全文获取类型
收费全文 | 195篇 |
免费 | 0篇 |
国内免费 | 3篇 |
专业分类
系统科学 | 3篇 |
教育与普及 | 1篇 |
理论与方法论 | 2篇 |
现状及发展 | 49篇 |
研究方法 | 15篇 |
综合类 | 124篇 |
自然研究 | 4篇 |
出版年
2018年 | 2篇 |
2017年 | 1篇 |
2016年 | 1篇 |
2015年 | 1篇 |
2014年 | 2篇 |
2013年 | 1篇 |
2012年 | 9篇 |
2011年 | 15篇 |
2010年 | 4篇 |
2009年 | 1篇 |
2008年 | 9篇 |
2007年 | 5篇 |
2006年 | 13篇 |
2005年 | 8篇 |
2004年 | 10篇 |
2003年 | 6篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 5篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 7篇 |
1970年 | 8篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 3篇 |
1966年 | 7篇 |
1965年 | 3篇 |
1963年 | 1篇 |
1958年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有198条查询结果,搜索用时 618 毫秒
131.
King RD Whelan KE Jones FM Reiser PG Bryant CH Muggleton SH Kell DB Oliver SG 《Nature》2004,427(6971):247-252
The question of whether it is possible to automate the scientific process is of both great theoretical interest and increasing practical importance because, in many scientific areas, data are being generated much faster than they can be effectively analysed. We describe a physically implemented robotic system that applies techniques from artificial intelligence to carry out cycles of scientific experimentation. The system automatically originates hypotheses to explain observations, devises experiments to test these hypotheses, physically runs the experiments using a laboratory robot, interprets the results to falsify hypotheses inconsistent with the data, and then repeats the cycle. Here we apply the system to the determination of gene function using deletion mutants of yeast (Saccharomyces cerevisiae) and auxotrophic growth experiments. We built and tested a detailed logical model (involving genes, proteins and metabolites) of the aromatic amino acid synthesis pathway. In biological experiments that automatically reconstruct parts of this model, we show that an intelligent experiment selection strategy is competitive with human performance and significantly outperforms, with a cost decrease of 3-fold and 100-fold (respectively), both cheapest and random-experiment selection. 相似文献
132.
Tudosa I Stamm C Kashuba AB King F Siegmann HC Stöhr J Ju G Lu B Weller D 《Nature》2004,428(6985):831-833
In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either 'up' or 'down'. In such devices, the fastest and most efficient recording method involves precessional switching: when a magnetic field B(p) is applied as a write pulse over a period tau, the magnetization vector precesses about the field until B(p)tau reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse B(p) might therefore substantially shorten the required switching time tau and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording. 相似文献
133.
Cytoskeletal molecular motors belonging to the kinesin and dynein families transport cargos (for example, messenger RNA, endosomes, virus) on polymerized linear structures called microtubules in the cell. These 'nanomachines' use energy obtained from ATP hydrolysis to generate force, and move in a step-like manner on microtubules. Dynein has a complex and fundamentally different structure from other motor families. Thus, understanding dynein's force generation can yield new insight into the architecture and function of nanomachines. Here, we use an optical trap to quantify motion of polystyrene beads driven along microtubules by single cytoplasmic dynein motors. Under no load, dynein moves predominantly with a mixture of 24-nm and 32-nm steps. When moving against load applied by an optical trap, dynein can decrease step size to 8 nm and produce force up to 1.1 pN. This correlation between step size and force production is consistent with a molecular gear mechanism. The ability to take smaller but more powerful strokes under load--that is, to shift gears--depends on the availability of ATP. We propose a model whereby the gear is downshifted through load-induced binding of ATP at secondary sites in the dynein head. 相似文献
134.
Zusammenfassung Die Befunde zeigen, dass am Abbau von Hefenukleinsäure durch Rattenleberextrakte zwei Fermente wirksam sind:a) eine Ribonuklease, welche 3-Nukleotide als Spaltprodukte liefert, undb) eine Phosphodiesterase, welche zyklische Nukleotide zu 2-Nukleotiden abbant.
This work was supported by a grant (C-2459C2) from the U. S. Department of Public Health. 相似文献
This work was supported by a grant (C-2459C2) from the U. S. Department of Public Health. 相似文献
135.
Mutations in NOTCH1 cause aortic valve disease 总被引:2,自引:0,他引:2
Garg V Muth AN Ransom JF Schluterman MK Barnes R King IN Grossfeld PD Srivastava D 《Nature》2005,437(7056):270-274
136.
Although mutations in cell cycle regulators or spindle proteins can perturb chromosome segregation, the causes and consequences of spontaneous mitotic chromosome nondisjunction in human cells are not well understood. It has been assumed that nondisjunction of a chromosome during mitosis will yield two aneuploid daughter cells. Here we show that chromosome nondisjunction is tightly coupled to regulation of cytokinesis in human cell lines, such that nondisjunction results in the formation of tetraploid rather than aneuploid cells. We observed that spontaneously arising binucleated cells exhibited chromosome mis-segregation rates up to 166-fold higher than the overall mitotic population. Long-term imaging experiments indicated that most binucleated cells arose through a bipolar mitosis followed by regression of the cleavage furrow hours later. Nondisjunction occurred with high frequency in cells that became binucleated by furrow regression, but not in cells that completed cytokinesis to form two mononucleated cells. Our findings indicate that nondisjunction does not directly yield aneuploid cells, but rather tetraploid cells that may subsequently become aneuploid through further division. The coupling of spontaneous segregation errors to furrow regression provides a potential explanation for the prevalence of hyperdiploid chromosome number and centrosome amplification observed in many cancers. 相似文献
137.
Barthelmy SD Chincarini G Burrows DN Gehrels N Covino S Moretti A Romano P O'Brien PT Sarazin CL Kouveliotou C Goad M Vaughan S Tagliaferri G Zhang B Antonelli LA Campana S Cummings JR D'Avanzo P Davies MB Giommi P Grupe D Kaneko Y Kennea JA King A Kobayashi S Melandri A Meszaros P Nousek JA Patel S Sakamoto T Wijers RA 《Nature》2005,438(7070):994-996
Two short (< 2 s) gamma-ray bursts (GRBs) have recently been localized and fading afterglow counterparts detected. The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual gamma-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2-3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts--the coalescence of binary systems of neutron stars or a neutron star-black hole pair--are the progenitors of short bursts. 相似文献
138.
Bruce LJ Robinson HC Guizouarn H Borgese F Harrison P King MJ Goede JS Coles SE Gore DM Lutz HU Ficarella R Layton DM Iolascon A Ellory JC Stewart GW 《Nature genetics》2005,37(11):1258-1263
We identified 11 human pedigrees with dominantly inherited hemolytic anemias in both the hereditary stomatocytosis and spherocytosis classes. Affected individuals in these families had an increase in membrane permeability to Na and K that is particularly marked at 0 degrees C. We found that disease in these pedigrees was associated with a series of single amino-acid substitutions in the intramembrane domain of the erythrocyte band 3 anion exchanger, AE1. Anion movements were reduced in the abnormal red cells. The 'leak' cation fluxes were inhibited by SITS, dipyridamole and NS1652, chemically diverse inhibitors of band 3. Expression of the mutated genes in Xenopus laevis oocytes induced abnormal Na and K fluxes in the oocytes, and the induced Cl transport was low. These data are consistent with the suggestion that the substitutions convert the protein from an anion exchanger into an unregulated cation channel. 相似文献
139.
140.
Valency investigations of iron dextran ("Imferon") 总被引:1,自引:0,他引:1