全文获取类型
收费全文 | 48618篇 |
免费 | 4137篇 |
国内免费 | 7篇 |
出版年
2023年 | 240篇 |
2021年 | 385篇 |
2020年 | 333篇 |
2019年 | 330篇 |
2018年 | 929篇 |
2017年 | 956篇 |
2016年 | 1067篇 |
2015年 | 980篇 |
2014年 | 1154篇 |
2013年 | 1992篇 |
2012年 | 3184篇 |
2011年 | 3518篇 |
2010年 | 1835篇 |
2009年 | 1226篇 |
2008年 | 2888篇 |
2007年 | 2947篇 |
2006年 | 2741篇 |
2005年 | 2514篇 |
2004年 | 2412篇 |
2003年 | 2250篇 |
2002年 | 2241篇 |
2001年 | 1594篇 |
2000年 | 1791篇 |
1999年 | 1036篇 |
1998年 | 495篇 |
1997年 | 405篇 |
1996年 | 468篇 |
1995年 | 407篇 |
1994年 | 432篇 |
1993年 | 383篇 |
1992年 | 525篇 |
1991年 | 441篇 |
1990年 | 426篇 |
1989年 | 399篇 |
1988年 | 371篇 |
1987年 | 378篇 |
1986年 | 338篇 |
1985年 | 407篇 |
1984年 | 455篇 |
1983年 | 403篇 |
1982年 | 367篇 |
1981年 | 336篇 |
1980年 | 305篇 |
1979年 | 314篇 |
1978年 | 303篇 |
1977年 | 286篇 |
1976年 | 279篇 |
1975年 | 336篇 |
1974年 | 273篇 |
1973年 | 256篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
42.
Chromosomal location and cloning of the gene (trmD) responsible for the synthesis of tRNA (m1G) methyltransferase in Escherichia coli K-12 总被引:5,自引:0,他引:5
Summary The trmD gene, which governs the formation of 1-methyl-guanosine (m1G) in transfer ribonucleic acid (tRNA), has been located by phage P1 transduction at 56 min on the chromosomal map of Escherichia coli. Cotransduction to tyrA at 56 min is 80%. From the Clarke and Carbon collection a ColE1-tyrA
+ hybrid plasmid was isolated, which carried the trmD
+ gene and was shown to over-produce the tRNA (m1G)methyltransferase. By subcloning restriction enzyme fragments in vitro, the trmD
+ gene was located to a 3.4 kb DNA fragment 6.5 kb clockwise from the tyrA
+ gene. The mutation trmD1, which renders the tRNA (m1G) methyltransferase temperaturesensitive both in vivo and in vitro could be complemented by trmD
+ plasmids. These results suggest that the gene trmD
+ is the structural gene for the tRNA (m1G)methyltransferase (EC 2.1.1.3.1). 相似文献
43.
The membrane topology of proton-pumping nicotinamide-nucleotide transhydrogenase from Escherichia coli was determined by site-specific chemical labeling. A His-tagged cysteine-free transhydrogenase was used to introduce unique cysteines in positions corresponding to potential membrane loops. The cysteines were reacted with fluorescent reagents, fluorescein 5-maleimide or 2-[(4'-maleimidyl)anilino]naphthalene-6-sulfonic acid, in both intact cells and inside-out vesicles. Labeled transhydrogenase was purified with a small-scale procedure using a metal affinity resin, and the amount of labeling was measured as fluorescence on UV-illuminated acrylamide gels. The difference in labeling between intact cells and inside-out vesicles was used to discriminate between a periplasmic and a cytosolic location of the residues. The membrane region was found to be composed of 13 helices (four in the alpha-subunit and nine in the beta-subunit), with the C terminus of the alpha-subunit and the N terminus of the beta-subunit facing the cytosolic and periplasmic sides, respectively. These results differ from previous models with regard to both number of helices and the relative location and orientation of certain helices. This study constitutes the first in which all transmembrane segments of transhydrogenase have been experimentally determined and provides an explanation for the different topologies of the mitochondrial and E. coli transhydrogenases. 相似文献
44.
A bacterial clone carrying sequences coding for elongation factor EF-1 alpha from Artemia 总被引:2,自引:0,他引:2
A bacterial cDNA clone was identified carrying one third of the nucleotides coding for elongation factor EF-1 alpha from the brine shrimp Artemia. The sequence of codons corresponds with the known sequence of amino acids of EF-1 alpha in the region involved. 相似文献
45.
H. Wiksell V. Ekstrand C. Wadström G. Auer 《Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)》2009,25(3):128-132
A large and increasing number of women in the western world will at some point during their life be investigated morphologically for some type of breast lesion. Fine Needle Aspiration (FNA) is one morphological method which is considered to be the fastest, cheapest and the most patient-friendly approach. However, the frequency of conclusive samples using this method varies and is often too low, especially when performed by unexperienced operators. In this study we have developed and tested a new semi-automated instrument (“CytoTest”) designed for FNA which is intended to improve the efficacy of the technique by increasing the percentage of conclusive samples. A total of 443 consecutive aspiration procedures on palpable breast lesions were performed to compare this new “CytoTest” equipment with the standard protocol using the same type of needles. We conclude that by increasing the extent and frequency of the reciprocatory motions used by an experienced sampling operator as well as enhancing the ejection pressure, the cellular yield can be increased almost three folded compared to the standard protocol. For cases with high amounts of non-diagnostic material (such as blood or cystic fluid) which were discarded, up to four times more sample could be obtained. Furthermore, the frequency of sparse samples under 1 mg was halved with use of the “CytoTest”. 相似文献
46.
47.
48.
49.
50.
Plant Reactions to Inoculation of Roots with Fungi and Bacteria 总被引:1,自引:0,他引:1
The potential of 120 isolates of fungi and bacteria from plant rhizospheres to interfere with plant development and growth was studied in greenhouse experiments. The pure cultured isolates were obtained from plant roots in the field and applied as suspensions to the roots of eight test plant species. 10–20% of the isolates caused distinct symptoms on shoots, growth retardations without other symptoms or growth promotions. Responses of treated plants ranged from death of plants soon after treatment to up to about 40% higher shoot fresh weight than in control plants. Two bacterial isolates induced strong reactions in most of the plant species tested while other isolates showed a more or less pronounced specificity by giving reactions in only some of the plant species tested. 相似文献