全文获取类型
收费全文 | 2308篇 |
免费 | 225篇 |
国内免费 | 1篇 |
出版年
2023年 | 9篇 |
2022年 | 35篇 |
2021年 | 75篇 |
2020年 | 42篇 |
2019年 | 39篇 |
2018年 | 43篇 |
2017年 | 52篇 |
2016年 | 67篇 |
2015年 | 110篇 |
2014年 | 127篇 |
2013年 | 134篇 |
2012年 | 164篇 |
2011年 | 158篇 |
2010年 | 99篇 |
2009年 | 87篇 |
2008年 | 104篇 |
2007年 | 88篇 |
2006年 | 79篇 |
2005年 | 72篇 |
2004年 | 77篇 |
2003年 | 84篇 |
2002年 | 63篇 |
2001年 | 55篇 |
2000年 | 60篇 |
1999年 | 50篇 |
1998年 | 23篇 |
1997年 | 18篇 |
1996年 | 12篇 |
1995年 | 29篇 |
1994年 | 23篇 |
1993年 | 25篇 |
1992年 | 39篇 |
1991年 | 30篇 |
1990年 | 34篇 |
1989年 | 26篇 |
1988年 | 36篇 |
1987年 | 29篇 |
1986年 | 23篇 |
1985年 | 31篇 |
1984年 | 18篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1979年 | 17篇 |
1978年 | 10篇 |
1975年 | 8篇 |
1974年 | 11篇 |
1973年 | 14篇 |
1972年 | 11篇 |
1969年 | 7篇 |
1967年 | 7篇 |
排序方式: 共有2534条查询结果,搜索用时 31 毫秒
41.
Alexander Schouten Jan Roosien Fred A. van Engelen G. A. M. de Jong A. W. M. Borst-Vrenssen Jacoline F. Zilverentant Dirk Bosch Willem J. Stiekema Fred J. Gommers Arjen Schots Jaap Bakker 《Plant molecular biology》1996,30(4):781-793
The effects of subcellular localization on single-chain antibody (scFv) expression levels in transgenic tobacco was evaluated using an scFv construct of a model antibody possessing different targeting signals. For translocation into the secretory pathway a secretory signal sequence preceded the scFv gene (scFv-S). For cytosolic expression the scFv antibody gene lacked such a signal sequence (scFv-C). Also, both constructs were provided with the endoplasmic reticulum (ER) retention signal KDEL (scFv-SK and scFv-CK, respectively). The expression of the different scFv constructs in transgenic tobacco plants was controlled by a CaMV 35S promoter with double enhancer. The scFv-S and scFv-SK antibody genes reached expression levels of 0.01% and 1% of the total soluble protein, respectively. Surprisingly, scFv-CK transformants showed considerable expression of up to 0.2% whereas scFv-C transformants did not show any accumulation of the scFv antibody. The differences in protein expression levels could not be explained by the steady-state levels of the mRNAs. Transient expression assays with leaf protoplasts confirmed these expression levels observed in transgenic plants, although the expression level of the scFv-S construct was higher. Furthermore, these assays showed that both the secretory signal and the ER retention signal were recognized in the plant cells. The scFv-CK protein was located intracellularly, presumably in the cytosol. The increase in scFv protein stability in the presence of the KDEL retention signal is discussed. 相似文献
42.
The structural and functional basis for the kirromycin resistance of mutant EF-Tu species in Escherichia coli. 总被引:5,自引:4,他引:1
下载免费PDF全文
![点击此处可从《The EMBO journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J R Mesters L A Zeef R Hilgenfeld J M de Graaf B Kraal L Bosch 《The EMBO journal》1994,13(20):4877-4885
A structural and functional understanding of resistance to the antibiotic kirromycin in Escherichia coli has been sought in order to shed new light on the functioning of the bacterial elongation factor Tu (EF-Tu), in particular its ability to act as a molecular switch. The mutant EF-Tu species G316D, A375T, A375V and Q124K, isolated by M13mp phage-mediated targeted mutagenesis, were studied. In this order the mutant EF-Tu species showed increasing resistance to the antibiotic as measured by poly(U)-directed poly(Phe) synthesis and intrinsic GTPase activities. The K'd values for kirromycin binding to mutant EF-Tu.GTP and EF-Tu.GDP increased in the same order. All mutation sites cluster in the interface of domains 1 and 3 of EF-Tu.GTP, not in that of EF-Tu.GDP. Evidence is presented that kirromycin binds to this interface of wild-type EF-Tu.GTP, thereby jamming the conformational switch of EF-Tu upon GTP hydrolysis. We conclude that the mutations result in two separate mechanisms of resistance to kirromycin. The first inhibits access of the antibiotic to its binding site on EF-Tu.GTP. A second mechanism exists on the ribosome, when mutant EF-Tu species release kirromycin and polypeptide chain elongation continues. 相似文献
43.
Hanna E. Abboud Giuseppe Grandaliano Massimo Pinzani Thomas Knauss Glenn F. Pierce Fatima Jaffer 《Journal of cellular physiology》1994,158(1):140-150
Platelet-derived growth factor (PDGF) occurs as homodimers or heterodimers of related polypeptide chains PDGF-BB, -AA, and -AB. There are two receptors that bind PDGF, termed alpha and beta. The beta receptor recognizes PDGF B chain and is dimerized in response to PDGF BB. The alpha receptor recognizes PDGF B as well as A chains and can be dimerized by the three dimeric forms of PDGF AA, AB, and BB. To characterize PDGF receptor signaling mechanisms and biologic activities in human mesangial cells (MC), we explored the effects of the three PDGF isoforms on DNA synthesis, phospholipase C activation, and PDGF protooncogene induction. PDGF-BB homodimer and AB heterodimer induced a marked increase in DNA synthesis, activation of phsopholipase C, and autoinduction of PDGF A and B chain mRNAs, whereas PDGF-AA homodimer was without effect. The lack of response to PDGF AA could be accounted for by down regulation of the PDGF-alpha receptor since preincubation of MC with suramin restored PDGF AA-induced DNA synthesis. Ligand binding studies demonstrate specific binding of labeled PDGF BB and AB and to a lower extent PDGF AA isoforms to mesangial cells. These results are consistent with predominant expression of PDGF beta receptor in MC, which is linked to phospholipase-C activation. The potent biologic effects of PDGF-AB heterodimer in cells that express very few alpha receptors and do not respond to PDGF AA are somewhat inconsistent with the currently accepted model of PDGF receptor interaction and suggest the presence of additional mechanisms for PDGF isoform binding and activation. © 1994 Wiley-Liss, Inc. 相似文献
44.
Insect resistance of transgenic plants that express modified Bacillus thuringiensis cryIA(b) and cryIC genes: a resistance management strategy 总被引:7,自引:0,他引:7
45.
Insertion of N-linked glycosylation sites in the variable regions of the human immunodeficiency virus type 1 surface glycoprotein through AAT triplet reiteration. 总被引:7,自引:6,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)
Variable regions with sequence length variation in the human immunodeficiency virus type 1 envelope exhibit an unusual pattern of codon usage with AAT, ACT, and AGT together composing > 70% of all codons used. We postulate that this distribution is caused by insertion of AAT triplets followed by point mutations and selection. Accumulation of the encoded amino acids (asparagine, serine, and threonine) leads to the creation of new N-linked glycosylation sites, which helps the virus to escape from the immune pressure exerted by virus-neutralizing antibodies. 相似文献
46.
Geertruida N. Jonges Ilse M. C. Vogels Klazina S. Bosch Koert P. Dingemans Cornelis J. F. Van Noorden 《Histochemistry and cell biology》1993,100(1):41-51
Metastases in rat liver were generated experimentally by intraportal injection of colon cancer cells to investigate the effects of cancerous growth on the metabolism of surrounding liver tissue. Maximum activities (capacity) of glucose-6-phosphate dehydrogenase, phosphogluconate dehydrogenase, lactate dehydrogenase, succinate dehydrogenase, alkaline phosphatase, 5-nucleotidase, xanthine oxidoreductase, purine nucleoside phosphorylase and adenosine triphosphatase have been determined. Two types of metastases were found, a small type surrounded by stroma and a larger type in direct contact with hepatocytes. Both types affected the adjacent tissue in a similar way suggesting that the interactions were not mediated by stroma. High capacity of the degradation pathway of extracellular purines released from dead cells of either tumours or host tissue was found in stroma and sinusoidal cells. Metastases induced both an increase in the number of Kupffer cells and proliferation of hepatocytes. The distribution pattern in the liver lobulus of most enzymes investigated did not change distinctly. However, activity of alkaline phosphatase, succinate dehydrogenase and phosphogluconate dehydrogenase was increased in hepatocytes directly surrounding metastases. These data imply that the overall metabolic zonation in liver lobuli is not dramatically disturbed by the presence of cancer cells despite the fact that various metabolic processes in liver cells are affected.In honour of Prof. Dr. Z. Lojda for his 65th birthday 相似文献
47.
Imane Nait Irahal Dounia Darif Ismail Guenaou Fouzia Hmimid Fatima azzahra Lahlou Fatima Ez-zahra Ousaid Fatima Abdou-Allah Lamiaa Aitsi Khadija Akarid Noureddine Bourhim 《化学与生物多样性》2023,20(3):e202201169
Type 1 diabetes is characterized by insulin deficiency due to the destruction of pancreatic β cells, leading to hyperglycemia, which in turn induces vascular complications. In the current study, we investigated the effect of intraperitoneal administration of clove essential oil (CEO: 20 mg/kg body weight) on certain oxidative stress and glucose metabolism enzymes, as well as the expression of proinflammatory mediators. Administration of CEO to diabetic rats showed a significant decline in blood glucose levels, total cholesterol, and xanthine oxidase, compared to the streptozotocin group. Furthermore, these treated rats elicited a notable attenuation in the levels of lipid peroxides, and thiols groups in both liver and brain tissues. The activities of antioxidant and metabolic enzymes were reverted to normality in diabetic upon CEO administration. In addition to its protective effects on red blood cell hemolysis, CEO is a potent α-amylase inhibitor with an IC50=298.0±2.75 μg/mL. Also, treatment of diabetic rats with CEO significantly reduced the iNOS expression in the spleen. Our data showed that CEO has potential beneficial effects on diabetes, which can possibly prevent the pathogenesis of diabetic micro- and macrovascular complications. 相似文献
48.
49.
Mapping of genes involved in macromolecular synthesis on the chromosome of Streptomyces coelicolor A3(2). 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
G P van Wezel M J Buttner E Vijgenboom L Bosch D A Hopwood H M Kieser 《Journal of bacteriology》1995,177(2):473-476
The genes for the beta, beta', and seven sigma factor subunits of RNA polymerase, for elongation factors EF-Tu1 and EF-Tu3, and for six rRNA operons were mapped on the combined genetic and physical map of the Streptomyces coelicolor chromosome. Like the previously mapped tRNA genes, the RNA polymerase and rRNA genes map to scattered positions. The lack of rRNA operons in the immediate vicinity of the origin of replication (oriC) and the absence of tRNA genes in any of the rRNA operons are novel features of the Streptomyces chromosome. 相似文献
50.
It is shown that Shigella flexneri maintains genetic control over the modal chain length of the O-antigen polysaccharide chains of its lipopolysaccharide (LPS) molecules because such a distribution is required for virulence. The effect of altering O-antigen chain length on S. flexneri virulence was investigated by inserting a kanamycin (Km)-resistance cassette into the rol gene (controlling the modal O-antigen chain length distribution), and into the rfbD gene, whose product is needed for synthesis of dTDP-rhamnose (the precursor of rhamnose in the O-antigen). The mutations had the expected effect on LPS structure. The rol ::Km mutation was impaired in the ability to elicit keratoconjunctivitis, as determined by the Serény test. The rol ::Km and rfbD ::Km mutations prevented plaque formation on HeLa cells, but neither mutation affected the ability of S. flexneri to invade and replicate in HeLa cells. Microscopy of bacteria-infected HeLa cells stained with fluorescein isothiocyanate (FITC)-phalloidin demonstrated that both the rol ::Km and rfbD ::Km mutants were defective in F-actin tail formation: the latter mutant showed distorted F-actin tails. Plasma-membrane protrusions were occasionally observed. Investigation of the location of IcsA (required for F-actin tail formation) on the cell surface by immunofluorescence and immunogold electron microscopy showed that while most rol mutant bacteria produced little or no cell-surface IcsA, 10% resembled the parental bacterial cell (which had IcsA at one cell pole; the rfbD mutant had IcsA located over its entire cell surface although it was more concentrated at one end of the cell). That the O-antigen chains of the rol ::Km mutant did not mask the IcsA protein was demonstrated by using the endorhamnosidase activity of Sf6c phage to digest the O-antigen chains, and comparing untreated and Sf6c-treated cells by immunofluorescence with anti-IcsA serum. 相似文献