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   22628篇
  免费   1618篇
  国内免费   1639篇
  2024年   55篇
  2023年   403篇
  2022年   556篇
  2021年   727篇
  2020年   715篇
  2019年   1053篇
  2018年   909篇
  2017年   612篇
  2016年   637篇
  2015年   795篇
  2014年   1463篇
  2013年   1752篇
  2012年   1079篇
  2011年   1515篇
  2010年   1147篇
  2009年   1194篇
  2008年   1208篇
  2007年   1213篇
  2006年   1036篇
  2005年   954篇
  2004年   792篇
  2003年   690篇
  2002年   616篇
  2001年   430篇
  2000年   386篇
  1999年   343篇
  1998年   315篇
  1997年   262篇
  1996年   241篇
  1995年   214篇
  1994年   233篇
  1993年   184篇
  1992年   196篇
  1991年   181篇
  1990年   146篇
  1989年   124篇
  1988年   105篇
  1987年   118篇
  1986年   98篇
  1985年   158篇
  1984年   154篇
  1983年   120篇
  1982年   122篇
  1981年   130篇
  1980年   113篇
  1979年   90篇
  1978年   73篇
  1977年   54篇
  1976年   62篇
  1974年   39篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
191.
对长春和北京地区连续12年(1976年冬至1988年春)引起小儿肺炎的3、7型腺病毒102株标本,进行了限制性内切酶核酸电泳图谱分析。56株7型腺病毒经BamHⅠ、BclⅠ、BglⅠ、XbaⅠ、SmaⅠ、HindⅢ分析后,表现为两个基因组型——Ad7 b和Ad7 d。46株3型腺病毒被Bg1 Ⅱ、BamHⅠ酶解后,表现为 3个基因组型——Ad 3Ⅰ、Ad 3Ⅱ、Ad 3Ⅲ。各基因组型的分布情况是:56株7型腺病毒中,43株为Ad 7 b(76.8%),流行于1976年冬至1986年春;13株是Ad 7 d(23.2%),出现于1982年,与Ad 7 b共同流行;1986年~1988年分析的5株病毒都是Ad 7d。43株3型腺病毒中,Ad3Ⅰ42株(91.0%),分布于12年中;Ad 3Ⅱ、Ad 3Ⅲ各2株,散在分布。此结果表明,国内这12年中引起小儿肺炎的3型腺病毒至少有3个基因组型,7型腺病毒至少有两个基因组型。Ad3Ⅰ和Ad7 b是流行优势基因组型。但自80年代初开始出现Ad7 d以来,有逐年增多的趋势,最近两年的标本又都是Ad7 d,很可能它将取代Ad7 b而成为流行的优势基因组型.  相似文献   
192.
 分别从人肝及鼠肝中高度纯化L型和M_1型丙酮酸激酶(Pyk),制备相应的免抗血清。从抗血清中纯化IgG并水解成F(ab')_2,进而还原成Fab',后者与辣根过氧物酶(HRP)交联成Fab'-HRP,再利用这两种复合物建立了各自的高灵敏夹心ELISA来免疫定量L及M_2型(与鼠M_1-Pyk有交叉免疫)Pyk,结果发现:正常人肝中95%的Pyk为L型,M_2-Pyk仅占5%,而肝细胞癌中L-Pyk降至正常肝的3.5%,M_2-Pyk却增至正常的14.6倍,以致M_2-Pyk占总量的95.5%。免疫组化研究进一步证实定量的结果,正常人肝L-Pyk染色呈强阳性。M_2-Pyk染色较弱,而肝细胞癌恰好相反,L-Pyk染色明显减退,而M_2-Pyk不仅癌组织染色很深,癌周组织也明显深于正常,而且愈近癌巢,染色愈深。测定血浆Pyk,发现正常人L-Pyk占89.9%,M-Pyk(以M_2计)仅占10.1%。肝细胞癌患者血浆L-Pyk略见降低,为正常值的79.6%,p值在0.02~0.05之间,但血浆M型Pyk明显升高至正常的4.59倍,占Pyk总量的39.6%,并证明主要来源于肝癌组织中的M_2,故M_2-Pyk有可能成为肝细胞癌诊断的新指标。  相似文献   
193.
Abstract. The Clements' Deterministic model of plant communities implies that a change in one species, especially a change in the physiognomic dominant, would have profound effects on the remainder of the community. Gleason's Individualistic model suggests there would be little effect. These alternative models are tested by examining the forest composition on both sides of a boundary along which the tree Nothofagus menziesii is slowly invading. Classification of forest composition, excluding N. menziesii, gave little evidence of an effect of N. menziesii on the lower strata: understorey vegetation types were distributed across the boundary. Some differences were found at individual sites, but these were often inconsistent between sites. Ordination, also excluding N. menziesii, similarly showed that none of the first three understorey axes reflected any effect of N. menziesii. The fourth axis was correlated with the presence of N. menziesii, but only when canopy trees were included. It is concluded that specific composition of the canopy in these montane New Zealand evergreen forests has little effect on the understorey, supporting Gleason's Individualistic concept of the community.  相似文献   
194.
Cytochalasin B (CB) is known to interfere reversibly with the cytoplasmic contractile filamental network of mammalian cells. The role of the microfilament system in the mechanism of the reactive oxygen intermediates release of polymorphonuclear leukocytes (PMNL) was studied for different kinds of stimuli. PMNL from fresh human blood were treated with CB and stimulated by adherence on plastic surfaces, by opsonized zymosan, by phorbol myristate acetate and by N-formylmethionyl-phenylalaline. The production of reactive oxygen species were monitored by simultaneous detection of native, luminol-independent, luminescence (NL) and luminol-dependent luminescence (LDL) using a method of spectral discrimination. Different influences of CB on NL with respect to LDL as well stimuli-dependent influences of CB on the luminescence response of PMNL were observed. Especially phagocytosis-associated activation of PMNL was strongly inhibited by CB, whereas LDL was reduced to a much greater extent in comparison with NL. A firm involvement of the microfilament system is indicated, but it depends on the kind of stimulus engaged.  相似文献   
195.
A chemiluminescent assay for hepatitis-B surface antigen is described which used an isoluminol derivative as the label. The assay is precise intra-assay CV, 1.96-2.45%; inter-assay CV, 5.26-8.11% and has a lower detection limit for hepatitis-B surface antigen of 1.3U/I.  相似文献   
196.
Abstract: Hyperphosphorylated τ proteins are the principal fibrous component of the neurofibrillary tangle pathology in Alzheimer's disease. The possibility that τ phosphorylation is controlled by cell surface neurotransmitter receptors was examined in PC12 cells transfected with the gene for the rat m1 muscarinic acetylcholine receptor. Stimulation of m1 receptor in these cells with two acetylcholine agonists, carbachol and AF102B, decreased τ phosphorylation, as indicated by specific τ monoclonal antibodies that recognize phosphorylation-dependent epitopes and by alkaline phosphatase treatment. The muscarinic effect was both time and dose dependent. In addition, a synergistic effect on τ phosphorylation was found between treatments with muscarinic agonists and nerve growth factor. These studies provide the first evidence for a link between the cholinergic signal transduction system and the neuronal cytoskeleton that can be mediated by regulated phosphorylation of τ microtubule-associated protein.  相似文献   
197.
198.
Thea mating type locus ofUstilago maydis contains the structural genes for a pheromone-based cell recognition system that governs fusion of haploid cells. The locus exists in two alleles, termeda1 anda2. We have completed the analysis of the nucleotide sequences unique toa1 anda2. Within these dissimilar regions we find two short patches of DNA sequence similarity. Interestingly, one of these segments corresponds to the transcribed region of thea1 pheromone precursor. As a result of multiple nucleotide exchanges this sequence does not code for a functional product. The existence of a second pheromone gene in thea2 allele suggests that the present locus had a multiallelic ancestor. In addition, we describe the presence of two additional genes in thea2 allele. We have investigated the role of these genes during mating and pathogenic development and speculate that they might affect mitochondrial inheritance.  相似文献   
199.
In order to expand tumor-infiltrating lymphocytes (TIL) efficiently and in order to use them for immunotherapy, we utilized lipopolysaccharide-activated B cells (LPS blasts) as costimulatory-signal-providing cells in an in vitro culture system. TIL, prepared from subcutaneously inoculated B16 melanoma, failed to expand when cultured with anti-CD3 monoclonal antibody (mAb) alone followed by a low dose of interleukin(IL)-2. In contrast, such TIL did expand efficiently in culture with both anti-CD3 mAb and LPS blasts followed by culture with IL-2. These findings suggest that the presence of LPS blasts in the initial culture was essential for the cell expansion. The expansion of TIL was partially blocked by the addition of CTLA4 Ig, which is an inhibitor of costimulatory molecules such as CD80 and CD86, and was almost blocked by the addition of anti-(Fc receptor II)mAb. These findings thus indicate that such molecules, in conjunction with the receptor on the LPS blasts, participate in the efficient expansion of TIL. The B16-derived TIL, which expanded in our culture system, were predominantly CD8+T cells and showed a higher level of cytolytic activity against B16 melanoma than either lymphokine-activated killer cells or TIL cultured with a high dose of IL-2. In addition, the in vitro expanded B16-derived TIL produced interferon , but not IL-4, in response to B16 melanoma. What is more important, the adoptive transfer of such TIL had a significant antitumor effect against pulmonary metastasis in B16 melanoma, even without the concurrent administration of IL-2. Collectively, our results thus indicate the therapeutic efficacy of the protocol presented here for antitumor immunotherapy with TIL.This work was supported in part by a grant from the Ministry of Education, Science and Culture  相似文献   
200.
星花绣线菊中的一个新二萜——绣线菊内酯B   总被引:4,自引:1,他引:3  
从星花绣线菊(Spiraeajaponicavar.stellaris)根中得到1个新的Atisine型二萜,命名为绣线菊内酯B(1)。通过光谱分析测定了其结构,此外还得到了9个已知化合物,包括1个atisine型二萜-spiraminol(2)和8个二萜生物碱,spiramineA,B,C,D,F,H,P,Q。  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