全文获取类型
收费全文 | 32117篇 |
免费 | 117篇 |
国内免费 | 287篇 |
专业分类
系统科学 | 496篇 |
丛书文集 | 645篇 |
教育与普及 | 110篇 |
理论与方法论 | 123篇 |
现状及发展 | 12563篇 |
研究方法 | 1332篇 |
综合类 | 16513篇 |
自然研究 | 739篇 |
出版年
2013年 | 385篇 |
2012年 | 694篇 |
2011年 | 1441篇 |
2010年 | 440篇 |
2009年 | 424篇 |
2008年 | 753篇 |
2007年 | 885篇 |
2006年 | 898篇 |
2005年 | 830篇 |
2004年 | 698篇 |
2003年 | 567篇 |
2002年 | 569篇 |
2001年 | 946篇 |
2000年 | 962篇 |
1999年 | 668篇 |
1992年 | 527篇 |
1991年 | 451篇 |
1990年 | 471篇 |
1989年 | 430篇 |
1988年 | 391篇 |
1987年 | 434篇 |
1986年 | 447篇 |
1985年 | 509篇 |
1984年 | 443篇 |
1983年 | 345篇 |
1982年 | 314篇 |
1981年 | 321篇 |
1980年 | 404篇 |
1979年 | 910篇 |
1978年 | 695篇 |
1977年 | 719篇 |
1976年 | 530篇 |
1975年 | 619篇 |
1974年 | 885篇 |
1973年 | 755篇 |
1972年 | 736篇 |
1971年 | 861篇 |
1970年 | 1105篇 |
1969年 | 844篇 |
1968年 | 746篇 |
1967年 | 868篇 |
1966年 | 727篇 |
1965年 | 542篇 |
1959年 | 300篇 |
1958年 | 474篇 |
1957年 | 362篇 |
1956年 | 296篇 |
1955年 | 281篇 |
1954年 | 264篇 |
1948年 | 196篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
At the end of mitosis, daughter cells are separated from each other by cytokinesis. This process involves equal partitioning
and segregation of cytoplasm between the two cells. Despite years of study, the mechanism driving cytokinesis in animal cells
is not fully understood. Actin and myosin are major components of the contractile ring, the structure at the equator between
the dividing cells that provides the force necessary to constrict the cytoplasm. Despite this, there are also tantalizing
results suggesting that cytokinesis can occur in the absence of myosin. It is unclear what the roles are of the few other
contractile ring components identified to date. While it has been difficult to identify important proteins involved in cytokinesis,
it has been even more challenging to pinpoint the regulatory mechanisms that govern this vital process. Cytokinesis must be
precisely controlled both spatially and temporally; potential regulators of these parameters are just beginning to be identified.
This review discusses the recent progress in our understanding of cytokinesis in animal cells and the mechanisms that may
regulate it.
Received 24 August 1998; received after revision 9 October 1998; accepted 9 October 1998 相似文献
992.
Images, calculated from electron micrographs, show the three-dimensional structures of microtubules and tubulin sheets decorated stoichiometrically with motor protein molecules. Dimeric motor domains (heads) of kinesin and ncd, the kinesin-related protein that moves in the reverse direction, each appeared to bind to tubulin in the same way, by one of their two heads. The second heads show an interesting difference in position that seems to be related to the directions of movement of the two motors. X-ray crystallographic results showing the structures of kinesin and ncd to be very similar at atomic resolution, and homologous also to myosin, suggest that the two motor families may use mechanisms that have much in common. Nevertheless, myosins and kinesins differ kinetically. Also, whereas conformational changes in the myosin catalytic domain are amplified by a long lever arm that connects it to the stalk domain, kinesin and ncd do not appear to possess a structure with a similar function but may rely on biased diffusion in order to move along microtubules. 相似文献
993.
Hyaluronan synthases: fascinating glycosyltransferases from vertebrates, bacterial pathogens, and algal viruses 总被引:14,自引:0,他引:14
DeAngelis PL 《Cellular and molecular life sciences : CMLS》1999,56(7-8):670-682
Hyaluronan (or hyaluronic acid or hyaluronate; HA) is a polysaccharide found in the extracellular matrix of vertebrate tissues and in the surface coating of certain Streptococcus and Pasteurella bacterial pathogens. At least one algal virus directs its host to produce HA on the cell surface early in infection. HA synthases (HASs) are the enzymes that polymerize HA using uridine diphospho-sugar precursors. In all known cases, HA is secreted out of the cell; therefore, HASs are normally found in the outer membranes of the organism. In the last 6 years, the HASs have been molecularly cloned from all the above sources. They were the first class of glycosyltransferases identified in which a single polypeptide species catalyzes the transfer of two different monosaccharides; this finding is in contrast to the usual 'single enzyme, single sugar' dogma of glycobiology. There appear to be two distinct classes of HASs based on differences in amino acid sequence, topology in the membrane, and reaction mechanism. This review discusses the current state of knowledge surrounding the molecular details of HA biosynthesis and summarizes the possible evolutionary history of the HASs. 相似文献
994.
The hypothalamus regulates diverse physiological functions, including the control of energy metabolism, circadian rhythms, stress and anxiety, sexual and reproductive behaviors. An overview of the most prevalent hypothalamus-enriched mRNAs revealed that this area of the brain specializes in producing intercellular signaling molecules. Two new secreted peptides derived from a single neuropeptide precursor, named hypocretins and orexins by two different groups, are synthesized in a small set of neurons in the perifornical area of the hypothalamus. Intracerebroventricular injection of the hypocretins/orexins increases food consumption in rats. Here we review recent progress in identifying the role of the hypocretins/orexins in the control of energy balance and in other physiological systems. 相似文献
995.
Immune responses to DNA vaccines 总被引:16,自引:0,他引:16
DNA vaccines, based on plasmid vectors expressing an antigen under the control of a strong promoter, have been shown to induce
protective immune responses to a number of pathogens, including viruses, bacteria and parasites. They have also displayed
efficacy in treatment or prevention of cancer, allergic diseases and autoimmunity. Immunologically, DNA vaccines induce a
full spectrum of immune responses that include cytolytic T cells, T helper cells and antibodies. The immune response to DNA
vaccines can be enhanced by genetic engineering of the antigen to facilitate its presentation to B and T cells. Furthermore,
the immune response can be modulated by genetic adjuvants in the form of vectors expressing biologically active determinants
or by more traditional adjuvants that facilitate uptake of DNA into cells. The ease of genetic manipulation of DNA vaccines
invites their use not only as vaccines but also as research tools for immunologists and microbiologists.
Received 26 October 1998; received after revision 3 December 1998; accepted 3 December 1998 相似文献
996.
997.
MAP kinases in plant signal transduction 总被引:10,自引:0,他引:10
Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular
targets in all eukaryotes. Distinct MAPK pathways are regulated by different extracellular stimuli and are implicated in a
wide variety of biological processes. In plants there is evidence for MAPKs playing a role in the signaling of abiotic stresses,
pathogens and plant hormones. The large number and divergence of plant MAPKs indicates that this ancient mechanism of bioinformatics
is extensively used in plants and may provide a new molecular handle on old questions. 相似文献
998.
Wójcik C 《Cellular and molecular life sciences : CMLS》1999,56(11-12):908-917
The proteasome (multicatalytic proteinase complex, prosome) is a major cytoplasmic proteolytic enzyme, responsible for degradation of the vast majority of intracellular proteins. Proteins degraded by the proteasome are usually tagged with multiple ubiquitin moieties, conjugated to the substrates by a complicated cascade of enzymes. Over the last years, evidence has accumulated that changes in the expression and activity of the different components of the ubiquitin-proteasome system occur during apoptosis. Proteasome inhibitors have been used to induce apoptosis in various cell types, whereas in others, these compounds were able to prevent apoptosis induced by different stimuli. The proteasome mediated step(s) in apoptosis is located upstream of mitochondrial changes and caspase activation, and can involve in different systems Bcl-2, Jun N-terminal kinase, heat shock proteins, Myc, p53, polyamines and other factors. 相似文献
999.
Sarti P Silver RB Paroli L Nikonorov I Blanck TJ 《Cellular and molecular life sciences : CMLS》1999,56(11-12):1061-1069
Rat heart myocytes undergoing progressive damage demonstrate morphological changes of shortening and swelling followed by the formation of intracellular vacuoles and plasma membrane blebbing. The damaged myocytes displayed impaired N,N'-tetramethyl-p-phenyldiamine (TMPD) ascorbate-stimulated respiratory activity which was restored by the addition of reduced cytochrome c to the cell culture medium. To clarify the role played by cytochrome c in the impairment of cell respiration, polarographic, spectrophotometric and fluorescence as well as electron microscopy imaging experiments were performed. TMPD/ascorbate-stimulated respiratory activity returned to control levels, at approximately 20 microM cytochrome c, establishing the threshold below which the turnover rate by cytochrome c oxidase in the cell depends on cytochrome concentration. Mildly damaged cardiac myocytes, as indicated by cell shortening, retention of visible striations and free-fluorescein exclusion, together with the absence of lactate dehydrogenase leakage and exclusion of trypan blue, were able to oxidize exogenous cytochrome c and were permeable to fluorescein-conjugated cytochrome c. The results, while consistent with an early cytochrome c release observed at the beginning of cell death, elucidate the role played by cytochrome c in the kinetic control of mitochondrial electron transfer under pathological conditions, particularly those involving the terminal part of the respiratory chain. These data are the first to demonstrate that the sarcolemma of cardiac myocytes, damaged but still viable, is permeable to cytochrome c. 相似文献
1000.
Are elicitins cryptograms in plant-Oomycete communications? 总被引:13,自引:0,他引:13
Ponchet M Panabières F Milat M-L Mikes V Montillet JL Suty L Triantaphylides C Tirilly Y Blein JP 《Cellular and molecular life sciences : CMLS》1999,56(11-12):1020-1047
Stimulation of plant natural defenses is an important challenge in phytoprotection prospects. In that context, elicitins, which are small proteins secreted by Phytophthora and Pythium species, have been shown to induce a hypersensitive-like reaction in tobacco plants. Moreover, these plants become resistant to their pathogens, and thus this interaction constitutes an excellent model to investigate the signaling pathways leading to plant resistance. However, most plants are not reactive to elicitins, although they possess the functional signaling pathways involved in tobacco responses to elicitin. The understanding of factors involved in this reactivity is needed to develop agronomic applications. In this review, it is proposed that elicitins could interact with regulating cell wall proteins before they reach the plasma membrane. Consequently, the plant reactivity or nonreactivity status could result from the equilibrium reached during this interaction. The possibility of overexpressing the elicitins directly from genomic DNA in Pichia pastoris allows site-directed mutagenesis experiments and structure/function studies. The recent discovery of the sterol carrier activity of elicitins brings a new insight on their molecular activity. This constitutes a crucial property, since the formation of a sterol-elicitin complex is required to trigger the biological responses of tobacco cells and plants. Only the elicitins loaded with a sterol are able to bind to their plasmalemma receptor, which is assumed to be an allosteric calcium channel. Moreover, Phytophthora and Pythium do not synthesize the sterols required for their growth and their fructification, and elicitins may act as shuttles trapping the sterols from the host plants. Sequence analysis of elicitin genes from several Phytophthora species sheds unexpected light on the phylogenetic relationships among the genus, and suggests that the expression of elicitins is under tight regulatory control. Finally, general involvement of these lipid transfer proteins in the biology of Pythiaceae, and in plant defense responses, is discussed. A possible scheme for the coevolution between Phytophthora and tobacco plants is approached. 相似文献