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   22441篇
  免费   56篇
  国内免费   53篇
系统科学   225篇
丛书文集   493篇
教育与普及   45篇
理论与方法论   60篇
现状及发展   9487篇
研究方法   878篇
综合类   10991篇
自然研究   371篇
  2013年   129篇
  2012年   276篇
  2011年   665篇
  2010年   101篇
  2008年   325篇
  2007年   376篇
  2006年   400篇
  2005年   390篇
  2004年   375篇
  2003年   383篇
  2002年   306篇
  2001年   708篇
  2000年   706篇
  1999年   414篇
  1992年   410篇
  1991年   363篇
  1990年   387篇
  1989年   326篇
  1988年   368篇
  1987年   366篇
  1986年   342篇
  1985年   484篇
  1984年   363篇
  1983年   303篇
  1982年   238篇
  1981年   256篇
  1980年   343篇
  1979年   687篇
  1978年   580篇
  1977年   551篇
  1976年   472篇
  1975年   531篇
  1974年   663篇
  1973年   579篇
  1972年   588篇
  1971年   701篇
  1970年   925篇
  1969年   726篇
  1968年   624篇
  1967年   653篇
  1966年   596篇
  1965年   433篇
  1964年   113篇
  1959年   264篇
  1958年   400篇
  1957年   303篇
  1956年   269篇
  1955年   238篇
  1954年   263篇
  1948年   176篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
Triosephosphate isomerase: a highly evolved biocatalyst   总被引:1,自引:0,他引:1  
Triosephosphate isomerase (TIM) is a perfectly evolved enzyme which very fast interconverts dihydroxyacetone phosphate and d-glyceraldehyde-3-phosphate. Its catalytic site is at the dimer interface, but the four catalytic residues, Asn11, Lys13, His95 and Glu167, are from the same subunit. Glu167 is the catalytic base. An important feature of the TIM active site is the concerted closure of loop-6 and loop-7 on ligand binding, shielding the catalytic site from bulk solvent. The buried active site stabilises the enediolate intermediate. The catalytic residue Glu167 is at the beginning of loop-6. On closure of loop-6, the Glu167 carboxylate moiety moves approximately 2 Å to the substrate. The dynamic properties of the Glu167 side chain in the enzyme substrate complex are a key feature of the proton shuttling mechanism. Two proton shuttling mechanisms, the classical and the criss-cross mechanism, are responsible for the interconversion of the substrates of this enolising enzyme.  相似文献   
972.
Human bone marrow-derived mesenchymal stem cells (MSC) home to injured tissues and have regenerative capacity. In this study, we have investigated in vitro the influence of apoptotic and necrotic cell death, thus distinct types of tissue damage, on MSC migration. Concordant with an increased overall motility, MSC migrated towards apoptotic, but not vital or necrotic neuronal and cardiac cells. Hepatocyte growth factor (HGF) was expressed by the apoptotic cells only. MSC, in contrast, revealed expression of the HGF-receptor, c-Met. Blocking HGF bioactivity resulted in significant reduction of MSC migration. Moreover, recombinant HGF attracted MSC in a dose-dependent manner. Thus, apoptosis initiates chemoattraction of MSC via the HGF/c-Met axis, thereby linking tissue damage to the recruitment of cells with regenerative potential.  相似文献   
973.
Since its discovery in the 1920s, relaxin has enjoyed a reputation as a peptide hormone of pregnancy. However, relaxin and other relaxin family peptides are now associated with numerous non-reproductive physiologies and disease states. The new millennium bought with it the sequence of the human genome and subsequently new directions for relaxin research. In 2002, the ancestral relaxin gene RLN3 was identified from genome databases. The relaxin-3 peptide is highly expressed in a small region of the brain and in species from teleost to primates and has both conserved sequence and sites of expression. Combined with the discovery of the relaxin family peptide receptors, interest in the role of the relaxin family peptides in the central nervous system has been reignited. This review explores the relaxin family peptides that are expressed in or act upon the brain, the receptors that mediate their actions, and what is currently known of their functions.  相似文献   
974.
The majority rule has been a popular method for producing a consensus classification from several different classifications, when the classifications are all on the same set of objects and are structured as hierarchies. In this note, a new axiomatic characterization is proved for this consensus method on hierarchies.  相似文献   
975.
研究了以Faddeev—Hahn方程半经典形式描述量子力学系统中3个带电粒子交换反应的可能性.为了解所考虑的方程,我们运用了一个考虑Stark线性效应的改进的强耦合道方法.  相似文献   
976.
977.
R Aharoni  D Teitelbaum  R Arnon  J Puri 《Nature》1991,351(6322):147-150
Autoimmune diseases occur when T lymphocytes become activated on recognizing self antigen linked to the autologous class II molecule of the major histocompatibility complex (MHC). The resulting complex of antigen MHC T-cell receptor could be a target for treatment of autoimmune diseases. Studies in which each component is blocked separately might be limited by interference in non-relevant immune responses that either use the same set of T-cell-receptor V gene segments or are linked to the same MHC. We report here an attack by a specific antibody on the unique antigenic site formed by the binding of two components of the trimolecular complex, the autoantigen bound to the self MHC. We tested its effect in experimental allergic encephalomyelitis, an acute neurological autoimmune disease which is widely regarded as a model for autoimmune disorders and which is mediated by CD4+ T cells recognizing myelin basic protein (BP), or its peptides, in association with self Ia. We made monoclonal antibodies which bound only the complex of BP and I-As. These antibodies blocked the proliferative response in vitro to the encephalitogenic determinant of BP and reduced the response to intact BP, without affecting the response to a nonrelevant antigen-purified protein derivative of tuberculin presented on syngeneic macrophages. They also inhibited experimental allergic encephalomyelitis in H-2s mice. Hence, antibodies directed specifically to the autoantigen-Ia complex, may offer a highly selective and effective treatment in autoimmune diseases.  相似文献   
978.
Effects of the steel gene product on mouse primordial germ cells in culture.   总被引:21,自引:0,他引:21  
I Godin  R Deed  J Cooke  K Zsebo  M Dexter  C C Wylie 《Nature》1991,352(6338):807-809
Mutations at the steel (sl) and dominant white spotting (W) loci in the mouse affect primordial germ cells (PGC), melanoblasts and haemopoietic stem cells. The W gene encodes a cell-surface receptor of the tyrosine kinase family, the proto-oncogene c-kit. In situ analysis has shown c-kit messenger RNA expression in PGC in the early genital ridges. The Sl gene encodes the ligand for this receptor, a peptide growth factor, called here stem cell factor (SCF). SCF mRNA is expressed in many regions of the early mouse embryo, including the areas of migration of these cell types. It is important now to identify the role of the Sl-W interaction in the development of these migratory embryonic stem cell populations. Using an in vitro assay system, we show that SCF increases both the overall numbers and colony sizes of migratory PGC isolated from wild-type mouse embryos, and cultured on irradiated feeder layers of STO cells (a mouse embryonic fibroblast line). In the absence of feeder cells, SCF causes a large increase in the initial survival and apparent motility of PGC in culture. But labelling with bromodeoxyuridine shows that SCF is not, by itself, a mitogen for PGC. SCF does not exert a chemotropic effect on PGC in in vitro assays. These results suggest that SCF in vivo is an essential requirement for PGC survival. This demonstrates the control of the early germ-line population by a specific trophic factor.  相似文献   
979.
Assembly of class I major histocompatibility complex (MHC) molecules involves the interaction of two distinct polypeptides (the heavy and light chains) with peptide antigen. Cell lines synthesizing both chains but expressing low levels of MHC class I molecules on their surface as a result of a failure in assembly and transport have been identified. We now report that although the apparent steady-state distribution in these cells of class I molecules is in the endoplasmic reticulum (ER), the molecules in fact are recycled between the ER and Golgi, rather than retained in the ER. This explains the failure of class I molecules to negotiate the secretory pathway. Class I molecules do not seem to be modified by Golgi enzymes, suggesting that the proteins do not reach the Golgi apparatus during recycling. But morphological and subcellular fractionation evidence indicates that they pass through the cis Golgi or a Golgi-associated organelle, which we postulate to be the recycling organelle. This compartment, which we call the 'cis-Golgi network', would thereby be a sorting organelle that selects proteins for return to the ER.  相似文献   
980.
v-Src and EJ Ras alleviate repression of c-Jun by a cell-specific inhibitor   总被引:17,自引:0,他引:17  
V R Baichwal  A Park  R Tjian 《Nature》1991,352(6331):165-168
  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