全文获取类型
收费全文 | 543974篇 |
免费 | 56536篇 |
国内免费 | 1152篇 |
专业分类
601662篇 |
出版年
2018年 | 15003篇 |
2017年 | 13636篇 |
2016年 | 12727篇 |
2015年 | 9128篇 |
2014年 | 9855篇 |
2013年 | 13705篇 |
2012年 | 19465篇 |
2011年 | 27546篇 |
2010年 | 21488篇 |
2009年 | 17156篇 |
2008年 | 22578篇 |
2007年 | 24495篇 |
2006年 | 12877篇 |
2005年 | 12886篇 |
2004年 | 13032篇 |
2003年 | 12514篇 |
2002年 | 11757篇 |
2001年 | 21957篇 |
2000年 | 21997篇 |
1999年 | 17493篇 |
1998年 | 6308篇 |
1997年 | 6552篇 |
1996年 | 6369篇 |
1995年 | 5770篇 |
1994年 | 5810篇 |
1993年 | 5645篇 |
1992年 | 13718篇 |
1991年 | 13045篇 |
1990年 | 12689篇 |
1989年 | 12547篇 |
1988年 | 11220篇 |
1987年 | 10828篇 |
1986年 | 9907篇 |
1985年 | 9689篇 |
1984年 | 8228篇 |
1983年 | 7121篇 |
1982年 | 5550篇 |
1981年 | 5032篇 |
1980年 | 4675篇 |
1979年 | 7715篇 |
1978年 | 5943篇 |
1977年 | 5440篇 |
1976年 | 5076篇 |
1975年 | 5394篇 |
1974年 | 5837篇 |
1973年 | 5688篇 |
1972年 | 5331篇 |
1971年 | 5026篇 |
1970年 | 3943篇 |
1969年 | 3856篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Acquisition of Resistance to Extended-Spectrum Cephalosporins by Salmonella enterica subsp. enterica Serovar Newport and Escherichia coli in the Turkey Poult Intestinal Tract 下载免费PDF全文
992.
993.
Chien-Hung Liu Wen-Ming Chen Jo-Shu Chang 《World journal of microbiology & biotechnology》2007,23(5):633-640
Acidic lipase finds its commercial values in medical applications and bioremediation of food wastes. In this work, approaches
for rapid screening of lipase-producing bacteria were developed and the feasibility assessment of the screening methods was
performed. From food waste samples, the proposed screening procedures allowed isolation of sixteen pure bacterial strains
expressing higher lipase activity at acidic pH (pH 6.0) than at alkaline pH (pH 9.0). To enhance the accuracy of lipase activity
determination under acidic conditions, a novel assay procedure was also developed by deactivating lipase activity by microwave
treatment prior to back titration. This additional step could minimize interferences arising from residual lipase activity
during conventional direct back-titration methods in measuring lipase activity at acidic pH. Using the four strategies proposed
in this work, the best acidic-lipase-producing isolate was obtained by strategy C (SSC) and was identified as Aeromonas sp. C14, displaying an optimal lipase activity of 0.7 U/ml at an acidic pH of 6.0. 相似文献
994.
K R Chintalacharuvu J F Piskurich M E Lamm C S Kaetzel 《Journal of cellular physiology》1991,148(1):35-47
The HT-29 human colon carcinoma cell line differentiates in glucose-free medium to an enterocytic phenotype. We previously isolated a series of HT-29 subclones selected for high levels of expression of secretory component (SC), the epithelial receptor for polymeric immunoglobulins. To develop a model system for studying effects of cell polarity on SC expression and release from the cell surface, the HT-29.74 subclone was induced to differentiate in glucose-free medium. Expression of SC was induced by glucose deprivation in both the parental HT-29 cell line and, to an even greater extent, in the HT-29.74 subclone. Prolonged glucose deprivation of HT-29.74 cells resulted in morphological changes consistent with enterocytic differentiation. Metabolic radiolabeling of SC in differentiated HT-29.74 cells indicated that proteolytic cleavage of membrane-bound to free SC occurred both on the cell surface and intracellularly, possibly in a vacuolar apical compartment or intrapeithelial lumen. To study effects of cell polarity on SC release, differentiated HT-29.74 cells were depolarized by culturing in low calcium medium. Within 2 hours after transfer of the cells into low calcium medium, a burst of SC release was observed concomitant with cell depolarization. Subsequently, release of SC declined significantly and remained low as long as cells were maintained in a depolarized state. The extent of cell depolarization could be controlled by varying the extracellular calcium concentration or by substituting the divalent cation Sr++, which partially prevents depolarization, for Ca++. In either case, the magnitude of the initial burst and subsequent decline in release of SC was proportional to the extent of cell depolarization. We conclude that cell polarity plays an important role in controlling the release of SC in intestinal epithelial cells, most likely by regulating the distribution of membrane-bound SC and SC protease, which are on the basolateral and apical cell surfaces, respectively, in differentiated cells. 相似文献
995.
996.
997.
Transmission electron microscopic studies on the mitotic cycle of nucleolar proteins impregnated with silver 总被引:2,自引:0,他引:2
Hela cells were impregnated with silver according to Paweletz et al. (1967). In cells in mitosis not only the nucleolar organizer regions (NORs) are strongly impregnated but also part of the nucleolar material, which accumulates in and around the chromosomes. The treatment with adenosine, which in interphase cells spreads the nucleolar material within the nucleus, also distributes the argentophilic material in and around the chromosomes. During the reconstruction phase this material reassembles around the NORs to form parts of the new nucleolus. The silver impregnation technique clearly demonstrates that two main components are responsible for the argentophily of the nucleolus. This is in agreement with the results obtained by Lischwe et al. (1979). 相似文献
998.
Sensitivity to and requirement for iron in Plantago species 总被引:1,自引:0,他引:1
999.
G. S. C. BUSO Z. P. S. AMARAL R. P. V. BRONDANI M. E. FERREIRA 《Molecular ecology resources》2006,6(1):252-254
Efforts to develop molecular tools for genetic analysis and breeding of common bean in the tropics are still limited. The number of microsatellite markers available for the crop is small compared to other crops of similar social and economic importance. As part of a project to broaden the use of molecular tools in bean breeding, a genomic library enriched for AG/TC repeat sequences was constructed for Phaseolus vulgaris. Twenty microsatellite markers were initially developed and 10 were characterized using a panel of 85 representative accessions of the bean gene bank. The number of alleles per marker ranged from three to 10. The polymorphism information content (PIC) varied from 0.23 to 0.80. The results indicate that the new markers can be readily used in genetically analysis of common bean. 相似文献
1000.
Fátima H. Vaz Patrícia M. Machado Rita D. Brand?o Cátia T. Laranjeira Joana S. Eugénio Aires H. Fernandes Saudade P. André 《The journal of histochemistry and cytochemistry》2007,55(11):1105-1113
Only 20-25% of families screened for BRCA1/2 mutations are found positive. Because only a positive result is informative, we studied the role of BRCA1/2 immunohistochemistry as an additional method for patient selection. From 53 high-risk-affected probands, 18 (34%) had available paraffin blocks of their tumors and were selected for this study. Mutation screening was done by conformation-sensitive gel electrophoresis and multiplex ligation-dependent probe amplification. For immunohistochemistry, 21 neoplastic specimens (15 breast carcinomas, 5 ovary neoplasms, and 1 rectal adenocarcinoma) were analyzed with BRCA1 (monoclonal antibody, Ab-1, oncogene) and BRCA2 (polyclonal antibody, Ab-2, oncogene) antibodies. Absence of the BRCA1 protein was confirmed in negative tumors by Western blotting. Seven patients were positive for BRCA1/2 mutations: 5 for BRCA1 and 2 for BRCA2. Four out of five positive patients had tumors negative for BRCA1 immunostaining, and the remaining 13 BRCA1-negative patients had positive BRCA1 immunostaining in all tumor samples. Sensitivity to predict for BRCA1 mutation carriers was 80%, and specificity was 100%, with a positive predictive value of 100% and a negative predictive value of 93%. This correlation was statistically significant (p=0.001). No correlation was observed for BRCA2. If larger studies confirm these results, high-risk patients with BRCA1-negative tumors should be screened first for this gene. 相似文献