首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   12742篇
  免费   1093篇
  国内免费   6篇
  13841篇
  2023年   72篇
  2022年   143篇
  2021年   275篇
  2020年   147篇
  2019年   178篇
  2018年   266篇
  2017年   233篇
  2016年   401篇
  2015年   625篇
  2014年   796篇
  2013年   864篇
  2012年   1197篇
  2011年   1080篇
  2010年   706篇
  2009年   652篇
  2008年   826篇
  2007年   802篇
  2006年   755篇
  2005年   754篇
  2004年   667篇
  2003年   597篇
  2002年   571篇
  2001年   119篇
  2000年   76篇
  1999年   113篇
  1998年   124篇
  1997年   86篇
  1996年   88篇
  1995年   66篇
  1994年   56篇
  1993年   66篇
  1992年   49篇
  1991年   39篇
  1990年   32篇
  1989年   33篇
  1988年   24篇
  1987年   14篇
  1986年   18篇
  1985年   13篇
  1984年   21篇
  1983年   22篇
  1982年   12篇
  1981年   14篇
  1980年   17篇
  1979年   13篇
  1978年   10篇
  1977年   11篇
  1976年   12篇
  1973年   7篇
  1972年   7篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
p24 proteins are assumed to play an important role in the transport of secreted and transmembrane proteins into membranes. However, only few cargo proteins are known that partially, but in no case completely require p24 proteins for membrane transport. Here, we show that two p24 proteins are essential for dorsoventral patterning of Drosophila melanogaster embryo. Mutations in the genes, eclair (eca) and baiser (bai), encoding two p24 proteins reduce signalling by the TGF-beta homologue, Dpp, in early embryos. This effect is strictly maternal and specific to early embryogenesis, as Dpp signalling in other contexts is not notably affected. We provide genetic evidence that in the absence of eca or bai function in the oocyte, the maternally expressed type I TGF-beta receptor Tkv is not active. We propose that during early embryogenesis eca and bai are specifically required for the activity of the maternal Tkv, while the zygotic Tkv is not affected in the mutant embryos. Mutations in either eca or bai are sufficient for the depletion of Tkv activity and no enhancement of the phenotypes was observed in embryos derived from oocytes mutant for both genes. The dependence of maternal Tkv protein on the products of p24 genes may serve as an in vivo model for studying p24 proteins.  相似文献   
52.
To elucidate the function of the two cytokine-binding modules (CBM) of the leukemia inhibitory factor receptor (LIFR), receptor chimeras of LIFR and the interleukin-6 receptor (IL-6R) were constructed. Either the NH(2)-terminal (chimera RILLIFdeltaI) or the COOH-terminal LIFR CBM (chimera RILLIFdeltaII) were replaced by the structurally related CBM of the IL-6R which does not bind LIF. Chimera RILLIFdeltaI is functionally inactive, whereas RILLIFdeltaII binds LIF and mediates signalling as efficiently as the wild-type LIFR. Deletion mutants of the LIFR revealed that both the NH(2)-terminal CBM and the Ig-like domain of the LIFR are involved in LIF binding, presumably via the LIF site III epitope. The main function of the COOH-terminal CBM of the LIFR is to position the NH(2)-terminal CBM and the Ig-like domain, so that these can bind to LIF. In analogy to a recently published model of the IL-6R complex, a model of the active LIFR complex is suggested which positions the COOH-terminal CBM at LIF site I and the NH(2)-terminal CBM and the Ig-like domain at site III. An additional contact is postulated between the Ig-like domain of gp130 and the NH(2)-terminal CBM of the LIFR.  相似文献   
53.
Recent advances in chlorophyll biosynthesis and breakdown in higher plants   总被引:18,自引:0,他引:18  
Chlorophyll (Chl) has unique and essential roles in photosynthetic light-harvesting and energy transduction, but its biosynthesis, accumulation and degradation is also associated with chloroplast development, photomorphogenesis and chloroplast-nuclear signaling. Biochemical analyses of the enzymatic steps paved the way to the identification of their encoding genes. Thus, important progress has been made in the recent elucidation of almost all genes involved in Chl biosynthesis and breakdown. In addition, analysis of mutants mainly in Arabidopsis, genetically engineered plants and the application of photo-reactive herbicides contributed to the genetic and regulatory characterization of the formation and breakdown of Chl. This review highlights recent progress in Chl metabolism indicating highly regulated pathways from the synthesis of precursors to Chl and its degradation to intermediates, which are not longer photochemically active.  相似文献   
54.
Evidence for specific and direct bacterial product recognition through toll-like receptors (TLRs) has been emphasized recently. We analyzed lipopeptide analogues and enterobacterial lipopolysaccharide (eLPS) for their potential to activate cells through TLR2 and TLR4. Whereas bacterial protein palmitoylated at its N-terminal cysteine and N-terminal peptides derived thereof are known to induce TLR2-mediated cell activation, a synthetic acylhexapeptide mimicking a bacterial lipoprotein subpopulation for which N-terminal trimyristoylation is characteristic (Myr(3)CSK(4)) activated cells not only through TLR2 but also through TLR4. Conversely, highly purified eLPS triggered cell activation through overexpressed TLR2 in the absence of TLR4 expression if CD14 was coexpressed. Accordingly, TLR2(-/-) macrophages prepared upon gene targeting responded to Myr(3)CSK(4) challenge, whereas TLR2(-/-)/TLR4(d/d) cells were unresponsive. Through interferon-gamma (IFNgamma) priming, macrophages lacking expression of functional TLR4 and/or MD-2 acquired sensitivity to eLPS, whereas TLR2/TLR4 double deficient cells did not. Not only TLR2(-/-) mice but also TLR4(-/-) mice were resistant to Myr(3)CSK(4) challenge-induced fatal shock. d-Galactosamine-sensitized mice expressing defective TLR4 or lacking TLR4 expression acquired susceptibility to eLPS-driven toxemia upon IFNgamma priming, whereas double deficient mice did not. Immunization toward ovalbumin using Myr(3)CSK(4) as adjuvant was ineffective in TLR2(-/-)/TLR4(-/-) mice yet effective in wild-type, TLR2(-/-), or TLR4(-/-) mice as shown by analysis of ovalbumin-specific serum Ig concentration. A compound such as Myr(3)CSK(4) whose stimulatory activity is mediated by both TLR2 and TLR4 might constitute a preferable adjuvant. On the other hand, simultaneous blockage of both of the two TLRs might effectively inhibit infection-induced pathology.  相似文献   
55.
Ligand binding to the extracellular domain of the thrombopoietin receptor (TpoR) imparts a specific orientation on the transmembrane (TM) and intracellular domains of the receptors that is required for physiologic activation via receptor dimerization. To map the inactive and active dimeric orientations of the TM helices, we performed asparagine (Asn)-scanning mutagenesis of the TM domains of the murine and human TpoR. Substitution of Asn at only one position (S505N) activated the human receptor, whereas Asn substitutions at several positions activated the murine receptor. Second site mutational studies indicate that His499 near the N terminus of the TM domain is responsible for protecting the human receptor from activation by Asn mutations. Structural studies reveal that the sequence preceding His499 is helical in the murine receptor but non-helical in peptides corresponding to the TM domain of the inactive human receptor. The activating S505N mutation and the small molecule agonist eltrombopag both induce helix in this region of the TM domain and are associated with dimerization and activation of the human receptor. Thus, His499 regulates the activation of human TpoR and provides additional protection against activating mutations, such as oncogenic Asn mutations in the TM domain.  相似文献   
56.
Mycobacterium bovis bacillus Calmette-Guérin (BCG) is a live vaccine that has been used in routine vaccination against tuberculosis for nearly 80 years. However, its efficacy is controversial. The failure of BCG vaccination may be at least partially explained by the induction of poor or inappropriate host responses. Dendritic cells (DCs) are likely to play a key role in the induction of immune response to mycobacteria by polarizing the reactivity of T lymphocytes toward a Th1 profile, contributing to the generation of protective cellular immunity against mycobacteria. In this study we aimed to investigate the production of Th1 and Th2 cytokines by naive CD4+ T cells to mycobacterial antigen-pulsed DCs in the group of young, healthy BCG vaccinated volunteers. The response of naive helper T cells was compared with the response of total blood lymphocytes. Our present results clearly showed that circulating naive CD45RA+CD4+ lymphocytes from BCG-vaccinated subjects can become effector helper cells producing IFN-gamma and IL-5 under the stimulation by autologous dendritic cells presenting mycobacterial protein antigen-PPD or infected with live M. bovis BCG bacilli.  相似文献   
57.
ErbB-2 and met reciprocally regulate cellular signaling via plexin-B1   总被引:3,自引:0,他引:3  
Sema4D-induced activation of plexin-B1 has been reported to evoke different and sometimes opposing cellular responses. The mechanisms underlying the versatility of plexin-B1-mediated effects are not clear. Plexin-B1 can associate with the receptor tyrosine kinases ErbB-2 and Met. Here we show that Sema4D-induced activation and inactivation of RhoA require ErbB-2 and Met, respectively. In breast carcinoma cells, Sema4D can have pro- and anti-migratory effects depending on the presence of ErbB-2 and Met, and the exchange of the two receptor tyrosine kinases is sufficient to convert the cellular response to Sema4D from pro- to anti-migratory and vice versa. This work identifies a novel mechanism by which plexin-mediated signaling can be regulated and explains how Sema4D can exert different biological activities through the differential association of its receptor with ErbB-2 and Met.  相似文献   
58.
59.
Antibiotic resistance has been reported since the introduction of synthetic antibiotics. Bacteria, such as one of the most common nosocomial pathogens P. aeruginosa, adapt quickly to changing environmental conditions, due to their short generation time. Thus microevolutional changes can be monitored in situ. In this study, the microevolutional process of Pseudomonas aeruginosa PAO1 resistance against a recently developed novel antibacterial zinc Schiff-base (ZSB) was investigated at the proteome level. After extended exposure to ZSB the passaged strain differed in tolerance against ZSB, with the adapted P. aeruginosa PAO1 exhibiting 1.6 times higher minimal inhibitory concentration. Using Two-dimensional Difference Gel Electrophoresis, the changes in the proteome of ZSB adapted P. aeruginosa PAO1 were examined by comparison with the non-adapted P. aeruginosa PAO1. The proteome of the adapted P. aeruginosa PAO1 strain differed significantly from the non-adapted in the abundance of two proteins when both strains were grown under stressing conditions. One protein could be identified as the outer membrane protein D that plays a role in uptake of basic amino acids as well as in carbapeneme resistance. The second protein has been identified as alkyl peroxide reductase subunit F. Our data indicated a slight increase in abundance of alkyl peroxide reductase F (AhpF) in the case of ZSB passaged P. aeruginosa PAO1. Higher abundance of Ahp has been discussed in the literature as a promoter of accelerated detoxification of benzene derivatives. The observed up-regulated AhpF thus appears to be connected to an increased tolerance against ZSB. Changes in the abundance of proteins connected to oxidative stress were also found after short-time exposure of P. aeruginosa PAO1 to the ZSB. Furthermore, adapted P. aeruginosa PAO1 showed increased tolerance against hydrogen peroxide and, in addition, showed accelerated degradation of ZSB, as determined by HPLC measurements.  相似文献   
60.
We compared the expression of a functional recombinant TMVspecific fullsize antibody (rAb29) in both the apoplast and cytosol of tobacco plants and a single chain antibody fragment (scFv29), derived from rAb29, was expressed in the cytosol. Cloned heavy and light chain cDNAs of fullsize rAb29, which binds to TMV coat protein monomers, were integrated into the plant expression vector pSS. The fullsize rAb29 was expressed in the cytosol and targeted to the apoplast by including the original murine antibody leader sequences. Levels of functional fullsize rAb29 expression were high in the apoplast (up to 8.5g per gram leaf tissue), whereas cytosolic expression was low or at the ELISA detection limit. Sequences of the variable domains of rAb29 light and heavy chain were used to generate the single chain antibody scFv29, which was expressed in the periplasmic space of E.coli and showed the same binding specificity as fullsize rAb29. In addition, scFv29 was functionally expressed in the cytosol of tobacco plants and plant derived scFv29 maintained same binding specificity to TMVcoat protein monomers as rAb29.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号