全文获取类型
收费全文 | 397029篇 |
免费 | 32760篇 |
国内免费 | 275篇 |
出版年
2021年 | 3724篇 |
2019年 | 3378篇 |
2018年 | 6501篇 |
2017年 | 6139篇 |
2016年 | 8270篇 |
2015年 | 10493篇 |
2014年 | 11671篇 |
2013年 | 16088篇 |
2012年 | 18593篇 |
2011年 | 18490篇 |
2010年 | 12276篇 |
2009年 | 10032篇 |
2008年 | 15433篇 |
2007年 | 15434篇 |
2006年 | 14492篇 |
2005年 | 13461篇 |
2004年 | 12993篇 |
2003年 | 12233篇 |
2002年 | 11658篇 |
2001年 | 15154篇 |
2000年 | 15044篇 |
1999年 | 11757篇 |
1998年 | 3853篇 |
1997年 | 3669篇 |
1996年 | 3533篇 |
1995年 | 3316篇 |
1994年 | 3188篇 |
1993年 | 2962篇 |
1992年 | 8793篇 |
1991年 | 8443篇 |
1990年 | 8373篇 |
1989年 | 8092篇 |
1988年 | 7519篇 |
1987年 | 7053篇 |
1986年 | 6296篇 |
1985年 | 6653篇 |
1984年 | 5416篇 |
1983年 | 4608篇 |
1982年 | 3283篇 |
1981年 | 3033篇 |
1979年 | 4856篇 |
1978年 | 3741篇 |
1977年 | 3391篇 |
1976年 | 3277篇 |
1975年 | 3839篇 |
1974年 | 4104篇 |
1973年 | 4148篇 |
1972年 | 3548篇 |
1971年 | 3320篇 |
1970年 | 2988篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
Morphological pathway of flagellar assembly in Salmonella typhimurium. 总被引:14,自引:0,他引:14
The process of flagellar assembly was investigated in Salmonella typhimurium. Seven types of flagellar precursors produced by various flagellar mutants were purified by CsCl density gradient protocol. They were characterized morphologically by electron microscopy, and biochemically by two-dimensional gel electrophoresis. The MS ring is formed in the absence of any other flagellar components, including the switch complex and the putative export apparatus. Four proteins previously identified as rod components, FlgB, FlgC, FlgF, FlgG, and another protein, FliE, assemble co-operatively into a stable structure. The hook is formed in two distinct steps; formation of its proximal part and elongation. Proximal part formation occurs, but elongation does not occur, in the absence of the LP ring. FlgD is necessary for hook formation, but not for LP-ring formation. A revised pathway of flagellar assembly is proposed based on these and other results. 相似文献
972.
Using gas chromatography, the authors studied the development of the proportion of 25 saturated and non-saturated fatty acids in the plasma of Wistar rats of their own breed aged 5, 10, 14 and 25 days and adult (both sexes), and in three age groups (5 and 14 day and adult) they determined the effect of acute altitude hypoxia. Fatty acids were determined as methylesters (FAME) (Base 1978), using the method already described by Smídová et al. (1984a), in mixed blood plasma obtained from decapitated control and experimental animals. Altitude hypoxia, which corresponded to 9 000 m, lasted 20 min. The temperature was kept stable at 25 degrees C. Marked changes occurred during ontogenesis in the proportion of total fatty acids (TFA) in the plasma; the proportion of sigma non-saturated FA fell significantly during maturation, chiefly in favour of monoene FA, with a resultant significant drop in their reciprocal index (sigma saturated/sigma monoene). The proportion of all saturated FA amounted in 5-day-old rats to 54% of the total quantity, but in adult rats to only 40%. Acute hypoxia had a demonstrable effect in all three age categories. In 5-day-old rats it led to significant changes in 7 FA and in adult rats in 10. In both 5-day-old and adult animals it produced an increase in the arachidonic acid level. The results are discussed in relation to previous findings on the effect of the same hypoxia on FA proportion in various parts of the CNS of rats of different ages. 相似文献
973.
The myc family of proto-oncogenes consists of several members that possess regions of sequence homology and some have known similarities in structure and function. We have isolated an 8.8-kb EcoRI fragment from a human genomic library by hybridization to a 28-base oligonucleotide probe derived from a region of the second exon of MYC, which is highly conserved in the myc gene family. Sequence analysis of this myc-like (MYCLK1) DNA fragment has revealed the existence of a region with 85% homology to the 28-base oligonucleotide probe. An open reading frame of 207 nucleotides containing the region of homology was found. We have mapped MYCLK1 to human chromosome 7 at band p15 by chromosome in situ hybridization; this site is distinct from the map location of previously characterized myc genes. Whether MYCLK1 represents a new functional member of the myc family of proto-oncogenes remains to be determined. 相似文献
974.
975.
976.
977.
R. Brändén T. Nilsson S. Styring 《Biochemical and biophysical research communications》1980,92(4):1297-1305
A sensitive and reliable method to determine the stereochemical composition of 3-phosphoglyceric acid is presented. Results obtained with this method show that 3-phosphoglyceric acid formed in the ribulose-1,5-bisphosphate carboxylase reaction is a mixture of 10% L-3-PGA and 90% D-3-PGA. 相似文献
978.
979.
The activity of superoxide dismutase (SOD) from bovine erythrocytes was measured by the inhibition of nitrotetrazolium blue reduction rate in superoxide anion radical generation systems--xanthine/xanthine oxidase of NADH/phenazine methasulfate. The enzyme activity increases in the presence of compounds acting as electron donors in radical-involving reactions and decreased in the presence of compounds possessing the properties of electron acceptors. Activation of SOD by electron donors and its inhibition by electron acceptors was dependent on the concentration of the above compounds. In the absence of SOD electron donors and acceptors did not change the rate of tetrazolium blue reduction by superoxide anion radicals. The role of the new type of SOD regulation for the enzyme functioning in the cell is discussed. 相似文献
980.
A comparison of liquid-holding recovery and photoreactivation in halophilic and non-halophilic bacteria 总被引:2,自引:0,他引:2
The ability of the extreme halophile Halobacterium cutirubrum to recover from the effects of ultraviolet radiation during liquid holding in the dark in non-nutrient medium has been compared with that of (i) a moderately halophilic bacterium (NRC 41227) and (ii) Escherichia coli B. The photoreactivabilities of all three bacteria have also been studied. The extreme halophile was incapable of liquid-holding recovery in these conditions, in marked contrast to both E. coli B and the moderate halophile, and also failed to recover when held in nutrient medium in the dark. These results strongly support the hypothesis that H. cutirubrum lacks DNA excision repair. It was also found that ultraviolet-irradiated H. cutirubrum could be almost completely photoreactivated from any level of survival in the range 10(-4)-80%, provided exposure to visible light was not delayed, whereas the moderate halophile resembled E. coli B and had a comparatively limited capacity for photoreactivation. 相似文献