全文获取类型
收费全文 | 3246篇 |
免费 | 242篇 |
国内免费 | 3篇 |
专业分类
3491篇 |
出版年
2023年 | 11篇 |
2022年 | 31篇 |
2021年 | 41篇 |
2020年 | 18篇 |
2019年 | 35篇 |
2018年 | 41篇 |
2017年 | 36篇 |
2016年 | 61篇 |
2015年 | 112篇 |
2014年 | 101篇 |
2013年 | 171篇 |
2012年 | 241篇 |
2011年 | 233篇 |
2010年 | 138篇 |
2009年 | 117篇 |
2008年 | 169篇 |
2007年 | 215篇 |
2006年 | 179篇 |
2005年 | 155篇 |
2004年 | 172篇 |
2003年 | 185篇 |
2002年 | 163篇 |
2001年 | 36篇 |
2000年 | 22篇 |
1999年 | 41篇 |
1998年 | 37篇 |
1997年 | 40篇 |
1996年 | 34篇 |
1995年 | 32篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 36篇 |
1990年 | 38篇 |
1989年 | 32篇 |
1988年 | 27篇 |
1987年 | 16篇 |
1986年 | 39篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 26篇 |
1982年 | 25篇 |
1981年 | 24篇 |
1980年 | 21篇 |
1979年 | 22篇 |
1978年 | 13篇 |
1977年 | 18篇 |
1975年 | 15篇 |
1974年 | 13篇 |
1968年 | 10篇 |
排序方式: 共有3491条查询结果,搜索用时 0 毫秒
991.
We measured the kinetics of DNA bending by M.EcoRI using DNA labeled at both 5'-ends and observed changes in fluorescence resonance energy transfer. Although known to bend its cognate DNA site, energy transfer is decreased upon enzyme binding. This unanticipated effect is shown to be robust because we observe the identical decrease with different dye pairs, when the dye pairs are placed on the respective 3'-ends, the effect is cofactor- and protein-dependent, and the effect is observed with duplexes ranging from 14 through 17 base pairs. The same labeled DNA shows the anticipated increased energy transfer with EcoRV endonuclease, which also bends this sequence, and no change in energy transfer with EcoRI endonuclease, which leaves this sequence unbent. We interpret these results as evidence for an increased end-to-end distance resulting from M.EcoRI binding, mediated by a mechanism novel for DNA methyltransferases, combining DNA bending and an overall expansion of the DNA duplex. The M.EcoRI protein sequence is poorly accommodated into well defined classes of DNA methyltransferases, both at the level of individual motifs and overall alignment. Interestingly, M.EcoRI has an intercalation motif observed in the FPG DNA glycosylase family of repair enzymes. Enzyme-dependent changes in anisotropy and fluorescence resonance energy transfer have similar rate constants, which are similar to the previously determined rate constant for base flipping; thus, the three processes are nearly coincidental. Similar fluorescence resonance energy transfer experiments following AdoMet-dependent catalysis show that the unbending transition determines the steady state product release kinetics. 相似文献
992.
993.
Escherichia coli DNA adenine methyltransferase (EcoDam) methylates the N-6 position of the adenine in the sequence 5'-GATC-3' and plays vital roles in gene regulation, mismatch repair, and DNA replication. It remains unclear how the small number of critical GATC sites involved in the regulation of replication and gene expression are differentially methylated, whereas the approximately 20,000 GATCs important for mismatch repair and dispersed throughout the genome are extensively methylated. Our prior work, limited to the pap regulon, showed that methylation efficiency is controlled by sequences immediately flanking the GATC sites. We extend these studies to include GATC sites involved in diverse gene regulatory and DNA replication pathways as well as sites previously shown to undergo differential in vivo methylation but whose function remains to be assigned. EcoDam shows no change in affinity with variations in flanking sequences derived from these sources, but methylation kinetics varied 12-fold. A-tracts immediately adjacent to the GATC site contribute significantly to these differences in methylation kinetics. Interestingly, only when the poly(A) is located 5' of the GATC are the changes in methylation kinetics revealed. Preferential methylation is obscured when two GATC sites are positioned on the same DNA molecule, unless both sites are surrounded by large amounts of nonspecific DNA. Thus, facilitated diffusion and sequences immediately flanking target sites contribute to higher order specificity for EcoDam; we suggest that the diverse biological roles of the enzyme are in part regulated by these two factors, which may be important for other enzymes that sequence-specifically modify DNA. 相似文献
994.
Julia Wallmeier Diana Bracht Hessa S. Alsaif Gerard W. Dougherty Heike Olbrich Sandra Cindric Mark Dzietko Christoph Heyer Norbert Teig Charlotte Thiels Eissa Faqeih Aqeela Al-Hashim Sameena Khan Ibrahim Mogarri Mohammed Almannai Wadha Al Otaibi Fowzan S. Alkuraya Cordula Koerner-Rettberg Heymut Omran 《American journal of human genetics》2021,108(7):1318-1329
995.
Derived from the Monod-model and regulating principles a regulation model of the rotifer development in chemostats was developed. The model was validated in continuous cultures of Brachionus angularis both in steady-states, when undisturbed, and in transient-states after perturbations by step changes of dilution rate or input substrate concentration. Simulations of the simple model monotonically approached steady-states, but cultures show overshoots and damped oscillations before reaching this state. After introducing time-lags into the model it depends on the size of the time lag if model rotifer densities reach stable steady-state values (at low time lags) or stable limit cycles with periodic oscillations (at high time lags). At even higher time lags chaotic conditions occur in the model with final extinction of the rotifers. 相似文献
996.
Colin D Lancon A Delmas D Lizard G Abrossinow J Kahn E Jannin B Latruffe N 《Biochimie》2008,90(11-12):1674-1684
Resveratrol is a well known polyphenol largely produced in grapevine. It is a strong antioxidant and a free radical scavenger. It exhibits several beneficial effects for health including cancer. Resveratrol antioxidant activity is essential in the prevention of chemical-induced cancer by inhibiting initiation step of carcinogenesis process but it is also considered to inhibit cancer promotion and progression steps. While the effects of resveratrol on cancer cells are widely described, the data available on the antiproliferative potential of resveratrol derivatives remain weak. Nevertheless, resveratrol analogs could exhibit stronger potentials than the parent molecule. So, we compared the cellular effects of trans-resveratrol, trans-epsilon-viniferin and their respective acetate derivatives, as well as a polyphenol mixture extracted from grapevine shoots, called vineatrol. We studied their abilities to interfere with cell proliferation, their uptake and their effects on parameters of cellular state in human hepatoma cells (HepG2). Cell growth experiments show that resveratrol triacetate presents a slightly better antiproliferative potential than resveratrol. The dimer epsilon-viniferin,as well as its pentaacetate analog, is less powerful than resveratrol, although a similar uptake kinetics in cells. Interestingly, among the tested polyphenols, vineatrol is the most potent solution, indicating a possible synergistic effect of both resveratrol and epsilon-viniferin. We took advantage of the fluorescence properties of these compounds to evidence cellular uptake by using flow cytometry. In addition, by competition assay, we demonstrate that resveratrol triacetate enters in hepatic HepG2 cells by the same way as resveratrol. By autofluorescence in situ measurement we observed that resveratrol and related compounds induce deep changes in cells activity. These changes occur mainly by increasing NADPH cell content and the number of green fluorescent cytoplasmic granular structures which may be related to an induction of detoxifying enzyme mechanisms. 相似文献
997.
Lilian Quero Marina Klawitter Anja Schmaus Melanie Rothley Jonathan Sleeman André N Tiaden Juergen Klasen Norbert Boos Michael O Hottiger Karin Wuertz Peter J Richards 《Arthritis research & therapy》2013,15(4):R94
Introduction
Intervertebral disc (IVD) degeneration is characterized by extracellular matrix breakdown and is considered to be a primary cause of discogenic back pain. Although increases in pro-inflammatory cytokine levels within degenerating discs are associated with discogenic back pain, the mechanisms leading to their overproduction have not yet been elucidated. As fragmentation of matrix components occurs during IVD degeneration, we assessed the potential involvement of hyaluronic acid fragments (fHAs) in the induction of inflammatory and catabolic mediators.Methods
Human IVD cells isolated from patient biopsies were stimulated with fHAs (6 to 12 disaccharides) and their effect on cytokine and matrix degrading enzyme production was assessed using quantitative real-time polymerase chain reaction (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA). The involvement of specific cell surface receptors and signal transduction pathways in mediating the effects of fHAs was tested using small interfering RNA (siRNA) approaches and kinase inhibition assays.Results
Treatment of IVD cells with fHAs significantly increased mRNA expression levels of interleukin (IL)-1β, IL-6, IL-8, cyclooxygenase (COX)-2, matrix metalloproteinase (MMP)-1 and -13. The stimulatory effects of fHAs on IL-6 protein production were significantly impaired when added to IVD cells in combination with either Toll-like receptor (TLR)-2 siRNA or a TLR2 neutralizing antibody. Furthermore, the ability of fHAs to enhance IL-6 and MMP-3 protein production was found to be dependent on the mitogen-activated protein (MAP) kinase signaling pathway.Conclusions
These findings suggest that fHAs may have the potential to mediate IVD degeneration and discogenic back pain through activation of the TLR2 signaling pathway in resident IVD cells. 相似文献998.
Jozef Gašparík Łukasz J. Binkowski Andrej Jahnátek Peter Šmehýl Milan Dobiaš Norbert Lukáč Martyna Błaszczyk Magdalena Semla Peter Massanyi 《Biological trace element research》2017,176(2):258-269
Urine lead level is one of the most employed measures of lead exposure and risk. The urine samples used in this study were obtained from ten healthy male cyclists. Dispersive liquid–liquid microextraction combined with ultraviolet and visible spectrophotometry was utilized for preconcentration, extraction, and determination of lead in urine samples. Optimization of the independent variables was carried out based on chemometric methods in three steps. According to the screening and optimization study, 133 μL of CCl4 (extracting solvent), 1.34 mL ethanol (dispersing solvent), pH 2.0, 0.00 % of salt, and 0.1 % O,O-diethyl dithiophosphoric (chelating agent) were used as the optimum independent variables for microextraction and determination of lead. Under the optimized conditions, R 2 was 0.9991, and linearity range was 0.01–100 μg L?1. Precision was evaluated in terms of repeatability and intermediate precision, with relative standard deviations being <9.1 and <15.3 %, respectively. The accuracy was estimated using urine samples of cyclists as real samples and it was confirmed. The relative error of ≤5 % was considered significant in the method specificity study. The lead concentration mean for the cyclists was 3.79 μg L?1 in urine samples. As a result, the proposed method is a robust technique to quantify lead concentrations higher than 11.6 ng L?1 in urine samples. 相似文献
999.
AtSTP6, a new pollen-specific H+-monosaccharide symporter from Arabidopsis 总被引:6,自引:0,他引:6 下载免费PDF全文
This paper describes the molecular, kinetic, and physiological characterization of AtSTP6, a new member of the Arabidopsis H(+)/monosaccharide transporter family. The AtSTP6 gene (At3g05960) is interrupted by two introns and encodes a protein of 507 amino acids containing 12 putative transmembrane helices. Expression in yeast (Saccharomyces cerevisiae) shows that AtSTP6 is a high-affinity (K(m) = 20 microM), broad-spectrum, and uncoupler-sensitive monosaccharide transporter that is targeted to the plasma membrane and that can complement a growth deficiency resulting from the disruption of most yeast hexose transporter genes. Analyses of AtSTP6-promoter::GUS plants and in situ hybridization experiments detected AtSTP6 expression only during the late stages of pollen development. A transposon-tagged Arabidopsis mutant was isolated and homozygous plants were analyzed for potential effects of the Atstp6 mutation on pollen viability, pollen germination, fertilization, and seed production. However, differences between wild-type and mutant plants could not be observed. 相似文献
1000.
To understand the molecular basis and to prevent diseases such as Alzheimer's disease (AD), the targets of the triggering agent have to be elucidated. beta-Amyloid peptide (Abeta) is the major component of extracellular senile plaques characteristic of AD. For a very long time, the aggregated form of the Abeta was supposed to be responsible for the neurodegeneration that occurs in AD. Recently, the attention has been diverted to the monomeric or oligomeric forms of Abeta and their interaction with cellular targets. In our investigation, the physiological and medically important insertion of externally applied Abeta monomers into the bilayer of lipid vesicles is demonstrated. Abeta(25-35) has been localized in the region of the lipid alkyl chain, and it has a severe disordering effect on the lamellar order of the lipid bilayer. Both of these results are of biomedical relevance. 相似文献