全文获取类型
收费全文 | 4037篇 |
免费 | 385篇 |
国内免费 | 2篇 |
专业分类
4424篇 |
出版年
2023年 | 10篇 |
2022年 | 30篇 |
2021年 | 61篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 49篇 |
2017年 | 52篇 |
2016年 | 98篇 |
2015年 | 146篇 |
2014年 | 202篇 |
2013年 | 241篇 |
2012年 | 341篇 |
2011年 | 285篇 |
2010年 | 233篇 |
2009年 | 169篇 |
2008年 | 259篇 |
2007年 | 261篇 |
2006年 | 224篇 |
2005年 | 225篇 |
2004年 | 220篇 |
2003年 | 170篇 |
2002年 | 189篇 |
2001年 | 53篇 |
2000年 | 48篇 |
1999年 | 67篇 |
1998年 | 62篇 |
1997年 | 40篇 |
1996年 | 40篇 |
1995年 | 29篇 |
1994年 | 35篇 |
1993年 | 31篇 |
1992年 | 43篇 |
1991年 | 33篇 |
1990年 | 47篇 |
1989年 | 43篇 |
1988年 | 33篇 |
1987年 | 19篇 |
1986年 | 37篇 |
1985年 | 18篇 |
1984年 | 25篇 |
1983年 | 18篇 |
1982年 | 18篇 |
1981年 | 12篇 |
1980年 | 13篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 14篇 |
1975年 | 8篇 |
1973年 | 16篇 |
排序方式: 共有4424条查询结果,搜索用时 0 毫秒
21.
Résumé L'étude en différentes conditions de la migration verticale deTetranychus urticae dans de fortes infestations, a permis de mettre en éviduence un rythme circadien caractérisé par un mouvement alterné de
montée et de descente, la population migrant vers le haut des plantes en début d'après-midi. Ce rythme est en partie endogène
puisqu'il se maintient plusieurs jours dans des conditions constantes.
相似文献
22.
A cyclic AMP response element mediates repression of tyrosine aminotransferase gene transcription by the tissue-specific extinguisher locus Tse-1 总被引:30,自引:0,他引:30
Tyrosine aminotransferase (TAT) gene expression is liver specific and inducible by glucocorticoids and via the cAMP signaling pathway. In fibroblasts and other nonliver cells the gene is subject to negative control by the trans-dominant tissue-specific extinguisher locus Tse-1. We identified a hepatocyte-specific enhancer that is repressed by Tse-1. Two distinct sequence motifs are absolutely essential for function of this enhancer: a cAMP response element (CRE), which is the target for repression by Tse-1, and a hepatocyte-specific element. The specificity of the enhancer is generated by the combination of these two essential elements, which are fully interdependent. In vivo footprinting indicates that Tse-1 acts by affecting protein binding at the CRE. A direct antagonism between Tse-1 and the cAMP signaling pathway suggests that Tse-1 plays a role in control of developmental activation of the TAT gene. 相似文献
23.
D A Brenner J M Didier F Frasier S R Christensen G A Evans H A Dailey 《American journal of human genetics》1992,50(6):1203-1210
Protoporphyria is generally an autosomal dominant disease that is characterized clinically by photosensitivity and hepatobiliary disease and that is characterized biochemically by elevated protoporphyrin levels. The enzymatic activity of ferrochelatase, which catalyzes the last step in the heme biosynthetic pathway, is deficient in all tissues of patients with protoporphyria. In this study, sequencing of ferrochelatase cDNAs from a patient with protoporphyria revealed a single point mutation in the cDNAs resulting in the conversion of a Phe(TTC) to a Ser(TCC) in the carboxy-terminal end of the protein, F417S. Further, the human ferrochelatase gene was mapped to chromosome 18q21.3 by chromosomal in situ suppression hybridization. Finally, expression of recombinant ferrochelatase in Escherichia coli demonstrated a marked deficiency in activity of the mutant ferrochelatase protein and of mouse-human mutant ferrochelatase chimeric proteins. Therefore, a point mutation in the coding region of the ferrochelatase gene is the genetic defect in some patients with protoporphyria. 相似文献
24.
Selection of field populations originating from several countries allowed us to isolate 13 strains ofDrosophila melanogaster resistant to parathion.In vitro studies of acetylcholinesterase inhibition by paraoxon have been carried out on purified enzymes: most of the resistant strains harbor an altered acetylcholinesterase. Enzymes with higher resistance levels have been characterized with respect to their cross-resistance toward several insecticides. The patterns obtained have permitted us to group them and to delineate four categories. The existence of four distinct types of protein suggests that several mutations of acetylcholinesterase are responsible for insecticide resistance inDrosophila. 相似文献
25.
E Didier P Didier N Fargeix J Guillot J P Thiery 《The International journal of developmental biology》1990,34(4):421-431
The expression of end-chain sugar residues and of oligosaccharidic sequences has been investigated in chick germ cells at critical stages during the migration, proliferation and sexual differentiation of these cells. Fluorescent lectins and indirect immunofluorescence studies using the NC-1/HNK-1 monoclonal antibody indicate a remarkable control of glycosylation during germ cell embryonal life. Besides a retained expression of glucose/mannose residues, it was found that alpha- and beta-galactose residues, N-acetyllactosamine and N-N' diacetylchitobiose sequences as well as the sulfated trisaccharidic NC-1 epitope were detectable in a stage-specific pattern. Present at a very high density in the cytoplasm and on the surface of the early germ cells at premigrative and migratory stages, the staining for these carbohydrate sequences gradually disappeared when the germ cells settled and proliferated in the developing gonadal primordia. The disaccharide Gal beta 1----3 Gal NAc was exclusively detected in migrating PGCs. In sexualized gonads, acetyllactosamine and/or diacetylchitobiose were similarly reexpressed in both oogonia and spermatogonia. Spermatogonia displayed beta-galactose residues and a high immunoreactivity with the NC1 Mab, indicating modulations in PGC glycosylations related to the acquisition of sexual phenotypes. In addition NC-1 was found to be expressed in the somatic component of the undifferentiated gonad and in the testis interstitial gland. 相似文献
26.
Identification and expression of water stress- and abscisic acid-regulated genes in a drought-tolerant sunflower genotype 总被引:17,自引:0,他引:17
27.
The synthesis of a variety of important biochemicals involves multistep enzyme-catalyzed reactions. In many cases, the optimal operating pH is much different for the individual enzymatic steps of such synthesis reactions. Yet, it may be beneficial if such reaction steps are combined or paired, allowing them to occur simultaneously, in proximity to one another, and at their respective optimal pH. This can be achieved by separating the micro-environments of the two steps of a reaction pathway using a thin urease layer that catalyzes an ammonia-forming reaction. In this article, the pH control system in a commercial immobilized glucose (xylose) isomerase pellet, which has an optimal pH of 7.5, is demonstrated. This system allows the glucose isomerase to have near its optimal pH activity when immersed in a bulk solution of pH 4.6. A theoretical analysis is also given for the effective fraction of the immobilized glucose isomerase, which remains active when the bulk pH is at 4.6 in the presence of 20 mM urea versus when the bulk pH is at its optimal pH of 7.5. Both theoretical and experimental results show that this pH control system works well in this case. (c) 1996 John Wiley & Sons, Inc. 相似文献
28.
Claude Bouchard Angelo Tremblay Jean-Pierre Desprs Germain Thriault Andr Nadeauf Paul J. Lupien Sital Moorjani Denis Prudhomme Guy Fournier 《Obesity (Silver Spring, Md.)》1994,2(5):400-410
Seven pairs of young adult male identical twins completed a negative energy balance protocol during which they exercised on cycle ergometers twice a day, 9 out of 10 days, over a period of 93 days while being kept on a constant daily energy and nutrient intake. The total energy deficit caused by exercise above the estimated energy cost of body weight maintenance reached 244 ± 9.8 MJ (Mean ± SEM). Baseline energy intake was estimated over a period of 17 days preceding the negative energy balance protocol. Mean body weight loss was 5.0 kg (SEM = 0.6) (p <0.001) and it was entirely accounted for by the loss of fat mass (p <0.001). Fat-free mass was unchanged. Body energy losses reached 191 MJ (SEM = 24) (p <0.001) which represented about 78% of the estimated energy deficit. Subcutaneous fat loss was slightly more pronounced on the trunk than on the limbs as estimated from skinfolds, circumferences, and computed tomography (CT). The reduction in CT-assessed abdominal visceral fat was quite striking, from 81 cm2 (SEM = 5) to 52 cm2 (SEM = 6) (p <0.001). At the same submaximal power output level, subjects oxidized more lipids than carbohydrates after the program as indicated by the changes in the respiratory exchange ratio (p <0.05). Intrapair resemblance was observed for the changes in body weight (p <0.05), fat mass (P <0.01), percent fat (p <0.01), body energy content (p <0.01), sum of 10 skinfolds (p <0.01), abdominal visceral fat (p <0.01), fasting plasma triglycerides (p <0.05) and cholesterol (p <0.05), maximal oxygen uptake (p <0.05), and respiratory exchange ratio during submaximal work (p <0.01). We conclude that even though there were large individual differences in response to the negative energy balance and exercise protocol, subjects with the same genotype were more alike in responses than subjects with different genotypes particularly for body fat, body energy, and abdominal visceral fat changes. High lipid oxidizers and low lipid oxidizers during sub-maximal exercise were also seen despite the fact that all subjects had experienced the same exercise and nutritional conditions for about three months. 相似文献
29.
30.