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   347篇
  免费   1篇
  国内免费   2篇
系统科学   5篇
丛书文集   1篇
教育与普及   1篇
现状及发展   71篇
研究方法   34篇
综合类   226篇
自然研究   12篇
  2018年   3篇
  2017年   3篇
  2016年   1篇
  2015年   2篇
  2014年   3篇
  2013年   2篇
  2012年   25篇
  2011年   44篇
  2010年   15篇
  2009年   3篇
  2008年   11篇
  2007年   15篇
  2006年   24篇
  2005年   10篇
  2004年   10篇
  2003年   6篇
  2002年   7篇
  2001年   10篇
  2000年   11篇
  1999年   7篇
  1997年   1篇
  1996年   3篇
  1995年   1篇
  1994年   1篇
  1992年   11篇
  1991年   4篇
  1990年   7篇
  1989年   6篇
  1987年   10篇
  1986年   5篇
  1985年   4篇
  1984年   4篇
  1983年   3篇
  1982年   4篇
  1981年   4篇
  1980年   3篇
  1979年   9篇
  1978年   4篇
  1977年   1篇
  1976年   5篇
  1975年   2篇
  1974年   4篇
  1973年   7篇
  1971年   4篇
  1970年   9篇
  1969年   3篇
  1968年   5篇
  1967年   7篇
  1966年   1篇
  1965年   5篇
排序方式: 共有350条查询结果,搜索用时 78 毫秒
341.
Legumes (Fabaceae or Leguminosae) are unique among cultivated plants for their ability to carry out endosymbiotic nitrogen fixation with rhizobial bacteria, a process that takes place in a specialized structure known as the nodule. Legumes belong to one of the two main groups of eurosids, the Fabidae, which includes most species capable of endosymbiotic nitrogen fixation. Legumes comprise several evolutionary lineages derived from a common ancestor 60 million years ago (Myr ago). Papilionoids are the largest clade, dating nearly to the origin of legumes and containing most cultivated species. Medicago truncatula is a long-established model for the study of legume biology. Here we describe the draft sequence of the M. truncatula euchromatin based on a recently completed BAC assembly supplemented with Illumina shotgun sequence, together capturing ~94% of all M. truncatula genes. A whole-genome duplication (WGD) approximately 58 Myr ago had a major role in shaping the M. truncatula genome and thereby contributed to the evolution of endosymbiotic nitrogen fixation. Subsequent to the WGD, the M. truncatula genome experienced higher levels of rearrangement than two other sequenced legumes, Glycine max and Lotus japonicus. M. truncatula is a close relative of alfalfa (Medicago sativa), a widely cultivated crop with limited genomics tools and complex autotetraploid genetics. As such, the M. truncatula genome sequence provides significant opportunities to expand alfalfa's genomic toolbox.  相似文献   
342.
Zoltowski BD  Vaidya AT  Top D  Widom J  Young MW  Crane BR 《Nature》2011,480(7377):396-399
The cryptochrome/photolyase (CRY/PL) family of photoreceptors mediates adaptive responses to ultraviolet and blue light exposure in all kingdoms of life. Whereas PLs function predominantly in DNA repair of cyclobutane pyrimidine dimers (CPDs) and 6-4 photolesions caused by ultraviolet radiation, CRYs transduce signals important for growth, development, magnetosensitivity and circadian clocks. Despite these diverse functions, PLs/CRYs preserve a common structural fold, a dependence on flavin adenine dinucleotide (FAD) and an internal photoactivation mechanism. However, members of the CRY/PL family differ in the substrates recognized (protein or DNA), photochemical reactions catalysed and involvement of an antenna cofactor. It is largely unknown how the animal CRYs that regulate circadian rhythms act on their substrates. CRYs contain a variable carboxy-terminal tail that appends the conserved PL homology domain (PHD) and is important for function. Here, we report a 2.3-? resolution crystal structure of Drosophila CRY with an intact C terminus. The C-terminal helix docks in the analogous groove that binds DNA substrates in PLs. Conserved Trp?536 juts into the CRY catalytic centre to mimic PL recognition of DNA photolesions. The FAD anionic semiquinone found in the crystals assumes a conformation to facilitate restructuring of the tail helix. These results help reconcile the diverse functions of the CRY/PL family by demonstrating how conserved protein architecture and photochemistry can be elaborated into a range of light-driven functions.  相似文献   
343.
以锥尾伞滑刃线虫(Bursaphelenchus conicaudatus)作为对照,在韩国不同地域和寄主中采集了15个松材线虫分离物、3 个拟松材线虫的分离物及5个未鉴定的该属的分离物,并对其进行了ITS 和 D2D3 rDNA序列分析。以单条雌线虫的DNA为模板,ITS 和 D2D3 区由PCR仪扩增并克隆其序列。结果表明,所有松材线虫的ITS 和 D2D3 区序列相同,没有种内变异。然而2个采自黑松和红松的拟松材线虫分离物的基因型分别是亚洲型和欧洲型。5个未鉴定线虫的数据表明,它们分别接近B. tusciae、B. lini、B. thailandae、B. doui和 B. hylobianum,该结果同时被形态学结果所支持。利用5种酶(Hinf I、Alu I、Msp I、Hae III、Rsa),PCR产物克隆并测序的数据也可以区分不同种和基因型。  相似文献   
344.
现代服务业的发展水平是一个区域综合实力的重要体现,加快发展现代服务业对于转变经济增长方式、优化产业结构、缓解就业压力,具有重要的现实意义.上饶市服务业发展已实现了从小到大、由传统向现代的历史转变,构建了优势较明显、特色较鲜明、功能较完善的服务业发展格局.但与发达地区和发展要求相比,仍然存在发展规划相对滞后、辐射带动力不强、产业层次不高、管理体制不顺等许多亟待解决的问题和不足.上饶市必须加快提升现代服务业发展的层次和水平,要坚持立足当前与谋划长远相结合,重点发展与全面提升相结合,构建现代服务业体系,拓展商务区空间,加强保障机制建设.  相似文献   
345.
Long JA  Trinajstic K  Young GC  Senden T 《Nature》2008,453(7195):650-652
The extinct placoderm fishes were the dominant group of vertebrates throughout the Middle Palaeozoic era, yet controversy about their relationships within the gnathostomes (jawed vertebrates) is partly due to different interpretations of their reproductive biology. Here we document the oldest record of a live-bearing vertebrate in a new ptyctodontid placoderm, Materpiscis attenboroughi gen. et sp. nov., from the Late Devonian Gogo Formation of Australia (approximately 380 million years ago). The new specimen, remarkably preserved in three dimensions, contains a single, intra-uterine embryo connected by a permineralized umbilical cord. An amorphous crystalline mass near the umbilical cord possibly represents the recrystallized yolk sac. Another ptyctodont from the Gogo Formation, Austroptyctodus gardineri, also shows three small embryos inside it in the same position. Ptyctodontids have already provided the oldest definite evidence for vertebrate copulation, and the new specimens confirm that some placoderms had a remarkably advanced reproductive biology, comparable to that of some modern sharks and rays. The new discovery points to internal fertilization and viviparity in vertebrates as originating earliest within placoderms.  相似文献   
346.
Yang YD  Cho H  Koo JY  Tak MH  Cho Y  Shim WS  Park SP  Lee J  Lee B  Kim BM  Raouf R  Shin YK  Oh U 《Nature》2008,455(7217):1210-1215
Calcium (Ca(2+))-activated chloride channels are fundamental mediators in numerous physiological processes including transepithelial secretion, cardiac and neuronal excitation, sensory transduction, smooth muscle contraction and fertilization. Despite their physiological importance, their molecular identity has remained largely unknown. Here we show that transmembrane protein 16A (TMEM16A, which we also call anoctamin 1 (ANO1)) is a bona fide Ca(2+)-activated chloride channel that is activated by intracellular Ca(2+) and Ca(2+)-mobilizing stimuli. With eight putative transmembrane domains and no apparent similarity to previously characterized channels, ANO1 defines a new family of ionic channels. The biophysical properties as well as the pharmacological profile of ANO1 are in full agreement with native Ca(2+)-activated chloride currents. ANO1 is expressed in various secretory epithelia, the retina and sensory neurons. Furthermore, knockdown of mouse Ano1 markedly reduced native Ca(2+)-activated chloride currents as well as saliva production in mice. We conclude that ANO1 is a candidate Ca(2+)-activated chloride channel that mediates receptor-activated chloride currents in diverse physiological processes.  相似文献   
347.
Chromosome 17 is unusual among the human chromosomes in many respects. It is the largest human autosome with orthology to only a single mouse chromosome, mapping entirely to the distal half of mouse chromosome 11. Chromosome 17 is rich in protein-coding genes, having the second highest gene density in the genome. It is also enriched in segmental duplications, ranking third in density among the autosomes. Here we report a finished sequence for human chromosome 17, as well as a structural comparison with the finished sequence for mouse chromosome 11, the first finished mouse chromosome. Comparison of the orthologous regions reveals striking differences. In contrast to the typical pattern seen in mammalian evolution, the human sequence has undergone extensive intrachromosomal rearrangement, whereas the mouse sequence has been remarkably stable. Moreover, although the human sequence has a high density of segmental duplication, the mouse sequence has a very low density. Notably, these segmental duplications correspond closely to the sites of structural rearrangement, demonstrating a link between duplication and rearrangement. Examination of the main classes of duplicated segments provides insight into the dynamics underlying expansion of chromosome-specific, low-copy repeats in the human genome.  相似文献   
348.
Chung Y  Klimanskaya I  Becker S  Marh J  Lu SJ  Johnson J  Meisner L  Lanza R 《Nature》2006,439(7073):216-219
The most basic objection to human embryonic stem (ES) cell research is rooted in the fact that ES cell derivation deprives embryos of any further potential to develop into a complete human being. ES cell lines are conventionally isolated from the inner cell mass of blastocysts and, in a few instances, from cleavage stage embryos. So far, there have been no reports in the literature of stem cell lines derived using an approach that does not require embryo destruction. Here we report an alternative method of establishing ES cell lines-using a technique of single-cell embryo biopsy similar to that used in pre-implantation genetic diagnosis of genetic defects-that does not interfere with the developmental potential of embryos. Five putative ES and seven trophoblast stem (TS) cell lines were produced from single blastomeres, which maintained normal karyotype and markers of pluripotency or TS cells for up to more than 50 passages. The ES cells differentiated into derivatives of all three germ layers in vitro and in teratomas, and showed germ line transmission. Single-blastomere-biopsied embryos developed to term without a reduction in their developmental capacity. The ability to generate human ES cells without the destruction of ex utero embryos would reduce or eliminate the ethical concerns of many.  相似文献   
349.
350.
Bacterial infection remains a serious threat to human lives because of emerging resistance to existing antibiotics. Although the scientific community has avidly pursued the discovery of new antibiotics that interact with new targets, these efforts have met with limited success since the early 1960s. Here we report the discovery of platensimycin, a previously unknown class of antibiotics produced by Streptomyces platensis. Platensimycin demonstrates strong, broad-spectrum Gram-positive antibacterial activity by selectively inhibiting cellular lipid biosynthesis. We show that this anti-bacterial effect is exerted through the selective targeting of beta-ketoacyl-(acyl-carrier-protein (ACP)) synthase I/II (FabF/B) in the synthetic pathway of fatty acids. Direct binding assays show that platensimycin interacts specifically with the acyl-enzyme intermediate of the target protein, and X-ray crystallographic studies reveal that a specific conformational change that occurs on acylation must take place before the inhibitor can bind. Treatment with platensimycin eradicates Staphylococcus aureus infection in mice. Because of its unique mode of action, platensimycin shows no cross-resistance to other key antibiotic-resistant strains tested, including methicillin-resistant S. aureus, vancomycin-intermediate S. aureus and vancomycin-resistant enterococci. Platensimycin is the most potent inhibitor reported for the FabF/B condensing enzymes, and is the only inhibitor of these targets that shows broad-spectrum activity, in vivo efficacy and no observed toxicity.  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