全文获取类型
收费全文 | 10024篇 |
免费 | 854篇 |
国内免费 | 1251篇 |
专业分类
12129篇 |
出版年
2024年 | 39篇 |
2023年 | 171篇 |
2022年 | 456篇 |
2021年 | 606篇 |
2020年 | 449篇 |
2019年 | 547篇 |
2018年 | 493篇 |
2017年 | 306篇 |
2016年 | 420篇 |
2015年 | 663篇 |
2014年 | 801篇 |
2013年 | 789篇 |
2012年 | 986篇 |
2011年 | 889篇 |
2010年 | 495篇 |
2009年 | 499篇 |
2008年 | 584篇 |
2007年 | 457篇 |
2006年 | 403篇 |
2005年 | 336篇 |
2004年 | 266篇 |
2003年 | 215篇 |
2002年 | 158篇 |
2001年 | 131篇 |
2000年 | 110篇 |
1999年 | 143篇 |
1998年 | 90篇 |
1997年 | 101篇 |
1996年 | 71篇 |
1995年 | 63篇 |
1994年 | 53篇 |
1993年 | 48篇 |
1992年 | 54篇 |
1991年 | 41篇 |
1990年 | 43篇 |
1989年 | 32篇 |
1988年 | 25篇 |
1987年 | 20篇 |
1986年 | 29篇 |
1985年 | 20篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
To reveal the difference of molecular property between structurallysimilar odorless and odoriferous musk compounds, 10 pairs ofbenzenoids (monocyclic-, dicyclic- and tricyclic-) were examined.Molecular structures of all compounds were optimized by MNDO(modified neglect of diatomic differential overlap) consideringconformation. Parameters effective in discriminating two groups,group A of 10 odorless compounds and group B of 10 musk odorcompounds, were searched from 34 candidate parameters by adaptiveleast squares. The best three parameters found were log P value(octanol/water partition coefficient), the longest side lengthof hexahedron circumscribing a molecule, and the parameter whichexpresses structural hindrance to the functional group whena molecule approaches the receptor site. The two groups of compoundswere completely discriminated using these three parameters. 相似文献
2.
Lycopene beta-cyclase (Lyc-B) is the key enzyme in the catalysis of linear lycopene to form cyclic beta-carotene, an indispensable part of the photosynthetic apparatus and an important source of vitamin A in human and animal nutrition. Studies showing that the microalga Dunaliella salina can accumulate a high level of beta-carotene are lacking. We hypothesize that D. salina is closely involved with the catalytic mechanism of Lyc-B and the molecular regulation of its gene. In this study, we used RT-PCR and RACE-PCR to isolate a 2475 bp cDNA with a 1824 bp open reading frame, encoding a putative Lyc-B, from D. salina. Homology studies showed that the deduced amino acid sequence had a significant overall similarity with sequences of other green algae and higher plants, and that it shared the highest sequence identity, up to 64%, with Lyc-B of Chlamydomonas reinhardtii. Codon analysis showed that synonymous codon usage in the enzyme has a strong bias towards codons ending with adenosine. Two motifs were found in the Lyc-B sequence, one at the N terminus, for binding the hypothetical cofactor FAD, and the other was a substrate carrier motif in oxygenic organisms shared by an earlier carotenogenesis enzyme, phytoene desaturase, and Lyc-B. A tertiary structure prediction suggested that the catalytic or binding site structure within LycB from D. salina is superior to that of both H. pluvialis and C. reinhardtii. The LycB protein from D. salina was quite removed from that of H. pluvialis and C. reinhardtii in the phylogenetic tree. Taken as a whole, this information provides insight into the regulatatory mechanism of Lyc-B at the molecular level and the high level of beta-carotene accumulation in the microalga D. salina. 相似文献
3.
4.
5.
Muscarinic acetylcholine receptor regulates phosphatidylcholine phospholipase D in canine brain 总被引:10,自引:0,他引:10
The hydrolytic activity of phosphatidylcholine phospholipase D in the synaptosomes from canine brain was examined using a radiochemical assay with 1,2-dipalmitoyl-sn-glycerol-3-phosphoryl[3H]choline as the exogenous substrate. The involvement of G protein(s) in regulation of this enzyme was demonstrated by a 2- to 3-fold stimulation of the basal activity (4.81 +/- 0.44 nmol choline released/mg protein/h) with guanosine 5'-(3-O-thiol)triphosphate (GTP gamma S), guanyl-5'-yl-(beta, gamma-methylene)diphosphonate, aluminum fluoride, or cholera toxin. The stimulation of phospholipase D hydrolytic activity by GTP gamma S was inhibited by 2 mM guanosine 5'-(2-O-thiol)diphosphate. GTP gamma S at the maximum stimulatory concentration (10 microM) had an additive effect on the maximum cholera toxin stimulation of phospholipase D activity. However, the reverse was not true, thus indicating the possibility that more than one G protein may be involved. Furthermore, cholinergic agonists, including acetylcholine, carbachol, and muscarine, were able to increase the phospholipase D hydrolytic activity at low but not maximally stimulatory concentrations of guanine nucleotide. These cholinergic stimulations were antagonized by atropine, a muscarinic blocker. In addition, O-tetradecanoylphorbol 13-acetate, a protein kinase C activator, was able to stimulate the hydrolytic activity of phospholipase D more than 300% in the presence of 0.2 microM GTP gamma S. However, in the absence of GTP gamma S, stimulation was less than 60%. Our results not only indicate that the receptor-G protein-regulated phospholipase D may be directly responsible for the rapid accumulation of choline and phosphatidic acid in the central nervous system but also reveal that muscarinic acetylcholine receptor-G protein-regulated phospholipase D is a novel signal transduction process coupling the neuronal muscarinic receptor to cellular responses. 相似文献
6.
Expression of the human retinoblastoma gene product pp110RB in insect cells using the baculovirus system 总被引:4,自引:0,他引:4
The product of the retinoblastoma susceptibility gene (RB) was overproduced in cultured insect cells using the baculovirus expression system. Upon insertion of the cloned human RB complementary DNA sequence into the viral genome downstream of the promoter of the polyhedrin gene, full-length RB protein with an apparent molecular weight of 110,000 was expressed in the insect cells. This protein was found to be phosphorylated, located in the nuclei of the infected cells, and immunologically indistinguishable from pp110RB of human cells as assayed by several anti-RB antibodies. Following cell disruption and a one-step immunoaffinity chromatographic purification, 6-12 mg of soluble pp110RB with approximately 95% purity were obtained per liter of infected suspension culture. Characterization of the two known biochemical properties of RB protein showed that this purified protein from insect cells behaved similarly to the authentic human pp110RB. First, it bound to DNA, and second, it could form a specific complex with SV40 T antigen in vitro. Prompt translocation of the protein from cytoplasm to nucleus after microinjection further indicated that the purified RB protein may be active. The availability of soluble, intact, and presumably active pp110RB in large quantity represents a significant advance for studying the biochemical and biophysical properties of the RB gene product as well as its potential biological function in cancer suppression. 相似文献
7.
H. -X. Lin H. -R. Qian J. -Y. Zhuang J. Lu S. -K. Min Z. -M. Xiong N. Huang K. -L. Zheng 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1996,92(8):920-927
Quantitative triat loci (QTLs) for yield and related traits in rice were mapped based on RFLP maps from two indica/indica F2 populations, Tesanai 2/CB and Waiyin 2/CB. In Tesanai 2/CB, 14 intervals carrying QTLs for eight traits were detected, including 3 for grain weight per plant (GWT), 2 for number of panicles per plant (NP), 2 for number of grains per panicle (NG), 1 for total number of spikelets per panicle (TNS), 1 for spikelet fertility (SF), 3 for 1000-grain weight (TGWT), 1 for spikelet density (SD), and 1 for number of first branches per main panicle. The 3 QTLs for GWT were located on chromosomes 1, 2, and 4, with 1 in each chromosome. The additive effect of the single locus ranged from 2.0 g to 9.1 g. A major gene (np4) for NP was detected on chromosome 4 within the interval of RG143–RG214, about 4cM for RG143, and this locus explained 26.1% of the observed phenotypic variance for NP. The paternal allele of this locus was responsible for reduced panicles per plant (3 panicles per plant). In another population, Waiyin 2/CB, 12 intervals containing QTLs for six of the above-mentioned traits were detected, including 3 for GWT, 2 for each of NP, TNS, TGWT and SD, 1 for SF. Three QTLs for GWT were located on chromosome 1, 4, and 5, respectively. The additive effect of the single locus for GWT ranged from 6.7 g to 8.8 g, while the dominance effect was 1.7–11.5 g. QTL mapping in two populations with a common male parent is compared and discussed. 相似文献
8.
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
9.
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
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
10.
【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