全文获取类型
收费全文 | 38006篇 |
免费 | 116篇 |
国内免费 | 146篇 |
专业分类
系统科学 | 179篇 |
丛书文集 | 611篇 |
教育与普及 | 65篇 |
理论与方法论 | 154篇 |
现状及发展 | 16674篇 |
研究方法 | 1583篇 |
综合类 | 18307篇 |
自然研究 | 695篇 |
出版年
2013年 | 302篇 |
2012年 | 575篇 |
2011年 | 1249篇 |
2010年 | 246篇 |
2008年 | 682篇 |
2007年 | 769篇 |
2006年 | 733篇 |
2005年 | 781篇 |
2004年 | 837篇 |
2003年 | 672篇 |
2002年 | 702篇 |
2001年 | 1098篇 |
2000年 | 1034篇 |
1999年 | 747篇 |
1992年 | 697篇 |
1991年 | 507篇 |
1990年 | 566篇 |
1989年 | 555篇 |
1988年 | 537篇 |
1987年 | 542篇 |
1986年 | 586篇 |
1985年 | 764篇 |
1984年 | 549篇 |
1983年 | 444篇 |
1982年 | 418篇 |
1981年 | 420篇 |
1980年 | 492篇 |
1979年 | 1142篇 |
1978年 | 903篇 |
1977年 | 870篇 |
1976年 | 726篇 |
1975年 | 751篇 |
1974年 | 1054篇 |
1973年 | 920篇 |
1972年 | 988篇 |
1971年 | 1091篇 |
1970年 | 1474篇 |
1969年 | 1182篇 |
1968年 | 1089篇 |
1967年 | 1076篇 |
1966年 | 1007篇 |
1965年 | 732篇 |
1964年 | 249篇 |
1959年 | 405篇 |
1958年 | 762篇 |
1957年 | 510篇 |
1956年 | 430篇 |
1955年 | 369篇 |
1954年 | 456篇 |
1948年 | 301篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex 总被引:36,自引:0,他引:36
Do new synapses form in the adult cortex to support experience-dependent plasticity? To address this question, we repeatedly imaged individual pyramidal neurons in the mouse barrel cortex over periods of weeks. We found that, although dendritic structure is stable, some spines appear and disappear. Spine lifetimes vary greatly: stable spines, about 50% of the population, persist for at least a month, whereas the remainder are present for a few days or less. Serial-section electron microscopy of imaged dendritic segments revealed retrospectively that spine sprouting and retraction are associated with synapse formation and elimination. Experience-dependent plasticity of cortical receptive fields was accompanied by increased synapse turnover. Our measurements suggest that sensory experience drives the formation and elimination of synapses and that these changes might underlie adaptive remodelling of neural circuits. 相似文献
952.
Respiration in the open ocean 总被引:11,自引:0,他引:11
A key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon. This will depend on the production of organic matter relative to the decomposition due to biological respiration. Estimates of respiration are available for the top layers, the mesopelagic layer, and the abyssal waters and sediments of various ocean regions. Although the total open ocean respiration is uncertain, it is probably substantially greater than most current estimates of particulate organic matter production. Nevertheless, whether the biota act as a net source or sink of carbon remains an open question. 相似文献
953.
Bennett IM Farfano HM Bogani F Primak A Liddell PA Otero L Sereno L Silber JJ Moore AL Moore TA Gust D 《Nature》2002,420(6914):398-401
Transport of calcium ions across membranes and against a thermodynamic gradient is essential to many biological processes, including muscle contraction, the citric acid cycle, glycogen metabolism, release of neurotransmitters, vision, biological signal transduction and immune response. Synthetic systems that transport metal ions across lipid or liquid membranes are well known, and in some cases light has been used to facilitate transport. Typically, a carrier molecule located in a symmetric membrane binds the ion from aqueous solution on one side and releases it on the other. The thermodynamic driving force is provided by an ion concentration difference between the two aqueous solutions, coupling to such a gradient in an auxiliary species, or photomodulation of the carrier by an asymmetric photon flux. Here we report a different approach, in which active transport is driven not by concentration gradients, but by light-induced electron transfer in a photoactive molecule that is asymmetrically disposed across a lipid bilayer. The system comprises a synthetic, light-driven transmembrane Ca2+ pump based on a redox-sensitive, lipophilic Ca2+-binding shuttle molecule whose function is powered by an intramembrane artificial photosynthetic reaction centre. The resulting structure transports calcium ions across the bilayer of a liposome to develop both a calcium ion concentration gradient and a membrane potential, expanding Mitchell's concept of a redox loop mechanism for protons to include divalent cations. Although the quantum yield is relatively low (approximately 1 per cent), the Ca2+ electrochemical potential developed is significant. 相似文献
954.
Analysis of the mouse transcriptome based on functional annotation of 60,770 full-length cDNAs 总被引:6,自引:0,他引:6
Okazaki Y Furuno M Kasukawa T Adachi J Bono H Kondo S Nikaido I Osato N Saito R Suzuki H Yamanaka I Kiyosawa H Yagi K Tomaru Y Hasegawa Y Nogami A Schönbach C Gojobori T Baldarelli R Hill DP Bult C Hume DA Quackenbush J Schriml LM Kanapin A Matsuda H Batalov S Beisel KW Blake JA Bradt D Brusic V Chothia C Corbani LE Cousins S Dalla E Dragani TA Fletcher CF Forrest A Frazer KS Gaasterland T Gariboldi M Gissi C Godzik A Gough J Grimmond S Gustincich S Hirokawa N Jackson IJ Jarvis ED Kanai A 《Nature》2002,420(6915):563-573
955.
The genome sequence and structure of rice chromosome 1 总被引:2,自引:0,他引:2
Sasaki T Matsumoto T Yamamoto K Sakata K Baba T Katayose Y Wu J Niimura Y Cheng Z Nagamura Y Antonio BA Kanamori H Hosokawa S Masukawa M Arikawa K Chiden Y Hayashi M Okamoto M Ando T Aoki H Arita K Hamada M Harada C Hijishita S Honda M Ichikawa Y Idonuma A Iijima M Ikeda M Ikeno M Ito S Ito T Ito Y Ito Y Iwabuchi A Kamiya K Karasawa W Katagiri S Kikuta A Kobayashi N Kono I Machita K Maehara T Mizuno H Mizubayashi T Mukai Y Nagasaki H Nakashima M Nakama Y Nakamichi Y Nakamura M Namiki N 《Nature》2002,420(6913):312-316
The rice species Oryza sativa is considered to be a model plant because of its small genome size, extensive genetic map, relative ease of transformation and synteny with other cereal crops. Here we report the essentially complete sequence of chromosome 1, the longest chromosome in the rice genome. We summarize characteristics of the chromosome structure and the biological insight gained from the sequence. The analysis of 43.3 megabases (Mb) of non-overlapping sequence reveals 6,756 protein coding genes, of which 3,161 show homology to proteins of Arabidopsis thaliana, another model plant. About 30% (2,073) of the genes have been functionally categorized. Rice chromosome 1 is (G + C)-rich, especially in its coding regions, and is characterized by several gene families that are dispersed or arranged in tandem repeats. Comparison with a draft sequence indicates the importance of a high-quality finished sequence. 相似文献
956.
957.
958.
959.
Rajagopalan H Bardelli A Lengauer C Kinzler KW Vogelstein B Velculescu VE 《Nature》2002,418(6901):934
Genes of the RAF family encode kinases that are regulated by Ras and mediate cellular responses to growth signals. Activating mutations in one RAF gene, BRAF, have been found in a high proportion of melanomas and in a small fraction of other cancers. Here we show that BRAF mutations in colorectal cancers occur only in tumours that do not carry mutations in a RAS gene known as KRAS, and that BRAF mutation is linked to the proficiency of these tumours in repairing mismatched bases in DNA. Our results not only provide genetic support for the idea that mutations in BRAF and KRAS exert equivalent effects in tumorigenesis, but also emphasize the role of repair processes in establishing the mutation spectra that underpin human cancer. 相似文献
960.