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   848篇
  免费   79篇
  927篇
  2022年   9篇
  2021年   13篇
  2020年   10篇
  2018年   7篇
  2016年   15篇
  2015年   24篇
  2014年   25篇
  2013年   31篇
  2012年   29篇
  2011年   26篇
  2010年   24篇
  2009年   17篇
  2008年   33篇
  2007年   25篇
  2006年   34篇
  2005年   31篇
  2004年   36篇
  2003年   25篇
  2002年   26篇
  2001年   28篇
  2000年   37篇
  1999年   22篇
  1998年   21篇
  1997年   17篇
  1996年   9篇
  1995年   11篇
  1994年   15篇
  1993年   11篇
  1992年   19篇
  1991年   16篇
  1990年   12篇
  1989年   20篇
  1988年   12篇
  1987年   11篇
  1986年   7篇
  1985年   13篇
  1983年   6篇
  1982年   13篇
  1981年   6篇
  1980年   7篇
  1979年   19篇
  1978年   13篇
  1977年   11篇
  1976年   13篇
  1975年   7篇
  1974年   9篇
  1973年   6篇
  1970年   5篇
  1969年   7篇
  1968年   7篇
排序方式: 共有927条查询结果,搜索用时 15 毫秒
1.
Literary scholars are generally suspicious of the concept of universals: there are presently no candidates for literary universals that a high proportion of literary scholars would accept as valid. This paper reports results from a content analysis of patterns of characterization in folktales from 48 culture areas, aimed at identifying patterns of characterization that apply across regions of the world and levels of cultural complexity. The search for these patterns was guided by evolutionary theory and the findings are consistent with previous research on patterns of altruism, sex differences in mate preferences, sex differences in reproductive strategy, and differing emphases on male and female physical attractiveness. World literature, especially originally oral literature, represents a vast and neglected repository of information that researchers can use to more precisely map the contours of human nature. Jonathan Gottschall received his Ph.D. in English from Binghamton University and now teaches at St. Lawrence University in Canton, New York. His research focuses on integrating Darwinian approaches to human behavior and psychology with literary studies. The other authors are undergraduate students at St. Lawrence University.  相似文献   
2.
3.
The type II CRISPR/Cas system from Streptococcus pyogenes and its simplified derivative, the Cas9/single guide RNA (sgRNA) system, have emerged as potent new tools for targeted gene knockout in bacteria, yeast, fruit fly, zebrafish and human cells. Here, we describe adaptations of these systems leading to successful expression of the Cas9/sgRNA system in two dicot plant species, Arabidopsis and tobacco, and two monocot crop species, rice and sorghum. Agrobacterium tumefaciens was used for delivery of genes encoding Cas9, sgRNA and a non-fuctional, mutant green fluorescence protein (GFP) to Arabidopsis and tobacco. The mutant GFP gene contained target sites in its 5′ coding regions that were successfully cleaved by a CAS9/sgRNA complex that, along with error-prone DNA repair, resulted in creation of functional GFP genes. DNA sequencing confirmed Cas9/sgRNA-mediated mutagenesis at the target site. Rice protoplast cells transformed with Cas9/sgRNA constructs targeting the promoter region of the bacterial blight susceptibility genes, OsSWEET14 and OsSWEET11, were confirmed by DNA sequencing to contain mutated DNA sequences at the target sites. Successful demonstration of the Cas9/sgRNA system in model plant and crop species bodes well for its near-term use as a facile and powerful means of plant genetic engineering for scientific and agricultural applications.  相似文献   
4.
5.
6.
In many bacterial species, including the aquatic bacterium and human pathogen Vibrio cholerae, the second messenger cyclic diguanylate (c-di-GMP) modulates processes such as biofilm formation, motility, and virulence factor production. By interacting with various effectors, c-di-GMP regulates gene expression or protein function. One type of c-di-GMP receptor is the class I riboswitch, representatives of which have been shown to bind c-di-GMP in vitro. Herein, we examined the in vitro and in vivo function of the putative class I riboswitch in Vibrio cholerae, Vc1, which lies upstream of the gene encoding GbpA, a colonization factor that contributes to attachment of V. cholerae to environmental and host surfaces containing N-acetylglucosamine moieties. We provide evidence that Vc1 RNA interacts directly with c-di-GMP in vitro, and that nucleotides conserved among this class of riboswitch are important for binding. Yet the mutation of these conserved residues individually in the V. cholerae chromosome inconsistently affects the expression of gbpA and production of the GbpA protein. By isolating the regulatory function of Vc1, we show that the Vc1 element positively regulates downstream gene expression in response to c-di-GMP. Together these data suggest that the Vc1 element responds to c-di-GMP in vivo. Positive regulation of gbpA expression by c-di-GMP via Vc1 may influence the ability of V. cholerae to associate with chitin in the aquatic environment and the host intestinal environment.  相似文献   
7.
Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) syndrome is a rare, multisystem disorder characterized clinically by ptosis, progressive external ophthalmoplegia, gastrointestinal dysmotility, leukoencephalopathy, thin body habitus, and myopathy. Laboratory studies reveal defects of oxidative-phosphorylation and multiple mtDNA deletions frequently in skeletal muscle. We studied four ethnically distinct families affected with this apparently autosomal recessive disorder. Probands from each family were shown, by Southern blot, to have multiple mtDNA deletions in skeletal muscle. We mapped the MNGIE locus to 22q13.32-qter, distal to D22S1161, with a maximum two-point LOD score of 6.80 at locus D22S526. Cosegregation of MNGIE with a single chromosomal region in families with diverse ethnic backgrounds suggests that we have mapped an important locus for this disorder. We found no evidence to implicate three candidate genes in this region, by using direct sequence analysis for DNA helicase II and by assaying enzyme activities for arylsulfatase A and carnitine palmitoyltransferase.  相似文献   
8.
The taxonomic impediment to biodiversity studies may be influenced radically by the application of new technology, in particular, desktop image analysers and neural networks. The former offer an opportunity to automate objective feature measurement processes, and the latter provide powerful pattern recognition and data analysis tools which are able to 'learn' patterns in multivariate data. The coupling of these technologies may provide a realistic opportunity for the automation of routine species identifications. The potential benefits and limitations of these technologies, along with the development of automated identification systems are reviewed.  相似文献   
9.
10.
Sibley and Ahlquist compared the single-copy nuclear DNA sequences of the hominoid primates using DNA-DNA hybridization. From this data set they estimated a phylogeny that clusters man and chimpanzees using a distance Wagner procedure. However, no assessment of statistical confidence in this estimated phylogeny was made, despite the fact that their data set contains internal inconsistencies concerning the correct branching order. This paper presents a modification of Pielou's Q- statistic that allows one to make nonparametric tests of phylogenetic relationship from distance data. The results of this analysis indicate that the estimated phylogeny of Sibley and Ahlquist is without statistical significance owing to the internal inconsistencies of the data set. A survey and additional analyses of other types of molecular data indicate that the phylogeny that clusters chimpanzees and gorillas and has the human lineage splitting off earlier is statistically consistent with all the molecular data (including the DNA-DNA hybridization data), whereas the phylogeny estimated by Sibley and Ahlquist can be rejected at the 5% level using the data on restriction- endonuclease sites in the mitochondrial genome.   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