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91.
92.
Mutations can arise in static populations of cells that are subjected to nonlethal selective pressure, a phenomenon that has
been called ‘adaptive mutation’. This phenomenon has been extensively studied in FC40, a strain ofEscherichia coli that cannot metabolize lactose (Lac−) but that reverts to lactose utilization (Lac+) when lactose is its sole energy and carbon source. The adaptive Lac+ mutations arise by two mutational processes: a recombination-dependent process that is highly active on the episome carrying
the Lac− allele, and an unknown process that affects the whole genome. Most of the Lac+ mutations are due to the first process, which also produces nonselected mutations on the F′ episome. However, about 10% of
the Lac+ mutations arise in a subpopulation of cells that experience a period of transient hypermutation. Although minor contributors
to any one type of mutation, the hypermutators account for nearly all cases of multiple mutations. The evolutionary implications
of these results are: (i) DNA synthesis associated with recombination may be an important source of spontaneous mutation,
particularly in cells that are not actively growing; (ii) the efficient mutational mechanism that occurs on the episome could
result in the horizontal transfer of new alleles among species that carry and exchange conjugal plasmids; and (iii) a subpopulation
of transient hypermutators could be a source of multiple mutations that would allow for rapid adaptive evolution under adverse
conditions. 相似文献
93.
94.
GFP标记的肿瘤生长和转移的整体荧光成像 总被引:6,自引:0,他引:6
Fugene 6脂质体介导pEGFP-C1转染人源肺癌细胞(SPC-A1),经G418抗性筛选和96孔板有限稀释获得稳定高表达GFP的单克隆细胞株SPC-A1-EGFP。裸鼠腹腔注射SPC-A1-EGFP细胞建立自发转移模型;裸鼠尾静脉注射SPC-A1-EGFP细胞建立实验转移模型。利用整体光学成像系统(wllole-body optical imaging system)对荷瘤鼠整体荧光成像。结果表明,整体光学成像系统可实时非侵入监测腹腔肿瘤生长和扩散过程,通过胸腔皮瓣窗chest—wall skin-flap window)可低侵入检测肺转移。该研究为在体监测原位移植瘤的自发转移和发现抗肿瘤新药物提供了良好实验平台。 相似文献
95.
Hierarchically Structured 3D Integrated Electrodes by Galvanic Replacement Reaction for Highly Efficient Water Splitting 下载免费PDF全文
A NiFe‐based integrated electrode is fabricated by the spontaneous galvanic replacement reaction on an iron foam. Driven by the different electrochemical potentials between Ni and Fe, the dissolution of surface Fe occurs with electroless plating of Ni on iron foam with no need to access instrumentation and input energy. A facile cyclic voltammetry treatment is subsequently applied to convert the metallic NiFe to NiFeOx . A series of analytical methods indicates formation of a NiFeOx film of nanosheets on the iron foam surface. This hierarchically structured three dimensional electrode displays high activity and durability against water oxidation. In 1 m KOH, a current density of 1000 mA cm?2 is achieved at an overpotential of only 300 mV. This method is readily extended to fabricate CoFe or NiCoFe‐based integrated electrodes for water oxidation. Phosphorization of the bimetallic oxide (NiFeOx ) generates the bimetallic phosphide (NiFe‐P), which can act as an excellent electrocatalyst for hydrogen production in 1 m KOH. An alkaline electrolyzer is constructed using NiFeOx and NiFe‐P coated iron foams as anode and cathode, which can realize overall water splitting with a current density of 100 mA cm?2 at an overpotential of 630 mV. 相似文献
96.
Bhardwaj A Walker-Kopp N Wilkens S Cingolani G 《Protein science : a publication of the Protein Society》2008,17(9):1475-1485
A common objective in protein engineering is the enhancement of the thermodynamic properties of recombinant proteins for possible applications in nanobiotechnology. The performance of proteins can be improved by the rational design of chimeras that contain structural elements with the desired properties, thus resulting in a more effective exploitation of protein folds designed by nature. In this paper, we report the design and characterization of an ultra-stable self-refolding protein fiber, which rapidly reassembles in solution after denaturation induced by harsh chemical treatment or high temperature. This engineered protein fiber was constructed on the molecular framework of bacteriophage P22 tail needle gp26, by fusing its helical core to the foldon domain of phage T4 fibritin. Using protein engineering, we rationally permuted the foldon upstream and downstream from the gp26 helical core and characterized gp26-foldon chimeras by biophysical analysis. Our data demonstrate that one specific protein chimera containing the foldon immediately downstream from the gp26 helical core, gp26(1-140)-F, displays the highest thermodynamic and structural stability and refolds spontaneously in solution following denaturation. The gp26-foldon chimeric fiber remains stable in 6.0 M guanidine hydrochloride, or at 80 degrees C, rapidly refolds after denaturation, and has both N and C termini accessible for chemical/biological modification, thereby representing an ideal platform for the design of self-assembling nanoblocks. 相似文献
97.
98.
This study aimed to define the karyotype of the recently described Iberian endemic Iberochondrostoma almacai, to revisit the previously documented chromosome polymorphisms of its sister species I.lusitanicum using C-, Ag-/CMA(3) and RE-banding, and to compare the two species genome sizes. A 2n = 50 karyotype (with the exception of a triploid I.lusitanicum specimen) and a corresponding haploid chromosome formula of 7M:15SM:3A (FN = 94) were found. Multiple NORs were observed in both species (in two submetacentric chromosome pairs, one of them clearly homologous) and a higher intra and interpopulational variability was evidenced in I.lusitanicum. Flow cytometry measurements of nuclear DNA content showed some significant differences in genome size both between and within species: the genome of I. almacai was smaller than that of I.lusitanicum (mean values 2.61 and 2.93 pg, respectively), which presented a clear interpopulational variability (mean values ranging from 2.72 to 3.00 pg). These data allowed the distinction of both taxa and confirmed the existence of two well differentiated groups within I. lusitanicum: one that includes the populations from the right bank of the Tejo and Samarra drainages, and another that reunites the southern populations. The peculiar differences between the two species, presently listed as "Critically Endangered", reinforced the importance of this study for future conservation plans. 相似文献
99.
The equilibrium theory for lipid membranes is used to describe the structure of nuclear pores and the membrane shapes accompanying
endocytosis. The commonly used variant of the theory contains a fixed parameter called the spontaneous curvature which accounts
for asymmetry in the bending response of the membrane. This is replaced here by a variable distribution of spontaneous curvature
representing the influence of attached proteins. The required adjustments to the standard theory are described and the resulting
model is applied to the study of membrane morphology at the cites of protein-assisted nuclear pore formation and endocytosis. 相似文献
100.