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   107111篇
  免费   146931篇
  国内免费   41755篇
  2021年   2715篇
  2020年   2764篇
  2019年   4498篇
  2018年   3590篇
  2017年   3419篇
  2016年   3610篇
  2015年   4642篇
  2014年   5544篇
  2013年   5285篇
  2012年   6870篇
  2011年   7197篇
  2010年   7771篇
  2009年   12978篇
  2008年   7423篇
  2007年   6882篇
  2006年   5814篇
  2005年   5276篇
  2004年   4707篇
  2003年   3909篇
  2002年   4509篇
  2001年   5236篇
  2000年   2891篇
  1999年   7232篇
  1998年   9185篇
  1997年   9298篇
  1996年   8661篇
  1995年   8898篇
  1994年   8269篇
  1993年   7947篇
  1992年   7824篇
  1991年   7833篇
  1990年   8669篇
  1989年   7903篇
  1988年   7215篇
  1987年   6291篇
  1986年   5823篇
  1985年   5278篇
  1984年   4089篇
  1983年   3278篇
  1982年   3631篇
  1981年   3261篇
  1980年   3182篇
  1979年   3269篇
  1978年   2978篇
  1977年   2915篇
  1976年   2741篇
  1974年   2466篇
  1973年   2480篇
  1972年   2816篇
  1971年   2594篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
72.
73.
Many studies have examined the association between the FABP2 (rs1799883) Ala54Thr gene polymorphism and type 2 diabetes mellitus risk (T2DM) in various populations, but their results have been inconsistent. To assess this relationship more precisely, A HuGE review and meta‐analysis were performed. The PubMed and CNKI database was searched for case‐control studies published up to April 2014. Data were extracted and pooled odds ratios (OR) with 95% confidence intervals (CI) were calculated. Ultimately, 13 studies, comprising 2020 T2DM cases and 2910 controls were included. Overall, for the Thr carriers (Ala/Thr and Thr/Thr) versus the wild‐type homozygotes (Ala/Ala), the pooled OR was 1.18 (95% CI = 1.04–1.34, P = 0.062 for heterogeneity), for Thr/Thr versus Ala/Ala the pooled OR was 1.17 (95% CI = 1.05–1.41 P = 0.087 for heterogeneity). In the stratified analysis by ethnicity, the significantly risks were found among Asians but not Caucasians. This meta‐analysis suggests that the FABP2 (rs1799883) Ala54Thr polymorphisms are associated with increased susceptibility to T2DM risk among Asians but not Caucasians.  相似文献   
74.
Functional organization of the bacteriophage PRD1 genome.   总被引:5,自引:3,他引:2       下载免费PDF全文
  相似文献   
75.
76.
The ability of a series of DNA-damaging agents to induce homologous intrachromosomal recombination between duplicated genes in the chromosome of mouse cells was investigated. The target cells were the thymidine kinase-deficient mouse L-cell strain 333M, which contains a single integrated copy of a plasmid with two herpes simplex virus thymidine kinase (Htk) genes, each containing an 8-base-pair XhoI linker inserted at a unique site. Expression of a functional Htk enzyme requires a productive recombinational event between the two nonfunctional genes. The spontaneous rate of recombination in this strain is 3 per 10(6) cells per generation. The agents tested represent physical carcinogens (UV and ionizing radiation), a simple alkylating agent (N-methyl-N'-nitro-N-nitrosoguanidine), an alkylating cross-linking agent (mitomycin C), and a reactive metabolite of a polycyclic aromatic hydrocarbon ((+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene [BPDE] ). The background frequency of tk+ recombinants in the untreated population averaged 18 X 10(-6) +/- 5 X 10(-6). Ionizing radiation had little or no effect on recombination; exposure to mitomycin C, N-methyl-N'-nitro-N-nitrosoguanidine, BPDE, or UV, at doses that lowered the survival to between 90 and 10% of the control, caused a dose-dependent increase in frequency of recombinants, reaching 50 X 10(-6) to 100 X 10(-6). No tk+ cells could be generated with a control cell line that contained only one mutant copy of the Htk gene. Molecular hybridization analysis showed that 85 to 90% of the tk+ recombinants retained the Htk gene duplication, consistent with nonreciprocal transfer of wild-type genetic information, gene conversion. In the rest, only a single copy of the Htk gene remained, reflecting a single reciprocal exchange within a chromatid or a single unequal exchange between sister chromatids. Each recombinant tested contained an XhoI-resistant (wild-type) Htk gene.  相似文献   
77.
Saccharomyces cerevisiae CDC8 gene and its product.   总被引:14,自引:6,他引:8  
  相似文献   
78.
79.
80.
Extracts from the stems and leaves of Pitavia punctata Mol. were examined. The neutral fraction yielded β-sitosterol, daucosterin, quercetin, avicularin, and the previously undescribed quercetin 3-rhamnosylarabinoside. Braylin was co-extracted with the basic constituents, dictamnine, skimmianine and γ-fagarine. Acid hydrolysis of the leaves yielded cyanidin and delphinidin.  相似文献   
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