首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   525804篇
  免费   37684篇
  国内免费   98篇
  563586篇
  2018年   5483篇
  2017年   5326篇
  2016年   6976篇
  2015年   7932篇
  2014年   9929篇
  2013年   14189篇
  2012年   16133篇
  2011年   16736篇
  2010年   11585篇
  2009年   10230篇
  2008年   14810篇
  2007年   15456篇
  2006年   14603篇
  2005年   13918篇
  2004年   14098篇
  2003年   13602篇
  2002年   13430篇
  2001年   21915篇
  2000年   21714篇
  1999年   17242篇
  1998年   5843篇
  1997年   5822篇
  1996年   5541篇
  1995年   5245篇
  1994年   5192篇
  1993年   5004篇
  1992年   14291篇
  1991年   14084篇
  1990年   13914篇
  1989年   13675篇
  1988年   12772篇
  1987年   12125篇
  1986年   10981篇
  1985年   11275篇
  1984年   9211篇
  1983年   7917篇
  1982年   5931篇
  1981年   5178篇
  1980年   5001篇
  1979年   8969篇
  1978年   6831篇
  1977年   6305篇
  1976年   5989篇
  1975年   6718篇
  1974年   7412篇
  1973年   7422篇
  1972年   6746篇
  1971年   6317篇
  1970年   5382篇
  1969年   5238篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
Contrary to highly selected commercial breeds, indigenous domestic breeds are composed of semi-wild or feral populations subjected to reduced levels of artificial selection. As a consequence, many of these breeds have become locally adapted to a wide range of environments, showing high levels of phenotypic variability and increased fitness under natural conditions. Genetic analyses of three loci associated with milk production (alpha(S1)-casein, kappa-casein and prolactin) and the locus BoLA-DRB3 of the major histocompatibility complex indicated that the Argentinean Creole cattle (ACC), an indigenous breed from South America, maintains high levels of genetic diversity and population structure. In contrast to the commercial Holstein breed, the ACC showed considerable variation in heterozygosity (H(e)) and allelic diversity (A) across populations. As expected, bi-allelic markers showed extensive variation in He whereas the highly polymorphic BoLA-DRB3 showed substantial variation in A, with individual populations having 39-74% of the total number of alleles characterized for the breed. An analysis of molecular variance (AMOVA) of nine populations throughout the distribution range of the ACC revealed that 91.9-94.7% of the total observed variance was explained by differences within populations whereas 5.3-8.1% was the result of differences among populations. In addition, the ACC breed consistently showed higher levels of genetic differentiation among populations than Holstein. Results from this study emphasize the importance of population genetic structure within domestic breeds as an essential component of genetic diversity and suggest that indigenous breeds may be considered important reservoirs of genetic diversity for commercial domestic species.  相似文献   
5.
Intraperitoneal administration of marinobufagenin resulted in a reliable and dose-dependent suppression of ethanol self-administration in drug- and experimentally naive DBA/2 mice. The findings suggest that Na/K-ATPase contributes to both mediation of the ethanol reinforcing properties and the mood regulation.  相似文献   
6.
Methods of synthesis of coproporphyrin and uroporphyrin by using bacteria of the genus Arthrobacter are proposed. Metal complexes of coproporphyrin and uroporphyrin with Pt, Pd, and Zn were synthesized. Their structures were identified by spectrophotometry, IR spectrometry, 1H-NMR, mass spectrometry, and HPLC. Data showing the possibility to use coproporphyrin III-metal complexes as luminophores for fluorescence detection of tumors. The current and prospective uses of metal complexes of water-soluble natural porphyrins in advanced immunofluorescence assays are discussed.  相似文献   
7.
The Major Facilitator Superfamily lactose transport protein (LacS) undergoes reversible self-association in the detergent-solubilized state, and is present in the membrane as a dimer. We determined the functional unit for proton motive force (Deltap)-driven lactose uptake and lactose/methyl-beta-D-galactopyranoside equilibrium exchange in a proteoliposomal system in which a single cysteine mutant, LacS-C67, defective in Deltap-driven uptake, was co-reconstituted with fully functional cysteine-less protein, LacS-cl. From the quadratic relationship between the uptake activity and the ratio of LacS-C67/LacS-cl, we conclude that the dimeric state of LacS is required for Deltap-driven uptake. N-ethylmaleimide (NEM) treatment of proteoliposomes abolished the LacS-C67 exchange activity but left the LacS-cl unaffected. After NEM treatment, the exchange activity decreased linearly with increasing ratios of LacS-C67/LacS-cl, suggesting that the monomeric state of LacS is sufficient for this mode of transport. We propose that the two subunits of LacS are functionally coupled in the step associated with conformational reorientation of the empty binding site, a step unique for Deltap-driven uptake.  相似文献   
8.
PYCNOGENOL is an antioxidant phytochemical shown to have antiinflammatory activity in both the in vitro and in vivo models. This study compared the effects of chewing gums with and without PYCNOGENOL on gingival bleeding and plaque formation in 40 human subjects. In this double-blind study, subjects were assigned randomly to receive either control gums without PYCNOGENOL or experimental gums containng 5 mg PYCNOGENOL. Subjects used chewing gums for 14 days. Gingival bleeding and plaque scores were taken before and after the experiment. PYCNOGENOL chewing gums significantly reduced gingival bleeding, while no changes were noted in bleeding indexes in control subjects who used regular chewing gums. Subjects using regular control gums had significant increases of dental plaque accumulation during the two-week period. No increases in plaque accumulation were noted in subjects using PYCNOGENOL chewing gums. The data of this study suggest that the use of Pycnogenol chewing gums can minimize gingival bleeding and plaque accumulation.  相似文献   
9.
10.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号