全文获取类型
收费全文 | 781篇 |
免费 | 64篇 |
国内免费 | 4篇 |
专业分类
849篇 |
出版年
2021年 | 12篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 15篇 |
2015年 | 20篇 |
2014年 | 30篇 |
2013年 | 32篇 |
2012年 | 49篇 |
2011年 | 37篇 |
2010年 | 37篇 |
2009年 | 22篇 |
2008年 | 35篇 |
2007年 | 41篇 |
2006年 | 33篇 |
2005年 | 29篇 |
2004年 | 26篇 |
2003年 | 28篇 |
2002年 | 18篇 |
2001年 | 21篇 |
2000年 | 17篇 |
1999年 | 19篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 11篇 |
1972年 | 10篇 |
1971年 | 6篇 |
1968年 | 5篇 |
1967年 | 7篇 |
排序方式: 共有849条查询结果,搜索用时 0 毫秒
1.
AAA ATPases form a functionally diverse superfamily of proteins. Most members form homo-hexameric ring complexes, are catalytically active only in the fully assembled state, and show co-operativity among the six subunits. The mutual dependence among the subunits is clearly evidenced by the fact that incorporation of mutated, inactive subunits can decrease the activity of the remaining wild type subunits. For the first time, we develop here models to describe this form of allostery, evaluate them in a simulation study, and test them on experimental data. We show that it is important to consider the assembly reactions in the kinetic model, and to define a formal inhibition scheme. We simulate three inhibition scenarios explicitly, and demonstrate that they result in differing outcomes. Finally, we deduce fitting formulas, and test them on real and simulated data. A non-competitive inhibition formula fitted experimental and simulated data best. To our knowledge, our study is the first one that derives and tests formal allosteric schemes to explain the inhibitory effects of mutant subunits on oligomeric enzymes. 相似文献
2.
3.
The Subcellular Origin of Bioluminescence in Noctiluca miliaris 总被引:4,自引:2,他引:2
The light emitted by Noctiluca has its origin in 1 to 5 x 104 organelles ("microsources") which are scattered throughout the perivacuolar cytoplasm, and which appear to be the elementary functional units of bioluminescence. Microscopical techniques, image intensification, and microphotometry were employed in their investigation. Microsources are fluorescent, strongly phase-retarding, and range widely in diameter below 1.5 microns. The number of quanta emitted in a flash from a microsource ("microflash") is of the order of 105 photons. However, microflashes show a wide range of intensities, which are correlated with the size of the organelles from which they arise. Each organelle responds repetitively and with reproducible time course to a succession of invading triggering potentials. Reversible changes in the intensity of the flash emitted by the whole cell ("macroflash") occur because of graduations in intensity of microflashes rather than as a result of changes in the number of responsive organelles. The shape of the flash emitted by individual microsources resembles that of the macroflash except for slightly shorter rise and decay times. It is concluded that the macroflash results from somewhat asynchronous, but otherwise parallel summation of microflashes. 相似文献
4.
Envelope protein(s) derived from influenza virus 总被引:1,自引:0,他引:1
E A Eckert 《Journal of bacteriology》1966,91(5):1907-1910
Eckert, Edward A. (University of Michigan, Ann Arbor). Envelope protein(s) derived from influenza virus. J. Bacteriol. 91:1907-1910. 1966.-Lipids were extracted from influenza virus, strain PR8, with methanol-chloroform, and the protein residue was dissolved in 67% glacial acetic acid. Hemagglutinating activity and complement-fixing reactivity were markedly reduced or lost during lipid extraction, and then increased after acetic acid treatment and subsequent dialysis. Evidence is presented that the envelope protein(s) responsible for these activities is dissociated in acetic acid and reassociated at neutral pH. 相似文献
5.
Topology of the transposon Tn10-encoded tetracycline resistance protein within the inner membrane of Escherichia coli 总被引:15,自引:0,他引:15
The transposon Tn10-encoded tetracycline resistance protein TetA is an integral membrane protein responsible for the export of tetracycline from the cytoplasmic to the periplasmic side of the inner membrane of Gram-negative bacteria. From a plot of the average hydrophobicity along the sequence of this protein, a two-dimensional membrane topology with 12 transmembrane domains may be predicted. Using plasmid-bearing Escherichia coli maxicells we specifically radiolabeled the TetA protein. The amino terminus of this membrane protein was shown not to be processed, and its location on the inner side of the cytoplasmic membrane was demonstrated by a newly developed use of a chemical method. Spheroplasts and inside-out vesicles of the TetA protein synthesizing maxicells were subjected to limited digestion by proteases of different specificities. The TetA protein was not accessible to proteases from the periplasmic side. On the inner side of the cytoplasmic membrane, the carboxyl terminus and four sites accessible to endoproteases could be identified. The cleavage sites are proposed to be localized between amino acid residues 60-70, 110-130, 180-200, and at amino acid 327. These results allow the definition of a model for the two-dimensional topology of the TetA protein. 相似文献
6.
Paul A. Stevenson Hans-Joachim Pflüger Manfred Eckert Jürgen Rapus 《Cell and tissue research》1994,275(2):299-308
Using a well characterized anti-serum, the distribution of octopamine-like immunoreactive neurones is described in the locust seventh abdominal (A7) and terminal ganglia (TG), which are associated with genital organs. Apart from 4 paired ventral somata occasionally observed in the TG, all labelled cells could be identified as efferent dorsal- and ventral unpaired median (DUM/VUM) neurones by virtue of the characteristic large size and position of their somata, projections of their primary neurites in DUM-cell tracts, and bifurcating axons which arise from dorsal T-junctions and enter peripheral nerves. For the examined ganglia our data indicate that the whole population of efferent DUM and VUM-cells, defined here as progeny of the segment specific unpaired median neuroblast with peripheral axons, are octopaminergic, and that equal numbers of these cells occur in both sexes: 8 in A7 and 11 in TG. Sex-specific differences are probably restricted to the axonal projections of 5 octopamine-like immunoreactive DUM-somata in A7, and 5 in TG, which in females project into their segment specific sternal nerves, but in males into the genital nerve of the TG. Numerous intersegmentally projecting octopamine-like immunoreactive fibres traverse both ganglia. The majority probably stem from previously described octopamine-like immunoreactive neurones in the thoracic and suboesophageal ganglia. 相似文献
7.
Sheila Andreatta-Van Leyen Joan R. Hembree Richard L. Eckert 《Journal of cellular physiology》1994,160(2):265-274
Insulin-like growth factors (IGFs) are important regulators of epithelial cell growth. The mitogenic activity of these factors is influenced by the levels of extracellular IGF binding proteins, including insulin-like growth factor binding protein 3 (IGFBP-3). In the present report we study the effects of epidermal growth factor (EGF) and all-trans-retinoic acid (RA) on IGFBP-3 RNA and protein levels in human papillomavirus-immortalized cervical epithelial cells. Treatment of ECE16-1 cells with 3–20 ng/ml EGF causes a marked reduction in IGFBP-3 levels. In contrast, 1 μM RA increases IGFBP-3 mRNA and protein levels in the presence or absence of 20 ng/ml EGF. The response is concentration dependent with a half-maximal increase observed at 1 nM RA. RA is able to reverse the EGF suppression when added simultaneously or 3 days after initiation of EGF treatment. Conversely, when cells are treated with RA, IGFBP-3 levels increase within 24 h and subsequent addition of EGF is without effect. Thus, the RA-dependent increase in IGFBP-3 levels is dominant over the EGF suppression. The increased IGFBP-3 levels are correlated with RA suppression of proliferation. Similar RA effects on IGFBP-3 mRNA levels were observed in other cervical epithelial cell lines (i.e., ECE16-D1, ECE16-D2, and CaSki). These results suggest that RA may act to inhibit cervical cell growth by increasing IGFBP-3 levels and reducing the extracellular concentration of free insulin-like growth factor I (IGFI) and/or, alternatively, IGFBP-3 may inhibit cell growth by direct effects on the cell, independent of IGFI. © 1994 Wiley-Liss, Inc. 相似文献
8.
The 688 nm absorption changes (ΔA688), indicating the photochemical turnover of chlorophyll aII (Chl aII) have been investigated under repetitive laser flash excitation conditions in spinach chlorplasts. It was found that under steady state conditions about 50–60% of the photo-oxidized primary donor of Photosystem II (PS II), Chl a+II, becomes re-reduced with a biphasic kinetics in the nanosecond time scale with half-life times of about 50 ns and 400 ns. The remaining Chl a+II becomes re-reduced in the microsecond range. 相似文献
9.
TERESA MARYAKA HALSZKA OSMÓLSKA 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1979,12(3):265-273
Supraorbital bones in Saurolophus angustirostris are described, and their presence in all hadrosaurs is suggested. Frontal-nasal and premaxillar-nasal fontanellae are distinguished in hadrosaurs; their presence is explained as connected with growth and considered to he responsible for the variability of crest structures. New data indicating the presence of a cartilaginous diverticulum nasi within the circumnarial depression in Saurobphus ongustirostris are presented. A physiological (respiratory and/or thermoregulatory) function of the nasal diverticulum is proposed. 相似文献
10.
Summary In the American cockroach, the distribution and connections of neuronal elements of the terminal ganglion-proctodeal nerve-hindgut system were investigated by means of immunohistochemical methods and axonal CoCl2 iontophoresis. Proctolinlike immunoreactivity was localized within neurons of the terminal ganglion projecting into the proctodeal nerve on the one hand, and in nerve cells without a direct connection to this system on the other. Immunohistochemically, in whole mount preparations fibres of the proctodeal nerve and terminal structures in the hindgut musculature exhibit strong proctolinlike immunoreactivity. At the light- and electron-microscopic levels the pathways of about 30 somata of the proctodeal neural system were characterized by cobalt chloride iontophoresis. The relationships of cobalt filled and immunoreactive neuronal structures are discussed.For the preparation of tritiated proctolin we thank Dr. S. Reißmann, WB Biochemie, Sektion Biologie, FSU JenaThe authors wish to thank G. Schörlitz, Film- und Bildstelle, FSU Jena, for photographs of whole mount preparations and Ms. A. Zinßer and Mrs. B. Cosack for excellent technical assistance. 相似文献