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11.
Gel formation by fibrin oligomers without addition of monomers 总被引:1,自引:0,他引:1
Soluble fibrin oligomers were formed by reacting fibrinogen with thrombin under fine clotting conditions where the action of thrombin is the rate-determining step for polymerization, and by inhibiting the reaction shortly before gelation. Oligomeric fibrin was separated from unreacted fibrinogen and small oligomers by gel permeation chromatography. Electron microscopy revealed that the largest soluble fibrin oligomers resemble the protofibrils present in fine clots, but are somewhat shorter and entirely lack the twisted, trifunctional junctions that contribute to the elastic properties of fine clots. When thrombin was added to the soluble fibrin oligomers, polymerization resumed and clots were formed at a more rapid rate than from fibrinogen at the same concentration and resulted in a less-opaque clot under coarse clotting conditions. The results confirm a prediction of a theory for the polymerization of fibrin and provide additional evidence that the final state of a coarse fibrin clot depends on the mobility of protofibrils during its formation. 相似文献
12.
Production and Consumption of H2 during Growth of Methanosarcina spp. on Acetate 总被引:10,自引:6,他引:4
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Methanosarcina sp. strain TM-1 and Methanosarcina acetivorans produced and consumed H2 to maintain H2 partial pressures of 16 to 92 Pa in closed cultures during growth on acetate. Strain TM-1 produced H2 continuously when H2 was continuously removed from the culture. The potential physiological significance of H2 in acetate metabolism to methane is discussed. 相似文献
13.
14.
Maintenance of the cellobiose utilization genes of Escherichia coli in a cryptic state 总被引:6,自引:1,他引:5
The genes for cellobiose utilization are normally cryptic in Escherichia
coli. The cellobiose system was used as a model to understand the process
by which silent genes are maintained in microbial populations. Previously
reported was (1) the isolation of a mutant strain that expresses the
cellobiose-utilization (Cel) genes and (2) that expression of those genes
allows utilization of three beta- glucoside sugars: cellobiose, arbutin,
and salicin. The Cel gene cluster has now been cloned from that mutant
strain. In the course of locating the Cel genes within the cloned DNA
segment, it was discovered that inactivation of the Cel-encoded hydrolase
rendered the host strain sensitive to all three beta-glucosides as potent
inhibitors. This sensitivity arises from the accumulation of the
phosphorylated beta- glucosides. Because even the fully active genes
conferred some degree of beta-glucoside sensitivity, the effects of
cellobiose on a series of five Cel+ mutants of independent origin were
investigated. Although each of those strains utilizes cellobiose as a sole
carbon and energy source, cellobiose also acts as a potent inhibitor that
reduces the growth rate on glycerol 2.5-16.5-fold. On the other hand,
wild-type strains that cannot utilize cellobiose are not inhibited. The
observation that the same compound can serve either as a nutrient or as an
inhibitor suggests that, under most conditions in which cellobiose will be
present together with other resources, there is a strong selective
advantage to having the cryptic (Cel0) allele. In those environments in
which cellobiose is the sole, or the best, resource, mutants that express
the genes (Cel+) will have a strong selective advantage. It is suggested
that temporal alternation between these two conditions is a major factor in
the maintenance of these genes in E. coli populations. This alternation of
environments and fitnesses was predicted by the model for cryptic-gene
maintenance that was previously published.
相似文献
15.
16.
Addition of telomere-associated HeT DNA sequences "heals" broken chromosome ends in Drosophila 总被引:21,自引:0,他引:21
H Biessmann J M Mason K Ferry M d'Hulst K Valgeirsdottir K L Traverse M L Pardue 《Cell》1990,61(4):663-673
Stocks of D. melanogaster X chromosomes carrying terminal deletions (RT chromosomes) have been maintained for several years. Some of the chromosomes are slowly losing DNA from the broken ends (as expected if replication is incomplete) and show no telomere-associated DNA added to the receding ends. Two stocks carry chromosomes that have become "healed" and are no longer losing DNA. In both stocks the broken chromosome end has acquired a segment of HeT DNA, a family of complex repeats found only at telomeres and in pericentric heterochromatin. Although the HeT family is complex, the HeT sequence joined to the broken chromosome end is the same in both stocks. In contrast, the two chromosomes are broken in different places and have no detectable sequence similarity at the junction with the new DNA. Sequence analysis suggests that the new telomere sequences have been added by a specific mechanism that does not involve homologous recombination. 相似文献
17.
The electron donor (component B) to the methyl coenzyme M methylreductase system from Methanosarcina thermophila was isolated as the 7-methyl derivative and characterized. Gas chromatography-mass spectrometry and 1H NMR analyses identified this derivative as 7-methylthioheptanoylthreonine phosphate (CH3-S-HTP), indicating that the original component B had the same structure (HS-HTP) as previously determined for component B from Methanobacterium thermoautotrophicum. The heterodisulfide of HS-HTP and coenzyme M (HS-CoM, 2-mercaptoethanesulfonate) was enzymatically reduced in cell extracts using electrons supplied by either H2 or CO, confirming that HS-HTP was a functional molecule in M. thermophila. 相似文献
18.
K. A. Buss C. Ingram-Smith J. G. Ferry D. A. Sanders M. S. Hasson 《Protein science : a publication of the Protein Society》1997,6(12):2659-2662
The unique biochemical properties of acetate kinase present a classic conundrum in the study of the mechanism of enzyme-catalyzed phosphoryl transfer. Large, single crystals of acetate kinase from Methanosarcina thermophila were grown from a solution of ammonium sulfate in the presence of ATP. The crystals diffract to beyond 1.7 A resolution. Analysis of X-ray data from the crystals is consistent with a space group of C2 and unit cell dimensions a = 181 A, b = 67 A, c = 83 A, beta = 103 degrees. Diffraction data have been collected from the crystals at 110 and 277 K. Data collected at 277 K extend to lower resolution, but are more reproducible. The orientation of a noncrystallographic two-fold axis of symmetry has been determined. Based on an analysis of the predicted amino acid sequences of acetate kinase from several organisms, we hypothesize that acetate kinase is a member of the sugar kinase/actin/hsp70 structural family. 相似文献
19.
Does the guanine nucleotide regulatory protein Ni mediate progesterone inhibition of Xenopus oocyte adenylate cyclase?
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In Xenopus laevis oocytes progesterone is able to inhibit directly the plasma membrane adenylate cyclase activity and induce reinitiation of meiotic maturation. To determine whether progesterone inhibition is mediated by the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase, Ni, the effect of the Bordetella pertussis toxin (IAP) and limited proteolysis on progesterone action in oocytes was investigated. Treatment of oocyte membranes with islet activating protein (IAP) in the presence of [32P]NAD led to incorporation of radiolabel into a 41 000-dalton membrane protein. However, exposure of isolated oocytes to 100 ng/ml IAP for up to 24 h, or oocyte membranes with concentrations of toxin as high as 100 micrograms/ml, had no effect on either progesterone inhibition of adenylate cyclase or induction of maturation. Similarly, limited alpha-chymotrypsin proteolysis of oocyte membranes failed to modify progesterone-induced inhibition of adenylate cyclase. In contrast, inhibition of human platelet adenylate cyclase by epinephrine, acting via a GTP-dependent, alpha 2-adrenergic receptor-mediated pathway, is almost completely abolished by both IAP treatment and limited proteolysis of platelet membranes. These data indicate that unlike attenuation of platelet enzyme activity, the inhibition of adenylate cyclase in oocyte membranes by progesterone does not occur via a classical Ni-mediated pathway. 相似文献
20.
Effect of Monensin on Growth and Methanogenesis of Methanobacterium formicicum 总被引:1,自引:0,他引:1
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Monensin inhibited methanogenesis from formate but not from H2-CO2 by resting-cell suspensions of Methanobacterium formicicum. The antibiotic severely inhibited growth on formate. The lag phase of H2-CO2-grown cultures was prolonged by monensin, but these cultures recovered from the initial inhibition. The recovery did not result from the development of a monensin-resistant population or inactivation of the antibiotic. 相似文献