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   12422篇
  免费   1219篇
  国内免费   6篇
  2021年   142篇
  2020年   105篇
  2019年   139篇
  2018年   151篇
  2017年   156篇
  2016年   263篇
  2015年   418篇
  2014年   482篇
  2013年   536篇
  2012年   685篇
  2011年   649篇
  2010年   415篇
  2009年   397篇
  2008年   513篇
  2007年   552篇
  2006年   522篇
  2005年   494篇
  2004年   458篇
  2003年   444篇
  2002年   475篇
  2001年   431篇
  2000年   415篇
  1999年   345篇
  1998年   181篇
  1997年   167篇
  1996年   164篇
  1995年   142篇
  1994年   132篇
  1993年   151篇
  1992年   237篇
  1991年   241篇
  1990年   223篇
  1989年   202篇
  1988年   181篇
  1987年   171篇
  1986年   196篇
  1985年   174篇
  1984年   130篇
  1983年   134篇
  1982年   108篇
  1981年   88篇
  1980年   75篇
  1979年   106篇
  1978年   109篇
  1977年   65篇
  1976年   64篇
  1975年   67篇
  1974年   80篇
  1972年   79篇
  1971年   72篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Summary Unstimulated IFN- and IL2-stimulated (NK) cell activities were investigated in patients with breast cancer who had received either local radiotherapy alone or adjuvant chemotherapeutic treatment with CMF combined with radiotherapy 12 to 18 months previously. When tested against the primarily NK-sensitive K562 cell line, patients who had received adjuvant chemotherapeutic treatment with CMF were shown to have a significantly decreased unstimulated and IFN-stimulated NK cell activity, as compared to both patients after radiotherapy only (P<0.002) and P<0.005, respectively) and healthy control persons (P<0.05). The former group of patients also had a significantly decreased IFN-stimulated NK cell activity, when tested against the primarily NK-insensitive Chang hepatoma cell line, as compared to patients after radiotherapy only (P<0.005) and healthy controls (P<0.05). Moreover, patients after radiotherapy only proved to have a significantly increased unstimulated (P<0.01) and IFN-stimulated NK cell activity (K562: P<0.05; Chang hepatoma cell line: P<0.05), as compared to healthy control individuals. In contrast, no difference in IL2-stimulated NK cell activity was detected. The investigation for the expression of CD3 and/or Leu 19 antigens as phenotypic markers of cells with non-MHC restricted cytotoxicity showed a significantly lower percentage of cells with the CD3+ phenotype in patients with breast cancer, irrespective of the chosen post-operative treatment, as compared to healthy controls (P<0.01). Finally, patients with breast cancer who had received radiotherapy only had a significant trend towards an increased percentage of CD3+/Leu 19+ PMNC, as compared to both patients after CMF treatment (P<0.05) and healthy controls (P<0.025). We conclude that patients with breast cancer vary on a long-term basis in their NK activity and in the phenotype of their PMNC depending on their post-operative adjuvant management.Abbreviations NK natural killer cells - IFN interferon - IL 2 interleukin 2 - CMF cyclophosphamide, methotrexate, fluorouracil - ER oestrogen receptor - PMNC peripheral blood mononuclear cells - MHC major histocompatibility complex - CTL cytotoxic T lymphocytes  相似文献   
112.
113.
Differential regulation of Ultrabithorax in two germ layers of Drosophila   总被引:25,自引:0,他引:25  
M Bienz  G Saari  G Tremml  J Müller  B Züst  P A Lawrence 《Cell》1988,53(4):567-576
The homeotic gene Ultrabithorax (Ubx) is expressed in specific parts of Drosophila embryos: in a single metamer in the visceral mesoderm and forming a complex pattern limited to a broad domain in the ectoderm and in the somatic mesoderm. Here we use a linked beta-galactosidase gene to identify cis-acting regulatory sequences. In the visceral mesoderm, correct expression of Ubx depends on localized upstream sequences. In the ectoderm, all galactosidase-positive transformants show the same characteristic pattern. The repeated elements of this basal pattern appear to be a sub-pattern of engrailed (en) expression; they depend on en function as well as on sequences in the Ubx RNA leader. We use a mutant (Haltere-mimic) to show that sequences that normally restrict segmental expression of Ubx in the ectoderm are located downstream from the RNA leader.  相似文献   
114.
Summary 4-Chlorobenzoate dehalogenase from Pseudomonas sp. CBS3 showed dehalogenating activity in various organic solvents. In alcohols like methanol (150%) or ethanol (120%) higher activities than in water (100%) were obtained. In apolar solvents like petroleum ether (5%) and nhexane (5%) only trace activities were observed. The solvents did not increase the stability of the enzyme. 4-Chlorobenzoic acid methylester, a substance not soluble in water, was not dehalogenated in organic solvents.  相似文献   
115.
Some basic proteins enable microtubule protein to form special assembly products in vitro, known as double-walled microtubules. Using histones (H1, core histones) as well as the human encephalitogenic protein to induce the formation of double-walled microtubules, we made the following electron microscopic observations: (1) Double-walled microtubules consist of an "inner" microtubule which is covered by electron-dense material, apparently formed from the basic protein, and by a second tubulin wall. (2) The tubulin of the second wall seems to be arranged as protofilaments, surrounding the inner microtubule in a helical or ring-like manner. (3) The surface of double-walled microtubules lacks the projections of microtubule-associated proteins, usually found on microtubules. (4) In the case of protofilament ribbons (incomplete microtubules), H1 binds exclusively to their convex sides that correspond to the surface of microtubules. Zn2+-induced tubulin sheets, consisting in contrast to microtubules of alternately arranged protofilaments, are covered by H1 on both surfaces. Furthermore, multilayered sheet aggregates appeared. The results indicate that the basic proteins used interact only with that protofilament side which represents the microtubule surface. In accordance with this general principle, models on the structure of double-walled microtubules and multilayered tubulin sheets were derived.  相似文献   
116.
In extracts from Arthrobacter sp. SU DSM 20407 an enzyme was detectable, that converted 4-chlorobenzoate into 4-hydroxybenzoate. This conversion was also observed when no oxygen was present in the reaction mixture. Boiling for 5 min destroyed the enzyme activity. 4-Bromo- and 4-iodobenzoate were substrates for the enzyme too, but not 4-fluorobenzoate, 4-chlorophenylacetate and 4-chlorocinnamic acid. The enzyme showed optimum activity at 16 degrees C and at pH 7-7.5. The specific activity in the extracts varied between 0.5 and 5 mU/mg of protein. Zn2+ and Cu2+ inhibited the enzyme, while H2O2 slightly activated. In contrast to all other 4-chlorobenzoate dehalogenases described before the enzyme was not inhibited by EDTA, nor was it activated by Mn2+. Other divalent ions also had no effect. The molecular mass of the enzyme was 45,000 +/- 5,000 Da as judged by gel-filtration.  相似文献   
117.
Caudoxirene (cis-3-(1,2-trans-epoxy-but-3-enyl)-4-vinyl-cyclopentene) is a new gamete releasing factor from Perithalia caudata (Sporochnales). Its threshold concentration is found at 30 pmol for gamete release. Multifidene, viridiene and a Z-isomer of caudoxirene were identified as by-products or trace constituents.  相似文献   
118.
The specific binding protein for substance P (SP) was solubilized in an active form from the crude mitochondrial (P2) fraction of bovine brainstem. After incubation with 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS) and 0.1 M NaCl at 0 degrees C for 30 min, the SP binding to the supernatant fraction (100,000 g, 60 min) was determined by the glass fiber filtration method reported by Bruns et al. (1983). The specific [3H]SP binding to the solubilized fraction was highly specific for SP and was displaced by nanomolar concentrations of SP and physalaemin, but only by micromolar concentrations of eledoisin. In addition, the binding was inhibited by GTP (approximately 40% of the specific binding decreased by 10 microM GTP) in both preparations. These results were virtually identical to those of P2 membrane preparations and suggested that this high-affinity SP binding site belongs to the SP-P type. Scatchard analyses of SP binding to the solubilized fraction revealed a single saturable component with a Bmax of 22.0 +/- 5.10 fmol/mg protein and a KD of 0.79 nM, and these values are almost the same as those obtained in the P2 fraction (Bmax = 31.3 +/- 3.56 fmol/mg protein, KD = 0.82 nM). Gel filtration analysis showed that the detergent-SP binding protein complex has two calculated molecular weights of greater than 1,000,000 and 55,000-60,000 (a corresponding Stokes radius of 35.5 nm).  相似文献   
119.
Glyoxysomal citrate synthase (gCS) was purified from crude extracts of watermelon (Citrullus vulgaris Schrad.) cotyledons, yielding a homogenous protein with a subunit MW of 48 kDa. The enzyme was selectively inhibited by 5,5-dithiobis-(2-nitrobenzoic acid), allowing quantification in the presence of the mitochondrial isoenzyme (mCS). Differences were also observed with respect to inhibition by ATP (k i=2.6 mmol · l-1 for gCS, k i=0.33 mmol · l-1 for mCS). The antibodies prepared against gCS did not cross-react with mCS. The immunocytochemical localization of gCS by the indirect protein A-gold procedure was restricted to the glyoxysomal membrane or the peripheral matrix of glyoxysomes. Other compartments, e.g. the endoplasmic reticulum, were not labeled. Xenopus oocytes were used for the translation of watermelon polyadenylated RNA (poly(A)+RNA). A translation product with a MW of 51 kDa was immunoprecipitated by the anti-gCS antibodies. It was absent in controls without poly(A)+RNA or with preimmune serum. A similar translation product was also immunoprecipitated after cell-free synthesis of watermelon poly(A)+RNA in a reticulocyte system, in contrast to the in-vivo labeled gCS (48 kDa). It was concluded that gCS is synthesized as a higher-molecular-weight precursor.Abbreviations DTNB 5,5-dithiobis-(2-nitrobenzoic acid) - gCS glyoxysomal citrate synthase - gMDH glyoxysomal malate dehydrogenase - k i inhibitor constant - mCS mitochondrial citrate synthase - OAA oxaloacetate - poly(A)+RNA polyadenylated RNA - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis  相似文献   
120.
Twelve loci form a continuous linkage map for human chromosome 18   总被引:7,自引:0,他引:7  
We have constructed a primary genetic map of human chromosome 18 consisting of 11 DNA markers and one serological marker (JK). Two of these loci define highly polymorphic VNTR systems. The markers define a continuous genetic linkage map of 97 cM in males and 205 cM in females; female genetic distances in a panel of 59 three-generation families were consistently about twice those observed in males. The high odds in support of the linear order of the markers on this recombination map, and the extent of coverage of chromosome 18, indicate that this map will permit efficient linkage studies of human genetic diseases that may be segregating on chromosome 18 and will provide anchor points for development of high-resolution maps for this chromosome.  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