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   1789篇
  免费   84篇
  1873篇
  2021年   12篇
  2020年   16篇
  2019年   13篇
  2018年   26篇
  2017年   13篇
  2016年   28篇
  2015年   40篇
  2014年   48篇
  2013年   109篇
  2012年   87篇
  2011年   75篇
  2010年   49篇
  2009年   51篇
  2008年   79篇
  2007年   84篇
  2006年   82篇
  2005年   77篇
  2004年   65篇
  2003年   61篇
  2002年   67篇
  2001年   47篇
  2000年   59篇
  1999年   55篇
  1998年   24篇
  1997年   12篇
  1996年   16篇
  1995年   20篇
  1994年   19篇
  1993年   12篇
  1992年   60篇
  1991年   41篇
  1990年   24篇
  1989年   32篇
  1988年   46篇
  1987年   38篇
  1986年   20篇
  1985年   24篇
  1984年   19篇
  1983年   24篇
  1981年   11篇
  1980年   12篇
  1979年   26篇
  1976年   11篇
  1975年   15篇
  1974年   14篇
  1973年   12篇
  1972年   12篇
  1971年   10篇
  1970年   13篇
  1968年   13篇
排序方式: 共有1873条查询结果,搜索用时 15 毫秒
181.
Female moths of Lyclene dharma dharma (Arctiidae, Lithosiinae) produced three pheromone components (I-III), which strongly stimulated male antennae. Using GC-MS analysis and chemical derivatizations, the following structures were estimated: 6-methyl-2-octadecanone (I), 14-methyl-2-octadecanone (II), and 6,14-dimethyl-2-octadecanone (III). While the stereochemistry of the chiral centers could not be determined because it was difficult to collect a sufficient amount of the natural pheromone, the plain structures of I and II were confirmed by synthesis of the racemic mixtures starting from diols. These methyl-branched ketones have not been identified as a natural product, indicating that they constitute a new chemical group of lepidopteran female sex pheromones.  相似文献   
182.
Neurochemical Research - Valproic acid (VPA) is a drug used for the treatment of epilepsy, seizures, migraines, and bipolar disorders. Cyclin-dependent kinase 5 (Cdk5) is a Ser/Thr kinase activated...  相似文献   
183.
184.
185.
186.
187.
Danforth'sshort-tail (Sd) mouse is a semi-dominant mutation affecting the development of the vertebral column. Although the notochord degenerates completely by embryonic day 9.5, the vertebral column exists up to the lumber region, suggesting that the floor plate can substitute for notochord function. We previously established the mutant mouse line, Skt(Gt), through gene trap mutagenesis and identified the novel gene, Skt, which was mapped 0.95cM distal to the Sd locus. Taking advantage of the fact that monitoring notochordal development and genotyping of the Sd locus can be performed using the Skt(Gt) allele, we assessed the development of the vertebra, notochord, somite, floor plate and sclerotome in +-+/+-Skt(Gt), Sd-+/+-+, Sd-Skt(Gt)/+-+, Sd-Skt(Gt)/+-Skt(Gt), Sd-+/Sd-+ and Sd-Skt(Gt)/Sd-Skt(Gt) embryos. In Sd homozygous mutants with a C57BL/6 genetic background, the vertebral column was truncated in the 6th thoracic vertebra, which was more severe than previously reported. The floor plate and sclerotome developed to the level of somite before notochord degeneration and the number of remaining vertebrae corresponded well with the level of development of the floor plate and sclerotome. Defects to the sclerotome and subsequent vertebral development were not due to failure of somitogenesis. Taken together, these results suggest that the notochord induced floor plate development before degeneration, and that the remaining floor plate is sufficient for maintenance of differentiation of the somite into the sclerotome and vertebra in the absence of the notochord.  相似文献   
188.
For Escherichia coli, it has been assumed that L-alanine is synthesized by alanine-valine transaminase (AvtA) in conjunction with an unknown alanine aminotransferase(s). We isolated alanine auxotrophs from a prototrophic double mutant deficient in AvtA and YfbQ, a novel alanine aminotransferase, by chemical mutagenesis. A shotgun cloning experiment identified two genes, uncharacterized yfdZ and serC, that complemented the alanine auxotrophy. When the yfdZ- or serC-mutation was introduced into the double mutant, one triple mutant (avtA yfbQ yfdZ) showed alanine auxotrophy, and another (avtA yfbQ serC), prototrophy. In addition, we found that four independent alanine auxotrophs possessed a point mutation in yfdZ but not in serC. We also found that yfdZ expression was induced in minimal medium. Furthermore, yfbQ-bearing plasmid conferred the ability to excrete alanine on the mutant lacking D-amino acid dehydrogenase-encoding gene, dadA. From these results, we concluded that E. coli synthesizes L-alanine by means of three aminotransferases, YfbQ, YfdZ, and AvtA.  相似文献   
189.
190.
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