全文获取类型
收费全文 | 235篇 |
免费 | 2篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 7篇 |
理论与方法论 | 1篇 |
现状及发展 | 48篇 |
研究方法 | 42篇 |
综合类 | 133篇 |
自然研究 | 8篇 |
出版年
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2018年 | 7篇 |
2017年 | 3篇 |
2016年 | 5篇 |
2015年 | 4篇 |
2014年 | 2篇 |
2013年 | 2篇 |
2012年 | 11篇 |
2011年 | 30篇 |
2010年 | 6篇 |
2009年 | 2篇 |
2008年 | 17篇 |
2007年 | 19篇 |
2006年 | 15篇 |
2005年 | 10篇 |
2004年 | 18篇 |
2003年 | 16篇 |
2002年 | 14篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 7篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1992年 | 5篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 7篇 |
1969年 | 4篇 |
1968年 | 1篇 |
排序方式: 共有239条查询结果,搜索用时 15 毫秒
161.
162.
163.
Mutations in the genes encoding 11beta-hydroxysteroid dehydrogenase type 1 and hexose-6-phosphate dehydrogenase interact to cause cortisone reductase deficiency 总被引:12,自引:0,他引:12
164.
Nicotinamide and PNC1 govern lifespan extension by calorie restriction in Saccharomyces cerevisiae 总被引:8,自引:0,他引:8
Calorie restriction extends lifespan in a broad range of organisms, from yeasts to mammals. Numerous hypotheses have been proposed to explain this phenomenon, including decreased oxidative damage and altered energy metabolism. In Saccharomyces cerevisiae, lifespan extension by calorie restriction requires the NAD+-dependent histone deacetylase, Sir2 (ref. 1). We have recently shown that Sir2 and its closest human homologue SIRT1, a p53 deacetylase, are strongly inhibited by the vitamin B3 precursor nicotinamide. Here we show that increased expression of PNC1 (pyrazinamidase/nicotinamidase 1), which encodes an enzyme that deaminates nicotinamide, is both necessary and sufficient for lifespan extension by calorie restriction and low-intensity stress. We also identify PNC1 as a longevity gene that is responsive to all stimuli that extend lifespan. We provide evidence that nicotinamide depletion is sufficient to activate Sir2 and that this is the mechanism by which PNC1 regulates longevity. We conclude that yeast lifespan extension by calorie restriction is the consequence of an active cellular response to a low-intensity stress and speculate that nicotinamide might regulate critical cellular processes in higher organisms. 相似文献
165.
166.
167.
168.
Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of 'dangling bonds' by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO(3)--a standard model for oxides with a perovskite structure--have been studied extensively. Here we report a solution of the 2 x 1 SrTiO(3) (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO(6-x) units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO(3). We suggest that this structural concept can be extended to perovskite surfaces in general. 相似文献
169.
Hall N Pain A Berriman M Churcher C Harris B Harris D Mungall K Bowman S Atkin R Baker S Barron A Brooks K Buckee CO Burrows C Cherevach I Chillingworth C Chillingworth T Christodoulou Z Clark L Clark R Corton C Cronin A Davies R Davis P Dear P Dearden F Doggett J Feltwell T Goble A Goodhead I Gwilliam R Hamlin N Hance Z Harper D Hauser H Hornsby T Holroyd S Horrocks P Humphray S Jagels K James KD Johnson D Kerhornou A Knights A Konfortov B Kyes S Larke N Lawson D Lennard N Line A Maddison M 《Nature》2002,419(6906):527-531
Since the sequencing of the first two chromosomes of the malaria parasite, Plasmodium falciparum, there has been a concerted effort to sequence and assemble the entire genome of this organism. Here we report the sequence of chromosomes 1, 3-9 and 13 of P. falciparum clone 3D7--these chromosomes account for approximately 55% of the total genome. We describe the methods used to map, sequence and annotate these chromosomes. By comparing our assemblies with the optical map, we indicate the completeness of the resulting sequence. During annotation, we assign Gene Ontology terms to the predicted gene products, and observe clustering of some malaria-specific terms to specific chromosomes. We identify a highly conserved sequence element found in the intergenic region of internal var genes that is not associated with their telomeric counterparts. 相似文献
170.
Massive gene decay in the leprosy bacillus 总被引:73,自引:0,他引:73
Cole ST Eiglmeier K Parkhill J James KD Thomson NR Wheeler PR Honoré N Garnier T Churcher C Harris D Mungall K Basham D Brown D Chillingworth T Connor R Davies RM Devlin K Duthoy S Feltwell T Fraser A Hamlin N Holroyd S Hornsby T Jagels K Lacroix C Maclean J Moule S Murphy L Oliver K Quail MA Rajandream MA Rutherford KM Rutter S Seeger K Simon S Simmonds M Skelton J Squares R Squares S Stevens K Taylor K Whitehead S Woodward JR Barrell BG 《Nature》2001,409(6823):1007-1011
Leprosy, a chronic human neurological disease, results from infection with the obligate intracellular pathogen Mycobacterium leprae, a close relative of the tubercle bacillus. Mycobacterium leprae has the longest doubling time of all known bacteria and has thwarted every effort at culture in the laboratory. Comparing the 3.27-megabase (Mb) genome sequence of an armadillo-derived Indian isolate of the leprosy bacillus with that of Mycobacterium tuberculosis (4.41 Mb) provides clear explanations for these properties and reveals an extreme case of reductive evolution. Less than half of the genome contains functional genes but pseudogenes, with intact counterparts in M. tuberculosis, abound. Genome downsizing and the current mosaic arrangement appear to have resulted from extensive recombination events between dispersed repetitive sequences. Gene deletion and decay have eliminated many important metabolic activities including siderophore production, part of the oxidative and most of the microaerophilic and anaerobic respiratory chains, and numerous catabolic systems and their regulatory circuits. 相似文献