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   42748篇
  免费   4181篇
  国内免费   13篇
  2022年   181篇
  2021年   682篇
  2020年   377篇
  2019年   508篇
  2018年   594篇
  2017年   537篇
  2016年   918篇
  2015年   1457篇
  2014年   1644篇
  2013年   2100篇
  2012年   2497篇
  2011年   2637篇
  2010年   1631篇
  2009年   1545篇
  2008年   2260篇
  2007年   2284篇
  2006年   2207篇
  2005年   2247篇
  2004年   2106篇
  2003年   2069篇
  2002年   1944篇
  2001年   575篇
  2000年   385篇
  1999年   515篇
  1998年   569篇
  1997年   421篇
  1996年   415篇
  1995年   349篇
  1994年   389篇
  1993年   372篇
  1992年   410篇
  1991年   361篇
  1990年   372篇
  1989年   322篇
  1988年   344篇
  1987年   317篇
  1986年   281篇
  1985年   374篇
  1984年   450篇
  1983年   374篇
  1982年   416篇
  1981年   467篇
  1980年   447篇
  1979年   304篇
  1978年   304篇
  1977年   318篇
  1976年   308篇
  1975年   201篇
  1974年   269篇
  1973年   228篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
Initiation of bacterial DNA replication at oriC is mediated by primosomal proteins that act cooperatively to melt an AT-rich region where the replicative helicase is loaded prior to the assembly of the replication fork. In Bacillus subtilis, the dnaD, dnaB and dnaI genes are essential for initiation of DNA replication. We established that their mRNAs are maintained in fast growing asynchronous cultures. DnaB is truncated at its C-terminus in a growth phase-dependent manner. Proteolysis is confined to cytosolic, not to membrane-associated DnaB, and affects oligomerization. Truncated DnaB is depleted at the oriC relative to the native protein. We propose that DNA-induced oligomerization is essential for its action at oriC and proteolysis regulates its localization at oriC. We show that DnaB has two separate ssDNA-binding sites one located within residues 1–300 and another between residues 365–428, and a dsDNA-binding site within residues 365–428. Tetramerization of DnaB is mediated within residues 1–300, and DNA-dependent oligomerization within residues 365–428. Finally, we show that association of DnaB with the oriC is asymmetric and extensive. It encompasses an area from the middle of dnaA to the end of yaaA that includes the AT-rich region melted during the initiation stage of DNA replication.  相似文献   
82.
Noradrenaline caused a prompt but transient increase in the rate of45Ca2+ efflux from isolated rat islets of Langerhans perifused in Ca2+ depleted medium. The response was modest in size and was unaffected by isosmotic replacement of NaCl with choline chloride or by inclusion of 0.5 mM dibutyryl cAMP in the perifusion medium, suggesting that it was not mediated by Na+: Ca2+ exchange nor by lowered cAMP. Despite its effect on45Ca2+ efflux, noradrenaline treatment did not alter the kinetics of45Ca2+ efflux in response to the muscarinic agonist, carbamylcholine, nor did it change the magnitude of the response to this agent. Simultaneous introduction of 20 mM glucose with noradrenaline prevented a rise in45Ca2+ efflux and indeed resulted in inhibition of45Ca2+ efflux. The data suggest that noradrenaline does not directly activate the mechanisms which regulate Ca2+ extrusion from islets cells, and they do not support a primary role for the Ca2+ efflux response in mediating adrenergic inhibition of insulin secretion.  相似文献   
83.
Ribulosebisphosphate carboxylase/oxygenase (EC 4.1.1.39) (rubisco) must be fully activated in order to catalyze the maximum rates of photosynthesis observed in plants. Activation of the isolated enzyme occurs spontaneously, but conditions required to observe full activation are inconsistent with those known to occur in illuminated chloroplasts. Genetic studies with a nutant of Arabidopsis thaliana incapable of activating rubisco linked two chloroplast polypeptides to the activation process in vivo. Using a reconstituted light activation system, it was possible to demonstrate the participation of a chloroplast protein in rubisco activation. These results indicate that a specific chloroplast enzyme, rubisco activase, catalyzes the activation of rubisco in vivo.  相似文献   
84.
Abstracts     
William A. R. Thomson 《CMAJ》1955,72(3):234-237
  相似文献   
85.
86.
87.
Thirty cases of intraspinal neoplasms occurring during the first two decades of life are reviewed. Histologic examination showed 13 of these to be astrocytomas, 6 neuroblastomas, 5 sarcomas, 3 ependymomas, 2 neurofibromas and 1 a schwannoma. Orthopedic deformities developed or worsened in 60 percent of patients surviving longer than a year after diagnosis. In five patients some form of endocrine deficiency developed after irradiation. For treatment of radiosensitive extradural malignant lesions, biopsy followed by irradiation is advocated.  相似文献   
88.
This study has been carried out in order to clarify various aspects of the phenomenon known as taurodontism. Samples of both taurodont and non-taurodont teeth were measured and statistical analyses were performed on the resultant data from which various conclusions may be drawn. The most important conclusion is that this phenomenon may be defined metrically. The distributions have shown that the taurodont trait grades into the normal and it is quite difficult to make a subjective judgment at the low end of the taurodont range. It is for this reason that an accurate mathematical means for performing this task is so invaluable. The question of the nature of cuneiform single rooted molars was also investigated, and they were found not to be true taurodonts although the statistical procedure classed them as such. It was demonstrated that through the construction of simple plots these teeth could be eliminated from any group classed by analysis as taurodonts. Various metrical attributes were evaluated with respect to their ability to distinguish taurodonts, and two were found to be satisfactory for this purpose in most cases. Certain changes in molar morphology regarding caries and age were also investigated.  相似文献   
89.
90.
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