全文获取类型
收费全文 | 1225436篇 |
免费 | 98986篇 |
国内免费 | 1355篇 |
专业分类
1325777篇 |
出版年
2021年 | 17615篇 |
2020年 | 12527篇 |
2019年 | 16085篇 |
2018年 | 17434篇 |
2017年 | 16242篇 |
2016年 | 27767篇 |
2015年 | 41883篇 |
2014年 | 49852篇 |
2013年 | 76062篇 |
2012年 | 31743篇 |
2011年 | 19485篇 |
2010年 | 42388篇 |
2009年 | 44140篇 |
2008年 | 19163篇 |
2007年 | 16896篇 |
2006年 | 22661篇 |
2005年 | 24078篇 |
2004年 | 23254篇 |
2003年 | 21019篇 |
2002年 | 19251篇 |
2001年 | 25897篇 |
2000年 | 23177篇 |
1999年 | 25273篇 |
1998年 | 24445篇 |
1997年 | 24129篇 |
1996年 | 23797篇 |
1995年 | 21930篇 |
1994年 | 21753篇 |
1993年 | 20810篇 |
1992年 | 22451篇 |
1991年 | 21158篇 |
1990年 | 19778篇 |
1989年 | 20974篇 |
1988年 | 19224篇 |
1987年 | 18181篇 |
1986年 | 17288篇 |
1985年 | 19457篇 |
1984年 | 20081篇 |
1983年 | 17807篇 |
1982年 | 19089篇 |
1981年 | 18494篇 |
1980年 | 17312篇 |
1979年 | 15975篇 |
1978年 | 15875篇 |
1977年 | 15302篇 |
1976年 | 14575篇 |
1975年 | 13943篇 |
1974年 | 14348篇 |
1973年 | 14746篇 |
1972年 | 12433篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
931.
932.
933.
934.
935.
936.
Combining experimental evolution with whole‐genome resequencing is a promising new strategy for investigating the dynamics of evolutionary change. Published studies that have resequenced laboratory‐selected populations of sexual organisms have typically focused on populations sampled at the end of an evolution experiment. These studies have attempted to associate particular alleles with phenotypic change and attempted to distinguish between different theoretical models of adaptation. However, neither the population used to initiate the experiment nor multiple time points sampled during the evolutionary trajectory are generally available for examination. In this issue of Molecular Ecology, Orozco‐terWengel et al. (2012) take a significant step forward by estimating genome‐wide allele frequencies at the start, 15 generations into and at the end of a 37‐generation Drosophila experimental evolution study. The authors identify regions of the genome that have responded to laboratory selection and describe the temporal dynamics of allele frequency change. They identify two common trajectories for putatively adaptive alleles: alleles either gradually increase in frequency throughout the entire 37 generations or alleles plateau at a new frequency by generation 15. The identification of complex trajectories of alleles under selection contributes to a growing body of literature suggesting that simple models of adaptation, whereby beneficial alleles arise and increase in frequency unimpeded until they become fixed, may not adequately describe short‐term response to selection. 相似文献
937.
N P Veselkin I V Batueva 《Rossi?skii fiziologicheski? zhurnal imeni I.M. Sechenova / Rossi?skaia akademiia nauk》1999,85(6):743-750
Glycine and GABA play the role of inhibitory transmitters in the lamprey spinal cord. The mechanisms of action of both amino acids to the membrane receptors producing the postsynaptic inhibition as well as role and mechanism of GABA action producing the presynaptic inhibition are considered in this paper. The data concerned with morphological substrates of both type inhibitions are discussed. 相似文献
938.
The crystal structure analysis of the Fe-type nitrile hydratase from Rhodococcus sp. N-771 revealed the unique structure of the enzyme composed of the alpha- and beta-subunits and the unprecedented structure of the non-heme iron active center [Nagashima, S., et al. (1998) Nat. Struct. Biol. 5, 347-351]. A number of hydration water molecules were identified both in the interior and at the exterior of the enzyme. The study presented here investigated the roles of the hydration water molecules in stabilizing the tertiary and the quaternary structures of the enzyme, based on the crystal structure and the results from a laser light scattering experiment for the enzyme in solution. Seventy-six hydration water molecules between the two subunits significantly contribute to the alphabeta heterodimer formation by making up the surface shape, forming extensive networks of hydrogen bonds, and moderating the surface charge of the beta-subunit. In particular, 20 hydration water molecules form the extensive networks of hydrogen bonds stabilizing the unique structure of the active center. The amino acid residues hydrogen-bonded to those hydration water molecules are highly conserved among all known nitrile hydratases and even in the homologous enzyme, thiocyanate hydrolase, suggesting the structural conservation of the water molecules in the NHase family. The crystallographic asymmetric unit contained two heterodimers connected by 50 hydration water molecules. The heterotetramer formation in crystallization was clearly explained by the concentration-dependent aggregation state of NHase found in the light scattering measurement. The measurement proved that the dimer-tetramer equilibrium shifted toward the heterotetramer dominant state in the concentration range of 10(-2)-1.0 mg/mL. In the tetramer dominant state, 50 water molecules likely glue the two heterodimers together as observed in the crystal structure. Because NHase exhibits a high abundance in bacterial cells, the result suggests that the heterotetramer is physiologically relevant. In addition, it was revealed that the substrate specificity of this enzyme, recognizing small aliphatic substrates rather than aromatic ones, came from the narrowness of the entrance channel from the bulk solvent to the active center. This finding may give a clue for changing the substrate specificity of the enzyme. Under the crystallization condition described here, one 1,4-dioxane molecule plugged the channel. Through spectroscopic and crystallographic experiments, we found that the molecule prevented the dissociation of the endogenous NO molecule from the active center even when the crystal was exposed to light. 相似文献
939.
940.
Bacopa monniera Wettst. (BM, syn. Herpestis monniera L; Scrophulariaceae), is an Ayurvedic drug used as a rasayana. Its fresh juice was earlier reported to have significant antiulcerogenic activity. In continuation, methanolic extract of BM (BME) standardized to bacoside-A content (percentage-38.0 ± 0.9), when given in the dose of 10–50 mg/kg, twice daily for 5 days, showed dose-dependent anti-ulcerogenic on various gastric ulcer models induced by ethanol, aspirin, 2 h cold restraint stress and 4 h pylorus ligation. BME in the dose of 20 mg/kg, given for 10 days, twice daily showed healing effects against 50% acetic acid-induced gastric ulcers. Further work was done to investigate the possible mechanisms of its action by studying its effect on various mucosal offensive acid-pepsin secretion and defensive factors like mucin secretion, mucosal cell shedding, cell proliferation and antioxidant activity in rats. BME 20 mg/kg showed no effect on acid-pepsin secretion, increased mucin secretion, while it decreased cell shedding with no effect on cell proliferation. BME showed significant antioxidant effect per se and in stressed animals. Thus, the gastric prophylactic and curative effects of BME may be due to its predominant effect on mucosal defensive factors. 相似文献