全文获取类型
收费全文 | 1902篇 |
免费 | 166篇 |
国内免费 | 72篇 |
出版年
2023年 | 19篇 |
2022年 | 22篇 |
2021年 | 42篇 |
2020年 | 20篇 |
2019年 | 43篇 |
2018年 | 31篇 |
2017年 | 41篇 |
2016年 | 43篇 |
2015年 | 99篇 |
2014年 | 97篇 |
2013年 | 106篇 |
2012年 | 148篇 |
2011年 | 143篇 |
2010年 | 78篇 |
2009年 | 76篇 |
2008年 | 98篇 |
2007年 | 87篇 |
2006年 | 82篇 |
2005年 | 68篇 |
2004年 | 90篇 |
2003年 | 76篇 |
2002年 | 74篇 |
2001年 | 69篇 |
2000年 | 50篇 |
1999年 | 43篇 |
1998年 | 24篇 |
1997年 | 18篇 |
1996年 | 23篇 |
1995年 | 11篇 |
1994年 | 16篇 |
1993年 | 14篇 |
1992年 | 22篇 |
1991年 | 14篇 |
1990年 | 25篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1981年 | 10篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 12篇 |
1975年 | 15篇 |
1974年 | 8篇 |
1972年 | 7篇 |
1969年 | 7篇 |
排序方式: 共有2140条查询结果,搜索用时 15 毫秒
21.
Binding sites for monoclonal antibodies and for mRNPs on SV40 large T-antigen determined with a cleavage map 总被引:3,自引:0,他引:3
Immune complexes of simian virus 40 large T-antigen with monoclonal papovavirus protein antibodies PAb 416, PAb 402, or PAb 423 were bound to protein-A-Sepharose and then cleaved into discrete fragments by limited tryptic proteolysis. PAb 402 protected a specific cleavage site, located approximately within amino acid residues 450-500, from tryptic proteolysis; PAb 423 protected another site within residues 675-699. As shown by immunoblotting, 125I-labeled PAb 416 was bound to a 17-kDa N-terminal fragment of large T-antigen (amino acid residues 1-130), and PAb 423 was bound to several overlapping fragments derived from the C terminus of large T-antigen. These monoclonal antibodies were then used as accessibility probes to study the interaction of mRNPs with cytoplasmic large T-antigen. Whereas small T-antigen and nuclear large T-antigen were fully immunoreactive, cytoplasmic large T-antigen reacted poorly with PAb 402 or polyclonal antibodies unless the mRNP moiety was removed by treatment with EDTA/RNase A. In contrast, mRNP/T-antigen complexes were fully immunoreactive with PAb 416 or PAb 423 and did not require treatment with EDTA/RNase A. The results suggest that the binding site of PAb 402 is blocked due to the interaction with mRNPs whereas the N-terminal binding site of PAb 416 and the C-terminal binding site of PAb 423 remain accessible to antibodies. 相似文献
22.
Tai Te Wu 《Bulletin of mathematical biology》1969,31(2):395-402
In view of the possible changes of the structure of DNA at different relative humidities, a similar model for tRNA is proposed. In the native form, the sugar phosphate backbone folds repeatedly on itself resulting in a circular rod structure of about 50 Å in length and about 25 Å in diameter. Both the amino-acyl end and the anticodon site are surrounded by segments of base sequences common to different tRNA’s. 相似文献
23.
Cold-sensitive Mutations in Salmonella typhimurium Which Affect Ribosome Synthesis 总被引:31,自引:14,他引:17 下载免费PDF全文
A number of mutations (45) expressed as cold-sensitive conditional lethal pheno-types were screened by transduction for their linkage to the streptomycin-resistance locus; 7 showed such linkage. Of these, two were studied in greater detail. The sedimentation profiles of ribosomes from cultures grown at low temperature differed from wild type and from one another. Both mutants lost ribonucleic acid control at low temperature. It is suggested that a high proportion of mutants expressing a cold-sensitive phenotype harbor mutations in genes affecting ribosome synthesis or regulation. 相似文献
24.
Tai Te Wu 《Bulletin of mathematical biology》1968,30(4):687-700
The conformation of a deoxyribonucleotide unit in a deoxyribonucleic acid molecule can be defined by six angles, each of which
specifies the relative orientations between two groups of atoms adjacent to a covalent bond. With the assumption that these
atoms are hard spheres with fixed van der Waal radii, conformations are sought to minimize their overlapping. The additional
requirement of the polymer having a periodic structure further reduces the allowable conformations. 相似文献
25.
Tai C. Chen Norman P. Curthoys Carl F. Lagenaur Jules B. Puschett 《In vitro cellular & developmental biology. Plant》1989,25(8):714-722
Summary Proximal tubules were prepared from rat kidney cortex by collagenase digestion and purified by percoll gradient centrifugation.
Their enrichment was estimated by comparing the specific activities of various cell-specific enzymes in homogenates of renal
cortex and of the isolated tubules. The tubules were cultured in a 50:50 mixture of Dulbecco’s modified Eagle’s and Ham’s
F12 media supplemented with insulin, transferrin, epidermal growth factor, hydrocortisone, and prostaglandin E1. After 2 to 3 d an extensive outgrowth of epithelial cells developed from the attached tubules. After 5 to 7 d near confluent
monolayers were obtained. Hormonal responsiveness, marker enzyme activities, and transport properties were determined to further
characterize the primary cultures. The cultured cells exhibited increased cyclic AMP production in response to parathyroid
hormone but not calcitonin or vasopressin, consistent with the absence of cells derived from distal and collecting tubules.
The cells also retained significant levels of 25-hydroxyvitamin D3-lα-hydroxylase, alkaline phosphatase, and ψ-glytamyltranspeptidase, three enzymes that are primarily associated with the
proximal tubule. The cultured epithelial cells also exhibit a Na+-dependent phosphate and glucose transport systems. Therefore, the cells retain many functional properties that are characteristic
of proximal tubules. Thus, the primary cultures should be suitable for the study of processes that occur specifically within
this segment of the rat nephron.
This work was supported in part by the Veterans Administration (JBP), Washington, DC, by grant DK-37124 (NPC) from the National
Institutes of Health, Bethesda, MD, and by grant BNS-86-17004 (CFL) from the National Science Foundation, Washington, DC. 相似文献
26.
Recent years have seen the convergence of both genetic and biochemical approaches in the study of protein translocation inE. coli. The powerful combination of these approaches is exemplified in the use of anin vitro protein synthesis-protein translocaltion system to analyze the role of genetically defined components of the protein translocation machinery. We describe in this review recent results focusing on the function of thesecA, secB, andsecY gene products and the demonstration of their requirement forin vitro protein translocation. The SecA protein was recently shown to possess ATPase activity and was proposed to be a component of the translocation ATPase. We present a speculative working model whereby the translocator complex is composed of the integral membrane proteins SecY, SecD, SecE, and SecF, forming an aqueous channel in the cytoplasmic membrane, and the tightly associated peripheral membrane protein SecA functioning as the catalytic subunit of the translocator or protein-ATPase. 相似文献
27.
Y.-H. Tai J. Flick S.A. Levine J.L. Madara G.W.G. Sharp M. Donowitz 《The Journal of membrane biology》1996,149(1):71-79
Elevation in intracellular Ca2+ acting via protein kinase C (PKC) is shown to regulate tight junction resistance in T84 cells, a human colon cancer line and a model Cl− secretory epithelial cell. The Ca2+ ionophore A23187, which was used to increase the intracellular Ca2+ concentration, caused a decrease in tight junction resistance in a concentration- and time-dependent manner. Dual Na+/mannitol serosal-to-mucosal flux analysis performed across the T84 monolayers treated with 2 μm A23187 revealed that A23187 increased both fluxes and that in the presence of ionophore there was a linear relationship between
the Na+ and mannitol fluxes with a slope of 56.4, indicating that the decrease in transepithelial resistance was due to a decrease
in tight junction resistance. Whereas there was no effect of 0.1 μm A23187, 1 or 2 μm produced a 55% decrease in baseline resistance in 1 hr and 10 μm decreased resistance more than 80%. The A23187-induced decrease in tight junction resistance was partially reversible by
washing 3 times with a Ringer's-HCO3 solution containing 1% BSA. The A23187 effect on resistance was dependent on intracellular Ca2+; loading the T84 cells with the intracellular Ca2+ chelator BAPTA significantly reduced the decrease in tight junction resistance caused by A23187. This intracellular Ca2+ effect was mediated by protein kinase C and not calmodulin. While the protein kinase C antagonist H-7 totally prevented the
action of A23187 on tight junction resistance, the Ca2+/calmodulin inhibitor W13 did not have any effect. Sphingosine, another inhibitor of PKC, partially reduced the A23187-induced
decline in tight junction resistance. The PKC agonist PMA mimicked the A23187 effect on resistance, although the effect was
delayed up to 1 hr after exposure. In addition, however, PMA also caused an earlier increase in resistance, indicating it
had an additional effect in addition to mimicking the effect of elevating Ca2+. The effects of a phospholipase inhibitor (mepacrine) and of inhibitors of arachidonic acid metabolism (indomethacin for
the cyclooxygenase pathway, NDGA for the lipoxygenase pathway, and SKF 525A for the epoxygenase pathway) on the A23187 action
were also examined. None of these agents altered the A23187-induced decrease in resistance. Monolayers exposed to 2 μm A23187 for 1 hr were stained with fluorescein conjugated phalloidin, revealing that neighboring cells did not part one from
another and that A23187 did not have a detectable effect on distribution of F-actin in the perijunctional actomyosin ring.
The results indicate that elevation in intracellular Ca2+ decreases tight junction resistance in the T84 monolayer, acting through protein kinase C by a mechanism which does not involve visible changes in the perijunctional actomyosin
ring.
Received: 14 July 1995/Revised: 25 September 1995 相似文献
28.
Toshio Ariga Shama Bhat Takashi Kanda Masanaga Yamawaki Tadashi Tai Yasunori Kushi Takeshi Kasama Shizuo Handa Robert K. Yu 《Glycoconjugate journal》1996,13(2):135-145
We analysed the glycolipid composition of glioma cells (N-370 FG cells), which are derived from a culture of transformed human fetal glial cells. The neutral and acidic glycolipid fractions were isolated by column chromatography on DEAE-Sephadex and analysed by high-performance thin-layer chromatography (HPTLC). The neutral glycolipid fraction contained 1.6 µg of lipid-bound glucose/galactose per mg protein and consisted of GlcCer (11.4% of total neutral glycolipids), GalCer (21.5%), LacCer (21.4%), Gb4 (21.1%), and three unknown neutral glycolipids (23%). These unknown glycolipids were characterized as Lewisx (fucosylneolactonorpentaosyl ceramide; Lex), difucosylneolactonorhexaosyl ceramide (dimeric Lex), and neolactonorhexaosyl ceramide (nLc6) by an HPTLC-overlay method for glycolipids using specific mouse anti-glycolipid antibodies against glycolipid and/or liquid-secondary ion (LSI) mass spectrometry. The ganglioside fraction contained 0.6 µg of lipid-bound sialic acid per mg protein with GD1a as the predominant ganglioside species (83% of the total gangliosides) and GM3, GM2, and GM1 as minor components. Trace amounts of sialyl-Lex and the complex type of sialyl-Lex derivatives were also present. Immunocytochemical studies revealed that GD1a and GalCer were primarily localized on the surface of cell bodies. Interestingly, Lex glycolipids and sialyl-Lex were localized not only on the cell bodies but also on short cell processes. Especially, sialyl-Lex glycolipid was located on the tip of fine cellular processes. The unique localization of the Lex glycolipids suggests that they may be involved in cellular differentiation and initiation of cellular growth in this cell line. 相似文献
29.
Al-sensitive (als) mutants of Arabidopsis were isolated and characterized with the aim of defining mechanisms of Al toxicity and resistance. Most als mutants selected on the basis of root growth sensitivity to Al were recessive, and together the mutants constituted eight complementation groups. Also, in most als mutants, Al sensitivity appeared to be specific for Al relative to La (another trivalent cation), except als2, which was more sensitive to La than wild type. The tendency of roots on mutant seedlings to accumulate Al was examined by staining with morin and hematoxylin, dyes used to indicate the presence of Al. A significant increase in morin staining was observed in als5, consistent with its increased sensitivity to Al. Unexpectedly, als7 and als4 showed less morin staining, suggesting that the roots on these mutants accumulate less Al than wild type seedlings after exposure to Al-containing solutions. Roots of wild-type seedlings produce callose in response to AlCl3 concentrations that inhibit root growth. Only als5 accumulated more callose than wild type in response to low levels (25 mu M) of AICI3 However, als4 and als7 did not accumulate callose at this AlCl3 concentration even though root growth was significantly inhibited. The lack of callose accumulation in als4 and als7 suggests that there is not an obligatory relationship between callose deposition and Al-induced inhibition of root growth. 相似文献
30.
3-Ketoacyl-acyl carrier protein synthase III from spinach (Spinacia oleracea) is not similar to other condensing enzymes of fatty acid synthase. 总被引:5,自引:3,他引:2 下载免费PDF全文
A cDNA clone encoding spinach (Spinacia oleracea) 3-ketoacyl-acyl carrier protein synthase III (KAS III), which catalyzes the initial condensing reaction in fatty acid biosynthesis, was isolated. Based on the amino acid sequence of tryptic digests of purified spinach KAS III, degenerate polymerase chain reaction (PCR) primers were designed and used to amplify a 612-bp fragment from first-strand cDNA of spinach leaf RNA. A root cDNA library was probed with the PCR fragment, and a 1920-bp clone was isolated. Its deduced amino acid sequence matched the sequences of the tryptic digests obtained from the purified KAS III. Northern analysis confirmed that it was expressed in both leaf and root. The clone contained a 1218-bp open reading frame coding for 405 amino acids. The identity of the clone was confirmed by expression in Escherichia coli BL 21 as a glutathione S-transferase fusion protein. The deduced amino acid sequence was 48 and 45% identical with the putative KAS III of Porphyra umbilicalis and KAS III of E. coli, respectively. It also had a strong local homology to the plant chalcone synthases but had little homology with other KAS isoforms from plants, bacteria, or animals. 相似文献