全文获取类型
收费全文 | 3746篇 |
免费 | 266篇 |
国内免费 | 434篇 |
专业分类
4446篇 |
出版年
2024年 | 31篇 |
2023年 | 83篇 |
2022年 | 99篇 |
2021年 | 87篇 |
2020年 | 116篇 |
2019年 | 149篇 |
2018年 | 132篇 |
2017年 | 96篇 |
2016年 | 136篇 |
2015年 | 106篇 |
2014年 | 231篇 |
2013年 | 289篇 |
2012年 | 176篇 |
2011年 | 258篇 |
2010年 | 178篇 |
2009年 | 205篇 |
2008年 | 256篇 |
2007年 | 265篇 |
2006年 | 173篇 |
2005年 | 200篇 |
2004年 | 144篇 |
2003年 | 134篇 |
2002年 | 125篇 |
2001年 | 97篇 |
2000年 | 73篇 |
1999年 | 67篇 |
1998年 | 39篇 |
1997年 | 40篇 |
1996年 | 39篇 |
1995年 | 31篇 |
1994年 | 45篇 |
1993年 | 24篇 |
1992年 | 28篇 |
1991年 | 20篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1985年 | 19篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 24篇 |
1981年 | 20篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1975年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有4446条查询结果,搜索用时 15 毫秒
951.
We have determined the specificity profile of the homing endonuclease I-AniI and compared it to the conservation of its host gene. Homing endonucleases are encoded within intervening sequences such as group I introns. They initiate the transfer of such elements by cleaving cognate alleles lacking the intron, leading to their transfer via homologous recombination. Each structural homing endonuclease family has arrived at an appropriate balance of specificity and fidelity that avoids toxicity while maximizing target recognition and invasiveness. I-AniI recognizes a strongly conserved target sequence in a host gene encoding apocytochrome B and has fine-tuned its specificity to correlate with wobble versus nonwobble positions across that sequence and to the amount of degeneracy inherent in individual codons. The physiological target site in the host gene is not the optimal substrate for recognition and cleavage: at least one target variant identified during a screen is bound more tightly and cleaved more rapidly. This is a result of the periodic cycle of intron homing, which at any time can present nonoptimal combinations of endonuclease specificity and insertion site sequences in a biological host. 相似文献
952.
953.
本文应用聚合酶链反应技术对30例人生殖器尖锐湿疣 组织中HPV6,11DNA的存在进行了检测研究。结果发现HPV6,HPV11DNA的阳性率分别为60×,90%HPV6,HPV11DNA双重检出率为53.3%,总阳性率达96.67%,本文结果证实HPV6,11的感染和尖锐湿疣的发生密切相关。本文检测方法具有快速、灵敏、特异性强的优点,为HPVDNA的检测及其分型研究提供了有效的手段。作者还对两例女阴假性湿疣进行PCR扩增,结果似乎不支持女阴假性湿疣的HPV感染的病因学。 相似文献
954.
Bacteria adhere to environmental surfaces in multicellular assemblies described as biofilms. Plant-associated bacteria interact with host tissue surfaces during pathogenesis and symbiosis, and in commensal relationships. Observations of bacteria associated with plants increasingly reveal biofilm-type structures that vary from small clusters of cells to extensive biofilms. The surface properties of the plant tissue, nutrient and water availability, and the proclivities of the colonizing bacteria strongly influence the resulting biofilm structure. Recent studies highlight the importance of these structures in initiating and maintaining contact with the host by examining the extent to which biofilm formation is an intrinsic component of plant-microbe interactions. 相似文献
955.
Internalization and intracellular survival of Mycoplasma pneumoniae by non-phagocytic cells 总被引:4,自引:0,他引:4
Current theory holds that mycoplasmas remain attached to the surface of epithelial cells although some mycoplasmas have evolved mechanisms for entering host cells that are not naturally phagocytic. The ability of Mycoplasma pneumoniae strain M129 to invade and survive within host cells was studied using a HeLa cell line and a human lung carcinoma cell line (A549). The invasion process into the eukaryotic cells was studied qualitatively by confocal laser scanning microscopy and quantitatively by the gentamicin resistance assay. Internalization was found with A549 cells but not with HeLa cells. Internalization was dependent on the duration of the infection and on temperature. The organism, detected in the cytoplasm and perinuclear regions, survived within the host cells for prolonged periods of time. The intracellular location of M. pneumoniae is obviously a privileged niche, well protected from the immune system and from the action of many antibiotics and may explain the pathogenic potential of this organism. 相似文献
956.
David Bristow Russell Richman Adam Kirsh Christopher A. Kennedy Kim D. Pressnail 《Journal of Industrial Ecology》2011,15(3):381-393
The seasonal and hourly variation of electricity grid emissions and building operational energy use are generally not accounted for in carbon footprint analyses of buildings. This work presents a technique for and results of such an analysis and quantifies the errors that can be encountered when these variations are not appropriately addressed. The study consists of an hour‐by‐hour analysis of the energy used by four different variations of a five‐story condominium building, with a gross floor area of approximately 9,290 square meters (m2), planned for construction in Markham, Ontario, Canada. The results of the case studied indicate that failure to account for variation can, for example, cause a 4% error in the carbon footprint of a building where ground source heat pumps are used and a 6% and 8% error in accounting for the carbon savings of wind and photovoltaic systems, respectively. After the building envelope was enhanced and sources of alternative energy were incorporated, the embodied greenhouse gas (GHG) emissions were more than 50% of the building's operational emissions. This work illustrates the importance of short‐time‐scale GHG analysis for buildings. 相似文献
957.
Different scaffold proteins play distinct roles in various signaling pathways by recruiting different downstream molecules. Here, using MKK4(-/-) and MKK4(-/-)/7(-/-) murine embryonic fibroblast cells, we examined differential employment of MKK4 and MKK7 by scaffold proteins Axin, Dvl, and Epstein-Barr virus latent membrane protein-1 (LMP-1) in mediating JNK activation. We present evidence that Axin depends mainly on MKK7 for activation of JNK, while Dvl depends almost equally on MKK4 and MKK7 for JNK activation, In contrast, LMP-1-induced JNK activation is primarily dependent on MKK4. Our results demonstrate that Axin, Dvl, and LMP-1 differentially utilize MKK4 and MKK7 for JNK activation. 相似文献
958.
Fates and osteogenic differentiation potential of human mesenchymal stem cells in immunocompromised mice 总被引:1,自引:0,他引:1
Human mesenchymal stem cells (hMSCs) from bone marrow were genetically marked by using a murine leukaemia virus construct encoding enhanced green fluorescent protein (eGFP). The marked cells were either directly implanted into the tibialis anterior muscle or introduced into a variety of other tissue sites in immunocompromised mice (NOD/SCID and C.B-17 SCID/beige) to investigate their fates and differentiation potentials. It was observed that the hMSCs survived for up to 12 weeks and showed site-specific morphological phenotypes. hMSCs delivered by intravenous injection were found mainly in the lungs and were detected rarely in other organs. Histomorphometry showed that, after implantation of hMSCs into the tibialis anterior muscle juxtaskeletally, the areas of reactive host callus formation at 1 and 2 weeks and of ectopic human bone formation at 1 week were significantly increased compared with the control group. Expression of eGFP and human RUNX2, alkaline phosphatase, osteocalcin, osteopontin, and collagen type I mRNAs were detected in mice implanted with the labelled hMSCs but not in sham-treated samples. Active clearance of the reactive callus and ectopic calcified tissue by osteoclast-like tartrate-resistant acid phosphatase-positive cells was observed. We conclude that the eGFP-labelled hMSCs can survive and retain the potential to differentiate morphologically into a variety of apparent mesenchymal phenotypes in vivo. Absolute confirmation of differentiation capacity requires further study and is complicated by known possibilities of fusion of donor and host cells or limited transfer of genetic material. Nevertheless, the genetically marked hMSCs are shown to participate extensively in bone formation and turnover. Control of the host osteoclast/macrophage responses resulting in clearance of formed osteogenic tissue warrants further investigation to promote prolonged human osteogenesis in immunocompromised mice. Furthermore, any proposed general cytotherapeutic strategy for enhanced osteogenesis is likely to require supplementation of local bone-forming biological signals. 相似文献
959.
Chi‐Ping Day John Carter Carrie Bonomi Dominic Esposito Bruce Crise Betty Ortiz‐Conde Melinda Hollingshead Glenn Merlino 《Pigment cell & melanoma research》2009,22(3):283-295
Lentiviral vectors (LVs) are capable of labeling a broad spectrum of cell types, achieving stable expression of transgenes. However, for in vivo studies, the duration of marker gene expression has been highly variable. We have developed a series of LVs harboring different promoters for expressing reporter gene in mouse cells. Long‐term culture and colony formation of several LV‐labeled mouse melanoma cells showed that promoters derived from mammalian house‐keeping genes, especially those encoding RNA polymerase II (Pol2) and ferritin (FerH), provided the highest consistency for reporter expression. For in vivo studies, primary B16BL6 mouse melanoma were infected with LVs whose luciferase–green fluorescence protein fusion gene (Luc/GFP) was driven by either Pol2 or FerH promoters. When transplanted into syngeneic C57BL/6 mice, Luc/GFP‐labeled B16BL6 mouse melanoma cells can be monitored by bioluminescence imaging in vivo, and GFP‐positive cells can be isolated from the tumors by fluorescence‐activated cell sorter. Pol2‐Luc/GFP labeling, while lower in activity, was more sustainable than FerH‐Luc/GFP labeling in B16BL6 over consecutive passages into mice. We conclude that Pol‐2‐Luc/GFP labeling allows long‐term in vivo monitoring and tumor cell isolation in immunocompetent mouse melanoma models. 相似文献
960.