全文获取类型
收费全文 | 35867篇 |
免费 | 3182篇 |
国内免费 | 120篇 |
出版年
2022年 | 264篇 |
2021年 | 498篇 |
2020年 | 330篇 |
2019年 | 462篇 |
2018年 | 560篇 |
2017年 | 461篇 |
2016年 | 728篇 |
2015年 | 1151篇 |
2014年 | 1251篇 |
2013年 | 1759篇 |
2012年 | 1996篇 |
2011年 | 1839篇 |
2010年 | 1335篇 |
2009年 | 1101篇 |
2008年 | 1578篇 |
2007年 | 1477篇 |
2006年 | 1426篇 |
2005年 | 1284篇 |
2004年 | 1322篇 |
2003年 | 1248篇 |
2002年 | 1330篇 |
2001年 | 1149篇 |
2000年 | 1090篇 |
1999年 | 947篇 |
1998年 | 465篇 |
1997年 | 472篇 |
1996年 | 366篇 |
1995年 | 372篇 |
1994年 | 281篇 |
1993年 | 339篇 |
1992年 | 666篇 |
1991年 | 642篇 |
1990年 | 612篇 |
1989年 | 561篇 |
1988年 | 453篇 |
1987年 | 465篇 |
1986年 | 438篇 |
1985年 | 469篇 |
1984年 | 448篇 |
1983年 | 378篇 |
1982年 | 277篇 |
1981年 | 296篇 |
1980年 | 252篇 |
1979年 | 345篇 |
1978年 | 284篇 |
1977年 | 326篇 |
1976年 | 293篇 |
1975年 | 300篇 |
1974年 | 290篇 |
1973年 | 295篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
1.
In this work, incorporation of plasmid DNA, pre-complexed with PEI, into polyelectrolyte multilayers has been studied to further develop platforms for local gene delivery. Polyplex embedding in synthetic and naturally degradable architectures was efficient for transfection of human hepato-cellular carcinoma cells. 相似文献
2.
Temporal variability of top-down forces and their role in host choice evolution of phytophagous arthropods 总被引:1,自引:0,他引:1
The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account. 相似文献
3.
4.
5.
6.
Denise Risch Nicholas J. Gales Jason Gedamke Lars Kindermann Douglas P. Nowacek Andrew J. Read Ursula Siebert Ilse C. Van Opzeeland Sofie M. Van Parijs Ari S. Friedlaender 《Biology letters》2014,10(4)
For decades, the bio-duck sound has been recorded in the Southern Ocean, but the animal producing it has remained a mystery. Heard mainly during austral winter in the Southern Ocean, this ubiquitous sound has been recorded in Antarctic waters and contemporaneously off the Australian west coast. Here, we present conclusive evidence that the bio-duck sound is produced by Antarctic minke whales (Balaenoptera bonaerensis). We analysed data from multi-sensor acoustic recording tags that included intense bio-duck sounds as well as singular downsweeps that have previously been attributed to this species. This finding allows the interpretation of a wealth of long-term acoustic recordings for this previously acoustically concealed species, which will improve our understanding of the distribution, abundance and behaviour of Antarctic minke whales. This is critical information for a species that inhabits a difficult to access sea-ice environment that is changing rapidly in some regions and has been the subject of contentious lethal sampling efforts and ongoing international legal action. 相似文献
7.
8.
9.
D. Pangallo K. Chovanov H. Drahovska F. De Leo C. Urzì 《International biodeterioration & biodegradation》2009,63(7):868-872
The aim of this work was the evaluation of fluorescence ITS-PCR (f-ITS) as a molecular tool to analyze the microbial community involved in the biodeterioration of cultural heritage surfaces. As a case study we analyzed by f-ITS ninety-two bacterial strains isolated from a medieval fresco and the surrounding air environment. The internal transcribed spacer between the 16S and 23S rRNA genes was amplified, and then the fluorescently labeled PCR products were separated by capillary electrophoresis. Bacterial strains were identified by 16S rDNA sequencing. The f-ITS electropherograms showed different profiles coherent with the affiliation of the strains at the genus and species levels. Among the isolates obtained from the fresco surface, those belonging to the genus Bacillus were the most prevailing exhibiting 8 different f-ITS profiles. The airborne bacilli exhibited only 2 of these 8 profiles. Staphylococcus were mostly isolated from air and produced 4 different profiles. Pseudomonas isolates presented 3 different profiles, and one of them was typical of Pseudomonas putida. Members of the other genera produced their distinctive profiles. Our results show that f-ITS is a promising molecular tool for the rapid selection and clustering of strains isolated from different sources. 相似文献
10.
R Gill C Verma B Wallach B Urs? J Pitts A Wollmer P De Meyts S Wood 《Protein engineering》1999,12(4):297-303
Insulin-like growth factor-1 (IGF-1) is a serum protein which unexpectedly folds to yield two stable tertiary structures with different disulphide connectivities; native IGF-1 [18-61,6-48,47-52] and IGF-1 swap [18-61,6-47, 48-52]. Here we demonstrate in detail the biological properties of recombinant human native IGF-1 and IGF-1 swap secreted from Saccharomyces cerevisiae. IGF-1 swap had a approximately 30 fold loss in affinity for the IGF-1 receptor overexpressed on BHK cells compared with native IGF-1.The parallel increase in dose required to induce negative cooperativity together with the parallel loss in mitogenicity in NIH 3T3 cells implies that disruption of the IGF-1 receptor binding interaction rather than restriction of a post-binding conformational change is responsible for the reduction in biological activity of IGF-1 swap. Interestingly, the affinity of IGF-1 swap for the insulin receptor was approximately 200 fold lower than that of native IGF-1 indicating that the binding surface complementary to the insulin receptor (or the ability to attain it) is disturbed to a greater extent than that to the IGF-1 receptor. A 1.0 ns high-temperature molecular dynamics study of the local energy landscape of IGF-1 swap resulted in uncoiling of the first A-region alpha-helix and a rearrangement in the relative orientation of the A- and B-regions. The model of IGF-1 swap is structurally homologous to the NMR structure of insulin swap and CD spectra consistent with the model are presented. However, in the model of IGF-1 swap the C-region has filled the space where the first A-region alpha-helix has uncoiled and this may be hindering interaction of Val44 with the second insulin receptor binding pocket. 相似文献