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   104019篇
  国内免费   2044篇
  完全免费   20076篇
  综合类   126139篇
  2021年   7篇
  2020年   186篇
  2019年   413篇
  2018年   805篇
  2017年   758篇
  2016年   712篇
  2015年   709篇
  2014年   5563篇
  2013年   8872篇
  2012年   12967篇
  2011年   12831篇
  2010年   7726篇
  2009年   9755篇
  2008年   6844篇
  2007年   9875篇
  2006年   8506篇
  2005年   7529篇
  2004年   8082篇
  2003年   4427篇
  2002年   6209篇
  2001年   5626篇
  2000年   2582篇
  1999年   2218篇
  1998年   1242篇
  1997年   1137篇
  1996年   300篇
  1995年   51篇
  1994年   48篇
  1993年   50篇
  1992年   24篇
  1991年   15篇
  1990年   19篇
  1989年   4篇
  1988年   9篇
  1987年   11篇
  1986年   2篇
  1985年   3篇
  1981年   1篇
  1980年   1篇
  1972年   9篇
  1971年   1篇
  1970年   10篇
排序方式: 共有126139条查询结果,搜索用时 46 毫秒
1.
2.
Biodiversity hotspots for conservation priorities   总被引:165,自引:0,他引:165  
Conservationists are far from able to assist all species under threat, if only for lack of funding. This places a premium on priorities: how can we support the most species at the least cost? One way is to identify 'biodiversity hotspots' where exceptional concentrations of endemic species are undergoing exceptional loss of habitat. As many as 44% of all species of vascular plants and 35% of all species in four vertebrate groups are confined to 25 hotspots comprising only 1.4% of the land surface of the Earth. This opens the way for a 'silver bullet' strategy on the part of conservation planners, focusing on these hotspots in proportion to their share of the world's species at risk.  相似文献
3.
A dipole mode in the tropical Indian Ocean   总被引:156,自引:0,他引:156  
For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized, but in the Indian Ocean region a similar ocean-atmosphere interaction causing interannual climate variability has not yet been found. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air-sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Ni?o/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean--and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia--brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.  相似文献
4.
A protocol was established for simultaneous measurements of zircon U-Pb ages and trace elements by LA-ICP-MS at spot sizes of 16 32 μm.This was accomplished by introducing N 2 into ICP to increase the sensitivity.The obtained U-Pb ages for zircon standards GJ-1,TEMORA and SK10-2 are consistent with the preferred values within about 1% uncertainty (2σ) by simple external calibration against zircon standard 91500.Different data reduction softwares could yield different uncertainties for calculation of U-Pb ag...  相似文献
5.
Rho GTPases in cell biology   总被引:145,自引:0,他引:145  
Etienne-Manneville S  Hall A 《Nature》2002,420(6916):629-635
6.
Bmi-1 determines the proliferative capacity of normal and leukaemic stem cells   总被引:111,自引:0,他引:111  
Lessard J  Sauvageau G 《Nature》2003,423(6937):255-260
An emerging concept in the field of cancer biology is that a rare population of 'tumour stem cells' exists among the heterogeneous group of cells that constitute a tumour. This concept, best described with human leukaemia, indicates that stem cell function (whether normal or neoplastic) might be defined by a common set of critical genes. Here we show that the Polycomb group gene Bmi-1 has a key role in regulating the proliferative activity of normal stem and progenitor cells. Most importantly, we provide evidence that the proliferative potential of leukaemic stem and progenitor cells lacking Bmi-1 is compromised because they eventually undergo proliferation arrest and show signs of differentiation and apoptosis, leading to transplant failure of the leukaemia. Complementation studies showed that Bmi-1 completely rescues these proliferative defects. These studies therefore indicate that Bmi-1 has an essential role in regulating the proliferative activity of both normal and leukaemic stem cells.  相似文献
7.
齐次平衡原则及其应用   总被引:110,自引:3,他引:107  
叙述齐次平衡原则及其在非线性数学物理中的应用:根据该原则可导出一大类非线性PDE的非线性变换,并借之得到方程的精确解;利用计算机的符号计算系数获得许多非线性PDE的弧立皮解;对一些非线性方程的边值-初值问题也可得到其精确解的表达式。  相似文献
8.
Programmed cell death is a fundamental requirement for embryogenesis, organ metamorphosis and tissue homeostasis. In mammals, release of mitochondrial cytochrome c leads to the cytosolic assembly of the apoptosome-a caspase activation complex involving Apaf1 and caspase-9 that induces hallmarks of apoptosis. There are, however, mitochondrially regulated cell death pathways that are independent of Apaf1/caspase-9. We have previously cloned a molecule associated with programmed cell death called apoptosis-inducing factor (AIF). Like cytochrome c, AIF is localized to mitochondria and released in response to death stimuli. Here we show that genetic inactivation of AIF renders embryonic stem cells resistant to cell death after serum deprivation. Moreover, AIF is essential for programmed cell death during cavitation of embryoid bodies-the very first wave of cell death indispensable for mouse morphogenesis. AIF-dependent cell death displays structural features of apoptosis, and can be genetically uncoupled from Apaf1 and caspase-9 expression. Our data provide genetic evidence for a caspase-independent pathway of programmed cell death that controls early morphogenesis.  相似文献
9.
von Mering C  Krause R  Snel B  Cornell M  Oliver SG  Fields S  Bork P 《Nature》2002,417(6887):399-403
Comprehensive protein protein interaction maps promise to reveal many aspects of the complex regulatory network underlying cellular function. Recently, large-scale approaches have predicted many new protein interactions in yeast. To measure their accuracy and potential as well as to identify biases, strengths and weaknesses, we compare the methods with each other and with a reference set of previously reported protein interactions.  相似文献
10.
Active genes are tri-methylated at K4 of histone H3   总被引:91,自引:0,他引:91  
Lysine methylation of histones in vivo occurs in three states: mono-, di- and tri-methyl. Histone H3 has been found to be di-methylated at lysine 4 (K4) in active euchromatic regions but not in silent heterochromatic sites. Here we show that the Saccharomyces cerevisiae Set1 protein can catalyse di- and tri-methylation of K4 and stimulate the activity of many genes. Using antibodies that discriminate between the di- and tri-methylated state of K4 we show that di-methylation occurs at both inactive and active euchromatic genes, whereas tri-methylation is present exclusively at active genes. It is therefore the presence of a tri-methylated K4 that defines an active state of gene expression. These findings establish the concept of methyl status as a determinant for gene activity and thus extend considerably the complexity of histone modifications.  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