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   27673篇
  免费   95篇
  国内免费   112篇
系统科学   124篇
丛书文集   451篇
教育与普及   36篇
理论与方法论   90篇
现状及发展   12135篇
研究方法   1266篇
综合类   13308篇
自然研究   470篇
  2013年   215篇
  2012年   425篇
  2011年   893篇
  2010年   173篇
  2008年   489篇
  2007年   599篇
  2006年   535篇
  2005年   572篇
  2004年   622篇
  2003年   505篇
  2002年   530篇
  2001年   865篇
  2000年   825篇
  1999年   585篇
  1992年   560篇
  1991年   386篇
  1990年   422篇
  1989年   407篇
  1988年   403篇
  1987年   413篇
  1986年   455篇
  1985年   582篇
  1984年   403篇
  1983年   333篇
  1982年   294篇
  1981年   319篇
  1980年   372篇
  1979年   872篇
  1978年   681篇
  1977年   652篇
  1976年   521篇
  1975年   516篇
  1974年   771篇
  1973年   641篇
  1972年   708篇
  1971年   766篇
  1970年   1077篇
  1969年   843篇
  1968年   727篇
  1967年   752篇
  1966年   718篇
  1965年   529篇
  1964年   180篇
  1959年   267篇
  1958年   525篇
  1957年   361篇
  1956年   280篇
  1955年   255篇
  1954年   297篇
  1948年   186篇
排序方式: 共有10000条查询结果,搜索用时 5 毫秒
201.
Extraction of a weak climatic signal by an ecosystem   总被引:11,自引:0,他引:11  
Taylor AH  Allen JI  Clark PA 《Nature》2002,416(6881):629-632
The complexity of ecosystems can cause subtle and chaotic responses to changes in external forcing. Although ecosystems may not normally behave chaotically, sensitivity to external influences associated with nonlinearity can lead to amplification of climatic signals. Strong correlations between an El Ni?o index and rainfall and maize yield in Zimbabwe have been demonstrated; the correlation with maize yield was stronger than that with rainfall. A second example is the 100,000-year ice-age cycle, which may arise from a weak cycle in radiation through its influence on the concentration of atmospheric CO2 (ref. 5). Such integration of a weak climatic signal has yet to be demonstrated in a realistic theoretical system. Here we use a particular climatic phenomenon-the observed association between plankton populations around the UK and the position of the Gulf Stream-as a probe to demonstrate how a detailed marine ecosystem model extracts a weak signal that is spread across different meteorological variables. Biological systems may therefore respond to climatic signals other than those that dominate the driving variables.  相似文献   
202.
The p53 tumour suppressor gene   总被引:266,自引:0,他引:266  
A J Levine  J Momand  C A Finlay 《Nature》1991,351(6326):453-456
The cell cycle is composed of a series of steps which can be negatively or positively regulated by various factors. Chief among the negative regulators is the p53 protein. Alteration or inactivation of p53 by mutation, or by its interactions with oncogene products of DNA tumour viruses, can lead to cancer. These mutations seem to be the most common genetic change in human cancers.  相似文献   
203.
tRNase Z: the end is not in sight   总被引:1,自引:0,他引:1  
Although the enzyme tRNase Z has only recently been isolated, a plethora of data has already been acquired concerning the enzyme. tRNase Z is the endonuclease that catalyzes the removal of the tRNA 3′ trailer, yielding the mature tRNA 3′ end ready for CCA addition and aminoacylation. Another substrate cleaved by tRNase Z is the small chromogenic phosphodiester bis(p-nitrophenyl)phosphate (bpNPP), which is the smallest tRNase Z substrate known so far. Hitherto the biological function as tRNA 3′-end processing enzyme has been shown only in one prokaryotic and one eukaryotic organism, respectively. This review summarizes the present information concerning the two tRNase Z substrates pre-tRNA and bpNPP, as well as the metal requirements of tRNase Z enzymes. Received 29 March 2007; received after revision 15 May 2007; accepted 21 May 2007  相似文献   
204.
Resequencing genes provides the opportunity to assess the full spectrum of variants that influence complex traits. Here we report the first application of resequencing to a large population (n = 3,551) to examine the role of the adipokine ANGPTL4 in lipid metabolism. Nonsynonymous variants in ANGPTL4 were more prevalent in individuals with triglyceride levels in the lowest quartile than in individuals with levels in the highest quartile (P = 0.016). One variant (E40K), present in approximately 3% of European Americans, was associated with significantly lower plasma levels of triglyceride and higher levels of high-density lipoprotein cholesterol in European Americans from the Atherosclerosis Risk in Communities Study and in Danes from the Copenhagen City Heart Study. The ratio of nonsynonymous to synonymous variants was higher in European Americans than in African Americans (4:1 versus 1.3:1), suggesting population-specific relaxation of purifying selection. Thus, resequencing of ANGPTL4 in a multiethnic population allowed analysis of the phenotypic effects of both rare and common variants while taking advantage of genetic variation arising from ethnic differences in population history.  相似文献   
205.
Germline gain-of-function mutations in SOS1 cause Noonan syndrome   总被引:1,自引:0,他引:1  
Noonan syndrome, the most common single-gene cause of congenital heart disease, is characterized by short stature, characteristic facies, learning problems and leukemia predisposition. Gain-of-function mutations in PTPN11, encoding the tyrosine phosphatase SHP2, cause approximately 50% of Noonan syndrome cases. SHP2 is required for RAS-ERK MAP kinase (MAPK) cascade activation, and Noonan syndrome mutants enhance ERK activation ex vivo and in mice. KRAS mutations account for <5% of cases of Noonan syndrome, but the gene(s) responsible for the remainder are unknown. We identified missense mutations in SOS1, which encodes an essential RAS guanine nucleotide-exchange factor (RAS-GEF), in approximately 20% of cases of Noonan syndrome without PTPN11 mutation. The prevalence of specific cardiac defects differs in SOS1 mutation-associated Noonan syndrome. Noonan syndrome-associated SOS1 mutations are hypermorphs encoding products that enhance RAS and ERK activation. Our results identify SOS1 mutants as a major cause of Noonan syndrome, representing the first example of activating GEF mutations associated with human disease and providing new insights into RAS-GEF regulation.  相似文献   
206.
207.
Most agronomic traits of importance, whether physiological (such as nutrient use efficiency) or developmental (such as flowering time), are controlled simultaneously by multiple genes and their interactions with the environment. Here, we show that variation in sulfate content between wild Arabidopsis thaliana accessions Bay-0 and Shahdara is controlled by a major quantitative trait locus that results in a strong interaction with nitrogen availability in the soil. Combining genetic and biochemical results and using a candidate gene approach, we have cloned the underlying gene, showing how a single-amino acid substitution in a key enzyme of the assimilatory sulfate reduction pathway, adenosine 5'-phosphosulfate reductase, is responsible for a decrease in enzyme activity, leading to sulfate accumulation in the plant. This work illustrates the potential of natural variation as a source of new alleles of known genes, which can aid in the study of gene function and metabolic pathway regulation. Our new insights on sulfate assimilation may have an impact on sulfur fertilizer use and stress defense improvement.  相似文献   
208.
209.
210.
West Nile virus (WNV), first recognized in North America in 1999, has been responsible for the largest arboviral epiornitic and epidemic of human encephalitis in recorded history. Despite the well-described epidemiological patterns of WNV in North America, the basis for the emergence of WNV-associated avian pathology, particularly in the American crow (AMCR) sentinel species, and the large scale of the North American epidemic and epiornitic is uncertain. We report here that the introduction of a T249P amino acid substitution in the NS3 helicase (found in North American WNV) in a low-virulence strain was sufficient to generate a phenotype highly virulent to AMCRs. Furthermore, comparative sequence analyses of full-length WNV genomes demonstrated that the same site (NS3-249) was subject to adaptive evolution. These phenotypic and evolutionary results provide compelling evidence for the positive selection of a mutation encoding increased viremia potential and virulence in the AMCR sentinel bird species.  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