全文获取类型
收费全文 | 56316篇 |
免费 | 4720篇 |
国内免费 | 36篇 |
专业分类
61072篇 |
出版年
2023年 | 275篇 |
2022年 | 607篇 |
2021年 | 1098篇 |
2020年 | 805篇 |
2019年 | 864篇 |
2018年 | 1253篇 |
2017年 | 1049篇 |
2016年 | 1704篇 |
2015年 | 2453篇 |
2014年 | 2431篇 |
2013年 | 2994篇 |
2012年 | 3556篇 |
2011年 | 3298篇 |
2010年 | 2035篇 |
2009年 | 1937篇 |
2008年 | 2365篇 |
2007年 | 2347篇 |
2006年 | 2130篇 |
2005年 | 2422篇 |
2004年 | 2380篇 |
2003年 | 1995篇 |
2002年 | 1608篇 |
2001年 | 1468篇 |
2000年 | 1396篇 |
1999年 | 1251篇 |
1998年 | 576篇 |
1997年 | 544篇 |
1996年 | 593篇 |
1995年 | 462篇 |
1994年 | 469篇 |
1993年 | 438篇 |
1992年 | 935篇 |
1991年 | 837篇 |
1990年 | 770篇 |
1989年 | 765篇 |
1988年 | 759篇 |
1987年 | 690篇 |
1986年 | 649篇 |
1985年 | 626篇 |
1984年 | 586篇 |
1983年 | 443篇 |
1982年 | 339篇 |
1981年 | 337篇 |
1980年 | 319篇 |
1979年 | 438篇 |
1978年 | 363篇 |
1977年 | 299篇 |
1975年 | 299篇 |
1974年 | 302篇 |
1973年 | 301篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Hoogsteen (HG) base pairing is characterized by a 180° rotation of the purine base with respect to the Watson-Crick-Franklin (WCF) motif. Recently, it has been found that both conformations coexist in a dynamical equilibrium and that several biological functions require HG pairs. This relevance has motivated experimental and computational investigations of the base-pairing transition. However, a systematic simulation of sequence variations has remained out of reach. Here, we employ advanced path-based methods to perform unprecedented free-energy calculations. Our methodology enables us to study the different mechanisms of purine rotation, either remaining inside or after flipping outside of the double helix. We study seven different sequences, which are neighbor variations of a well-studied A⋅T pair in A6-DNA. We observe the known effect of A⋅T steps favoring HG stability, and find evidence of triple-hydrogen-bonded neighbors hindering the inside transition. More importantly, we identify a dominant factor: the direction of the A rotation, with the 6-ring pointing either towards the longer or shorter segment of the chain, respectively relating to a lower or higher barrier. This highlights the role of DNA’s relative flexibility as a modulator of the WCF/HG dynamic equilibrium. Additionally, we provide a robust methodology for future HG proclivity studies. 相似文献
972.
Bradley E. Hiller Yongjun Yin Yi-Chieh Perng Ítalo de Araujo Castro Lindsey E. Fox Marissa C. Locke Kristen J. Monte Carolina B. Lpez David M. Ornitz Deborah J. Lenschow 《PLoS pathogens》2022,18(6)
Influenza A virus (IAV) preferentially infects conducting airway and alveolar epithelial cells in the lung. The outcome of these infections is impacted by the host response, including the production of various cytokines, chemokines, and growth factors. Fibroblast growth factor-9 (FGF9) is required for lung development, can display antiviral activity in vitro, and is upregulated in asymptomatic patients during early IAV infection. We therefore hypothesized that FGF9 would protect the lungs from respiratory virus infection and evaluated IAV pathogenesis in mice that overexpress FGF9 in club cells in the conducting airway epithelium (FGF9-OE mice). However, we found that FGF9-OE mice were highly susceptible to IAV and Sendai virus infection compared to control mice. FGF9-OE mice displayed elevated and persistent viral loads, increased expression of cytokines and chemokines, and increased numbers of infiltrating immune cells as early as 1 day post-infection (dpi). Gene expression analysis showed an elevated type I interferon (IFN) signature in the conducting airway epithelium and analysis of IAV tropism uncovered a dramatic shift in infection from the conducting airway epithelium to the alveolar epithelium in FGF9-OE lungs. These results demonstrate that FGF9 signaling primes the conducting airway epithelium to rapidly induce a localized IFN and proinflammatory cytokine response during viral infection. Although this response protects the airway epithelial cells from IAV infection, it allows for early and enhanced infection of the alveolar epithelium, ultimately leading to increased morbidity and mortality. Our study illuminates a novel role for FGF9 in regulating respiratory virus infection and pathogenesis. 相似文献
973.
Protha Biswas Uttpal Anand Suchismita Chatterjee Saha Nishi Kant Tulika Mishra Harison Masih Ananya Bar Devendra Kumar Pandey Niraj
Kumar Jha Madhumita Majumder Neela Das Vijaykumar
Shivaji Gadekar Mahipal S. Shekhawat Manoj Kumar Radha Jarosaw Prokw Jos M. Prez de la Lastra Abhijit Dey 《Journal of cellular and molecular medicine》2022,26(11):3083
974.
Christopher I. Jarvis Amy Gimma Flavio Finger Tim P. Morris Jennifer A. Thompson Olivier le Polain de Waroux W. John Edmunds Sebastian Funk Thibaut Jombart 《PLoS computational biology》2022,18(5)
The fraction of cases reported, known as ‘reporting’, is a key performance indicator in an outbreak response, and an essential factor to consider when modelling epidemics and assessing their impact on populations. Unfortunately, its estimation is inherently difficult, as it relates to the part of an epidemic which is, by definition, not observed. We introduce a simple statistical method for estimating reporting, initially developed for the response to Ebola in Eastern Democratic Republic of the Congo (DRC), 2018–2020. This approach uses transmission chain data typically gathered through case investigation and contact tracing, and uses the proportion of investigated cases with a known, reported infector as a proxy for reporting. Using simulated epidemics, we study how this method performs for different outbreak sizes and reporting levels. Results suggest that our method has low bias, reasonable precision, and despite sub-optimal coverage, usually provides estimates within close range (5–10%) of the true value. Being fast and simple, this method could be useful for estimating reporting in real-time in settings where person-to-person transmission is the main driver of the epidemic, and where case investigation is routinely performed as part of surveillance and contact tracing activities. 相似文献
975.
Common regulation of apoptosis signaling induced by CD95 and the DNA-damaging stimuli etoposide and gamma-radiation downstream from caspase-8 activation. 总被引:13,自引:0,他引:13
J G Boesen-de Cock A D Tepper E de Vries W J van Blitterswijk J Borst 《The Journal of biological chemistry》1999,274(20):14255-14261
The death receptor CD95 (APO-1/Fas), the anticancer drug etoposide, and gamma-radiation induce apoptosis in the human T cell line Jurkat. Variant clones selected for resistance to CD95-induced apoptosis proved cross-resistant to etoposide- and radiation-induced apoptosis, suggesting that the apoptosis pathways induced by these distinct stimuli have critical component(s) in common. The pathways do not converge at the level of CD95 ligation or caspase-8 signaling. Whereas caspase-8 function was required for CD95-mediated cytochrome c release, effector caspase activation, and apoptosis, these responses were unaffected in etoposide-treated and irradiated cells when caspase-8 was inhibited by FLIPL. Both effector caspase processing and cytochrome c release were inhibited in the resistant variant cells as well as in Bcl-2 transfectants, suggesting that, in Jurkat cells, the apoptosis signaling pathways activated by CD95, etoposide, and gamma-radiation are under common mitochondrial control. All three stimuli induced ceramide production in wild-type cells, but not in resistant variant cells. Exogenous ceramide bypassed apoptosis resistance in the variant cells, but not in Bcl-2-transfected cells, suggesting that apoptosis signaling induced by CD95, etoposide, and gamma-radiation is subject to common regulation at a level different from that targeted by Bcl-2. 相似文献
976.
Andrushchenko VV van de Sande JH Wieser H Kornilova SV Blagoi YP 《Journal of biomolecular structure & dynamics》1999,17(3):545-560
The B-Z transition of the synthetic oligonucleotide, (dG-dC)20, induced by Mn2+ ions at room temperature, was investigated by absorption and Vibrational Circular Dichroism (VCD) spectroscopy in the range of 1800-800 cm(-1). Metal ion concentration was varied from 0 to 0.73 M Mn2+ (0 to 8.5 moles of Mn2+ per mole of oligonucleotide phosphate, [Mn]/[P]). While both types of spectra showed considerable changes as the Mn2+ concentrations were raised, differences between the two were often complementary in their expression and extent, those displayed by VCD being more clearly evident due to the inversion of the opposite helical sense from the right-handed to the left-handed conformation. The main phase of the transition occurred in the metal ion concentration between 0.8-1.1 [Mn]/[P]. Gradual changes that took place in the spectra were interpreted in terms of simultaneous processes that depended on metal ion concentration, namely B-Z transformation, binding of Mn2+ to phosphates and to nitrogen bases, and partial denaturation. Below approximately 0.6 [Mn]/[P], only a small portion of the oligonucleotide adopted the Z conformation within a 3 hour period, whereas conversion was completed in the same time interval for concentrations between 0.9-1.2 [Mn]/[P]. At [Mn]/[P] >1.7, complete transition to the Z-form took place immediately on adding Mn2+. Applying VCD spectroscopy in combination with conventional infrared absorption proved most useful for corroborating changes in the absorption spectra, and for detecting in an unique manner, not attainable by absorption methods, conformational changes that lead to the inversion of the helical sense of the oligonucleotide. 相似文献
977.
978.
Helicobacter pylori vacuolating toxin forms anion-selective channels in planar lipid bilayers: possible implications for the mechanism of cellular vacuolation 总被引:6,自引:0,他引:6 下载免费PDF全文
Tombola F Carlesso C Szabò I de Bernard M Reyrat JM Telford JL Rappuoli R Montecucco C Papini E Zoratti M 《Biophysical journal》1999,76(3):1401-1409
The Helicobacter pylori VacA toxin plays a major role in the gastric pathologies associated with this bacterium. When added to cultured cells, VacA induces vacuolation, an effect potentiated by preexposure of the toxin to low pH. Its mechanism of action is unknown. We report here that VacA forms anion-selective, voltage-dependent pores in artificial membranes. Channel formation was greatly potentiated by acidic conditions or by pretreatment of VacA at low pH. No requirement for particular lipid(s) was identified. Selectivity studies showed that anion selectivity was maintained over the pH range 4.8-12, with the following permeability sequence: Cl- approximately HCO3- > pyruvate > gluconate > K+ approximately Li+ approximately Ba2+ > NH4+. Membrane permeabilization was due to the incorporation of channels with a voltage-dependent conductance in the 10-30 pS range (2 M KCl), displaying a voltage-independent high open probability. Deletion of the NH2 terminus domain (p37) or chemical modification of VacA by diethylpyrocarbonate inhibited both channel activity and vacuolation of HeLa cells without affecting toxin internalization by the cells. Collectively, these observations strongly suggest that VacA channel formation is needed to induce cellular vacuolation, possibly by inducing an osmotic imbalance of intracellular acidic compartments. 相似文献
979.
F. Perestelo M. A. Falcón Ana Carnicero Ana Rodríguez G. de la Fuente 《Biotechnology letters》1994,16(3):299-302
Summary
Serratia marcescens was found to degrade kraft lignin by only 15%. When 14C-radiolabelled lignocelluloses and DHP lignins were used as substrates the bacterium mineralized to 14CO2 only 1.1–1.9% and 0.4–0.8% of the lignins respectively. However, some 44.4% of the 14C--DHP lignin was recovered as soluble radiolabelled products. 相似文献
980.
Effects of ionizing radiations on proteins. Evidence of non-random fragmentations and a caution in the use of the method for determination of molecular mass. 下载免费PDF全文
M Le Maire L Thauvette B de Foresta A Viel G Beauregard M Potier 《The Biochemical journal》1990,267(2):431-439
We have reinvestigated the use of ionizing radiations to measure the molecular mass of water-soluble or membrane proteins. The test was performed by using the most straightforward aspect of the technique, which consists of SDS/PAGE analysis of the protein-fragmentation process. We found that exposure of purified standard proteins to increasing doses of ionizing radiation causes progressive fragmentation of the native protein into defined peptide patterns. The coloured band corresponding to the intact protein was measured on the SDS gel as a function of dose to determine the dose (D37.t) corresponding to 37% of the initial amount of unfragmented protein deposited on the gel. This led to a calibration curve between 1/D37.t and the known molecular mass of the standard proteins whose best fit gave Mr = 1.77 x 10(6)/D37.t at -78 degrees C, i.e. 35% higher than the generally accepted value at that temperature obtained from inactivation studies. However, we have to conclude that this method is useless to determine the state of aggregation of a protein, since, for all the oligomers tested, the best fit was obtained by using the protomeric molecular mass, suggesting that there is no energy transfer between promoters. Furthermore, SDS greatly increases the fragmentation rate of proteins, which suggests additional calibration problems for membrane proteins in detergent or in the lipid bilayer. But the main drawback of the technique arises from our observation that some proteins behaved anomalously, leading to very large errors in the apparent target size as compared with true molecular mass (up to 100%). It is thus unreliable to apply the radiation method for absolute molecular-mass determination. We then focused on the novel finding that discrete fragmentation of proteins occurs at preferential sites, and this was studied in more detail with aspartate transcarbamylase. N-Terminal sequencing of several radiolysis fragments of the catalytic chain of the enzyme revealed that breaks along the polypeptide chains are localized close to the C-terminal end. Examination of the three-dimensional structure of aspartate transcarbamylase suggests that radiolysis sites (fragile bonds) might be localized in connecting loops. 相似文献