全文获取类型
收费全文 | 3210篇 |
免费 | 232篇 |
国内免费 | 1篇 |
专业分类
3443篇 |
出版年
2023年 | 26篇 |
2022年 | 55篇 |
2021年 | 124篇 |
2020年 | 59篇 |
2019年 | 92篇 |
2018年 | 73篇 |
2017年 | 67篇 |
2016年 | 124篇 |
2015年 | 193篇 |
2014年 | 199篇 |
2013年 | 218篇 |
2012年 | 285篇 |
2011年 | 237篇 |
2010年 | 147篇 |
2009年 | 120篇 |
2008年 | 145篇 |
2007年 | 141篇 |
2006年 | 144篇 |
2005年 | 135篇 |
2004年 | 121篇 |
2003年 | 121篇 |
2002年 | 106篇 |
2001年 | 13篇 |
2000年 | 18篇 |
1999年 | 18篇 |
1998年 | 20篇 |
1997年 | 16篇 |
1996年 | 21篇 |
1995年 | 15篇 |
1994年 | 12篇 |
1993年 | 13篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 22篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 21篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 14篇 |
1973年 | 11篇 |
1972年 | 8篇 |
1968年 | 7篇 |
1967年 | 8篇 |
1963年 | 9篇 |
1959年 | 6篇 |
排序方式: 共有3443条查询结果,搜索用时 15 毫秒
31.
Charlotte Helfrich-Förster 《Seminars in cell & developmental biology》1996,7(6):791-802
Theperiod(per) gene and thetimeless(tim) gene are essential components of the circadian clock inDrosophila melanogaster. Both gene products interact in interdependent feedback loops, producing a self-sustained cellular rhythmin situ. Several oscillating cells are combined to discrete pacemaker centers that control rhythmic behavior. This paper reviews the work on localizing the circadian pacemaker neurons controlling activity and eclosion, leading to questions about how these pacemaker cells are synchronized to the external light–dark cycle, and how they impose periodicity on behavior. The circadian system ofDrosophilais also compared with that of other arthropods. 相似文献
32.
Charlotte Schou Erik R. Ottosen Hans Jørgen Petersen Fredrik Björkling Scilla Latini Pernille V. Hjarnaa Erik Bramm Lise Binderup 《Bioorganic & medicinal chemistry letters》1997,7(24):616-3100
4-Pyridyl cyanoguanidines with hydrophobic aromatic side chains showed potent antiproliferative activity in the human breast and lung cancer cell lines MCF-7, NYH and H460. In vivo, treatment with N-(6-chlorophenoxyhexyl)-N′-cyano-N″-4-pyridylguanidine (18, 20 mg/kg/day po.), gave a complete remission of tumours in a model of NYH inoculated nude mice. 相似文献
33.
Temporal and regional aspects of early neural crest cell migration in relation to extracellular matrix (ECM) organization and distribution in the embryonic axolotl trunk were studied by light microscopy, TEM, and SEM. The dominating structure of the interstitial ECM is a complex network of fibrils, which are indicated by ruthenium red staining to consist of collagen in association with ruthenium red-positive components, probably including glycosaminoglycans. The ECM fibrils, which are largely used as substratum for locomotion by the crest cells, have a temporally and regionally specific organization and distribution. Increase in ECM fibrils on the neural tube, ahead of the crest cell front, is correlated with initiation of crest cell emigration, and it is suggested that the fibrils may stimulate this process by providing a suitable substratum for cell locomotion. An increase in ECM fibrils in extracellular spaces surrounding the crest cell population is correlated with an expansion of these spaces and with progressing crest cell migration into them. It is proposed that the spatial organization of the ECM fibrils influences crest cell shape and orientation during early migration. 相似文献
34.
Ana Maria Andia-Waltenbaugh Charlotte A. Tate Naomi Kraus Friedmann 《Molecular and cellular biochemistry》1981,36(3):177-184
Summary Previous work by this and other laboratories has shown that glucagon administration stimulates calcium uptake by subsequently
isolated hepatic mitochondria. This stimulation of hepatic mitochondrial Ca2+ uptake byin vivo administration of glucagon was further characterized in the present report. Maximal stimulation of mitochondrial Ca2+ accumulation was achieved between 6–10 min after the intravenous injection of glucagon into intact rats. Under control conditions,
Ca2+ uptake was inhibited by the presence of Mg2+ in the incubation medium. Glucagon treatment, however, appeared to obliterate the observed inhibition by Mg2+ of mitochondrial Ca2+ uptake. Kinetic experiments revealed the usual sigmoidicity associated with initial velocity curves for mitochondrial calcium
uptake. Glucagon treatment did not alter this sigmoidal relationship. Glucagon treatment significantly increased the Vmax for Ca2+ uptake from 292±22 to 377±34 nmoles Ca2+ /min per mg protein (n=8) but did not affect the K0.5, (6.5–8.6 μM). Since the major kinetic change in mitochondrial Ca2+ uptake evoked by glucagon is an increase in Vmax, the enhancement mechanism is likely to be an increase either in the number of active transport sites available to Ca2+ or in the rate of Ca2+ carrier movement across the mitochondrial membranes. 相似文献
35.
Silke Ruppel Charlotte Hecht-Buchholz Rainer Remus Ursula Ortmann Rita Schmelzer 《Plant and Soil》1992,145(2):261-273
The aim of this study was to investigate the ability of Pantoea agglomerans, a plant growth-promoting bacterium, to colonize various regions and tissues of the wheat plant (Triticum aestivum L.) by using different inoculation methods and inoculum concentrations. In addition, the enzyme-linked immunosorbent assay
(ELISA) and transmission electron microscopy (TEM) were used to determine: (a) the ability of the bacterial cells to grow
and survive both on the surface and within internal tissue of the plant and (b) the response of the plant to bacterial infection.
After inoculation, cells of the diazotrophic bacterial strain P. agglomerans were found to be located in roots, stems and leaves. Colony development of bacterial cells was only detected within intercellular
spaces of the root and on the root surface. However, single bacterial cells were observed in leaves and stems on the surface
of the epidermis, in the vicinity to stomatal cells, within intercellular spaces of the mesophyll and within xylem vessels.
Inoculated bacterial cells were found to be able to enter host tissues, to multiply in the plant and to maintain a delicate
relationship between endophyte and host. The density of bacterial settlement in the plant in all experiments was about 106 to 107 cells per mL root or shoot sap. Establishment was confirmed by a low coefficient of variation of ELISA means at these concentrations. 相似文献
36.
Prof. Dr. Charlotte Thielke 《Plant Systematics and Evolution》1982,140(2-3):191-205
The spindle apparatus ofCoprinus micaceus begins to develop from the diglobular polar body outside the nucleus. During both meiotic divisions it operates inside the
nuclear envelope and consists of two amorphous poles, a central bundle of interpolar microtubules, and chromosomal microtubules.
A metaphase plate cannot exist because the interpolar strand of fibers is persistent throughout the division process. Within
the spindle axis more than 100 microtubules can be estimated. They are encircled by a ring of chromatic structures. During
the telophase the former spindle pole is evaginated from the nuclear envelope and contacts the plasmalemma near the cell wall. 相似文献
37.
Role of sugars in nitrate utilization by roots of dwarf bean 总被引:4,自引:0,他引:4
Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m?3). When plants were decapitated after an 18 h NO3? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG?3 uptake. Presumably due to a difference in NO3? due to a difference in NO3? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of 14CO2 from freshly absorbed (U-14C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO3? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO3? uptake was not indicated. A constant level of NRA occurred in roots of NO3? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots. 相似文献
38.
Summary The rDNA content in Drosophila hydei has been compared in wild-type and in two translocation genotypes possessing only one nucleolus organizer. In highly polyploid salivary glands where rDNA is underreplicated, an independent polytenization of the rDNA occurs resulting in about the same rDNA level in each genotype independently of the number of nucleolous organizers present in the genome. Thus, the situation in the salivary glands of D. hydei is similar to that in D. melanogaster (Spear and Gall 1973).In tetraploid thoracic muscle where rDNA is not underreplicated, the rDNA percentage in the two translocation genotypes is also considerably increased, although the wild-type level is not completely attained. This result shows that rDNA replication is independently controlled even in a non-underreplicating tissue.In larval diploid brain the situation in the two translocation stocks is dissimilar: in one genotype the rDNA content remains unaltered whereas in the other it is increased. This demonstrates for the first time that a gene compensation does occur in a diploid tissue.Supported by a grant from the Deutsche Forschungsgemeinschaft (Ku 282/7) 相似文献
39.
40.
Anjana Radhakrishnan Chelsea M. Brown Collette S. Guy Charlotte Cooper Raul Pacheco-Gomez Phillip J. Stansfeld Elizabeth Fullam 《MedChemComm》2022,13(10):1225
Tuberculosis, caused by Mycobacterium tuberculosis, claims ∼1.5 million lives annually. Effective chemotherapy is essential to control TB, however the emergence of drug-resistant strains of TB have seriously threatened global attempts to control and eradicate this deadly pathogen. Trehalose recycling via the LpqY-SugABC importer is essential for the virulence and survival of Mtb and inhibiting or hijacking this transport system is an attractive approach for the development of novel anti-tubercular and diagnostic agents. Therefore, we interrogated the drug-like compounds in the open-source Medicines for Malaria Pathogen Box and successfully identified seven compounds from the TB, kinetoplastids and reference compound disease sets that recognise LpqY. The molecules have diverse chemical scaffolds, are not specific trehalose analogues, and may be used as novel templates to facilitate the development of therapeutics that kill Mtb with a novel mechanism of action via the mycobacterial trehalose LpqY-SugABC transport system.Interrogation of the Pathogen Box identified diverse chemical scaffolds against the mycobacterial trehalose transporter. 相似文献