全文获取类型
收费全文 | 34358篇 |
免费 | 3282篇 |
国内免费 | 4055篇 |
出版年
2024年 | 93篇 |
2023年 | 443篇 |
2022年 | 979篇 |
2021年 | 1774篇 |
2020年 | 1243篇 |
2019年 | 1563篇 |
2018年 | 1455篇 |
2017年 | 1095篇 |
2016年 | 1423篇 |
2015年 | 2201篇 |
2014年 | 2601篇 |
2013年 | 2799篇 |
2012年 | 3322篇 |
2011年 | 2938篇 |
2010年 | 1946篇 |
2009年 | 1769篇 |
2008年 | 1952篇 |
2007年 | 1775篇 |
2006年 | 1557篇 |
2005年 | 1353篇 |
2004年 | 1143篇 |
2003年 | 1007篇 |
2002年 | 835篇 |
2001年 | 570篇 |
2000年 | 477篇 |
1999年 | 474篇 |
1998年 | 311篇 |
1997年 | 258篇 |
1996年 | 237篇 |
1995年 | 199篇 |
1994年 | 189篇 |
1993年 | 139篇 |
1992年 | 162篇 |
1991年 | 171篇 |
1990年 | 126篇 |
1989年 | 127篇 |
1988年 | 117篇 |
1987年 | 97篇 |
1986年 | 93篇 |
1985年 | 105篇 |
1984年 | 73篇 |
1983年 | 60篇 |
1982年 | 50篇 |
1981年 | 44篇 |
1980年 | 29篇 |
1979年 | 39篇 |
1978年 | 31篇 |
1977年 | 28篇 |
1974年 | 27篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Cloning and DNA sequence of a plasmid-determined citrate utilization system in Escherichia coli. 总被引:13,自引:5,他引:8
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The citrate utilization determinant from a large 200-kilobase (kb) naturally occurring plasmid was previously cloned into the PstI site of plasmid vector pBR325 creating the Cit+ tetracycline resistance plasmid pWR61 (15 kb). Tn5 insertion mutagenesis analysis of plasmid pWR61 limited the segment responsible for citrate utilization to a 4.8-kb region bordered by EcoRI and PstI restriction nuclease sites. The 4.8-kb fragment was cloned into phage M13, and the DNA sequence was determined by the dideoxyribonucleotide method. Within this sequence was a 1,296-base-pair open reading frame with a preceding ribosomal binding site. The 431-amino-acid polypeptide that could be translated from this open reading frame would be highly hydrophobic. A second long open reading frame with the potential of encoding a 379-amino-acid polypeptide preceded the larger open reading frame. Portions of the 4.8-kb fragment were further subcloned with restriction endonucleases BglII and BamHI, reducing the minimum size needed for a citrate-positive phenotype to a 1.9-kb BamHI-BglII fragment (which includes the coding region for the 431-amino-acid polypeptide, but only the distal 2/3 of the reading frame for the 379-amino-acid polypeptide). Citrate utilization results from a citrate transport activity encoded by the plasmid. With the 4.8-kb fragment (as with larger fragments) the citrate transport activity was inducible by growth on citrate. On transfer from glucose, succinate, malate, or glycerol medium to citrate medium, the Cit+ Escherichia coli strains showed a delay of 36 to 48 h before growth. 相似文献
92.
93.
Yan Yongshan Qian Jin Xi Xiahui 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,70(5):529-535
Summary Two 30 g/ml BrdU-resistant sublines and two 60 g/ml BrdU-resistant sublines are induced from a Chinese hamster cell line Wg3h (HGPRT–) by one-step and two-step selections, respectively. By inoculating the cells into BrdU-free medium or by adding more BrdU into the culture medium for 26–27 h, it was found that the two BrdU-resistant sublines analysed have very clear sister chromatid differential (SCD) staining patterns. This indicates that some of the nuclear DNA of the BrdU-resistant cells incorporate with BrdU to reach a kinetic balance. Frequencies of sister chromatid exchange (SCE) of the resistant cells are twice to four times as high as those of the Wg3h cells, depending on which BrdU-resistant subline is analysed. The SCE frequencies of the resistant cells also increase with the BrdU concentration in the medium. Analysis of silver-stained nucleolar organizer regions (NORs) indicates that the NOR activity of three out of the four BrdU-resistant sublines is significantly suppressed, i.e., averages of the Ag-NOR number and number of the chromosomes bearing Ag-NORs per cell decrease significantly. The degree of suppression for different BrdU-resistant sublines may be quite different. The suppressed NOR activity of the resistant cells can gradually be restored when the cells are inoculated into BrdU-free medium, but the recovery speed is far lower than that of the Wg3h cells. The suppression of the NOR activity of the BrdU-resistant sublines should be due to BrdU toxicity. 相似文献
94.
95.
Summary A recently developed methodology of directly measuring the oxidation and incorporation patterns of carbon substrate in continuous cultures of RuMP-type methylotrophs is extended to batch cultures of the obligate methylotrophMethylomonas L3. The ratio of cyclic to total substrate oxidation varies with the initial methanol concentration from 0 to 68%. Formaldehyde, as a methanol cosubstrate, enhances the net substrate oxidation. The substrate oxidation and incorporation pattern is also affected by the state of the culture inoculum. 相似文献
96.
由开花前1—4天的向日葵子房中取出胚珠,在液体培养基上进行漂浮培养,诱导了未受精的卵细胞发育为单倍体的胚状体.亦诱导了珠被绒毡层产生胚状体。对两种胚状体的发生和发育过程及其形态发生特点作了显微观察与描述。 相似文献
97.
Are Chaoborus larvae more abundant in acidified than in non-acidified lakes in Central Canada? 总被引:2,自引:0,他引:2
Eriksson et al (1980) hypothesized that the abundance of certain macroinvertebrate predators, such as larvae of the phantom midge Chaoborus , should increase in acidified lakes because of the elimination of fish. To examine the influence of pH and presence of fish on Chaoborus abundance, we surveyed Chaoborus populations in 33 lakes in Ontario, Canada which ranged in pH from 4.5 to 7.4. Chaoborus larvae were not more abundant in the acidified lakes that were devoid of fish than in the remaining lakes. Therefore, we concluded that pH and presence of fish are not prime determinants of total Chaoborus abundance in Canadian Shield lakes. We hypothesized that significant increases in Chaoborus abundance should only be anticipated when fish populations are eliminated by acidification of relatively nutrient rich lakes. 相似文献
98.
99.
中国某些野生和栽培茶的核型研究 总被引:9,自引:0,他引:9
研究了茶的4个变种和广东野生毛叶茶等共12个材料的核型。所有材料的染色体数目均是2n=30,为二倍体。所有中国大叶变种(越南大叶除外)(Cametlia sinensis var. macrophylla)和阿萨姆大叶变种(C. sinensis var. assamica)均具比较对称或原始的“2A”核型;中国小叶变种(C.sinensis var.bohea)(“铁观音”品种除外),掸部变种(C.sinensis var. shan form)和广东野生毛叶茶(C. ptilophylla)均具较不对称或较进化的“2B”核型。根据核型特征,植物习性和地理分布,作者认为中国四川和云南可能是茶的起源中心,向东或北迁移,演变为中国小叶变种;向南移则演变为阿萨姆变种和掸部变种。 相似文献
100.