全文获取类型
收费全文 | 325篇 |
免费 | 43篇 |
国内免费 | 155篇 |
出版年
2023年 | 9篇 |
2022年 | 15篇 |
2021年 | 21篇 |
2020年 | 14篇 |
2019年 | 22篇 |
2018年 | 9篇 |
2017年 | 22篇 |
2016年 | 18篇 |
2015年 | 19篇 |
2014年 | 20篇 |
2013年 | 31篇 |
2012年 | 31篇 |
2011年 | 20篇 |
2010年 | 40篇 |
2009年 | 27篇 |
2008年 | 26篇 |
2007年 | 20篇 |
2006年 | 17篇 |
2005年 | 13篇 |
2004年 | 16篇 |
2003年 | 19篇 |
2002年 | 14篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 9篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有523条查询结果,搜索用时 15 毫秒
1.
2.
鲴亚科(Xenocyprininae)鱼类多为中小型鱼类,常见于江河湖泊等较宽阔的水域中,我国长江、黑龙江、黄河及珠江诸流域皆有分布,共有10种,隶属4个属(伍献文等,1964)。迄今尚未见有该亚科鱼类染色体组型的研究报道。本文是对其中三属四种鱼的染色体组型的观察结果。这四种鱼是银鲴(Xenocypris argentea)、黄尾鲴(Xenocypris davidi)、细鳞斜颌鲴(Plagiognathops microlepis)和逆鱼(Acanthobrama simoni)。其中黄尾鲴和细鳞斜颌鲴均为新的淡水养殖鱼(沈德长等,1981;陈楚星,1979)。 相似文献
3.
前文由柑桔枝条在不同低温下、不同冷冻时间的电解质外渗测定,提出胁强(stress)、作用时间与胁变(strain)之间关系的数学模型。在这个模型中共有3个参数:屈服点温度(yield point temperature),胁强敏感度(stress sensitivity)和作用时间敏感度(sensitivity to duration),用以描述植物的抗性。抗性强的植物应表现为屈服点温度较低,胁强敏感度或者时间敏感度较低。为验证此数学模型,本工作以经冷锻炼与未经冷锻炼的盆栽柑桔枝条为材料,作不同温度与时间处理的电解质外渗率的测定,研究了冷锻炼对于上述3个参数的影响。发现胁强敏感度和屈服点温度受冷锻炼影响而下降,时间敏感度未表现明显变化。对于田间柑桔、油桐与毛竹的定期测定,在固定冷冻时间下,得到了类似于盆栽柑桔的结果。入冬时,植物抗冻性提高,3种植物都表现出下列两种变化:1.胁强敏感度的明显下降;2.屈服点温度和/或时间敏感度亦下降。开春时的变化则相反。胁强敏感度的变化与后一种变化有各自的规律,且因植物种类而不同。拐点胁强(stress at inflection point)具有与半致死温度(50%killing point temperature)不同的意义,它的变化是上述两种变化的综合结果。本试验结果表明,冷锻炼对于植物胁强敏感度有明显影响,用本数学模型的3个抗性指标描述 相似文献
4.
S-腺苷甲硫氨酸(S-adenosyl-l-methionine, SAM)广泛存在于生物体内,主要参与生物体内的转甲基过程、转硫过程及转氨丙基过程,具有重要的生理功能,其生产备受重视。目前SAM生产的研究主要集中于微生物发酵法,该方法与化学合成法和酶催化法相比,成本较低且更容易实现工业化生产。随着需求量的迅速增加,通过菌种改良提高SAM产量备受关注。当前SAM生产菌种改良的主要策略包括常规育种和代谢工程。本文综述了提高微生物生产SAM能力的近期研究进展并探讨了SAM生产中的瓶颈问题及解决方法,以期为进一步提高SAM产量提供思路。 相似文献
5.
Husain D.R. Goutx M. Acquaviva M. Gilewicz M. Bertrand J.-C. 《World journal of microbiology & biotechnology》1997,13(5):587-590
Three hydrocarbon uptake modes (adherence, emulsification and solubilization) were identified and quantified in cells and supernatants of a mesophilic marine bacterium Pseudomonas nautica strain 617 grown on eicosane. The adherence capacity was related to the enrichment of cells with wax esters and glycolipids. The emulsifying activity was related to the presence of extracellular biosurfactants composed of proteins, carbohydrates and lipids (35:63:2). The intensity of substrate uptake modes was sensitive to temperatures currently found in the original environment of P. nautica (16°C, 20°C and 32°C). When temperature decreased, a significant increase in adherence and emulsifying activity was observed in relation to biochemical changes, whereas solubilizing activity decreased. The marine bacterium was able to degrade 53–59% eicosane at the end of exponential growth after 13, 5 and 3 days incubation at 16°C, 20°C and 32°C respectively. 相似文献
6.
Biochemical and genetic characterization of three hamster cell mutants resistant to diphtheria toxin 总被引:2,自引:0,他引:2 下载免费PDF全文
RK Draper D Chin D Eurey-Owens IE Scheffler MI Simon 《The Journal of cell biology》1979,83(1):116-125
We describe here three different hamster cell mutants which are resistant to diphtheria toxin and which provide models for investigating some of the functions required by the toxin inactivates elongation factor 2 (EF-2). Cell-free extracts from mutants Dtx(r)-3 was codominant. The evidence suggests that the codominant phenotype is the result of a mutation in a gene coding for EF-2. The recessive phenotype might arise by alteration of an enzyme which modifies the structure of EF-2 so that it becomes a substrate for reaction with the toxin. Another mutant, Dtx(r)-2, contained EF-2 that was sensitive to the toxin and this phenotype was recessive. Pseudomonas aeruginosa exotoxin is known to inactivate EF-2 as does diphtheria toxin and we tested the mutants for cross-resistance to pseudomonas exotoxin. Dtx(r)-1 and Dtx(r)-3 were cross-resistant while Dtx(r)-2 was not. It is known that diphtheria toxin does not penetrate to the cytoplasm of mouse cells and that these cell have a naturally occurring phenotype of diphtheria toxin resistance. We fused each of the mutants with mouse 3T3 cells and measured the resistance. We fused each of the mutants with mouse 3T3 cells and measured the resistance of the hybrid cells to diphtheria toxin. Intraspecies hybrids containing the genome of mutants Dtx(r)-1 and Dtx(r)-3 had some resistance while those formed with Dtx(r)-2 were as sensitive as hybrids derived from fusions between wild-type hamster cells and mouse 3T3 cells. 相似文献
7.
M J Gauthier B Lafay R Christen L Fernandez M Acquaviva P Bonin J C Bertrand 《International journal of systematic bacteriology》1992,42(4):568-576
On the basis of phenotypical characteristics and analysis of 16S rRNA sequence, a new species belonging to a new genus is described, and the name Marinobacter hydrocarbonoclasticus is proposed. This organism, isolated from Mediterranean seawater near a petroleum refinery, is a gram-negative, aerobic, rod-shaped bacterium. It grows at NaCl concentrations of 0.08 to 3.5 M and uses various hydrocarbons as the sole source of carbon and energy. Its DNA has a guanine-plus-cytosine content of 52.7 mol%. The 16S rRNA analysis shows a clear affiliation between M. hydrocarbonoclasticus and the gamma group of the phylum Proteobacteria. A close phylogenetic relationship appears among the species Marinomonas vaga, Oceanospirillum linum, Halomonas elongata, and Pseudomonas aeruginosa. Because of the impossibility of finding a single most closely related species, we suggest that this bacterium be assigned to a new genus, at least temporarily. The possibility of a revision of this status when new data appear is, however, not excluded. The type strain is M. hydrocarbonoclasticus SP.17 (= ATCC 49840). 相似文献
8.
Two plasmids containing rat thyroglobulin cDNA sequences have been constructed and characterized. A plasmid with a 500-bp insert (pRT6) was isolated and identified as thyroglobulin-specific on the basis of the tissue specificity of the inserted sequence and of its ability to retain thyroglobulin mRNA on a nitrocellulose filter. The cDNA insert in pRT6 was subsequently used to screen a rat thyroid cDNA library constructed with large cDNA. A plasmid was found containing a 1700-bp insert. The polarity and the fidelity of the insert is demonstrated by S1 mapping. 相似文献
9.
10.
本文对祁连山野生荷叶离褶伞Lyophyllum decastes子实体的化学成分和生物活性进行研究。采用硅胶色谱、高效液相色谱等多种方法进行分离纯化得到8个化合物,通过MS、NMR和电子圆二色谱 (ECD)等方法确定了化学结构,其中有4个为聚炔类化合物。化合物1作为天然产物系首次报道,其相绝对构型是通过比较ECD的方法确定。对所得聚炔类化合物应用细胞模型进行抗氧化活性(CAA)指标检测,化合物1-4均呈现一定抗氧化活性,其中化合物1的抗氧化活性最强,其EC50为(24.73±6.12)μmol/L。聚炔类化合物1-4为荷叶离褶伞首次报道成分,可作为祁连山野生荷叶离褶伞HPLC-DAD化学表征参考化合物。 相似文献