全文获取类型
收费全文 | 24256篇 |
免费 | 2405篇 |
国内免费 | 18篇 |
专业分类
26679篇 |
出版年
2021年 | 265篇 |
2020年 | 185篇 |
2019年 | 221篇 |
2018年 | 262篇 |
2017年 | 268篇 |
2016年 | 403篇 |
2015年 | 679篇 |
2014年 | 725篇 |
2013年 | 1024篇 |
2012年 | 1133篇 |
2011年 | 1153篇 |
2010年 | 882篇 |
2009年 | 694篇 |
2008年 | 871篇 |
2007年 | 913篇 |
2006年 | 869篇 |
2005年 | 867篇 |
2004年 | 974篇 |
2003年 | 903篇 |
2002年 | 842篇 |
2001年 | 705篇 |
2000年 | 699篇 |
1999年 | 651篇 |
1998年 | 344篇 |
1997年 | 355篇 |
1996年 | 349篇 |
1995年 | 315篇 |
1994年 | 255篇 |
1993年 | 283篇 |
1992年 | 608篇 |
1991年 | 499篇 |
1990年 | 535篇 |
1989年 | 534篇 |
1988年 | 436篇 |
1987年 | 433篇 |
1986年 | 380篇 |
1985年 | 382篇 |
1984年 | 409篇 |
1983年 | 315篇 |
1982年 | 285篇 |
1981年 | 276篇 |
1980年 | 245篇 |
1979年 | 327篇 |
1978年 | 266篇 |
1977年 | 283篇 |
1976年 | 213篇 |
1975年 | 236篇 |
1974年 | 242篇 |
1973年 | 210篇 |
1971年 | 184篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
142.
Migration of O-acetyl groups in N,O-acetylneuraminic acids 总被引:6,自引:0,他引:6
J P Kamerling R Schauer A K Shukla S Stoll H Van Halbeek J F Vliegenthart 《European journal of biochemistry》1987,162(3):601-607
Highly purified N-acetyl-4-O-acetylneuraminic acid (Neu4,5Ac2), N-acetyl-7-O-acetylneuraminic acid (Neu5,7Ac2) and N-acetyl-7,9-di-O-acetylneuraminic acid (Neu5,7,9Ac3) were used to study spontaneous migrations of acetyl groups between hydroxyl groups. The techniques applied involved thin-layer chromatography, gas-liquid chromatography/mass spectrometry, high-performance liquid chromatography and 360-MHz 1H-NMR spectroscopy. It was found that at pH values at which no significant de-O-acetylation is observed: (a) Neu5,7Ac2 can easily be transformed into Neu5,9Ac2, (b) Neu5,7,9Ac3 yields an equilibrium of Neu5,7,9Ac3 and Neu5,8,9Ac3 in a molar ratio of approximately 1:1, and (c) Neu4,5Ac2 does not give rise to O-acetyl migrations. The importance of these findings is discussed in terms of the biosynthesis of O-acetylated sialic acids. 相似文献
143.
Transfer of human genes conferring resistance to methylating mutagens, but not to UV irradiation and cross-linking agents, into Chinese hamster ovary cells. 总被引:2,自引:1,他引:1 下载免费PDF全文
B Kaina A A Van Zeeland C Backendorf H W Thielmann P Van de Putte 《Molecular and cellular biology》1987,7(5):2024-2030
Chinese hamster ovary cells were transfected by human DNA ligated to the bacterial gpt (xanthine-guanine-phosphoribosyltransferase) gene which was used either in its native form or after partial inactivation with methylnitrosourea. The gpt+ transfectants were screened for resistance to high doses of N-methyl-N'-nitro-N-nitrosoguanidine. Using this approach, we showed that Chinese hamster ovary cells can acquire N-methyl-N'-nitro-N-nitrosoguanidine resistance upon transfection with DNA from diploid human fibroblasts, that this resistance is transferable by secondary transfection and is specific for methylating mutagens, and that it is not caused by increased removal of O6-methylguanine, 3-methyladenine, and 7-methylguanine from DNA. 相似文献
144.
E P Beem J J W Lisman G J Van Steijn C J Van Der Wal L A W Trippelvitz B Overdijk H Van Halbeek J H G M Mutsaers J F G Vliegenthart 《Glycoconjugate journal》1987,4(1):33-42
Acid -l-fucosidase (EC 3.2.1.51) was obtained from human liver and purified to homogeneity. The enzyme consists of four subunits; each of these has a molecular mass of 50 kDa and bears oneN-linked carbohydrate chain. The structures of these chains were studied at the glycopeptide level by methylation analysis and 500-MHz1H-NMR spectroscopy. Oligomannoside-type chains andN-acetyllactosamine-type chains are present in an approximate ratio of 31. While the oligomannoside-type chains show some heterogeneity in size (Man5–8GlcNAc2), theN-acetyllactosaminetype chains are exclusively bi-(2–6)-sialyl, bi-antennary in their structure.These observations on the carbohydrate moieties of -l-fucosidase substantiate our hypothesis [Overdijket al. (1986) Glycoconjugate J 3:339–50] with respect to the relationship between the oligosaccharide structure of lysosomal enzymes and their residual intracellular activity in I-cell disease. For the series of enzymes examined so far, namely, -N-acetylhexosaminidase, -l-fucosidase and -galactosidase, the relative amount ofN-acetyllactosamine-type carbohydrate increases, while the residual intracellular activity in I-cell disease tissue decreases in this order. The system which is responsible for preferentially retaining hydrolases with (non-phosphorylated) oligomannoside-type chains both in I-cells and in normal cells has yet to be identified. 相似文献
145.
P J McAlpine N Van Cong C Boucheix A J Pakstis R C Doute T B Shows 《Cytogenetics and cell genetics》1987,46(1-4):29-101
146.
The distribution of low density lipoprotein (LDL) receptors marked with colloidal gold-conjugated low density lipoproteins has been mapped on the surfaces of cultured human skin fibroblasts and bovine aortic endothelial cells viewed whole in the transmission electron microscope. A dispersed or scattered population of LDL receptors, in addition to and clearly distinct from clustered receptors was detected on the surfaces of both fibroblasts and dividing endothelial cells. No LDL receptors could be detected on contact-inhibited endothelial cells. Clustered receptors imaged in whole-mount preparations were often arranged in rings with an approximate diameter of 250 nm. In ultra-thin sections of marked cells, clustered receptors were localised in coated pits while the few dispersed receptors seen were restricted to non-coated membrane regions. Clustered receptors often appeared localised on the rims of coated pits whose central areas were not marked. The dispersed population of receptors was usually distributed diffusely amongst the clusters on dividing endothelial cells and normal fibroblasts. Only the dispersed population appeared on LDL receptor internalisation-defective mutant fibroblasts. The marginal zones of both fibroblasts and dividing endothelial cells were populated by dispersed receptors. Clusters appeared further "inland" and were rarely seen near the cell margins. These results indicate that LDL receptors on dividing endothelial cells and fibroblasts may be dispersed on the cell surface upon or soon after their insertion during recycling. 相似文献
147.
In the present study, we have localized immunohistochemically S-100 protein, glial fibrillary acidic (GFA) protein, and neuron-specific enolase (NSE) by the unlabelled antibody peroxidase-antiperoxidase technique. Special attention was paid to the influence of fixation and of pretreatment of sections with proteolytic enzymes. It appeared that the final immunostaining of a given antigen largely depends on the fixative and on the species used. Moreover, pepsin pretreatment proved to be necessary to unmask S-100 protein in quail and GFA protein in rat. S-100 protein (rat, human) and GFA protein (human) immunoreactivities were detected in the folliculo-stellate (FS) cells. In quail, S-100 protein was also found in cells, which were not arranged around a follicular lumen and, in rat, the endothelial cells were immunostained for GFA protein. Clusters of granular cells were weakly immunostained for NSE in all species. An exclusive relationship between FS cells and S-100 protein could not be ascertained from this study. 相似文献
148.
149.
150.
M. W. A. Verstegen A. Siegerink W. Van der Hel R. Geers C. Brandsma 《Journal of thermal biology》1987,12(4):257-261
The aim of the experiments was to investigate the effect of air velocity on the temperature preferred by growing pigs 12–14 weeks old. Pigs displayed a temperature preference by means of operant supplemental heating. They pushed a button connected to heating lamps. Six experiments of three weeks each and with two treatments with a group of 8 pigs each were made. Animals were housed in groups and weighed 14–20 kg at the start of the experiments. Air velocity was 0.08, 0.25 and 0.40 m/s. At each air velocity four replicates were made. Mean temperatures preferred were 17.9°C at 0.08 m/s, 20.5°C at 0.25 m/s and 21.7°C at 0.40 m/s. Within a day temperature preference fluctuated with 5.7 K at 0.08 m/s, 4.3 K at 0.25 m/s and 4.2 K at 0.4 m/s. Temperatures preferred were highest during day time. 相似文献