全文获取类型
收费全文 | 13918篇 |
免费 | 1046篇 |
国内免费 | 1篇 |
专业分类
14965篇 |
出版年
2023年 | 104篇 |
2022年 | 192篇 |
2021年 | 351篇 |
2020年 | 255篇 |
2019年 | 286篇 |
2018年 | 377篇 |
2017年 | 370篇 |
2016年 | 493篇 |
2015年 | 806篇 |
2014年 | 782篇 |
2013年 | 1030篇 |
2012年 | 1218篇 |
2011年 | 1108篇 |
2010年 | 750篇 |
2009年 | 646篇 |
2008年 | 799篇 |
2007年 | 798篇 |
2006年 | 739篇 |
2005年 | 655篇 |
2004年 | 614篇 |
2003年 | 541篇 |
2002年 | 509篇 |
2001年 | 117篇 |
2000年 | 92篇 |
1999年 | 121篇 |
1998年 | 134篇 |
1997年 | 96篇 |
1996年 | 82篇 |
1995年 | 81篇 |
1994年 | 55篇 |
1993年 | 57篇 |
1992年 | 38篇 |
1991年 | 42篇 |
1990年 | 76篇 |
1989年 | 25篇 |
1988年 | 28篇 |
1987年 | 37篇 |
1986年 | 31篇 |
1985年 | 32篇 |
1984年 | 45篇 |
1983年 | 44篇 |
1982年 | 33篇 |
1981年 | 40篇 |
1980年 | 18篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1977年 | 18篇 |
1975年 | 18篇 |
1974年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
991.
Modulation of the sensitivity of FimB recombination to branched-chain amino acids and alanine in Escherichia coli K-12 总被引:3,自引:0,他引:3 下载免费PDF全文
Phase variation of type 1 fimbriae of Escherichia coli requires the site-specific recombination of a short invertible element. Inversion is catalyzed by FimB (switching in either direction) or FimE (inversion mainly from on to off) and is influenced by auxiliary factors integration host factor (IHF) and leucine-responsive regulatory protein (Lrp). These proteins bind to sites (IHF site II and Lrp sites 1 and 2) within the invertible element to stimulate recombination, presumably by bending the DNA to enhance synapses. Interaction of Lrp with a third site (site 3) cooperatively with sites 1 and 2 (termed complex 1) impedes recombination. Inversion is stimulated by the branched-chain amino acids (particularly leucine) and alanine, and according to a current model, the amino acids promote the selective loss of Lrp from site 3 (complex 2). Here we show that the central portion of the fim invertible element, situated between Lrp site 3 and IHF site II, is dispensable for FimB recombination but that this region is also required for full amino acid stimulation of inversion. Further work reveals that the region is likely to contain multiple regulatory elements. Lrp site 3 is shown to bind the regulatory protein with low affinity, and a mutation that enhances binding to this element is found both to diminish the stimulatory effects of IVLA on FimB recombination and to inhibit recombination in the absence of the amino acids. The results obtained emphasize the importance of Lrp site 3 as a control element but also highlight the complexity of the regulatory system that affects this site. 相似文献
992.
Torres JA Rodriguez MH Rodriguez MC de la Cruz Hernandez-Hernandez F 《Experimental parasitology》2005,111(4):255-259
Plasmodium berghei: The effect of five protease inhibitors, TPCK, TLCK, PMSF, leupeptin, and 1,10-phenanthroline on in vitro gametogenesis and early zygote development of P. berghei was investigated. PMSF and leupeptin showed no effect. Cysteine/serine protease inhibitors TPCK/TLCK at concentrations of 75 and 100 microM were effective on inhibiting exflagellation center formation, and this effect was reversible with the addition of l-cysteine. Exflagellation center formation was most effectively blocked by 1,10-phenanthroline (1mM), and exflagellation center numbers were restored by the addition of Zn(2+). A reduction of ookinete production was observed when TPCK/TLCK (100 microM) was added at 2h after gametogenesis, but no effect was observed with 1,10-phenanthroline (1mM). Our results suggest that proteolysis is important in both gametocyte activation and sexual development of P. berghei. 相似文献
993.
Ferrão-Gonzales AD Robbs BK Moreau VH Ferreira A Juliano L Valente AP Almeida FC Silva JL Foguel D 《The Journal of biological chemistry》2005,280(41):34747-34754
Aggregation of proteins and peptides has been shown to be responsible for several diseases known as amyloidoses, which include Alzheimer disease (AD), prion diseases, among several others. AD is a neurodegenerative disorder caused primarily by the aggregation of beta-amyloid peptide (Abeta). Here we describe the stabilization of small oligomers of Abeta by the use of sulfonated hydrophobic molecules such as AMNS (1-amino-5-naphthalene sulfonate); 1,8-ANS (1-anilinonaphthalene-8-sulfonate) and bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate). The experiments were performed with either Abeta-1-42 or with Abeta-13-23, a shorter version of Abeta that is still able to form amyloid fibrils in vitro and contains amino acid residues 16-20, previously shown to be essential to peptide-peptide interaction and fibril formation. All sulfonated molecules tested were able to prevent Abeta aggregation in a concentration dependent fashion in the following order of efficacy: 1,8-ANS < AMNS < bis-ANS. Size exclusion chromatography revealed that in the presence of bis-ANS, Abeta forms a heterogeneous population of low molecular weight species that proved to be toxic to cell cultures. Since the ANS compounds all have apolar rings and negative charges (sulfonate groups), both hydrophobic and electrostatic interactions may contribute to interpeptide contacts that lead to aggregation. We also performed NMR experiments to investigate the structure of Abeta-13-23 in SDS micelles and found features of an alpha-helix from Lys(16) to Phe(20). 1H TOCSY spectra of Abeta-13-23 in the presence of AMNS displayed a chemical-shift dispersion quite similar to that observed in SDS, which suggests that in the presence of AMNS this peptide might adopt a conformation similar to that reported in the presence of SDS. Taken together, our studies provide evidence for the crucial role of small oligomers and their stabilization by sulfonate hydrophobic compounds. 相似文献
994.
Baig S Damian RT Morales-Montor J Olecki P Talhouk J Hashmey R White AC 《The Journal of parasitology》2005,91(5):983-987
Cysticercosis is caused by Taenia spp. metacestodes, which must survive in the host tissues to complete their life cycle. Their survival depends on their control of host immune responses. Because many parasites use proteases to modulate host responses, we examined culture media from Taenia crassiceps metacestodes for protease activity using peptide substrates. We identified prominent aminopeptidase activity at neutral pH, which was inhibited by chelating agents and partially inhibited by the aminopeptidase inhibitor, bestatin. Endopeptidase substrates were optimally cleaved at slightly acidic pH and endopeptidase activity was inhibited by cysteine protease inhibitors. Gel filtration FPLC and subsequent visualization by silver staining revealed a metallo-aminopeptidase of molecular weight 21 kDa and cysteine proteases of Mr 70 and 64 kDA. Recombinant IL-2 was digested when incubated with parasite culture supernatants, but not with control media. IL-2 degradation was completely inhibited by 1,10 phenanthroline and partially inhibited by bestatin, suggesting that a metallo-aminopeptidase was responsible. Incubation of human IgG with culture supernatants resulted in complete degradation of IgG, which was blocked by cysteine protease inhibitors. These observations demonstrate that Taenia spp. metacestodes secrete a number of proteolytic enzymes, which may target molecules from the host immune system and assist in evasion of the host immune response. 相似文献
995.
Nigel E. Gapper Simon A. Coupe Marian J. McKenzie Ben K. Sinclair Ross E. Lill Paula E. Jameson 《Journal of Plant Growth Regulation》2005,24(3):153-165
Broccoli (Brassica oleracea var. italica) deteriorates rapidly following harvest. The two plant hormones ethylene and cytokinin are known to act antagonistically
on harvest-induced senescence in broccoli: ethylene by accelerating the process, and cytokinin by delaying it. To determine
the level at which these hormones influenced senescence, we isolated and monitored the expression of genes normally associated
with senescence in broccoli florets treated with exogenous 6-benzyl aminopurine (6-BAP), 1-aminocyclopropane-1-carboxylic
acid (ACC), a combination of 6-BAP and ACC, and sucrose, in the five days following harvest. Exogenous 6-BAP caused both a
reduction (BoACO) and an increase (BoACS) in ethylene biosynthetic gene expression. The expression of genes used as senescence markers, BoCP5 and BoMT1, was reduced, whereas BoCAB1 levels were maintained after harvest in response to exogenous 6-BAP. In addition, the expression of genes encoding sucrose
transporters (BoSUC1 and BoSUC2) and carbohydrate metabolizing enzymes (BoINV1 and BoHK1) was also reduced upon 6-BAP feeding. Interestingly, the addition of ACC prevented the 6-BAP-induced increase in expression
of BoACS, but 6-BAP negated the ACC-induced increase in expression of BoACO. The culmination of these results indicates a significant role for cytokinin in the delay of senescence. The implication
that cytokinin regulates postharvest senescence in broccoli by inhibiting ethylene perception and/or biosynthesis, thus regulating
carbohydrate transport and metabolism, as well as senescence-associated gene expression, is discussed and a model presented. 相似文献
996.
997.
A semiparametric estimator of the bivariate distribution function for censored gap times 总被引:1,自引:0,他引:1
Let (T(1), T(2)) be gap times corresponding to two consecutive events, which are observed subject to random right-censoring. In this paper, a semiparametric estimator of the bivariate distribution function of (T(1), T(2)) and, more generally, of a functional E [φ(T(1),T(2))] is proposed. We assume that the probability of censoring for T(2) given the (possibly censored) gap times belongs to a parametric family of binary regression curves. We investigate the conditions under which the introduced estimator is consistent. We explore the finite sample behavior of the estimator and of its bootstrap standard error through simulations. The main conclusion of this paper is that the semiparametric estimator may be much more efficient than purely nonparametric methods. Real data illustration is included. 相似文献
998.
Atorrasagasti C Aquino JB Hofman L Alaniz L Malvicini M Garcia M Benedetti L Friedman SL Podhajcer O Mazzolini G 《American journal of physiology. Gastrointestinal and liver physiology》2011,300(5):G739-G748
Liver fibrosis is an active process that involves changes in cell-cell and cell-extracellular matrix (ECM) interaction. Secreted protein, acidic and rich in cysteine (SPARC) is an ECM protein with many biological functions that is overexpressed in cirrhotic livers and upregulated in activated hepatic stellate cells (aHSCs). We have recently shown that SPARC downregulation ameliorates liver fibrosis in vivo. To uncover the cellular mechanisms involved, we have specifically knocked down SPARC in two aHSC lines [the CFSC-2G (rat) and the LX-2 (human)] and in primary cultured rat aHSCs. Transient downregulation of SPARC in hepatic stellate cells (HSCs) did not affect their proliferation and had only minor effects on apoptosis. However, SPARC knockdown increased HSC adhesion to fibronectin and significantly decreased their migration toward PDFG-BB and TGF-β(1). Interestingly, TGF-β(1) secretion by HSCs was reduced following SPARC small interfering RNA (siRNA) treatment, and preincubation with TGF-β(1) restored the migratory capacity of SPARC siRNA-treated cells through mechanisms partially independent from TGF-β(1)-mediated induction of SPARC expression; thus SPARC knockdown seems to exert its effects on HSCs partially through modulation of TGF-β(1) expression levels. Importantly, collagen-I mRNA expression was reduced in SPARC siRNA-transfected HSCs. Consistent with previous results, SPARC knockdown in aHSCs was associated with altered F-actin expression patterns and deregulation of key ECM and cell adhesion molecules, i.e., downregulation of N-cadherin and upregulation of E-cadherin. Our data together suggest that the upregulation of SPARC previously reported for aHSCs partially mediates profibrogenic activities of TGF-β(1) and PDGF-BB and identify SPARC as a potential therapeutic target for liver fibrosis. 相似文献
999.
Social cues of sperm competition influence accessory reproductive gland size in a promiscuous mammal
Lemaître JF Ramm SA Hurst JL Stockley P 《Proceedings. Biological sciences / The Royal Society》2011,278(1709):1171-1176
Theory predicts that males should increase overall investment in ejaculate expenditure with increasing levels of sperm competition. Since ejaculate production is costly, we may expect males to tailor their reproductive investment according to anticipated levels of sperm competition. Here, we investigate plasticity in ejaculate investment in response to cues of population average levels of sperm competition in a promiscuous mammal, the bank vole (Myodes glareolus). We manipulated the social experience of experimental subjects during sexual development via differential exposure to the odour of rival males, to simulate conditions associated with relatively high or low average levels of sperm competition. Males exposed to a high level of competition developed larger major accessory reproductive glands (seminal vesicles) than those that experienced a low level of competition, suggesting that an increased investment in the production of copulatory plugs and/or mating rate may be beneficial at relatively high sperm competition levels. However, investment in sperm production, testis size and sperm motility were not altered according to social experience. Our findings emphasize the importance of non-sperm components of the ejaculate in mammalian postcopulatory sexual selection, and add to the growing evidence linking plasticity in reproductive traits to social cues of sperm competition. 相似文献
1000.