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   4022篇
  免费   320篇
  2023年   23篇
  2022年   33篇
  2021年   58篇
  2020年   46篇
  2019年   58篇
  2018年   106篇
  2017年   104篇
  2016年   137篇
  2015年   206篇
  2014年   211篇
  2013年   262篇
  2012年   292篇
  2011年   280篇
  2010年   205篇
  2009年   111篇
  2008年   225篇
  2007年   208篇
  2006年   183篇
  2005年   161篇
  2004年   147篇
  2003年   121篇
  2002年   104篇
  2001年   61篇
  2000年   67篇
  1999年   46篇
  1998年   27篇
  1997年   33篇
  1996年   18篇
  1994年   17篇
  1993年   20篇
  1992年   26篇
  1991年   26篇
  1990年   20篇
  1988年   18篇
  1985年   19篇
  1984年   31篇
  1983年   17篇
  1982年   24篇
  1981年   26篇
  1980年   37篇
  1979年   17篇
  1978年   23篇
  1977年   18篇
  1976年   32篇
  1975年   28篇
  1974年   29篇
  1973年   21篇
  1970年   23篇
  1968年   19篇
  1965年   16篇
排序方式: 共有4342条查询结果,搜索用时 15 毫秒
1.
A promising producer of bioactive compounds isolated from a Brazilian tropical soil was tested for its range of antimicrobial activities. Strain 606, classified as Streptomyces sp., could not be identified up to species level, suggesting a possible new taxon. The supernatant and 10 extracts and fractions, obtained by extraction and chromatographic techniques, presented antimicrobial activity using antibiograms. The methanolic fraction was highly active against pathogenic bacteria, phytopathogenic fungi and the human pathogenic yeast Candida albicans. It also possessed high antiviral activity inhibiting the propagation of an acyclovir-resistant herpes simplex virus type 1 strain on HEp-2 cells at non-cytotoxic concentration. The strong cytotoxic effect suggests an antitumour action. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
2.
3.
Toxin gamma is a basic, low-molecular-weight, neurotoxic protein, isolated from the venom of the Brazilian scorpion, Tityus serrulatus. Raman spectra (400-1800 cm-1 region) of this toxin in both the lyophilized state and in 0.1 M acetate buffer (pH 4.5) and the infrared spectrum (700-4000 cm-1 region) of a solid film were investigated. From the vibrational spectra, it can be concluded that the polypeptide backbone of toxin gamma consists of a mixture of the different secondary structures, with predominance of beta-sheet, followed by unordered structure and alpha-helix, with some evidence of beta-turn structures. The four disulfide bridges assume the gauche-gauche-gauche conformation of the CCSSCC fragments. The intensity ratio of the doublet at 853 and 828 cm-1 suggests that four out of the five tyrosine residues are exposed. The three tryptophan residues are exposed on the surface, and the single methionine residue assume the gauche-gauche conformation. Toxin gamma retains full activity in the pH 4.5-7.5 range, but is almost completely inactivated at pH 11.5.  相似文献   
4.
5.
The recBC nuclease (also called exonuclease V) has been partially purified from Escherichia coli K-12 strains carrying the thermosensitive recB270, recC271, and recB270 recC271 mutations. Of the multiple activities associated with the enzyme, only the adenosine 5'-triphosphate-dependent exonucleolytic hydrolysis of duplex deoxyribonucleic acid (DNA) is abnormally thermolabile. The exo- and endonucleolytic degradation of single-stranded DNA is no more thermosensitive than that catalyzed by the wild-type enzyme. These results suggest that the defects in genetic recombination, DNA repair, and the maintenance of cell viability observed in recBC mutants in vivo result primarily from the specific loss of adenosine 5'-triphosphate-dependent exonuclease active on duplex DNA.  相似文献   
6.
Evidence has been accumulating to support the process of reinforcement as a potential mechanism in speciation. In many species, mate choice decisions are influenced by cultural factors, including learned mating preferences (sexual imprinting) or learned mate attraction signals (e.g., bird song). It has been postulated that learning can have a strong impact on the likelihood of speciation and perhaps on the process of reinforcement, but no models have explicitly considered learning in a reinforcement context. We review the evidence that suggests that learning may be involved in speciation and reinforcement, and present a model of reinforcement via learned preferences. We show that not only can reinforcement occur when preferences are learned by imprinting, but that such preferences can maintain species differences easily in comparison with both autosomal and sex-linked genetically inherited preferences. We highlight the need for more explicit study of the connection between the behavioral process of learning and the evolutionary process of reinforcement in natural systems.  相似文献   
7.
8.
9.
10.
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