全文获取类型
收费全文 | 19651篇 |
免费 | 2002篇 |
国内免费 | 11篇 |
专业分类
21664篇 |
出版年
2023年 | 79篇 |
2022年 | 158篇 |
2021年 | 338篇 |
2020年 | 223篇 |
2019年 | 247篇 |
2018年 | 304篇 |
2017年 | 329篇 |
2016年 | 447篇 |
2015年 | 794篇 |
2014年 | 875篇 |
2013年 | 972篇 |
2012年 | 1372篇 |
2011年 | 1317篇 |
2010年 | 895篇 |
2009年 | 815篇 |
2008年 | 1112篇 |
2007年 | 1216篇 |
2006年 | 968篇 |
2005年 | 1020篇 |
2004年 | 1029篇 |
2003年 | 970篇 |
2002年 | 923篇 |
2001年 | 323篇 |
2000年 | 322篇 |
1999年 | 285篇 |
1998年 | 263篇 |
1997年 | 173篇 |
1996年 | 134篇 |
1995年 | 130篇 |
1994年 | 150篇 |
1993年 | 127篇 |
1992年 | 201篇 |
1991年 | 198篇 |
1990年 | 158篇 |
1989年 | 164篇 |
1988年 | 157篇 |
1987年 | 158篇 |
1986年 | 144篇 |
1985年 | 166篇 |
1984年 | 160篇 |
1983年 | 120篇 |
1982年 | 93篇 |
1981年 | 110篇 |
1980年 | 92篇 |
1979年 | 95篇 |
1978年 | 99篇 |
1977年 | 70篇 |
1976年 | 84篇 |
1974年 | 90篇 |
1971年 | 74篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
Protein tyrosine phosphatase 1B (PTP1B) is involved in the down-regulation of insulin signaling and is a well-validated therapeutic target for the treatment of diabetes and obesity. Key to the design of potent inhibitors of PTP1B is a moiety that effectively mimics the phosphate group of the natural phosphotyrosine substrate. Difluoromethylsulfonomethylphenylalanine (F(2)Smp) is one of the best monoanionic pTyr mimics reported to date. However, the difluoromethylenesulfonic acid (DFMS) group as a phosphate mimic has not been carefully evaluated in the context of a non-peptidyl platform. Here we present a careful examination of the DFMS group as a phosphate mimic. This was achieved by first constructing an analog of a previously reported high affinity, non-peptidyl PTP1B inhibitor (compound 2, IC(50)=8nM) in which a difluoromethylenephosphonic acid group is replaced with the DFMS moiety (compound 6). We also report the synthesis of its non-fluorinated methylenesulfonic analog (compound 7), as well as two other derivatives in which a distal sulfonamide moiety is replaced with a difluoromethylenesulfonamide group (compounds 8 and 9). Compounds 2 and 6-9 were examined as PTP1B inhibitors. Replacing the distal sulfonamide moiety with a difluoromethylenesulfonamide group had only a modest effect on inhibitor potency. However, compound 6 was approximately a 1000-fold poorer inhibitor than compound 2. Most significantly, inhibition studies with compound 7 and a peptide bearing sulfonomethylphenylalanine revealed that the fluorines have little effect on the potency of the DFMS-bearing inhibitors. This is in contrast to a previous assumption that the fluorines in DFMS-bearing inhibitors contributed significantly to their potency. This may in part explain the large difference in potency between the DFMS and DFMP-bearing compounds. These results also demonstrate that sulfonomethylphenylalanine, a pTyr mimic that is readily constructed, is a relatively good pTyr mimic in comparison to most others that have been reported when examined in the context of the DADE-X-LNH(2) peptide platform. 相似文献
872.
We describe a method for imaging individual mRNA molecules in fixed cells by probing each mRNA species with 48 or more short, singly labeled oligonucleotide probes. This makes each mRNA molecule visible as a computationally identifiable fluorescent spot by fluorescence microscopy. We demonstrate simultaneous detection of three mRNA species in single cells and mRNA detection in yeast, nematodes, fruit fly wing discs, and mammalian cell lines and neurons. 相似文献
873.
Alexander JJ Anderson AJ Barnum SR Stevens B Tenner AJ 《Journal of neurochemistry》2008,107(5):1169-1187
The complement cascade has long been recognized to play a key role in inflammatory and degenerative diseases. It is a 'double edged' sword as it is necessary to maintain health, yet can have adverse effects when unregulated, often exacerbating disease. The contrasting effects of complement, depending on whether in a setting of health or disease, is the price paid to achieve flexibility in scope and degree of a protective response for the host from infection and injury. Loss or even decreased efficiency of critical regulatory control mechanisms can result in aggravated inflammation and destruction of self-tissue. The role of the complement cascade is poorly understood in the nervous system and neurological disorders. Novel studies have demonstrated that the expression of complement proteins in brain varies in different cell types and the effects of complement activation in various disease settings appear to differ. Understanding the functioning of this cascade is essential, as it has therapeutic implications. In this review, we will attempt to provide insight into how this complex cascade functions and to identify potential strategic targets for therapeutic intervention in chronic diseases as well as acute injury in the CNS. 相似文献
874.
We tested the hypothesis that sex-biased natal dispersal reduces close inbreeding in American black bears, a solitary species that exhibits nearly complete male dispersal and female philopatry. Using microsatellite DNA and spatial data from reproductively mature bears (>or= 4 years old), we examined the spatial genetic structure of two distinct populations in New Mexico from 1993 to 2000. As predicted, relatedness (r) and the frequency of close relationships (parent-offspring or full siblings) decreased with distance among female dyads, but little change was observed among male or opposite-sex dyads. Neighbouring females were more closely related than neighbouring males. The potential for inbreeding was low. Most opposite-sex pairs that lived sufficiently close to facilitate mating were unrelated, and few were close relatives. We found no evidence that bears actively avoided inbreeding in their selection of mates from this nearby pool, as mean r and relationship frequencies did not differ between potential and actual mating pairs (determined by parentage analysis). These basic patterns were apparent in both study areas despite a nearly two-fold difference in density. However, the sex bias in dispersal was less pronounced in the lower-density area, based on proportions of bears with male and female relatives residing nearby. This result suggests that male bears may respond to reduced competition by decreasing their rate or distance of dispersal. Evidence supports the hypothesis that inbreeding avoidance is achieved by means of male-biased dispersal but also indicates that competition (for mates or resources) modifies dispersal patterns. 相似文献
875.
Atoui A Bao D Kaur N Grayburn WS Calvo AM 《Applied and environmental microbiology》2008,74(11):3596-3600
The Aspergillus nidulans putative mitogen-activated protein kinase encoded by mpkB has a role in natural product biosynthesis. An mpkB mutant exhibited a decrease in sterigmatocystin gene expression and low mycotoxin levels. The mutation also affected the expression of genes involved in penicillin and terrequinone A synthesis. mpkB was necessary for normal expression of laeA, which has been found to regulate secondary metabolism gene clusters. 相似文献
876.
Jennifer L. Bankers-Fulbright Kathleen R. Bartemes Gail M. Kephart Hirohito Kita Scott M. O’Grady 《The Journal of membrane biology》2009,228(2):99-109
Human eosinophils spontaneously adhere to various substrates in the absence of exogenously added activators. In the present
study a method was developed for characterizing eosinophil adhesion by measuring changes in impedance. Impedance measurements
were performed in HCO3-buffered HybriCare medium maintained in a humidified 5% CO2 incubator at 37°C. Impedance increased by more than 1 kΩ within minutes after eosinophils made contact with the substrate,
reaching a peak within 20 min. Blocking mobilization of intracellular [Ca2+] that precedes adhesion with BAPTA-AM (10 μM) completely inhibited the rise in impedance as well as the changes in cell shape
typically observed in adherent cells. However, lowering the extracellular [Ca2+] with 2.5 mM EGTA did not inhibit the increase in impedance. Pretreatment with anti-CD18 antibody to block substrate interactions
with β2-integrins, or jasplakinolide (2 μM) to block actin reorganization, abolished the increase in impedance and adherent morphology
of the cells. Exposure of eosinophils to the phosphatidylinositol 3 kinase inhibitor LY294002 (5 μM) or treatment with protein
kinase C zeta pseudosubstrate to competitively inhibit activity of the enzyme significantly reduced the increase in impedance
and inhibited the cell spreading associated with adhesion. These results demonstrate a novel method for measuring eosinophil
adhesion and showed that, following formation of a tethered attachment, a rapid increase in intracellular [Ca2+] precedes the cytoskeletal rearrangements required for cell shape changes and plasma membrane-substrate interactions associated
with adhesion. 相似文献
877.
Saccharomyces cerevisiae and some related yeasts are unusual in that two of the enzyme activities (galactose mutarotase and UDP-galactose 4-epimerase) required for the Leloir pathway of d-galactose catabolism are contained within a single protein-Gal10p. The recently solved structure of the protein shows that the two domains are separate and have similar folds to the separate enzymes from other species. The biochemical properties of Gal10p have been investigated using recombinant protein expressed in, and purified from, Escherichia coli. Protein-protein crosslinking confirmed that Gal10p is a dimer in solution and this state is unaffected by the presence of substrates. The steady-state kinetic parameters of the epimerase reaction are similar to those of the human enzyme, and are not affected by simultaneous activity at the mutarotase active site. The mutarotase active site has a strong preference for galactose over glucose, and is not affected by simultaneous epimerase activity. This absence of reciprocal kinetic effects between the active sites suggests that they act independently and do not influence or regulate each other. 相似文献
878.
Eastern equine encephalitis virus (EEEV) causes sporadic but often severe cases of human and equine neurological disease in North America. To determine how EEEV may evade innate immune responses, we screened individual EEEV proteins for the ability to rescue the growth of a Newcastle disease virus expressing green fluorescent protein (NDV-GFP) from the antiviral effects of interferon (IFN). Only expression of the EEEV capsid facilitated NDV-GFP replication. Inhibition of the antiviral effects of IFN by the capsid appears to occur through a general inhibition of cellular gene expression. For example, the capsid inhibited the expression of several reporter genes under the control of RNA polymerase II promoters. In contrast, capsid did not inhibit expression from a T7 RNA polymerase promoter construct, suggesting that the inhibition of gene expression is specific and is not a simple manifestation of toxicity. The inhibition correlated both with capsid-induced phosphorylation of eukaryotic initiation factor 2 alpha and with capsid-mediated inhibition of cellular mRNA accumulation. Mapping analysis identified the N terminus as the region important for the inhibition of host gene expression, suggesting that this inhibition is independent of capsid protease activity. Finally, when cell lines containing EEEV replicons encoding capsid were selected, replicons consistently acquired mutations that deleted all or part of the capsid, for example, amino acids 18 to 135. Given that the amino terminus of the capsid is required to inhibit host cell gene expression, these data suggest that capsid expression from the replicons is ultimately toxic to host cells, presumably because of its ability to inhibit gene expression. 相似文献
879.
Miller KA Barrow J Collinson JM Davidson S Lear M Hill RE Mackenzie A 《Developmental biology》2007,311(2):665-678
The product of the Msx1 gene is a potent inhibitor of muscle differentiation. Msx1 is expressed in muscle precursor cells of the limb bud that also express Pax3. It is thought that Msx1 may facilitate distal migration by delaying myogenesis in these cells. Despite the role played by Msx1 in inhibiting muscle differentiation, nothing is known of the mechanisms that support the expression of the Msx1 gene within limb bud muscle precursor cells. In the present study we have used a combination of comparative genomics, mouse transgenic analysis, in situ hybridisation and immunohistochemistry to identify a highly conserved and tissue-specific regulatory sub-domain within the previously characterised Msx1 gene proximal enhancer element that supports the expression of the Msx1 gene in Pax3-expressing mouse limb pre-muscle masses. Furthermore, using a combination of in situ hybridisation, in vivo ChIP assay and transgenic explant culture analysis we provide evidence that Msx1 expression in limb bud muscle precursor cells is dependent on the canonical Wnt/TCF signalling pathway that is important in muscle shape formation. The results of these studies provide evidence of a mechanistic link between the Wnt/TCF and the Msx1/Pax3/MyoD pathways within limb bud muscle precursor cells. 相似文献
880.
Katie L Mickle Sunita Ramanathan Adam Rosebrock Anna Oliva Amna Chaudari Chulee Yompakdee Donna Scott Janet Leatherwood Joel A Huberman 《BMC molecular biology》2007,8(1):1-28