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   29861篇
  免费   71篇
  国内免费   74篇
系统科学   138篇
丛书文集   363篇
教育与普及   57篇
理论与方法论   121篇
现状及发展   12815篇
研究方法   1182篇
综合类   14876篇
自然研究   454篇
  2013年   249篇
  2012年   368篇
  2011年   835篇
  2010年   157篇
  2008年   426篇
  2007年   510篇
  2006年   538篇
  2005年   512篇
  2004年   597篇
  2003年   481篇
  2002年   481篇
  2001年   996篇
  2000年   957篇
  1999年   582篇
  1992年   538篇
  1991年   433篇
  1990年   460篇
  1989年   487篇
  1988年   434篇
  1987年   434篇
  1986年   490篇
  1985年   572篇
  1984年   440篇
  1983年   423篇
  1982年   363篇
  1981年   363篇
  1980年   397篇
  1979年   967篇
  1978年   782篇
  1977年   739篇
  1976年   573篇
  1975年   695篇
  1974年   966篇
  1973年   766篇
  1972年   802篇
  1971年   960篇
  1970年   1204篇
  1969年   883篇
  1968年   910篇
  1967年   848篇
  1966年   764篇
  1965年   589篇
  1964年   156篇
  1959年   313篇
  1958年   523篇
  1957年   380篇
  1956年   293篇
  1955年   295篇
  1954年   282篇
  1948年   207篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Phosphoinositide 3-kinases produce 3'-phosphorylated phosphoinositides that act as second messengers to recruit other signalling proteins to the membrane. Pi3ks are activated by many extracellular stimuli and have been implicated in a variety of cellular responses. The Pi3k gene family is complex and the physiological roles of different classes and isoforms are not clear. The gene Pik3r1 encodes three proteins (p85 alpha, p55 alpha and p50 alpha) that serve as regulatory subunits of class IA Pi3ks (ref. 2). Mice lacking only the p85 alpha isoform are viable but display hypoglycaemia and increased insulin sensitivity correlating with upregulation of the p55 alpha and p50 alpha variants. Here we report that loss of all protein products of Pik3r1 results in perinatal lethality. We observed, among other abnormalities, extensive hepatocyte necrosis and chylous ascites. We also noted enlarged skeletal muscle fibres, brown fat necrosis and calcification of cardiac tissue. In liver and muscle, loss of the major regulatory isoform caused a great decrease in expression and activity of class IA Pi3k catalytic subunits; nevertheless, homozygous mice still displayed hypoglycaemia, lower insulin levels and increased glucose tolerance. Our findings reveal that p55 alpha and/or p50 alpha are required for survival, but not for development of hypoglycaemia, in mice lacking p85 alpha.  相似文献   
993.
In most mammals, male development is triggered by the transient expression of the Y-chromosome gene, Sry, which initiates a cascade of gene interactions ultimately leading to the formation of a testis from the indifferent fetal gonad. Several genes, in particular Sox9, have a crucial role in this pathway. Despite this, the direct downstream targets of Sry and the nature of the pathway itself remain to be clearly established. We report here a new dominant insertional mutation, Odsex (Ods), in which XX mice carrying a 150-kb deletion (approximately 1 Mb upstream of Sox9) develop as sterile XX males lacking Sry. During embryogenesis, wild-type XX fetal gonads downregulate Sox9 expression, whereas XY and XX Ods/+ fetal gonads upregulate and maintain its expression. We propose that Ods has removed a long-range, gonad-specific regulatory element that mediates the repression of Sox9 expression in XX fetal gonads. This repression would normally be antagonized by Sry protein in XY embryos. Our data are consistent with Sox9 being a direct downstream target of Sry and provide genetic evidence to support a general repressor model of sex determination in mammals.  相似文献   
994.
995.
The Galactic Centre is the most active and heavily processed region of the Milky Way, so it can be used as a stringent test for the abundance of deuterium (a sensitive indicator of conditions in the first 1,000 seconds in the life of the Universe). As deuterium is destroyed in stellar interiors, chemical evolution models predict that its Galactic Centre abundance relative to hydrogen is D/H = 5 x 10(-12), unless there is a continuous source of deuterium from relatively primordial (low-metallicity) gas. Here we report the detection of deuterium (in the molecule DCN) in a molecular cloud only 10 parsecs from the Galactic Centre. Our data, when combined with a model of molecular abundances, indicate that D/H = (1.7 +/- 0.3) x 10(-6), five orders of magnitude larger than the predictions of evolutionary models with no continuous source of deuterium. The most probable explanation is recent infall of relatively unprocessed metal-poor gas into the Galactic Centre (at the rate inferred by Wakker). Our measured D/H is nine times less than the local interstellar value, and the lowest D/H observed in the Galaxy. We conclude that the observed Galactic Centre deuterium is cosmological, with an abundance reduced by stellar processing and mixing, and that there is no significant Galactic source of deuterium.  相似文献   
996.
997.
Kolodner RD 《Nature》2000,407(6805):687, 689
  相似文献   
998.
999.
1000.
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