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排序方式: 共有117320条查询结果,搜索用时 113 毫秒
1.
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
2.
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
3.
Angiotensin-converting enzyme 2 is an essential regulator of heart function   总被引:131,自引:0,他引:131  
Cardiovascular diseases are predicted to be the most common cause of death worldwide by 2020. Here we show that angiotensin-converting enzyme 2 (ace2) maps to a defined quantitative trait locus (QTL) on the X chromosome in three different rat models of hypertension. In all hypertensive rat strains, ACE2 messenger RNA and protein expression were markedly reduced, suggesting that ace2 is a candidate gene for this QTL. Targeted disruption of ACE2 in mice results in a severe cardiac contractility defect, increased angiotensin II levels, and upregulation of hypoxia-induced genes in the heart. Genetic ablation of ACE on an ACE2 mutant background completely rescues the cardiac phenotype. But disruption of ACER, a Drosophila ACE2 homologue, results in a severe defect of heart morphogenesis. These genetic data for ACE2 show that it is an essential regulator of heart function in vivo.  相似文献
4.
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
5.
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
6.
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
7.
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
8.
9.
我国河流综合水质标识指数评价方法研究   总被引:76,自引:2,他引:74  
在单因子水质标识指数法基础上,提出一种全新的河流总体的综合水质评价方法——综合水质标识指数法.综合水质标识指数可以完整表达河流总体的综合水质信息,其特点是既能定性评价、也能定量评价;既不会因个别水质指标较差就否定综合水质,又能对河流综合水质做出合理的评价;既可以用于一条河流不同断面水质的客观比较,又可以用于不同河流水质的评价比较;既可以在同一类别中比较水质的优劣,也可以对劣Ⅴ类水比较污染的严重程度.将综合水质标识指数用于上海市河流的水质评价取得了令人信服的评价结果,并确定了我国河流水体黑臭的判别准则.  相似文献
10.
中国东南地壳增长与Nd同位素模式年龄   总被引:73,自引:0,他引:73  
中国东南变质基底岩石及各时代花岗岩的Nd同位素模式年龄的统计研究结果表明,该地区太古代—古元古代陆壳以横向增生方式形成,显生宙表现为局部的垂向增生.太古代时形成若干规模有限的陆核,地壳增长的主要时期为1.8~2.2Ga,1.5~1.8Ga期间的地壳增长主要发生在赣东北、皖南和浙东、闽东地区.晋宁期和古生代花岗岩主要由同一地区变质基底岩石重熔形成的,没有明显的幔源物质加入.但从赣东北到粤西存在一条北东向的低Nd模式年龄花岗岩带,其成因可能与加里东期裂陷槽有关.中生代花岗岩的Nd模式年龄比同一地区基底岩石和前中生代花岗岩的低,反映普遍的壳幔相互作用和陆壳的增长.在此背景上,闽浙沿海有一低Nd模式年龄花岗岩区,内陆存在数条北东向分布的低Nd模式年龄花岗岩带,表明有幔源物质的加入和地壳的增生.幔源物质的加入可能与这些地区中生代地壳拉张减薄作用有关,而这种地壳的拉张减薄及地幔物质上涌对华南地区一些重要的金属矿床的形成起了重要的作用  相似文献
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