全文获取类型
收费全文 | 12373篇 |
免费 | 1334篇 |
国内免费 | 1404篇 |
专业分类
15111篇 |
出版年
2024年 | 46篇 |
2023年 | 180篇 |
2022年 | 454篇 |
2021年 | 675篇 |
2020年 | 551篇 |
2019年 | 598篇 |
2018年 | 610篇 |
2017年 | 481篇 |
2016年 | 615篇 |
2015年 | 857篇 |
2014年 | 952篇 |
2013年 | 967篇 |
2012年 | 1114篇 |
2011年 | 989篇 |
2010年 | 683篇 |
2009年 | 541篇 |
2008年 | 637篇 |
2007年 | 542篇 |
2006年 | 507篇 |
2005年 | 389篇 |
2004年 | 409篇 |
2003年 | 403篇 |
2002年 | 379篇 |
2001年 | 249篇 |
2000年 | 174篇 |
1999年 | 199篇 |
1998年 | 117篇 |
1997年 | 106篇 |
1996年 | 96篇 |
1995年 | 99篇 |
1994年 | 74篇 |
1993年 | 58篇 |
1992年 | 55篇 |
1991年 | 48篇 |
1990年 | 41篇 |
1989年 | 44篇 |
1988年 | 27篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 8篇 |
1984年 | 13篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1977年 | 4篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
321.
Duwen Zeng Chenxi Qiu Yu Shen Jin Hou Zailu Li Jixiang Zhang Shuai Liu Jianli Shang Wensheng Qin Lili Xu Xiaoming Bao 《Microbial biotechnology》2020,13(6):2008-2019
Saccharomyces cerevisiae is the preferred source of RNA derivatives, which are widely used as supplements for foods and pharmaceuticals. As the most abundant RNAs, the ribosomal RNAs (rRNAs) transcribed by RNA polymerase I (Pol I) have no 5′ caps, thus cannot be translated to proteins. To screen high-nucleic-acid content yeasts more efficiently, a cap-independent protein expression system mediated by Pol I has been designed and established to monitor the regulatory changes of rRNA synthesis by observing the variation in the reporter genes expression. The elements including Pol I-recognized rDNA promoter, the internal ribosome entry site from cricket paralytic virus which can recruit ribosomes internally, reporter genes (URA3 and yEGFP3), oligo-dT and an rDNA terminator were ligated to a yeast episomal plasmid. This system based on the URA3 gene worked well by observing the growth phenotype and did not require the disruption of cap-dependent initiation factors. The fluorescence intensity of strains expressing the yEGFP3 gene increased and drifted after mutagenesis. Combined with flow cytometry, cells with higher GFP level were sorted out. A strain showed 58% improvement in RNA content and exhibited no sequence alteration in the whole expression cassette introduced. This study provides a novel strategy for breeding high-nucleic-acid content yeasts. 相似文献
322.
Xiaorui Lu Xingchao Liu Zhi Chen Jilun Li Gilles P. van Wezel Wei Chen Ying Wen 《Environmental microbiology》2020,22(12):5090-5108
Carbon catabolite repression (CCR) is a common phenomenon in bacteria that modulates expression of genes involved in uptake of alternative carbon sources. In the filamentous streptomycetes, which produce half of all known antibiotics, the precise mechanism of CCR is yet unknown. We report here that the ROK-family regulator Rok7B7 pleiotropically controls xylose and glucose uptake, CCR, development, as well as production of the macrolide antibiotics avermectin and oligomycin A in Streptomyces avermitilis. Rok7B7 directly repressed structural genes for avermectin biosynthesis, whereas it activated olmRI, the cluster-situated activator gene for oligomycin A biosynthesis. Rok7B7 also directly repressed the xylose uptake operon xylFGH, whose expression was induced by xylose and repressed by glucose. Both xylose and glucose served as Rok7B7 ligands. rok7B7 deletion led to enhancement and reduction of avermectin and oligomycin A production, respectively, relieved CCR of xylFGH, and increased co-uptake efficiency of xylose and glucose. A consensus Rok7B7-binding site, 5′-TTKAMKHSTTSAV-3′, was identified within aveA1p, olmRIp, and xylFp, which allowed prediction of the Rok7B7 regulon and confirmation of 11 additional targets involved in development, secondary metabolism, glucose uptake, and primary metabolic processes. Our findings will facilitate methods for strain improvement, antibiotic overproduction, and co-uptake of xylose and glucose in Streptomyces species. 相似文献
323.
324.
Min Cheng Yongyan Chen Dake Huang Wen Chen Weiping Xu Yin Chen Guodong Shen Tingjuan Xu Gan Shen Zhigang Tian Shilian Hu 《Aging cell》2020,19(2)
Cancer is an age‐associated disease, potentially related to the altered immune system of elderly individuals. However, cancer has gradually decreased incidence in the eldest globally such as the most common lung cancer, the mechanisms of which remain to be elucidated. In this study, it was found that the number of lung‐resident γδT cells was significantly increased with altered gene expression in aged mice (20–24 months) versus young mice (10–16 weeks). Aged lung Vγ4+ and Vγ6+ γδT cells predominantly produced interleukin‐17A (IL‐17A), resulting in increased levels in the serum and lungs. Moreover, the aged mice exhibited smaller tumors and reduced numbers of tumor foci in the lungs after challenge with intravenous injection of B16/F10 melanoma cells compared with the young mice. Aged lung Vγ4+ and Vγ6+ γδT cells were highly cytotoxic to B16/F10 melanoma cells with higher expression levels of CD103. The markedly longer survival of the challenged aged mice was dependent on γδT17 cells, since neutralization of IL‐17A or depletion of indicated γδT cells significantly shortened the survival time. Consistently, supplementation of IL‐17A significantly enhanced the survival time of young mice with lung melanoma. Furthermore, the anti‐tumor activity of aged lung γδT17 cells was not affected by alterations in the load and composition of commensal microbiota, as demonstrated through co‐housing of the aged and young mice. Intrinsically altered lung γδT17 cells underlying age‐dependent changes control lung melanoma, which will help to better understand the lung cancer progression in the elderly and the potential use of γδT17 cells in anti‐tumor immunotherapy. 相似文献
325.
326.
Wen Yongdiao Liu Shaobin Zhang Haifeng Xu Yan Yu Qiming Wang Lingling Frezza Fabrizio 《Plasmonics (Norwell, Mass.)》2020,15(4):1141-1149
Plasmonics - Herein, an integrated multifunctional design for simultaneous perfect absorption and polarization conversion is proposed that worked in the Ku band: the proposed structure consisted of... 相似文献
327.
328.
329.
330.
An exploration of the role of Sertoli cells on fetal testis development using cell ablation strategy
Yu‐Qian Wang Jin‐Mei Cheng Qing Wen Ji‐Xin Tang Jian Li Su‐Ren Chen Yi‐Xun Liu 《Molecular reproduction and development》2020,87(2):223-230
Sertoli cells (SCs) are presumed to be the center of testis differentiation because they provide both structural support and biological regulation for spermatogenesis. Previous studies suggest that SCs control germ cell (GC) count and Leydig cell (LC) development in mouse testes. However, the regulatory role of SCs on peritubular myoid (PTM) cell fate in fetal testis has not been clearly reported. Here, we employed Amh‐Cre; diphtheria toxin fragment A (DTA) mouse model to selectively ablate SCs from embryonic day (E) 14.5. Results found that SC ablation in the fetal stage caused the disruption of testis cords and the massive loss of GCs. Furthermore, the number of α‐smooth muscle actin‐labeled PTM cells was gradually decreased from E14.5 and almost lost at E18.5 in SC ablation testis. Interestingly, some Ki67 and 3β‐HSD double‐positive fetal LCs could be observed in Amh‐Cre; DTA testes at E16.5 and E18.5. Consistent with this phenomenon, the messenger RNA levels of Hsd3b1, Cyp11a1, Lhr, Star and the protein levels of 3β‐HSD and P450Scc were significantly elevated by SC ablation. SC ablation appears to induce ectopic proliferation of fetal LCs although the total LC number appeared reduced. Together, these findings bring us a better understanding of SCs’ central role in fetal testis development. 相似文献