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Cholesterol requirement for growth of mycoplasmas was tested in a serum-free medium supplemented with albumin, l-arginine, palmitic acid, and various concentrations of cholesterol dissolved in Tween 80. In cases in which Tween 80 was shown to inhibit growth, the test medium was supplemented with cholesterol dissolved in ethanol. Of the 31 species examined, all but Mycoplasma laidlawii, M. granularum, and Mycoplasma species strain S-743 exhibited a growth response to cholesterol. No requirement for cholesterol could be shown with the stable L-phase variants of Streptobacillus moniliformis and Proteus species. The results provide experimental support for the view that the large majority of the established Mycoplasma species require cholesterol for growth. 相似文献
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Formaldehyde fixation of cells is routinely used to study DNA-protein interactions in vivo. In these studies, DNA is often analyzed using a polymerase chain reaction technique. Although it is known that formaldehyde can damage DNA, no studies have been performed so far to compare the efficiency of DNA amplification between normal and fixed cells. Here we show that formaldehyde fixation results in a 15% to 20% reduction in the ability to amplify cellular DNA. The loss of amplifiability is independent of the length of the amplification region and the degree to which DNA is compacted on packaging into chromatin. 相似文献
4.
DNA fragments which specifically bind to isolated nuclear matrix in vitro interact with matrix-associated DNA topoisomerase II 总被引:2,自引:0,他引:2
Y S Vassetzky S V Razin G P Georgiev 《Biochemical and biophysical research communications》1989,159(3):1263-1268
A matrix-associated region (MAR)-containing fragment has been selected from the library of cloned chicken nuclear matrix-associated DNA fragments. Factors, which determine the specific binding of DNA fragments have been studied. Using topoisomerase II-specific inhibitor VM 26 we established that nuclear matrix-associated topoisomerase II interacted with the MAR-containing DNA fragment producing specific cleavage sites on DNA of the fragment. 相似文献
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Biological role of DNA methylation: sequence-specific single-strand breaks associated with hypomethylation of GATC sites in Escherichia coli DNA. 下载免费PDF全文
The effect of methylation of GATC sites in Escherichia coli DNA on the formation of single-strand breaks was studied with dam+, dam mutant, and Dam-overproducer strains. Single-strand breaks have been observed in dam mutant cells predominantly at TpT and, to a lesser extent, at CpC. In dam mutant cells harboring pTP166 (a plasmid containing the dam gene), no such nicks were observed. 相似文献
8.
M. T. Tosteson S. J. Holmes M. Razin D. C. Tosteson 《The Journal of membrane biology》1985,87(1):35-44
Summary This paper describes experiments designed to explore interactions between human red blood cell membranes and melittin, the main component of bee venom. We found that melittin binds to human red cell membranes suspended in isotonic NaCl at room temperature, with an apparent dissociation constant of 3×10–8
m and maximum binding capacity of 1.8×107 molecules/cell. When about 1% of the melittin binding sites are occupied, cell lysis can be observed, and progressive, further increases in the fraction of the total sites occupied lead to progressively greater lysis in a graded manner. 50% lysis occurs when there are about 2×106 molecules bound to the cell membrane. For any particular extent of melittin binding, lysis proceeds rapidly during the first few minutes but then slows and stops so that no further lysis occurs after one hour of exposure of cells to melittin. The graded lysis of erythrocytes by melittin is due to complete lysis of some of the cells, since both the density and the hemoglobin content of surviving, intact cells in a suspension that has undergone graded melittin lysis are similar to the values observed in the same cells prior to the addition of melittin. The cells surviving graded melittin lysis have an increased Na and reduced K, proportional to the extent of occupation of the melittin binding sites. Like lysis, Na accumulation and K loss proceed rapidly during the first few minutes of exposure to melittin but then stops so that Na, K and hemoglobin content of the cells remain constant after the first hour. These kinetic characteristics of both lysis and cation movements suggest that melittin modifies the permeability of the red cell membrane only for the first few minutes after the start of the interaction. Direct observation of cells by Nomarsky optics revealed that they crenate, become swollen and lyse within 10 to 30 sec after these changes in morphology are first seen. Taken together, these results are consistent with the idea that melittin produces lysis of human red cells at room temperature by a colloid osmotic mechanism. 相似文献
9.
Sequence and substrate specificity of isolated DNA methylases from Escherichia coli C 总被引:9,自引:5,他引:4 下载免费PDF全文
Two DNA methylase activities of Escherichia coli C, the mec (designates DNA-cytosine-methylase gene, which is also designated dcm) and dam gene products, were physically separated by DEAE-cellulose column chromatography. The sequence and substrate specificity of the two enzymes were studied in vitro. The experiments revealed that both enzymes show their expected sequence specificity under in vitro conditions, methylating symmetrically on both DNA strands. The mec enzyme methylates exclusively the internal cytosine residue of CCATGG sequences, and the dam enzyme methylates adenine residues at GATC sites. Substrate specificity experiments revealed that both enzymes methylate in vitro unmethylated duplex DNA as efficiently as hemimethylated DNA. The results of these experiments suggest that the methylation at a specific site takes place by two independent events. A methyl group in a site on one strand of the DNA does not facilitate the methylation of the same site on the opposite strand. With the dam methylase it was found that the enzyme is incapable of methylating GATC sites located at the ends of DNA molecules. 相似文献
10.
Mycoplasma gallisepticum cells were found to contain two different sugar transport systems, one for d-glucose and alpha-methyl-d-glucoside (alpha-MG) and the other for d-mannose and d-fructose. Both systems were noninducible, stereospecific, dependent on temperature and pH, and sensitive to sulfhydryl-blocking reagents. The rate of sugar uptake depended on its external concentration, obeying Michaelis-Menten kinetics. The sugar accumulated in the cells against a concentration gradient, and an energy requirement for accumulation was demonstrated with alpha-MG. Both transport systems thus meet the criteria of active transport. The exit of alpha-MG from the cells, like its entry, depended on temperature and was accelerated by energy supplied by the oxidizable d-mannose. d-Glucose accelerated alpha-MG exit, apparently by an exchange reaction. A method for measuring the intercellular space and intracellular free-water volume of Mycoplasma was devised, and several of its applications are described. 相似文献