全文获取类型
收费全文 | 54713篇 |
免费 | 4694篇 |
国内免费 | 6643篇 |
专业分类
66050篇 |
出版年
2024年 | 172篇 |
2023年 | 720篇 |
2022年 | 1724篇 |
2021年 | 2861篇 |
2020年 | 2035篇 |
2019年 | 2548篇 |
2018年 | 2382篇 |
2017年 | 1829篇 |
2016年 | 2487篇 |
2015年 | 3539篇 |
2014年 | 4278篇 |
2013年 | 4498篇 |
2012年 | 5317篇 |
2011年 | 4849篇 |
2010年 | 3074篇 |
2009年 | 2773篇 |
2008年 | 3123篇 |
2007年 | 2793篇 |
2006年 | 2438篇 |
2005年 | 2036篇 |
2004年 | 1629篇 |
2003年 | 1540篇 |
2002年 | 1276篇 |
2001年 | 859篇 |
2000年 | 794篇 |
1999年 | 753篇 |
1998年 | 469篇 |
1997年 | 424篇 |
1996年 | 407篇 |
1995年 | 351篇 |
1994年 | 332篇 |
1993年 | 237篇 |
1992年 | 289篇 |
1991年 | 217篇 |
1990年 | 186篇 |
1989年 | 151篇 |
1988年 | 114篇 |
1987年 | 86篇 |
1986年 | 86篇 |
1985年 | 92篇 |
1984年 | 40篇 |
1983年 | 44篇 |
1982年 | 43篇 |
1981年 | 30篇 |
1980年 | 14篇 |
1979年 | 22篇 |
1978年 | 10篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1971年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
52.
The availability of DNA structural probes that can be applied to living cells is essential for the analysis of biological functions of unusual DNA structures adopted in vivo. We have developed a chemical probe assay to detect and quantitate left-handed Z-DNA structures in recombinant plasmids in growing E. coli cells. Potassium permanganate selectively reacts with B-Z or Z-Z junction regions in supercoiled plasmids harbored in the cells. Restriction enzyme recognition sites located at these junctions are not cleaved by the corresponding endonuclease after modification with KMnO4. This inhibition of cleavage allows the determination of the relative amounts of B- and Z-forms of the cloned inserts inside the cell. We have successfully applied this method to monitor the extent of Z-DNA formation in E. coli as a function of the growth phase and mutated topoisomerase or gyrase activities. The assay can in principle be used for any unusual DNA structure that contains a restriction recognition site inside or near the structural alteration. It can be a useful tool to analyze in vivo correlations between DNA structure and gene regulatory events. 相似文献
53.
54.
Ailiang Jiang 《International journal of biometeorology》1988,32(4):280-282
According to the author's and his collaborators' investigations, the climate influences the growth of rubber trees (Hevea brasiliensis) in Xishuangbanna, the southern part of Yunnan Province, China, in at least four aspects: (1) The yield of latex per tapping and the final yield of dry rubber per tree per year or per unit area per year; (2) the growth rate, as expressed by increment of girth in cm; (3) the survival during the over-wintering period; (4) the initiation or suppression of certain diseases; In this paper the author would like to describe the influence of climatic elements on yield of latex and on survival during the over-wintering period. As for the other two aspects, only general comments are given. 相似文献
55.
56.
Degradation of Dehydrodivanillin by Anaerobic Bacteria from Cow Rumen Fluid 总被引:7,自引:7,他引:0 下载免费PDF全文
Dehydrodivanillin (DDV; 0.15 g/liter) was biodegradable at 37°C under strictly anaerobic conditions by microflora from cow rumen fluid to the extent of 25% within 2 days in a yeast extract medium. The anaerobes were acclimated on DDV for 2 weeks, leading to DDV-degrading microflora with rates of degradation eight times higher than those initially. Dehydrodivanillic acid and vanillic acid were detected in an ethylacetate extract of a DDV-enriched culture broth by thin-layer, gas, and high-performance liquid chromatographies and by mass spectrometry. 相似文献
57.
The natural products of both eremofortin C (EC) and PR toxin are secondary metabolites of Penicillium roqueforti. Because the chemical structures of EC and PR toxin are closely related to each other and differ only by a hydroxyl functional group in EC and an aldehyde functional group in PR toxin at the C-12 position, the chemical transformation of EC into PR toxin was investigated. Oxidation with a chromic anhydride-pyridine complex was found to be the most satisfactory method. 相似文献
58.
Using recombinant DNA techniques, the covalent structure was determined for three flagellar filament proteins produced by Salmonella serotypes with phase-1 antigens a, c and d. Comparison of the results obtained, together with previous results for antigen i, indicated an overall structure in which conservation of amino acid sequence was absolute at both ends of the molecule and proceeded inwards with progressively greater variation. Very few differences in nucleotide sequence were detected in regions of amino acid conservation, which suggested that these areas of the gene may be involved in regulatory functions. 相似文献
59.
T6 DNA topoisomerase has been purified from bacteriophage T6 infected Escherichia coli. Unlike the T4 DNA topoisomerase which has three subunits, it consists of two subunits of molecular weights 75,000 and 51,000. They are the products of T6 genes 39 and 52, respectively. The purified T6 enzyme can stimulate in vitro T6 DNA replication. It has an ATP-dependent DNA relaxation activity similar to the T4 enzyme. Either ATP or dATP can be used in both reactions. Using a "Western blotting" and radioimmuno-detection methods, we show that T6 39 subunit contains protein sequences specified by both the T4 39 and 60 genes. The 52-proteins of both phages appear to be identical. The T4 and T6 topoisomerase genes represent a naturally occurring example of gene separation or fusion. 相似文献
60.