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991.
Comparison between mutagenesis in normal and transformed syrian hamster fibroblasts: Difference in the temporal order of HPRT gene replication 总被引:1,自引:0,他引:1
Takeki Tsutsui Brian D. Crawford Paul O.P. Ts''o J.Carl Barrett 《Mutation research》1981,80(2):357-371
A highly tumorigenic subdiploid cell line, BP6T, derived in our laboratory from Syrian hamster embryo (SHE) cells, is amenable to studies of somatic mutation in vitro. Cellular and biochemical characterization of clonally derived BP6T cells resistant to 6-thioguanine (TGr) or ouabain (Ouar) demonstrated these mutants to be similar qualitatively to mutants of SHE cells characterized previously (Barrett et al., 1978). BP6T TGr mutants resistant to 6-thioguanine are cross-resistant to 8-azaguanine, lack HPRT activity, exhibit a low frequency of reversion and arise spontaneously at a rate of 5 × 10−7 mutants per cell per generation. BP6T Ouar mutants were shown to be highly resistant to ouabain-mediated inhibition of 86Rb influx, indicating an alteration in the Na+/K+ ATPase. These studies on the BP6T cell line provide the experimental basis for a comparative study of the mutagenic responses of normal, diploid SHE cells versus those of related, but transformed aneuploid cells. Highly synchronized cultures of these 2 cells were mutagenized by pulse treatment with BrdU during different periods of S phase, followed immediately by near-UV irradiation. The induced mutation frequencies so obtained provided information about the temporal order of replication of genes encoding HPRT and Na+/K+ ATPase in both SHE and BP6T cells. The temporal pattern of replication of Na+/K+ ATPase gene loci is similar in both cell types, but the temporal order of replication of the HPRT gene is significantly different between SHE and BP6T cells (mid-late S phase, versus early S phase, resp.). This observed difference emphasizes the caution required in the study of mutagenesis and DNA replication using transformed, aneuploid cells under the assumption that the underlying mechanisms are the same for normal, diploid cells. 相似文献
992.
Paul Jensén 《Physiologia plantarum》1981,52(4):437-441
Uptake of Rb+ from a complete nutrient solution with 2.0 mM Rb+ was studied in roots of spring wheat seedlings ( Triticum aestivum L. cv. Svenno) with different K+ levels. The relationship between Rb+ uptake and concentration of K+ in the roots indicated a negative feedback mechanism operating through allosteric control. The Rb+ uptake process in root cells was divided into two steps: (1) binding of the ion in the free space, and (ii) transmembrane transport into the cytoplasm. Metabolic and non-metabolic components of uptake were separated by addition of the metabolic inhibitor 2,4-dinitrophenol (DNP) to the nutrient solution. It is suggested that metabolic Rb+ uptake requires energy in two uptake steps (for binding to the carrier entity in the free space and for transmembrane transport) or in one step only (for transmembrane transport), dependent on the K+ status of the roots. The change from metabolic to non-metabolic binding in the free space is accomplished by changing the conformational state of the carrier (slow/fast transitions). There may be a hysteretic effect on metabolic Rb+ uptake through a slow transition between carrier states. This is superimposed on the negative cooperativity, strengthening further cooperativity at intermediate K+ levels in the roots. Non-metabolic Rb+ uptake probably consists of two components, a carrier-mediated (facilitated diffusion) and a parallel diffusive component. 相似文献
993.
994.
Regulation of asparaginase, glutamine synthetase, and glutamate dehydrogenase in response to medium nitrogen concentrations in a euryhaline chlamydomonas species
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The ammonium assimilatory enzymes glutamine synthetase (EC 6.3.1.2) and glutamate dehydrogenase (EC 1.4.1.3) were investigated for a possible role in the regulation of asparaginase (EC 3.5.1.1) in a Chlamydomonas species isolated from a marine environment. Cells grown under nitrogen limitation (0.1 millimolar NH(4) (+), NO(3) (-), or l-asparagine) possessed 6 times the asparaginase activity and approximately one-half the protein of cells grown at high nitrogen levels (1.5 to 2.5 millimolar). Biosynthetic glutamine synthetase activity was 1.5 to 1.8 times greater in nitrogen-limited cells than cells grown at high levels of the three nitrogen sources.Conversely, glutamate dehydrogenase (both NADH- and NADPH-dependent activities) was greatest in cells grown at high levels of asparagine or ammonium, while nitrate-grown cells possessed little activity at all concentrations employed. For all three nitrogen sources, glutamate dehydrogenase activity was correlated to the residual ammonium concentration of the media after growth (r = 0.88 and 0.94 for NADH- and NADPH-dependent activities, respectively).These results suggest that glutamate dehydrogenase is regulated in response to ambient ammonium levels via a mechanism distinct from asparaginase or glutamine synthetase. Glutamine synthetase and asparaginase, apparently repressed by high levels of all three nitrogen sources, are perhaps regulated by a common mechanism responding to intracellular nitrogen depletion, as evidenced by low cellular protein content. 相似文献
995.
Polarity shifts occur during organogenesis. The histological criterion for polarity is the direction of cell division. The biophysical criterion is the orientation of reinforcing cellulose microfibrils which lie normal to the organ axis and which determine the preferred growth direction. Using cell pattern to deduce cell lineage, and polarized light to study cellulose alignment, both aspects of polarity were examined in the epidermis of regenerating G. paraguayense. In this system new leaves and a stem arise from parallel cell files on a mature leaf. Large (90°) shifts in polarity occur in regions of the epidermis to give the new organs radial symmetry in the surface plane (files radiating from a pole). Study of the shifts in the epidermis showed that, during certain stages, shifts in the division direction are accompanied by shifts in the cellulose deposition direction, as expected. The new cellulose orientation is parallel to the new cross wall. During normal organ extension, however, shifts in division direction do not bring on changes in cellulose pattern. Thus the coupling between the two kinds of polarity is facultative. This variable relation is used in a biophysical model which can account for the reorganization of cell file pattern and cellulose reinforcement pattern into the radial symmetry of the new organ. 相似文献
996.
Axonal Transport of the Ca2+ -Dependent Protein Modulator of 3'':5''-Cyclic-AMP Phosphodiesterase in the Rabbit Visual System 总被引:2,自引:0,他引:2
Paul F. Erickson Kenneth B. Seamon Blake W. Moore Robert S. Lasher Lee N. Minier 《Journal of neurochemistry》1980,35(1):242-248
Water-soluble proteins were extracted from individual retinas, optic nerves, combined optic tracts and lateral geniculate bodies, and superior colliculi of rabbits at 1, 3, and 18 days after injection of [3H]leucine into the right eye. The Ca2+-dependent protein modulator of 3':5'-cyclic-AMP phosphodiesterase (calmodulin) was isolated from these samples by a two-step polyacrylamide gel electrophoresis procedure. An analysis of the radioactivity incorporated into the total soluble proteins and the calmodulin revealed that most of the calmodulin was axonally transported at a slow rate (2--4 mm/day) and represented about 0.45% of the total transported soluble protein. 相似文献
997.
Francis V. DeFeudis Lucienne Ossola Gaby Schmitt Paul Mandel 《Journal of neurochemistry》1980,34(4):845-849
Abstract: The effects of some GABA analogues and some drugs on the binding of [3H]muscimol (3.08 nM) to thoroughly washed subcellular particles prepared from a neuron-enriched culture of embryonic rat brain were examined using Na+-free Tris-citrate medium and a centrifugation method. Competition for [3H]muscimol binding sites by excess(10?5 M) unlabelled GABA provided estimates of “specific” binding. In accord with in vivo neuropharmacological studies on GABA receptors and with in vitro studies on cerebral membrane preparations, [3H]muscimol binding was potently inhibited by muscimol itself (IC50, 2.5 nM), GABA (1C50, 43 nM), isoguvacine (IC50, 61 nM), and 3-aminopropanesulphonic acid (IC50, 160 nM), and less potently inhibited by the GABA antagonist bicuculline methobromide (IC50, 800 nM). δ- Aminovaleric acid (IC50, 2.6 μM), the glycinelp-alanine antagonist strychnine (IC50, 6.6 μM), and the predominantly glial GABA uptake inhibitors β-alanine (IC50, 23 μM) and p-proline (IC50, 66 μM) also inhibited [3H]muscimol binding. Other inhibitors of Na+-dependent GABA uptake, (±)-nipecotic acid, L- 2,4-diaminobutyric acid, and guvacine, as well as picrotoxinin, were relatively inactive as inhibitors of [3H]muscimol binding (IC50≥ 1 mM). In addition to revealing that GABA receptors are present on neuronal membranes before the formation of most synapses, this binding of [3H]muscimol that occurs to neuronal, but not to glial, membranes might be useful as a “neuronal marker” and for the further characterization and isolation of GABA receptors. 相似文献
998.
Use of lambda pMu bacteriophages to isolate lambda specialized transducing bacteriophages carrying genes for bacterial chemotaxis. 总被引:3,自引:0,他引:3
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A general method for constructing lambda specialized transducing phages is described. The method, which is potentially applicable to any gene of Escherichia coli, is based on using Mu DNA homology to direct the integration of a lambda pMu phage near the genes whose transduction is desired. With this method we isolated a lambda transducing phage carrying all 10 genes in the che gene cluster (map location, 41.5 to 42.5 min). The products of the cheA and tar genes were identified by using transducing phages with amber mutations in these genes. It was established that tar codes for methyl-accepting chemotaxis protein II (molecular weight, 62,000) and that cheA codes for two polypeptides (molecular weights, 76,000 and 66,000). Possible origins of the two cheA polypeptides are discussed. 相似文献
999.
Different methods for the preparation of active lipoxygenase (LOX) extracts from apples were compared. Highest activities were obtained using a 0.25 M phosphate buffer (pH 7) containing 1% Triton X-100 and 10?2 M metabisulphite as extraction solvent. LOX activity during storage was investigated in the core, flesh, and peel. Activity was always highest in the core and peel. On storage, activity was increased in each part of the fruit but especially in the core and peel. Increase in LOX preceded the browning of the core. LOX may be responsible for the browning and may be concerned in the induction of superficial scald. 相似文献
1000.