全文获取类型
收费全文 | 6230篇 |
免费 | 444篇 |
国内免费 | 6篇 |
出版年
2023年 | 40篇 |
2022年 | 101篇 |
2021年 | 181篇 |
2020年 | 106篇 |
2019年 | 117篇 |
2018年 | 183篇 |
2017年 | 134篇 |
2016年 | 222篇 |
2015年 | 310篇 |
2014年 | 324篇 |
2013年 | 441篇 |
2012年 | 522篇 |
2011年 | 529篇 |
2010年 | 320篇 |
2009年 | 289篇 |
2008年 | 371篇 |
2007年 | 415篇 |
2006年 | 356篇 |
2005年 | 330篇 |
2004年 | 272篇 |
2003年 | 261篇 |
2002年 | 244篇 |
2001年 | 53篇 |
2000年 | 38篇 |
1999年 | 40篇 |
1998年 | 47篇 |
1997年 | 56篇 |
1996年 | 28篇 |
1995年 | 36篇 |
1994年 | 31篇 |
1993年 | 34篇 |
1992年 | 23篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 17篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1973年 | 5篇 |
排序方式: 共有6680条查询结果,搜索用时 864 毫秒
991.
Yaroslav Staroseletz Sergey Nechaev Elena Bichenkova Richard A. Bryce Catherine Watson Valentin Vlassov Marina Zenkova 《Biochimica et Biophysica Acta (BBA)/General Subjects》2018,1862(3):705-725
Background
While the RNA world hypothesis is widely accepted, it is still far from complete: the existence of self-replicating ribozyme, consisting of potentially hundreds of nucleotides, is a core assumption for the majority of RNA world models. The appearance of such long RNA molecules under prebiotic conditions is not self-evident. Recombination seems to be a plausible way of creating RNA diversity, resulting in the appearance of functional RNAs, capable of self-replicating.Methods
We report here on the study of recombination process modelled with two 96 nts RNA fragments. Detection of recombination products was performed with RT-PCR followed by TA-cloning and Sanger sequencing.Results
A wide range of recombinant products was detected. We found that (i) the most efficient ligation was observed for RNA species forming bulges or internal loops, with ligation partners located within the loop; (ii) a strong preference was observed for formation of a few types of major products with a large variety of minor products; (iii) ligation could occur with participation of either 2′,3′-cyclophosphate or 5′-ppp; (iv) the presence of key reaction components, i.e. 5′ppp-RNAs, enabled the formation of additional types of product; (v) molecular dynamics simulations of one of the most abundant products suggests that the ligation results in a preferable formation of 2′-5′- rather than 3′-5′-linkages.Conclusions
The study demonstrates regularities of new RNA molecules formation with non-enzymatic recombination process.General significance
Our findings provide new data supporting the RNA World hypothesis and show the way of new RNA sequences emergence under prebiotic conditions. 相似文献992.
993.
994.
Marina Espinasse Claudia Halsband Øystein Varpe Astthor Gislason Kristinn Gudmundsson Stig Falk-Petersen 《Marine Biology Research》2018,14(7):752-767
Phenological variations of the marine copepod Calanus finmarchicus were studied in Svalbard and northern Iceland, where samples were collected in summer and spring, respectively, over two decades. Four phenological indices, developed for copepodite stage-structured data, were used: the proportion of CV to total abundance (CVT), the population development index (PDI), the average weighted stage (AWS), and the average age in days (AAD). The variation of these indices was compared within and between locations to evaluate their suitability for the analysis of phenological effects. For both populations, phenology was related to local temperature and spring bloom dynamics, influenced by Atlantic water inflow. Large-scale climate was related to phenological variation only in the Svalbard population. C. finmarchicus phenology advanced under warmer conditions in both locations. We conclude that vertical phenological indices, i.e. based on interannual changes in copepodite stage structure, are useful to investigate zooplankton phenology, especially when data series covering the whole life cycle are unavailable. We suggest that AWS and AAD can be applied irrespective of sampling time, while PDI and CVT should be applied for early and late sampling seasons, respectively. When multiple phenological indices are needed, AAD in combination with either CVT or PDI should be preferred. 相似文献
995.
Marina V. Kovina Michael E. Krasheninnikov Tatiana G. Dyuzheva Michael I. Danilevsky Ilya D. Klabukov Maxim V. Balyasin Olga K. Chivilgina Alexey V. Lyundup 《Cytotherapy》2018,20(3):361-374
Background
Menstrual blood is only recently and still poorly studied, but it is an abundant and noninvasive source of highly proliferative mesenchymal stromal cells (MSCs). However, no appropriate isolation method has been reported due to its high viscosity and high content of clots and desquamated epithelium.Methods
We studied three different isolation approaches and their combinations: ammonium-containing lysing buffer, distilled water and gradient-density centrifugation. We tested the proliferative capacity, morphology, surface markers and pluripotency of the resulting cells.Results
Our isolation method yields up to four million nucleated cells per milliliter of initial blood, of which about 0.2–0.3% are colony-forming cells expressing standard mesenchymal markers CD90, CD105 and CD73, but not expressing CD45, CD34, CD117, CD133 or HLA-G. The cells have high proliferative potential (doubling in 26?h) and the ability to differentiate into adipocytes and osteocytes. Early endometrial MSCs (eMSCs) express epithelial marker cytokeratin 7 (CK7). CK7 is easily induced in later passages in a prohepatic environment. We show for the first time that a satisfactory and stable yield of eMSCs is observed throughout the whole menstrual period (5 consecutive days) of a healthy woman.Discussion
The new cost/yield adequate method allows isolation from menstrual blood a relatively homogenous pool of highly proliferative MSCs, which seem to be the best candidates for internal organ therapy due to their proepithelial background (early expression of CK7 and its easy induction in later passages) and for mass cryobanking due to their high yield and availability. 相似文献996.
Ignacio M. Hernández‐Agramonte María Semmartin Marina Omacini Martín Durante Pedro E. Gundel José De Battista 《Austral ecology》2018,43(2):172-179
Temperate grasses frequently acquire resistance to herbivores through a symbiosis with epichloid fungi that produces alkaloids of variable deterrent effects. However, in those cases without apparent endophyte negative effects on domestic herbivores, it is not clear whether plant consumption or preference is affected or not. We performed three experiments with 1‐year‐old steers (Bos taurus, Aberdeen Angus) and the annual grass Lolium multiflorum, infected or not by Epichloë occultans to evaluate preference and to identify the underlying tolerance mechanisms. The first experiment evaluated steer preference for L. multiflorum cultivated in plots with three endophyte infection frequencies (low, medium and high), and investigated the role of canopy structure and plant nutritional traits on preference. The second experiment evaluated preference for chopped grass, offered in individual trays with contrasting infection frequencies (low and high), to discard possible effects associated with canopy structure and to focus on nutritional traits. The third experiment was performed with a tray + basket design that separated visual and olfactory stimuli from nutritional traits. High endophyte infection frequencies reduced consistently animal preference, even after short (~10 min) feeding events. However, we did not find significant evidence of plant structural, nutritional, visual or olfactory traits. Our results discarded several potential mechanisms; therefore, the dissuasive effect of fungal endophytes on animal consumption might be related to other mechanisms, including, likely, alkaloids and changes on grass metabolome. 相似文献
997.
998.
Marina Gulyaeva Ivan Sobolev Kirill Sharshov Olga Kurskaya Alexander Alekseev Lidia Shestopalova Anna Kovner Yuhai Bi Weifeng Shi Michael Shchelkanov Alexander Shestopalov 《中国病毒学》2018,33(5):449-452
正Dear Editor Marine mammals are widely distributed and can be found almost in all coastal waters and coastlines around the world. The interface areas between marine and terrestrial environments provide natural habitats for aquatic and semiaquatic mammals as well as for reservoir species of avian influenza viruses (AIV)(Runstadler et al. 2013). Previous 相似文献
999.
Identification and characterization of the biotechnological potential of a wild strain of Paraconiothyrium sp 下载免费PDF全文
Marina Arredondo‐Santoyo Ma. Soledad Vázquez‐Garcidueñas Gerardo Vázquez‐Marrufo 《Biotechnology progress》2018,34(4):846-857
The isolation and characterization of fungal strains from poorly described taxa allows undercover attributes of their basic biology useful for biotechnology. Here, a wild fungal strain (CMU‐196) from recently described Paraconiothyrium genus was analyzed. CMU‐196 was identified as Paraconiothyrium brasiliense by phylogenetic analysis of the rDNA internal transcribed spacer region (ITS). CMU‐196 metabolized 57 out of 95 substrates of the Biolog FF microplates. Efficient assimilation of dextrins and glycogen indicates that CMU‐196 is a good producer of amylolytic enzymes. It showed a remarkably assimilation of α‐d ‐lactose, substrate described as inducer of cellulolytic activity but poorly assimilated by several fungi. Metabolically active mycelium of the strain decolorized broth supplemented with direct blue 71, Chicago sky blue and remazol brilliant blue R dyes. The former two dyes were also well removed from broth by mycelium inactivated by autoclaving. Both mycelia had low efficiency for removing fuchsin acid from broth and for decolorizing wastewater from the paper industry. CMU‐196 strain showed extracellular laccase activity when potato dextrose broth was supplemented with Cu+2, reaching a maximum activity of 46.8 (±0.33) U L?1. Studied strain antagonized phytopathogenic Colletotrichum spp. fungi and Phytophthora spp. oomycetes in vitro, but is less effective towards Fusarium spp. fungi. CMU‐196 antagonism includes overgrowing the mycelia of phytopathogens and growth inhibition, probably by hydrosoluble extracellular metabolites. The biotechnological potential of strain CMU‐196 here described warrants further studies to have a more detailed knowledge of the mechanisms associated with its metabolic versatility, capacity for environmental detoxification, extracellular laccase production, and antagonism against phytopathogens. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:846–857, 2018 相似文献
1000.
Pixel classification method in optical coherence tomography for tumor segmentation and its complementary usage with OCT microangiography 下载免费PDF全文
Alexander Moiseev Ludmila Snopova Sergey Kuznetsov Natalia Buyanova Vadim Elagin Marina Sirotkina Elena Kiseleva Lev Matveev Vladimir Zaitsev Felix Feldchtein Elena Zagaynova Valentin Gelikonov Natalia Gladkova Alex Vitkin Grigory Gelikonov 《Journal of biophotonics》2018,11(4)
A novel machine‐learning method to distinguish between tumor and normal tissue in optical coherence tomography (OCT) has been developed. Pre‐clinical murine ear model implanted with mouse colon carcinoma CT‐26 was used. Structural‐image‐based feature sets were defined for each pixel and machine learning classifiers were trained using “ground truth” OCT images manually segmented by comparison with histology. The accuracy of the OCT tumor segmentation method was then quantified by comparing with fluorescence imaging of tumors expressing genetically encoded fluorescent protein KillerRed that clearly delineates tumor borders. Because the resultant 3D tumor/normal structural maps are inherently co‐registered with OCT derived maps of tissue microvasculature, the latter can be color coded as belonging to either tumor or normal tissue. Applications to radiomics‐based multimodal OCT analysis are envisioned. 相似文献