全文获取类型
收费全文 | 5804篇 |
免费 | 470篇 |
国内免费 | 7篇 |
出版年
2023年 | 46篇 |
2022年 | 48篇 |
2021年 | 109篇 |
2020年 | 81篇 |
2019年 | 94篇 |
2018年 | 141篇 |
2017年 | 99篇 |
2016年 | 197篇 |
2015年 | 275篇 |
2014年 | 338篇 |
2013年 | 352篇 |
2012年 | 496篇 |
2011年 | 434篇 |
2010年 | 277篇 |
2009年 | 263篇 |
2008年 | 303篇 |
2007年 | 310篇 |
2006年 | 279篇 |
2005年 | 239篇 |
2004年 | 228篇 |
2003年 | 205篇 |
2002年 | 182篇 |
2001年 | 143篇 |
2000年 | 102篇 |
1999年 | 120篇 |
1998年 | 50篇 |
1997年 | 46篇 |
1996年 | 31篇 |
1995年 | 37篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 68篇 |
1991年 | 49篇 |
1990年 | 51篇 |
1989年 | 41篇 |
1988年 | 52篇 |
1987年 | 47篇 |
1986年 | 42篇 |
1985年 | 30篇 |
1984年 | 22篇 |
1983年 | 19篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 31篇 |
1978年 | 16篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 24篇 |
1972年 | 18篇 |
排序方式: 共有6281条查询结果,搜索用时 15 毫秒
991.
Tami Toroyan Ian Roberts Ann Oakley Gabrielle Laing Miranda Mugford Chris Frost 《BMJ (Clinical research ed.)》2003,327(7420):906
Objective To assess the effects of providing daycare facilities for young children on the health and welfare of disadvantaged families.Design Randomised controlled trial. Eligible children from the application list to a daycare facility were randomly allocated to receive a daycare place or not.Setting Early Years daycare centre in Borough of Hackney, London.Participants 120 mothers and 143 eligible children (aged between 6 months and 3.5 years).Intervention A place at the centre, which provided high quality day care. Control families used other child care that they secured for themselves.Main outcome measures Maternal paid employment, household income, child health and development.Results At 18 months'' follow up, 67% of intervention group mothers and 60% of control group mothers were in paid employment (adjusted risk ratio 1.23 (95% confidence interval 0.99 to 1.52)), but were no more likely to have a weekly household income of above £200 (risk ratio 0.88 (0.70 to 1.09)). Intervention group children had more otitis media with effusion (risk ratio 1.74 (1.02 to 2.96)) and used more health services (1.58 (1.05 to 2.38)), but both estimates were imprecise.Conclusion The provision of child day care may have increased maternal employment, but it did not seem to increase household income. The results suggest that providing day care may be insufficient as a strategy to reduce poverty. The study shows how random allocation can be used to ration and evaluate interventions where demand exceeds supply. 相似文献
992.
993.
Alexandre Keiji Tashima Marcel Ottens Luuk A.M. Van der Wielen Dennys E. Cintra José R. Pauli Pedro de Alcântara Pessôa Filho Everson Alves Miranda 《Biotechnology and bioengineering》2009,103(5):909-919
Recent works have pointed to the use of volatile electrolytes such as carbon dioxide (CO2) dissolved in aqueous solutions as a promising alternative to the precipitating agents conventionally used for protein recovery in the food and pharmaceutical industries. In this work we investigated experimental and theoretical aspects of the precipitation of porcine insulin, a biomolecule of pharmaceutical interest, using CO2 as an acid‐precipitating agent. The solubility of porcine insulin in NaHCO3 solutions in pressurized CO2 was determined as a function of temperature and pressure, with a minimum being observed close to the protein isoelectric point. A thermodynamic model was developed and successfully utilized to correlate the experimental data. Insulin was considered a polyelectrolyte in the model and its self‐association reactions were also taken into account. The biological activity of insulin was maintained after precipitation with CO2, although some activity can be lost if foam is formed in the depressurization step. Biotechnol. Bioeng. 2009;103: 909–919. © 2009 Wiley Periodicals, Inc. 相似文献
994.
995.
Analysis of individual remodeled nucleosomes reveals decreased histone–DNA contacts created by hSWI/SNF
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Karim Bouazoune Tina B. Miranda Peter A. Jones Robert E. Kingston 《Nucleic acids research》2009,37(16):5279-5294
Chromatin remodeling enzymes use the energy of ATP hydrolysis to alter histone–DNA contacts and regulate DNA-based processes in eukaryotes. Whether different subfamilies of remodeling complexes generate distinct products remains uncertain. We have developed a protocol to analyze nucleosome remodeling on individual products formed in vitro. We used a DNA methyltransferase to examine DNA accessibility throughout nucleosomes that had been remodeled by the ISWI and SWI/SNF families of enzymes. We confirmed that ISWI-family enzymes mainly created patterns of accessibility consistent with canonical nucleosomes. In contrast, SWI/SNF-family enzymes generated widespread DNA accessibility. The protection patterns created by these enzymes were usually located at the extreme ends of the DNA and showed no evidence for stable loop formation on individual molecules. Instead, SWI/SNF family proteins created extensive accessibility by generating heterogeneous products that had fewer histone–DNA contacts than a canonical nucleosome, consistent with models in which a canonical histone octamer has been ‘pushed’ off of the end of the DNA. 相似文献
996.
Rui-Cin Lyu Hui-Yu Hu Lih-Ying Kuo Huei-Fen Lo Ping-Lin Ong Hui-Ping Chang Long-Liu Lin 《Current microbiology》2009,59(2):101-106
Role of the conserved Thr399 and Thr417 residues of Bacillus licheniformis γ-glutamyltranspeptidase (BlGGT) was investigated by site-directed mutagenesis. Substitutions of Thr399 and Thr417 of BlGGT with Ser resulted in a dramatic reduction in enzymatic activity. A complete loss of the GGT activity was observed in T399A,
T399C, T417A, and T417K mutant enzymes. Furthermore, mutations on these two residues impaired the capability of autocatalytic
processing of the enzyme. In vitro maturation experiments showed that BlGGT mutant precursors, pro-T399S, pro-T417S, and pro-T417A, could precede a time-dependent autocatalytic process to generate
the 44.9- and 21.7-kDa subunits; however, the processed T417A had no enzymatic activity. Measurement of intrinsic tryptophan
fluorescence revealed alteration of the microenvironment of aromatic amino acid residues, while Far-UV circular dichroism
spectra were nearly identical for wild-type and mutant enzymes. These results suggest that residues Thr399 and Thr417 are
important for BlGGT in the enzymatic maturation and reaction.
An erratum to this article can be found at 相似文献
997.
Hsu-Feng Lu Jai-Sing Yang Kuang-Chi Lai Shu-Chun Hsu Shu-Ching Hsueh Yuan-Liang Chen Jo-Hua Chiang Chi-Cheng Lu Chyi Lo Mei-Due Yang Jing-Gung Chung 《Neurochemical research》2009,34(8):1491-1497
Curcumin is reported to be a potent inhibitor of the initiation and promotion of many cancer cells. We investigated to examine
whether or not curcumin induce DNA damage in mouse–rat hybrid retina ganglion cell line N18 cells. The Comet assay showed
that incubation of N18 cells with 10, 25 and 30 μM of curcumin led to a longer DNA migration smear (Comet tail). The DNA gel
electrophoresis showed that 20 μM of curcumin for 24 and 48 h treatment induced DNA damage and fragments in N18 cells. The
real time PCR analysis showed that 20 μM of curcumin for 48 h treatment decreased ATM, ATR, BRCA1, 14-3-3σ, DNA-PK and MGMT
mRNA, and ATM and MGMT mRNA expression were inhibited in a time-dependent manner. Our results indicate that curcumin caused
DNA damage and inhibited DNA repair genes which may be the factors for curcumin-inhibited cell growth.
H.-F. Lu and J.-S. Yang are contributed equally to this study. 相似文献
998.
Lai Hong Fu Yansong Miao Sze Wan Lo Tai Chi Seto Samuel S.M. Sun Zeng-Fu Xu Sabine Clemens Lorne A. Clarke Allison R. Kermode Liwen Jiang 《Plant science》2009,177(6):668-675
Lysosomal storage diseases (LSDs), that collectively represent over 50 disorders, are amenable to enzyme replacement therapies. However, the current methods used to commercially produce recombinant lysosomal enzymes for this purpose, most commonly Chinese Hamster Ovary cells and human fibroblasts, are prohibitively costly. Plant bioreactors hold great promise for economic production of functional human α-l-iduronidase (hIDUA; glycosaminoglycan α-l-iduronohydrolase; EC 3.2.1.76), the enzyme deficient in the human LSD, Mucopolysaccharidosis I. We have developed and tested an expression system using transgenic tobacco BY-2 cells to produce high amounts of active hIDUA. A plant signal peptide was essential for proper expression and secretion of the 78 kDa glycosylated hIDUA into the cultured media of transgenic BY-2 cells. The yield and activity of the secreted hIDUA from long-term cultures of transgenic BY-2 cell lines were as high as 10 μg/mL media and 53,000 pmol/min/mg proteins, respectively. Thus, this transgenic BY-2 cell line presents an attractive platform for economic production and easy downstream purification of hIDUA for enzyme replacement therapy. Furthermore, this system can be used for the production and purification of other human lysosomal enzymes or pharmaceuticals. 相似文献
999.
Miranda K. Hayworth Nels G. Johnson† Matthew E. Wilhelm† Robert P. Gove† Jackie D. Metheny & Olav Rueppell 《Ethology : formerly Zeitschrift fur Tierpsychologie》2009,115(7):698-706
Honey bee queens are exceptionally promiscuous. Early in life, queens perform one to five nuptial flights, mating with up to 44 drones. Many studies have documented potential benefits of multiple mating. In contrast, potential costs of polyandry and the sensitivity of queens to such costs have largely been ignored because they are difficult to address experimentally. To consider one aspect of mating costs to queens, the difficulty of flight, we compared flight behavior and success among a group of control queens and two experimental groups of queens that carried lead weights of two different sizes. For each queen, we assessed the number and duration of all flights and, after egg-laying commenced, the amount of stored sperm and the number of mates in terms of the offspring's patrilineal genetic diversity. Added weights quantitatively decreased the number of flights, the mean duration of flights and consequently the total time spent flying. Mating success in terms of sperm quantity and patrilines detected among the queens' offspring was also negatively impacted by the experimental manipulation. Thus, it can be concluded that the flight effort of honey bee queens during their mating period is adjusted in response to an experimentally increased cost of flying with multiple consequences for their mating success. Our results suggest that queen behavior is flexible and mating costs deserve more attention to explain the extreme polyandry in honey bees. 相似文献