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   523345篇
  免费   57430篇
  国内免费   1705篇
  2021年   3995篇
  2018年   10470篇
  2017年   10015篇
  2016年   10239篇
  2015年   9770篇
  2014年   10742篇
  2013年   14529篇
  2012年   19094篇
  2011年   23694篇
  2010年   16798篇
  2009年   15756篇
  2008年   19751篇
  2007年   21538篇
  2006年   13296篇
  2005年   14111篇
  2004年   13274篇
  2003年   12730篇
  2002年   12016篇
  2001年   22037篇
  2000年   22166篇
  1999年   17612篇
  1998年   6400篇
  1997年   6629篇
  1996年   6454篇
  1995年   5801篇
  1994年   5858篇
  1993年   5651篇
  1992年   13737篇
  1991年   13035篇
  1990年   12711篇
  1989年   12567篇
  1988年   11196篇
  1987年   10833篇
  1986年   9892篇
  1985年   9673篇
  1984年   8218篇
  1983年   7094篇
  1982年   5536篇
  1981年   5004篇
  1980年   4650篇
  1979年   7683篇
  1978年   5916篇
  1977年   5425篇
  1976年   5062篇
  1975年   5370篇
  1974年   5806篇
  1973年   5655篇
  1972年   5591篇
  1971年   5175篇
  1970年   3922篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
62.
The effect of the chromosomal ends of Tetrahymena thermophila on the stability of linear transforming molecules in the filamentous fungus Podospora anserina was tested. A derivative of an integrative vector for this fungus has been constructed, so that after linearization, the ends of the plasmid are the telomeric sequences of T. thermophila. After transformation, this linear molecule was maintained as an extrachromosomal plasmid with no integrated copies in about 50% of the transformants. Under selective conditions, there was approximately one linear molecule per 5 to 10 nuclei, and these extrachromosomal molecules were rapidly lost under nonselective conditions. The circular plasmid carrying an inverted repeat of T. thermophila telomeres could be linearized and processed in vivo.  相似文献   
63.
64.
65.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population.  相似文献   
66.
67.
68.
69.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory.  相似文献   
70.
β-Xylosidases are hydrolytic enzymes which play an important role in xylan degradation, hydrolyzing xylobiose and xylooligosaccharides to xylose from the non-reducing end. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they secrete β-xylosidases into the medium. Besides, fungal β-xylosidases are highly advantageous for their elevated activity levels and specificity. Interest in xylanolytic enzymes has been increasing, for their possible application in many biotechnological processes. This fact has driven the isolation, purification and characterization of several β-xylosidases. In this review, the mechanisms of action, substrate specificities, physicochemical characteristics, regulation at molecular level, molecular cloning and classification of filamentous fungal β-xylosidases are described. The potential industrial applications of fungal β-xylosidases will also be presented.  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