全文获取类型
收费全文 | 3121篇 |
免费 | 208篇 |
国内免费 | 68篇 |
出版年
2023年 | 16篇 |
2022年 | 41篇 |
2021年 | 63篇 |
2020年 | 30篇 |
2019年 | 44篇 |
2018年 | 50篇 |
2017年 | 51篇 |
2016年 | 82篇 |
2015年 | 108篇 |
2014年 | 132篇 |
2013年 | 146篇 |
2012年 | 228篇 |
2011年 | 184篇 |
2010年 | 144篇 |
2009年 | 128篇 |
2008年 | 169篇 |
2007年 | 191篇 |
2006年 | 168篇 |
2005年 | 170篇 |
2004年 | 149篇 |
2003年 | 164篇 |
2002年 | 144篇 |
2001年 | 42篇 |
2000年 | 43篇 |
1999年 | 45篇 |
1998年 | 54篇 |
1997年 | 29篇 |
1996年 | 33篇 |
1995年 | 26篇 |
1994年 | 34篇 |
1993年 | 31篇 |
1992年 | 37篇 |
1991年 | 34篇 |
1990年 | 29篇 |
1989年 | 37篇 |
1988年 | 34篇 |
1987年 | 17篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1976年 | 16篇 |
1975年 | 10篇 |
1974年 | 9篇 |
排序方式: 共有3397条查询结果,搜索用时 328 毫秒
1.
The dynamical structure of the RNA in alfalfa mosaic virus studied by 31P-nuclear magnetic resonance
The structure of the viral RNA in alfalfa mosaic virus (AlMV) was investigated by means of 31P-nuclear magnetic resonance (NMR). It was found that the 31P-NMR line width of AlMV Top a particles is significantly smaller than that of the larger Bottom particles. At low temperatures, the totational correlation time of the 31P nuclei essentially equals the tumbling rate of the virus particle, indicating that the RNA is contained rigidly inside the virion. At more elevated temperatures, the NMR line width sharpens more than expected on the basis of viscosity changes and the RNA exhibits internal mobility. The occurrence of internal mobility is paralleled by an increased internal mobility of the N-terminal part of the coat protein, as could be observed by 1H-NMR spectroscopy. The influence of EDTA on the 31P-NMR line width appeared to be negligible, which is in agreement with the idea that AlMV does not 'swell' like several other RNA-containing plant viruses. 相似文献
2.
3.
4.
The collective redox activities of transition‐metal (TM) cations and oxygen anions have been shown to increase charge storage capacity in both Li‐rich layered and cation‐disordered rock‐salt cathodes. Repeated cycling involving anionic redox is known to trigger TM migration and phase transformation in layered Li‐ and Mn‐rich (LMR) oxides, however, detailed mechanistic understanding on the recently discovered Li‐rich rock‐salt cathodes is largely missing. The present study systematically investigates the effect of oxygen redox on a Li1.3Nb0.3Mn0.4O2 cathode and demonstrates that performance deterioration is directly correlated to the extent of oxygen redox. It is shown that voltage fade and hysteresis begin only after initiating anionic redox at high voltages, which grows progressively with either deeper oxidation of oxygen at higher potential or extended cycling. In contrast to what is reported on layered LMR oxides, extensive TM reduction is observed but phase transition is not detected in the cycled oxide. A densification/degradation mechanism is proposed accordingly which elucidates how a unique combination of extensive chemical reduction of TM and reduced quality of the Li percolation network in cation‐disordered rock‐salts can lead to performance degradation in these newer cathodes with 3D Li migration pathways. Design strategies to achieve balanced capacity and stability are also discussed. 相似文献
5.
6.
Toxin–antitoxin (TA) systems are small genetic elements that typically encode a stable toxin and its labile antitoxin. These cognate pairs are abundant in prokaryotes and have been shown to regulate various cellular functions. Vibrio cholerae, a human pathogen that is the causative agent of cholera, harbors at least thirteen TA loci. While functional HigBA, ParDE have been shown to stabilize plasmids and Phd/Doc to mediate cell death in V. cholerae, the function of seven RelBE-family TA systems is not understood. In this study we investigated the function of the RelBE TA systems in V. cholerae physiology and found that six of the seven relBE loci encoded functional toxins in E. coli. Deletion analyses of each relBE locus indicate that RelBE systems are involved in biofilm formation and reactive oxygen species (ROS) resistance. Interestingly, all seven relBE loci are induced under the standard virulence induction conditions and two of the relBE mutants displayed a colonization defect, which was not due to an effect on virulence gene expression. Although further studies are needed to characterize the mechanism of action, our study reveals that RelBE systems are important for V. cholerae physiology. 相似文献
7.
8.
Two structural genes on different chromosomes are required for encoding the major subunit of human red cell glucose-6-phosphate dehydrogenase 总被引:6,自引:0,他引:6
Structural analysis revealed the existence of two types of subunits in human red cell glucose-6-phosphate dehydrogenase. The two subunits have the same COOH region consisting of 479 amino acid residues, but their NH2-terminal regions are different in size and sequence. The minor subunit can be fully encoded by the X-linked G6PD cDNA, but the NH2-terminal region of the major subunit cannot. The cDNA and the gene for the NH2-terminal region of the major subunit were cloned and characterized. Southern blot hybridization indicated that the gene for the NH2-terminal region is on chromosome 6, not on the X chromosome. Northern blot hybridization demonstrated an existence of two separate mRNA components, one for the COOH-terminal region and the other for the NH2-terminal region. Two separate structural genes, the X-linked and chromosome 6-linked genes, must be coresponsible for encoding the single chain subunit. Either cross-translation of two mRNAs, or transpeptidation, or some other mechanism must be involved in the synthesis of human red cell G6PD. 相似文献
9.
Absorbance changes induced by 25-ps laser flashes were measured in membranes of Heliobacterium chlorum at 15 K. Absorbance difference spectra, measured at various times after the flash showed negative bands in the Qy region at 812, 793 and 665 nm. The first of these bands was attributed to the formation of excited singlet states of a long-wavelength form of antenna bacteriochlorophyll g (BChl g 808). Absorbance changes of shorter wavelength absorbing antenna BChls g were at least an order of magnitude smaller, indicating rapid excitation energy transfer (i.e. within the time resolution of the apparatus) from these BChls to BChl g 808. Excited BChl g 808 showed a bi-exponential decay with time constants of 50 and 200 ps. The bands at 793 and 665 nm may be attributed to the primary charge separation and reflect the photooxidation of the primary electron donor P-798 and photoreduction of a primary electron acceptor absorbing near 670 nm, presumably a BChl c or Chl a-like pigment. The bleaching of this pigment reversed with a time constant of 300 ps at 15 K and of 800 ps at 300 K. This indicates that electron transfer from the primary to the secondary electron acceptor is approximately 2.5 times faster at 15 K than at room temperature.Abbreviations BChl
bacteriochlorophyll
- FWHM
full width at half maximum
- P-798
primary electron donor
- Tris
tris(hydroxymethyl)amino methane 相似文献
10.
J. A. L. van Kan M. D. van de Rhee D. Zuidema B. J. C. Cornelissen J. F. Bol 《Plant molecular biology》1989,12(2):153-155
Two tobacco genes encoding thaumatin-like proteins were cloned and sequenced. Both genes are expressed after infection of tobacco with tobacco mosaic virus (TMV). Comparison of the upstream sequences of these genes with those of other TMV-inducible tobacco genes revealed limited regions of homology. 相似文献