全文获取类型
收费全文 | 93491篇 |
免费 | 8296篇 |
国内免费 | 55篇 |
专业分类
101842篇 |
出版年
2023年 | 299篇 |
2022年 | 755篇 |
2021年 | 1654篇 |
2020年 | 941篇 |
2019年 | 1217篇 |
2018年 | 1544篇 |
2017年 | 1328篇 |
2016年 | 2279篇 |
2015年 | 3861篇 |
2014年 | 4285篇 |
2013年 | 5139篇 |
2012年 | 6917篇 |
2011年 | 6856篇 |
2010年 | 4387篇 |
2009年 | 4001篇 |
2008年 | 5737篇 |
2007年 | 5739篇 |
2006年 | 5527篇 |
2005年 | 5324篇 |
2004年 | 5177篇 |
2003年 | 4940篇 |
2002年 | 4674篇 |
2001年 | 961篇 |
2000年 | 713篇 |
1999年 | 1064篇 |
1998年 | 1326篇 |
1997年 | 886篇 |
1996年 | 817篇 |
1995年 | 717篇 |
1994年 | 684篇 |
1993年 | 728篇 |
1992年 | 642篇 |
1991年 | 620篇 |
1990年 | 551篇 |
1989年 | 471篇 |
1988年 | 484篇 |
1987年 | 422篇 |
1986年 | 385篇 |
1985年 | 460篇 |
1984年 | 613篇 |
1983年 | 475篇 |
1982年 | 586篇 |
1981年 | 558篇 |
1980年 | 515篇 |
1979年 | 361篇 |
1978年 | 403篇 |
1977年 | 356篇 |
1976年 | 331篇 |
1975年 | 262篇 |
1974年 | 307篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Rpn1 (109 kDa) and Rpn2 (104 kDa) are components of the 19S regulatory complex of the proteasome. The central portions of both proteins are predicted to have toroidal α-solenoid folds composed of 9-11 proteasome/cyclosome repeats, each ∼ 40 residues long and containing two α-helices and turns [A. V. Kajava, J. Biol. Chem. 277, 49791-49798, 2002]. To evaluate this prediction, we examined the full-length yeast proteins and truncated versions thereof consisting only of the repeat-containing regions by gel filtration, CD spectroscopy, and negative-staining electron microscopy (EM). All four proteins are monomeric in solution and highly α-helical, particularly the truncated ones. The EM data were analyzed by image classification and averaging techniques. The preponderant projections, in each case, show near-annular molecules 6-7 nm in diameter. Comparison of the full-length with the truncated proteins showed molecules similar in size and shape, indicating that their terminal regions are flexible and thus smeared to invisibility in the averaged images. We tested the toroidal model further by calculating resolution-limited projections and comparing them with the EM images. The results support the α-solenoid model, except that they indicate that the repeats are organized not as symmetrical circular toroids but in less regular horseshoe-like structures. 相似文献
82.
Michael A. Fox 《Bulletin of mathematical biology》1973,35(1-2):11-17
Complexity and stability are the ubiquitous characteristics of all biological phenomena. The theoretical suggestions presented
here acknowledge this and attempt to capitalize on the property of selectiveness that complexity induces. It is from this
that a belief in the existence of selection rules emerges. What is required of a viable theory is a linguistic means of maneuvering
known information so that these selection or limitation rules may be obtained, and thence, by their manipulation, to generate
the properties exhibited by biological systems. 相似文献
83.
84.
Madhusoodana P. Nambiar Tatsuya Oda Chaohua Chen Yasuo Kuwazuru Henry C. Wu 《Journal of cellular physiology》1993,154(2):222-228
The intracellular pathway following receptor-mediated endocytosis of cholera toxin was studied using brefeldin A (BFA), which inhibited protein secretion and induced dramatic morphological changes in the Golgi region. In both mouse Y1 adrenal cells and CHO cells, BFA at 1 μg/ml caused a 80–90% inhibition of the cholera toxin (CT)-elevation of intracellular cAMP. The inhibition of the cytotoxicity of CT by BFA was also observed in a rounding assay of Y1 adrenal cells. The inhibition of CT cytotoxicity by BFA was dose dependent, with the ID50 value similar to the LD50 of BFA in Y1 adrenal cells. Binding and internalization of [125I]-cholera toxin in Y1 adrenal cells was not affected by BFA. Unlike the BFA-sensitive cell lines such as Y1 adrenal and CHO cells, BFA at 1 μg/ml did not inhibit the cytotoxicity of CT in PtK1 cells, of which the Golgi structure was BFA-resistant. These results strongly suggest that a BFA-sensitive Golgi is required for the protection of CT cytotoxicity by BFA. In contrast, elevation of the intracellular cAMP by forskolin, which acts directly on the plasma membrane adenylate cyclase, was not affected by BFA. These observations indicate that the intoxication of target cells by CT requires an intact Golgi region for its intracellular trafficking and/or processing. In this respect, CT shares a common intracellular pathway with ricin, Pseudomonas toxin, and modeccin, even though their structures and modes of action are very different. © 1993 Wiley-Liss, Inc. 相似文献
85.
Michael Gewecke Martin Woike 《Ethology : formerly Zeitschrift fur Tierpsychologie》1978,47(3):293-298
Analysis of the flight behaviour of siskins (Carduelis spinus) was made possible by tethering them with a harness to a flight balance in a wind tunnel. It could be shown that the breast feathers work as an air-current sense organ influencing the siskins' flight pattern and the aerodynamic parameters. 相似文献
86.
Henry R. Mainwaring 《Archives of microbiology》1971,75(4):296-303
Summary The cytology of Podospora arizonensis, an apomictic pyrenomycete, has been studied from crozier formation to the first mitotic division in the ascus, using phase contrast light microscopy. Increase in ascus size is accompanied by morphological changes in the chromosomes which are reminiscent of the changes associated with meiosis in normal ascomycetes. The nucleolus is seen to increase in size during the phase of maximal ascus growth, and to decrease thereafter. The significance of these events is discussed. 相似文献
87.
A simple linear-operator model both describes and predicts the dynamics of choice that may underlie the matching relation. We measured inter-food choice within components of a schedule that presented seven different pairs of concurrent variable-interval schedules for 12 food deliveries each with no signals indicating which pair was in force. This measure of local choice was accurately described and predicted as obtained reinforcer sequences shifted it to favor one alternative or the other. The effect of a changeover delay was reflected in one parameter, the asymptote, whereas the effect of a difference in overall rate of food delivery was reflected in the other parameter, rate of approach to the asymptote. The model takes choice as a primary dependent variable, not derived by comparison between alternatives—an approach that agrees with the molar view of behaviour. 相似文献
88.
When expressed in Escherichia coli, a truncated form of phytochrome (oat PHYA AP3 residues 464-1129) self associates to form a series of products ranging in size from monomers to aggregates of greater than 20 subunits. When these same phytochrome sequences are coexpressed with the chaperonins GroEL and GroES, the truncated phytochrome migrates as a native-like dimer in size exclusion chromatography and no higher-order aggregates were detected. GroEL and GroES inhibition of phytochrome aggregation in E. coli presumably occurs via the suppression of folding pathways leading to incorrectly folded phytochrome. 相似文献
89.
90.
The Enterococcus faecalis conjugative plasmid pCF10 was used to introduce Tn925 into Acetobacterium woodii by filter mating. Tetracycline resistance was transferred at frequencies of about 10(-6) per donor, but no plasmid DNA was found in the transconjugants. DNA hybridization analyses of HindIII-digested chromosomal DNA demonstrated the insertion of Tn925 at a variety of locations, whereas wild type DNA showed no hybridization at all. The transconjugants were used as donor in mating experiments with tetracycline-sensitive Bacillus subtilis. Transfer of tetracycline resistance was observed at frequencies of 10(-8) per recipient. 相似文献