全文获取类型
收费全文 | 4612篇 |
免费 | 382篇 |
国内免费 | 1篇 |
专业分类
4995篇 |
出版年
2023年 | 25篇 |
2022年 | 43篇 |
2021年 | 98篇 |
2020年 | 51篇 |
2019年 | 97篇 |
2018年 | 95篇 |
2017年 | 87篇 |
2016年 | 170篇 |
2015年 | 230篇 |
2014年 | 241篇 |
2013年 | 308篇 |
2012年 | 312篇 |
2011年 | 283篇 |
2010年 | 202篇 |
2009年 | 203篇 |
2008年 | 221篇 |
2007年 | 218篇 |
2006年 | 203篇 |
2005年 | 209篇 |
2004年 | 189篇 |
2003年 | 161篇 |
2002年 | 142篇 |
2001年 | 110篇 |
2000年 | 120篇 |
1999年 | 81篇 |
1998年 | 64篇 |
1997年 | 64篇 |
1996年 | 44篇 |
1995年 | 41篇 |
1994年 | 32篇 |
1993年 | 40篇 |
1992年 | 64篇 |
1991年 | 50篇 |
1990年 | 50篇 |
1989年 | 47篇 |
1988年 | 53篇 |
1987年 | 42篇 |
1986年 | 27篇 |
1985年 | 38篇 |
1984年 | 31篇 |
1983年 | 17篇 |
1982年 | 34篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 19篇 |
1977年 | 14篇 |
1976年 | 11篇 |
1975年 | 16篇 |
1972年 | 6篇 |
排序方式: 共有4995条查询结果,搜索用时 15 毫秒
61.
62.
It is generally recognized that a fraction of all bacterioplanktoncells enumerated using conventional epifluorescence techniquesis neither growing, dividing nor metabolically active, but thevariation in the proportion of active cells among aquatic systemsis not well understood. Here, we hypothesize that the proportionof metabolically active cells increases systematically alonggradients of enrichment, and to test this hypothesis the numberand proportion of metabolically active planktonic bacteria wereinvestigated during the summer in a set of 24 temperate lakes,which span a considerable range in productivity. The tetrazoliumsalt 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was usedas an indicator of cells with an active electron transport system.The total number of bacteria ranged from 1.88x106 to 7.70x106ml1, whereas the number of active cells was more variableand ranged from 0.37x106 to 2.18x106 ml1 in the studylakes. The proportion of metabolically active cells ranged from15 to 33%, and tended to increase with nutrient and chlorophyllconcentrations, but not with dissolved organic carbon (DOC).Data on the number of total and active bacteria culled fromthe literature for marine, estuarine and freshwater systemsshow that the trends we measured in lakes are valid for pelagicsystems in general. Over a broad range of aquatic systems, thetotal number of bacteria varied by three orders of magnitude,whereas the number of active bacteria varied by four ordersof magnitude as system productivity increased. The proportionof active cells increased from ultraoligotrophic openocean areas(<5%) to highly productive estuaries (>50%). Our resultssuggest that in most aquatic systems there is a pool of rapidlygrowmg cells, embedded in a usually larger matrix of inactivebacteria, and that the relative size of the active and inactivepools varies systematically among bacterial populations alonggradients of enrichment. 相似文献
63.
Antonio del Castillo-Olivares Alicia Esteban del Valle Javier Márquez Ignacio NÚñez de Castro Miguel ángel Medina 《Journal of bioenergetics and biomembranes》1995,27(6):605-611
Ehrlich cell plasma membrane ferricyanide reductase activity increased in the presence of mastoparan, a generic activator of G proteins, using either whole cells or isolated plasma membrane fractions. Agents that increase intracellularcAMP also increased the rate of ferricyanide reduction by Ehrlich cells. For the first time, evidence is shown on a modulation of plasma membrane redox system bycGMP. In fact, permeant analogs ofcGMP, dibutyrylcGMP, and 8-bromo-cGMP increased the rate of ferricyanide reduction by the Ehrlich cell plasma membrane redox system. Furthermore, specific inhibition ofcGMP-phosphodiesterases by dipyridamole was also accompanied by an enhancement in the rate of ferricyanide reduction. On the other hand, treatments expected to increase cytoplasmic Ca2+ concentrations were accompanied by a remarkable stimulation of the reductase activity. Taking all these data together, it seems that the Ehrlich cell plasma membrane redox system is under a multiple and complex regulation by different signal transduction pathways involving G proteins, cyclic nucleotides, and Ca2+ ions. 相似文献
64.
Preferential recognition of hepatitis B nucleocapsid antigens by Th1 or Th2 cells is epitope and major histocompatibility complex dependent. 总被引:7,自引:3,他引:4 下载免费PDF全文
Regulatory T-helper (Th) cells have been categorized into two functional subsets, Th1 and Th2 cells, which produce distinct lymphokines. In general, Th1 cells mediate cellular immune responses and Th2 cells mediate humoral immunity. Recent serological studies suggest that the Th1-Th2 balance may be relevant in acute and chronic hepatitis B virus (HBV) infections. The purpose of this study was to determine the potential of the nucleocapsid antigens (Ags) (hepatitis B core and e Ags [HBc/eAg]) of HBV to preferentially elicit either a Th1 or a Th2 dominant response. For this purpose, H-2 congenic B10.S and B10 mice were immunized with HBc/eAg, and Ag-specific T-cell proliferative responses, T-cell helper function, and T-cell cytokine production were analyzed. The results indicated that B10.S mice preferentially develop a Th1-like response whereas B10 mice preferentially develop a Th2-like response after immunization with HBc/eAg. Furthermore, the preferential Th1 and Th2 response patterns were reproduced when 12-residue peptides representing the dominant HBc/eAg-specific T-cell sites for B10.S (peptide 120-131) and B10 (peptide 129-140) mice were used as immunogens. Therefore, the combination of the T-cell site recognized and the major histocompatibility complex restricting element can in large part determine the Th phenotype of the HBc/eAg-specific T-cell response. Other factors that influenced Th phenotype were the presence of exogenous cytokines, Ag structure, and tissue distribution. 相似文献
65.
Juventino J. Garcia María L. Hernndez Hugo Torrens Atilano Gutierrez Federico del Río 《Inorganica chimica acta》1995,230(1-2):173-176
By reacting [(C5Me5)M(SRF)2] (forM = Ir, Rf = C6F5 (1a) or C6F4H-p (1b); for M = Rh, Rf = C6F5 (2a) or C6F4H-p (2a)) in toluene with Na[AuCl4], ionic binuclear compounds with the general formula [(C5Me5)M(μ-SRF)2AuCl2]Cl for M = Ir, R = C6F5 (3a) or C6F4H-p (3a); for M = Rh, RF = C6F5 (4a) or C6F4H-p (4b) can be obtained, together with small amounts of [(C5Me5)2Rh2(μ-SRF)(μ-Cl)2]Cl (RF = C6F5 (5a) or C6F4H-p (5b)) as by-products when 2a and 2b were used. 相似文献
66.
In vitro recognition of the replication origin of pLS1 and of plasmids of the pLS1 family by the RepB initiator protein. 总被引:4,自引:1,他引:3 下载免费PDF全文
Rolling-circle replication of plasmid pLS1 is initiated by the plasmid-encoded RepB protein, which has nicking-closing (site-specific DNA strand transferase) enzymatic activity. The leading-strand origin of pLS1 contains two regions, (i) the RepB-binding site, constituted by three directly repeated sequences (iterons or the bind region), and (ii) the sequence where RepB introduces the nick to initiate replication (the nic region). A series of plasmids, belonging to the pLS1 family, show features similar to those of pLS1 and have DNA sequences homologous to the pLS1 nic region. In addition, they all share homologies at the level of their Rep proteins. However, the bind regions of these plasmids are, in general, not conserved. We tested the substrate specificity of purified RepB of pLS1. The RepB protein has a temperature-dependent nicking-closing action on supercoiled pLS1, as well as on recombinant plasmid DNAs harboring the pLS1 nic region. The DNA strand transferase activity of pLS1-encoded RepB was also assayed on two plasmids of the pLS1 family, namely, pE194 and pFX2. DNAs from both plasmids were relaxed by RepB, provided they had a proper degree of supercoiling; i.e., it was necessary to modulate the supercoiling of pE194 DNA to achieve RepB-mediated DNA relaxation. Single-stranded oligonucleotides containing the nic regions of various plasmids belonging to the pLS1 family, including those of pE194 and pFX2, were substrates for RepB. In vitro, the RepB protein does not need to bind to the iterons for its nicking-closing activity. 相似文献
67.
Manganese and lignin peroxidase (MnP, LiP) activities were measured in straw extracts from cultures of Phanerochaete chrysosporium. Out of six MnP substrates, the MBTH/DMAB (3-methyl-2-benzothiazolinone hydrazone/3-(dimethylamino)benzoic acid), gave the highest MnP activity. Detection of LiP activity as veratryl alcohol oxidation was inhibited by phenols in the straw culture extracts. Appropriate levels of veratryl alcohol and peroxide (4 mM and 0.4 mM, respectively), and a restricted sample volume (not larger than 10%) were necessary to detect activity. 相似文献
68.
69.
The Uba2 and Ufd1 proteins of Saccharomyces cerevisiae interact with poly(A) polymerase and affect the polyadenylation activity of cell extracts 总被引:2,自引:0,他引:2
Poly(A) polymerase is responsible for the addition of the adenylate tail to the 3′ ends of mRNA. Using the two-hybrid system,
we have identified two proteins which interact specifically with the Saccharomyces cerevisiae poly(A) polymerase, Pap1. Uba2 is a homolog of ubiquitin-activating (E1) enzymes and Ufd1 is a protein whose function is probably
also linked to the ubiquitin-mediated protein degradation pathway. These two proteins interact with Pap1 and with each other,
but not with eight other target proteins which were tested in the two-hybrid system. The last 115 amino acids of Uba2, which
contains an 82-amino acid region not present in previously characterized E1 enzymes, is sufficient for the interaction with
Pap1. Both Uba2 and Ufd1 can be co-immunoprecipitated from extracts with Pap1, confirming in vitro the interaction identified
by two-hybrid analysis. Depletion of Uba2 from cells produces extracts which polyadenylate precursor RNA with increased efficiency
compared to extracts from nondepleted cells, while depletion of Ufd1 yields extracts which are defective in processing. These
two proteins are not components of polyadenylation factors, and instead may have a role in regulating poly(A) polymerase activity.
Received: 6 January 1997 / Accepted: 27 February 1997 相似文献
70.
Plasmid rolling circle replication and its control 总被引:10,自引:0,他引:10
Manuel Espinosa Gloria del Solar Fernando Rojo Juan C. Alonso 《FEMS microbiology letters》1995,130(2-3):111-120
Abstract This review summarises current information on rolling circle replicating plasmids originally isolated from Gram-positive bacteria with a low guanine and cytosine content in their DNA. It focuses on the peculiar biological features of these small, high copy number plasmids that replicate via an asymmetric RC mechanism. The regulation of plasmid copy number is also discussed. 相似文献