全文获取类型
收费全文 | 8483篇 |
免费 | 840篇 |
国内免费 | 6篇 |
专业分类
9329篇 |
出版年
2023年 | 52篇 |
2022年 | 121篇 |
2021年 | 245篇 |
2020年 | 159篇 |
2019年 | 187篇 |
2018年 | 192篇 |
2017年 | 165篇 |
2016年 | 234篇 |
2015年 | 383篇 |
2014年 | 429篇 |
2013年 | 522篇 |
2012年 | 601篇 |
2011年 | 592篇 |
2010年 | 349篇 |
2009年 | 320篇 |
2008年 | 420篇 |
2007年 | 424篇 |
2006年 | 368篇 |
2005年 | 373篇 |
2004年 | 354篇 |
2003年 | 313篇 |
2002年 | 308篇 |
2001年 | 108篇 |
2000年 | 75篇 |
1999年 | 96篇 |
1998年 | 103篇 |
1997年 | 57篇 |
1996年 | 44篇 |
1995年 | 47篇 |
1994年 | 49篇 |
1993年 | 56篇 |
1992年 | 67篇 |
1991年 | 50篇 |
1990年 | 46篇 |
1989年 | 57篇 |
1988年 | 42篇 |
1987年 | 55篇 |
1986年 | 46篇 |
1985年 | 39篇 |
1984年 | 56篇 |
1982年 | 56篇 |
1981年 | 51篇 |
1980年 | 48篇 |
1978年 | 51篇 |
1977年 | 42篇 |
1975年 | 47篇 |
1974年 | 41篇 |
1973年 | 46篇 |
1972年 | 36篇 |
1971年 | 45篇 |
排序方式: 共有9329条查询结果,搜索用时 0 毫秒
11.
Structure-function analysis of epidermal growth factor: site directed mutagenesis and nuclear magnetic resonance 总被引:2,自引:0,他引:2
The role of leucine-47 in determining the structure and activity of human epidermal growth factor was examined using site-directed mutagenesis. Wild type protein and four variants in which Leu47 was replaced by valine, glutamate, aspartate and alanine were produced from yeast. 1H NMR experiments demonstrated that substitution of Leu47 had little effect on the protein structure. The observed reduction in receptor binding affinity caused by the substitutions could thus be attributed to perturbation of a residue directly involved in receptor interactions. 相似文献
12.
Susan J. Harrison-Uy Julie A. Siegenthaler Andrea Faedo John L. R. Rubenstein Samuel J. Pleasure 《PloS one》2013,8(3)
We examined the role of the orphan nuclear hormone receptor CoupTFI in mediating cortical development downstream of meningeal retinoic acid signaling. CoupTFI is a regulator of cortical development known to collaborate with retinoic acid (RA) signaling in other systems. To examine the interaction of CoupTFI and cortical RA signaling we utilized Foxc1-mutant mice in which defects in meningeal development lead to alterations in cortical development due to a reduction of RA signaling. By analyzing CoupTFI−/−;Foxc1H/L double mutant mice we provide evidence that CoupTFI is required for RA rescue of the ventricular zone and the neurogenic phenotypes in Foxc1-mutants. We also found that overexpression of CoupTFI in Foxc1-mutants is sufficient to rescue the Foxc1-mutant cortical phenotype in part. These results suggest that CoupTFI collaborates with RA signaling to regulate both cortical ventricular zone progenitor cell behavior and cortical neurogenesis. 相似文献
13.
Samuel M. Imathiu Rumiana V. Ray Matthew I. Back Martin C. Hare Simon G. Edwards 《Journal of Phytopathology》2013,161(7-8):553-561
Fusarium langsethiae is a toxigenic fungal species that has been reported in European small‐grain cereal crops such as oats, wheat and barley. Although its relative contribution to fusarium head blight (FHB) symptoms is not well understood, it is reported to contaminate these cereals with high levels of HT‐2 and T‐2 trichothecenes mycotoxins that are currently under consideration for legislation by the European Commission. Ten commercial oat fields in Shropshire and Staffordshire (two adjacent counties in the Midlands) in the UK were surveyed in the 2006/2007 growing season. Samples were taken from predetermined field locations at Zadoks growth stages 32/33, 69, 77‐85 and 90‐92 for F. langsethiae biomass and HT‐2 and T‐2 toxins quantification. The results from this study showed that oats can be heavily infected with F. langsethiae and have high concentrations of HT‐2 and T‐2 toxins with no apparent FHB symptoms. The regression of HT‐2 + T‐2 toxins on F. langsethiae DNA concentration was highly significant (P < 0.001, r2 = 0.55). The results indicated that although F. langsethiae had no direct effect on crop yield, it may result in indirect economic losses where the grain can be rejected or downgraded as a result of intolerable levels of HT‐2 and T‐2 toxins, which are of human food and animal feed safety concern. The influence of cultural field practices on the infection and HT‐2 and T‐2 toxins accumulation in oats was not clear and warrants further studies to identify the sources of F. langsethiae inoculum and conditions favourable for infection and mycotoxin production. 相似文献
14.
Mutations in selC, which reduce the 8-base pair aminoacyl-acceptor helix to the canonical 7-base pair length (tRNA(Sec)(delAc] or which replace the extra arm of tRNA(Sec) by that of a serine acceptor tRNA species (tRNA(Sec)(ExS), block the function in selenoprotein synthesis in vivo (Baron, C., Heider, J., and B?ck, A. (1990) Nucleic Acids Res. 18, 6761-6766). tRNA(Sec), tRNA(Sec)(delAc), and tRNA(Sec)(ExS) were purified and analyzed for their interaction with purified seryl-tRNA synthetase, selenocysteine synthase and translation factors SELB and EF-Tu. It was found that seryl-tRNA synthetase displays 10-fold impaired Km and Kcat values for tRNA(Sec) in comparison to tRNA(Ser), decreasing the overall charging efficiency (Kcat/Km) of tRNA(Sec) to 1% of that characteristic for tRNA(Ser). tRNA(Sec)(ExS) was a less efficient substrate for the enzyme (Kcat/Km 0.2% of the tRNA(Ser) value) whereas the tRNA(Ser)(delAc) variant was charged with an approximately 2-3-fold improved rate compared to wild-type tRNA(Sec). Both mutant tRNA variants, when charged with L-serine, were able to interact with selenocysteine synthase to give rise to selenocysteyl-tRNA with tRNA(Sec)(ExS) being as efficient as wild-type tRNA(Sec). Seryl-tRNA(Sec)(delAc), on the other hand, was selenylated very slowly. Reduction of the length of the aminoacyl-acceptor stem to 7 base pairs prevented the interaction with translation factor SELB but allowed binding to EF-Tu, irrespective of whether tRNA(Sec)(delAc) was charged with serine or selenocysteine. The aminoacyl-acceptor helix of tRNA(Sec), therefore, is a major determinant directing binding to SELB and precluding interaction with EF-Tu. 相似文献
15.
Dennis T. Villareal Caroline M. Apovian Robert F. Kushner Samuel Klein 《Obesity (Silver Spring, Md.)》2005,13(11):1849-1863
Obesity causes serious medical complications and impairs quality of life. Moreover, in older persons, obesity can exacerbate the age‐related decline in physical function and lead to frailty. However, appropriate treatment for obesity in older persons is controversial because of the reduction in relative health risks associated with increasing body mass index and the concern that weight loss could have potential harmful effects in the older population. This joint position statement from the American Society for Nutrition and NAASO, The Obesity Society reviews the clinical issues related to obesity in older persons and provides health professionals with appropriate weight‐management guidelines for obese older patients. The current data show that weight‐loss therapy improves physical function, quality of life, and the medical complications associated with obesity in older persons. Therefore, weight‐loss therapy that minimizes muscle and bone losses is recommended for older persons who are obese and who have functional impairments or medical complications that can benefit from weight loss. 相似文献
16.
J. H. Baron 《BMJ (Clinical research ed.)》1988,297(6664):1651-1653
17.
18.
Reinhold Benesch Samuel Waxman Ruth Benesch Charles Baugh 《Biochemical and biophysical research communications》1982,106(4):1359-1363
Pteroyl polyglutamates bind to hemoglobin with an affinity which leaves only a minor fraction of red cell folate in the free state. This could serve as a regulating mechanism for protein synthesis in the differentiating erythroblast. 相似文献
19.
The sensitivity of target cells to natural killer (NK) cell-mediated cytotoxicity was investigated. Five target cell lines were examined for susceptibility to killing by activated NK cells in a 4-hour cytotoxicity assay: one of them (K562) was highly sensitive, while the other four were resistant. However, the four NK-resistant target cell lines were fully susceptible to lysis when the assay was extended to 24 h. The cytotoxic cells that killed the NK-resistant target cells in a 24-hour assay were plastic- and nylon wool-nonadherent human peripheral blood mononuclear cells (PBMC) and their cytotoxicity was increased by interferon-alpha, interferon-gamma, and interleukin-2. Further, the cytotoxic activity of PBMC in the long-term assay was associated with large granular lymphocytes purified on a Percoll gradient, that killed the NK-sensitive cell line K562 in a 4-hour assay. All of the above are general criteria to qualify the cytotoxic cells as NK cells. Thus, the NK-resistant phenotype may not reflect absolute immunity to NK-mediated lysis, but it may reflect the different rates at which various target cell lines can be killed. 相似文献
20.
Biosynthesis of dihydromenaquinone-9 by Mycobacterium phlei 总被引:2,自引:0,他引:2