全文获取类型
收费全文 | 15735篇 |
免费 | 1739篇 |
国内免费 | 5289篇 |
出版年
2024年 | 106篇 |
2023年 | 373篇 |
2022年 | 767篇 |
2021年 | 1000篇 |
2020年 | 805篇 |
2019年 | 944篇 |
2018年 | 743篇 |
2017年 | 584篇 |
2016年 | 673篇 |
2015年 | 1026篇 |
2014年 | 1300篇 |
2013年 | 1219篇 |
2012年 | 1681篇 |
2011年 | 1472篇 |
2010年 | 995篇 |
2009年 | 1045篇 |
2008年 | 1213篇 |
2007年 | 1095篇 |
2006年 | 947篇 |
2005年 | 841篇 |
2004年 | 634篇 |
2003年 | 562篇 |
2002年 | 482篇 |
2001年 | 400篇 |
2000年 | 398篇 |
1999年 | 284篇 |
1998年 | 170篇 |
1997年 | 140篇 |
1996年 | 115篇 |
1995年 | 103篇 |
1994年 | 72篇 |
1993年 | 65篇 |
1992年 | 77篇 |
1991年 | 61篇 |
1990年 | 60篇 |
1989年 | 49篇 |
1988年 | 48篇 |
1987年 | 32篇 |
1986年 | 35篇 |
1985年 | 30篇 |
1984年 | 18篇 |
1983年 | 18篇 |
1982年 | 19篇 |
1981年 | 7篇 |
1976年 | 5篇 |
1964年 | 5篇 |
1957年 | 8篇 |
1955年 | 3篇 |
1954年 | 3篇 |
1950年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
John A. Lowe III Weimin Qian Pamela J. Scott Stafford McLean Dianne K. Bryce Rosemary T. Crawford Jon Bordner 《Bioorganic & medicinal chemistry letters》1994,4(24):2877-2882
A series of 5,7-diphenyl-3-ureidohexahydroazepin-2-one cholecystokinin-B (CCK-B) receptor antagonists was synthesized using Beckmann ring expansion of a suitable 2,4-diphenylcyclohexanone as a key step. SAR studies revealed the importance of the 5-aryl group for high and selective CCK-B receptor affinity, as illustrated in compound (−)-10i (CCK-B IC50 = 6.8 nM). 相似文献
62.
人工饲养的蒙新河猪其动情周期可划分为4个阶段:动情前期、动情期(发情交配期)、动情后期和停滞期(动情间隔期)。动情期间雌雄河狸均表现出活跃好动,浮躁不安,食量减小;发情交配期雌雄河狸均发出低沉嘶哑的叫声,雄性叫声频率比性略高,爬跨交配是动情期发情的高峰,动情周期各阶段的行为特征,在雌性与阴道涂片细胞学及外生殖器形态、色泽、阴道分泌物周期性变化特征相吻合,在雄性则与睾丸及阴茎形态特征吻合,在人工饲养 相似文献
63.
Gary L. Johnson Anne M. Gardner Carol Lange-Carter Nan-Xin Qian Marijane Russell Sim Winitz 《Journal of cellular biochemistry》1994,54(4):415-422
Serpentine receptors coupled to the heterotrimeric G protein, Gi2, are capable of stimulating DNA synthesis in a variety of cell types. A common feature of the Gi2-coupled stimulation of DNA synthesis is the activation of the mitogen-activated protein kinases (MAPKs). The regulation of MAPK activation by the Gi2-coupled thrombin and acetylcholine muscarinic M2 receptors occurs by a sequential activation of a network of protein kinases. The MAPK kinase (MEK) which phosphorylates and activates MAPK is also activated by phosphorylation. MEK is phosphorylated and activated by either Raf or MEK kinase (MEKK). Thus, Raf and MEKK converge at MEK to regulate MAPK. Gi2-coupled receptors are capable of activating MEK and MAPK by Raf-dependent and Raf-independent mechanisms. Pertussis toxin catalyzed ADP-ribosylation of αi2 inhibits both the Raf-dependent and-independent pathways activated by Gi2-coupled receptors. The Raf-dependent pathway involves Ras activation, while the Raf-independent activation of MEK and MAPK does not involve Ras. The Raf-independent activation of MEK and MAPK most likely involves the activation of MEKK. The vertebrate MEKK is homologous to the Ste11 and Byr2 protein kinases in the yeast Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively. The yeast Ste11 and Byr2 protein kinases are involved in signal transduction cascades initiated by pheromone receptors having a 7 membrane spanning serpentine structure coupled to G proteins. MEKK appears to be conserved in the regulation of G protein-coupled signal pathways in yeast and vertebrates. Raf represents a divergence in vertebrates from the yeast pheromone-responsive protein kinase system. Defining MEKK and Raf as a divergence in the MAPK regulatory network provides a mechanism for differential regulation of this system by Gi2-coupled receptors as well as other receptor systems, including the tyrosine kinases. 相似文献
64.
65.
66.
67.
68.
Mutations in acid beta-galactosidase cause GM1-gangliosidosis in American patients. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We describe four new mutations in the beta-galactosidase gene. These are the first mutations causing infantile and juvenile GM1-gangliosidosis to be described in American patients. Cell lines from two patients with juvenile and from six patients with infantile GM1-gangliosidosis were analyzed. Northern blot analysis showed the acid beta-galactosidase message to be of normal size and quantity in two juvenile and four infantile cases and of normal size but reduced quantity in two infantile cases. The mutations are distinct from the Japanese mutations. All are point mutations leading to amino acid substitutions: Lys577-->Arg, Arg590-->His, and Glu632-->Gly. The fourth mutation, Arg208-->Cys, accounts for 10 of 16 possible alleles. Two infantile cases from Puerto Rico of Spanish ancestry are homozygous for this mutation, suggesting that this allele may have come to South America and North America via Puerto Rico. That these mutations cause clinical disease was confirmed by marked reduction in catalytic activity of the mutant proteins in the Cos-1 cell expression system. 相似文献
69.
T cell receptor-beta mRNA splicing: regulation of unusual splicing intermediates. 总被引:10,自引:3,他引:7
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
L Qian L Theodor M Carter M N Vu A W Sasaki M F Wilkinson 《Molecular and cellular biology》1993,13(3):1686-1696
70.
【目的】葡聚糖酶是饲用添加剂的重要成分,本研究旨在从湖羊消化道微生物中挖掘性质优良的GH9家族葡聚糖酶基因,用于研发新型饲用酶制剂。【方法】从湖羊瘤胃微生物cDNA中扩增IDSGLUC9-25基因,在大肠杆菌中进行异源表达,对重组蛋白进行诱导表达和纯化,研究重组蛋白的酶学性质和底物水解模式。【结果】IDSGLUC9-25基因编码527个氨基酸,包含一个CelD_N结构和一个GH9家族催化结构域;重组蛋白rIDSGLUC9-25分子量约为62.7 kDa,最适反应温度和pH分别为40℃和6.0,在30-50℃下活性较高,在pH 4.0-8.0范围内能够保持较高的稳定性,经pH 4.0-8.0缓冲液处理1 h后残余活性均大于90%;底物谱分析表明,rIDSGLUC9-25能催化大麦β-葡聚糖、苔藓地衣多糖、魔芋胶和木葡聚糖,比活性分别为(443.55±24.48)、(65.56±5.98)、(122.37±2.85)和(159.16±7.73) U/mg;利用薄层色谱法(thin layer chromatography, TLC)和高效液相色谱法(high performance liquid chromatography, HPLC)分析水解产物发现,rIDSGLUC9-25降解大麦葡聚糖主要生成纤维三糖(占总还原糖64.19%±1.19%)和纤维四糖(占总还原糖26.24%±0.12%),催化地衣多糖主要生成纤维三糖(占总还原糖78.46%±0.89%)。【结论】本研究报道了一种来自密螺旋体属细菌的内切β-1,4-葡聚糖酶IDSGLUC9-25 (EC 3.2.1.4),能高效催化多糖底物生成纤维三糖和纤维四糖,为研发饲用酶制剂和制备低聚寡糖建立基础。 相似文献