboid
cell type resembling a plasmacyte after Wright staining. The neuronlike cells, contracting muscle cells, and tubule-forming
epithelium had normal karyotypes and displayed finite or undefined life spans upon long-term STO co-culture. The dome-forming
epithelium had an indefinite life span in STO co-culture and also retained a normal karyotype. These results demonstrate the
in vitro pluripotency of pig epiblast cells and indicate the epiblast can be a source for deriving various specialized cell
cultures or cell lines. 相似文献
86.
Strains of Salmonella enteritidis, Salm. typhimurium and Salm. virchow , carrying different numbers of plasmids, were examined for the ability to multiply in sera. Viable counts were performed to monitor the kinetics of growth of bacteria when in human, chicken and turkey sera. The presence of plasmids in Salm. enteritidis, Salm. typhimurium and Salm. virchow reduced considerably the ability of strains of these serotypes to multiply in serum. SDS-PAGE was used to show that growth of Salm. enteritidis in serum did not involve changes in outer membrane proteins or lipopolysaccharide. It was concluded that the carriage of plasmids may be disadvantageous for the survival in serum of certain common salmonella serotypes. 相似文献
87.
Robert J. Wall Caird E. Rexroad Jr. Anne Powell Avi Shamay Robert McKnight Lothar Hennighausen 《Transgenic research》1996,5(1):67-72
The synthesis of foreign proteins can be targeted to the mammary gland of transgenic animals, thus permitting commercial purification of otherwise unavailable proteins from milk. Genetic regulatory elements from the mouse whey acidic protein (WAP) gene have been used successfully to direct expression of transgenes to the mammary gland of mice, goats and pigs. To extend the practical usefulness of WAP promoter-driven fusion genes and further characterize WAP expression in heterologous species, we introduced a 6.8 kb DNA fragment containing the genomic form of the mouse WAP gene into sheep zygotes. Two lines of transgenic sheep were produced. The transgene was expressed in mammary tissue of both lines and intact WAP was secreted into milk at concentrations estimated to range from 100 to 500 mg/litre. Ectopic WAP gene expression was found in salivary gland, spleen, liver, lung, heart muscle, kidney and bone marrow of one founder ewe. WAP RNA was not detected in skeletal muscle and intestine. These data suggest that unlike pigs, sheep may possess nuclear factors in a variety of tissues that interact with WAP regulatory sequences. Though the data presented are based on only two lines, these findings suggest WAP regulatory sequences may not be suitable as control elements for transgenes in sheep bioreactors. 相似文献
88.
Hepatic nuclear factor 3- and hormone-regulated expression of the phosphoenolpyruvate carboxykinase and insulin-like growth factor-binding protein 1 genes. 总被引:11,自引:4,他引:7
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
89.
Selective Introgression of Paracentric Inversions between Two Sibling Species of the Anopheles Gambiae Complex 总被引:3,自引:0,他引:3
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The Anopheles gambiae complex includes the major vectors of malaria in sub-Saharan Africa where >80% of all world-wide cases occur. These mosquitoes are characterized by chromosomal inversions associated to the speciation process and to intraspecific ecological and behavioral flexibility. It has been postulated that introgressive hybridization has selectively transferred inversions on the second chromosome between A. gambiae and A. arabiensis, the two most important vectors of malaria. Here we directly test this hypothesis with laboratory experiments in which hybrid populations were established and the fate of chromosomal inversions were followed. Consistent with the hypothesis, ``foreign' X chromosomes were eliminated within two generations, while some ``foreign' second chromosomes persisted for the duration of the experiments and, judging from the excess of heterozygotes, established stable heterotic polymorphisms. Only those second chromosome inversions found naturally in the species could be introgressed. 相似文献
90.
Synonymous substitution rates in mitochondrial and nuclear genes of Drosophila were compared. To make accurate comparisons, we considered the following: (1) relative synonymous rates, which do not require
divergence time estimates, should be used; (2) methods estimating divergence should take into account base composition; (3)
only very closely related species should be used to avoid effects of saturation; (4) the heterogeneity of rates should be
examined. We modified the methods estimating synonymous substitution numbers to account for base composition bias. By using
these methods, we found that mitochondrial genes have 1.7–3.4 times higher synonymous substitution rates than the fastest
nuclear genes or 4.5–9.0 times higher rates than the average nuclear genes. The average rate of synonymous transversions was
2.7 (estimated from the melanogaster species subgroup) or 2.9 (estimated from the obscura group) times higher in mitochondrial genes than in nuclear genes. Synonymous transversions in mitochondrial genes occurred
at an approximately equivalent rate to those in the fastest nuclear genes. This last result is not consistent with the hypothesis
that the difference in turnover rates between mitochondrial and nuclear genomes is the major factor determining higher synonymous
substitution rates in mtDNA. We conclude that the difference in synonymous substitution rates is due to a combination of two
factors: a higher transitional mutation rate in mtDNA and constraints on nuclear genes due to selection for codon usage.
Received: 27 November 1996 / Accepted: 8 May 1997 相似文献