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The size distribution of chromatin fragments released by micrococcal nuclease digestion of liver chromatin at various ionic strengths was examined. Below 20 mM ionic strength, gradient profiles with a peak centered at 6 nucleosomes are generated, whereas between 20 and 50 mM the peak is always centered on 12 nucleosomes, and above 50 mM ionic strength the 30-nm fiber becomes less accessible to the nuclease and there is a corresponding increase in the size distribution of fragments in the gradients. However, extensive digestions always give profiles with a peak of 12 nucleosomes as nuclease-resistant dodecamers accumulate. All of these observations are consistent with the winding of the 10-nm polynucleosome chain into a helical coil commencing at about 20 mM ionic strength. The helical turns are stabilized by histone H1 interactions between 20 and 50 mM ionic strength producing stable dodecamers. Above 50 mM ionic strength the coil condenses longitudinally and the profiles are consistent with a random attack of this fiber by the nuclease. Consequently it is not necessary to invoke the existence of a subunit bead to explain the profiles. We further define the conditions at which specific structural transitions take place and provide methodology for the preparation of chromatin at various levels of condensation. 相似文献
4.
Chromatin structure. Evidence that the 30-nm fiber is a helical coil with 12 nucleosomes/turn 总被引:1,自引:0,他引:1
Sedimentation analysis has been used to compare the structure of 30-nm chromatin fibers, isolated and digested under conditions that maintain the native structure, with relaxed-refolded chromatin. The native chromatin fibers show sharp, ionic strength-dependent changes in sedimentation coefficient that are not apparent in relaxed-refolded fibers. The first transition at approximately 20 mM ionic strength reflects the organization of the 10-nm polynucleosome chain into a loose helically coiled 30-nm fiber. Between 20 and 60 mM ionic strength there is considerable interaction between nucleosomes within the coils to generate a stable helical array with 12 nucleosomes/turn. Above 60 mM ionic strength the helical coil continues to condense until it precipitates at ionic strengths slightly greater than those considered physiological, indicating that there is no end point in fiber formation. The data is incompatible with a solenoid model with 6 nucleosomes/turn and also rules out the existence of a beaded subunit structure. 相似文献
5.
An enzyme extract from apple(Pyrus malus Borb.) seeds which causes the disappearance of free indol-3-ylacetic acid (IAA) requires the presence of oxygen, but is not
inhibited by cyanide. Using 1-14C-IAA it has been demonstrated that the IAA transformation is not accompanied by its decarboxylation. Decarboxylating IAA
oxidase is absent during the whole period of apple seed cold stratification. Free IAA has not been detected in dormant apple
seeds and in seeds stratified at low temperature. It appears during stratification at 25 °C. Ethyl ester of IAA and indol-3-ylacetyl
aspartate have been identified in dormant and after-ripened seeds. Exogenous 1-14C IAA taken up by apple embryos is converted into conjugates with aspartate and short peptides containing an aspartate moiety. 相似文献
6.
Hydrogen cyanide and embryonal dormancy in apple seeds 总被引:3,自引:0,他引:3
Embryos of apple ( Malus domestica Borh. cv. Antonówka) were treated with 1 m M gaseous HCN for 6 h and cultured under a 12 h photoperiod. HCN pretreatment stimulated germination, increased the length of hypocotyls, shortened the main root and decreased the percentages of seedlings with asymmetrically grown as well as with asymmetrically greened cotyledons. High activity of β-cyanoalanine synthase (EC 4.4.1.9) and a sharp increase in cyanogen content during embryo culture suggested very low levels of endogenous HCN. despite the activity of HCN releasing enzymes. The obtained data allow us to postulate an important role for cyanide in the regulatory complex controlling dormancy in apple seeds. Experiments with respiratory inhibitors indicated, however, that HCN pretreatment affected neither the alternative electron transport pathway nor residual respiration. 相似文献
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The effect of fish predation on Cyclops life cycle 总被引:2,自引:2,他引:0
Two populations of Cyclops abyssorum tatricus studied in neighbouring alpine lakes in Tatra Mountains (southern Poland) differed in their life cycles. In the lake with planktivorous fishes, Cyclops was typically monocyclic, with highly synchronized reproduction during two winter months, while in the fishless lake its reproduction was asynchronous and continued for six months. Direct and indirect effects of fish predation on Cyclops life cycles are discussed. 相似文献
9.
Reinhard Bolli Katarzyna A. Nałecz Angelo Azzi 《Journal of bioenergetics and biomembranes》1986,18(4):277-284
Cytochromec oxidase fromParacoccus denitrificans was homogenously dispersed in Triton X-100. Using gel exclusion chromatography and sucrose gradient centrifugation analysis a molecular weight of the detergent-protein complex of 155,000 was determined. After subtraction of the bound detergent (111 mol/mol hemeaa
3) a molecular weight of 85,000 resulted, which agreed well with the model of a monomer containing two subunits. This monomer showed high cytochromec oxidase activity when measured spectrophotometrically in the presence of Triton X-100 (V
max=85 s–1). The molecular activity, plotted according to Eadie-Hofstee, was monophasic as a function of the cytochromec concentration. AK
m of 3.6×10–6 M was evaluated, similar to theK
m observed in the presence of dodecyl maltoside [Naeczet al. (1985).Biochim. Biophys. Acta
808, 259–272]. 相似文献
10.
Witold Lasek Anna Wańkowicz Katarzyna Kuc Wojciech Feleszko Jakub Golab Adam Giermasz Wiesŀaw Wiktor-J/cedrzejczak Marek Jakóbisiak 《Cancer immunology, immunotherapy : CII》1995,40(5):315-321
The efficacy of systemic infusion of recombinant human macrophage-colony-stimulating factor (M-CSF) in combination with local treatment with human recombinant tumor necrosis factor (TNF) and mouse recombinant interferon (IFN) was studied in vivo on a subclone of B16 melanoma (MmB16) in mice. Short-term intravenous administration of M-CSF at a dose of 106 units daily had no antitumor effect in vivo. Similarly, local treatment of tumor with TNF (5 g daily) did not produce any therapeutic effect. However, simultaneous administration of the same dose of TNF with IFN (1000 units daily) resulted in a synergistic effects manifested by the retardation of tumor growth. Addition of systemic infusion of M-CSF to the local therapy with TNF and IFN induced further augmentation of antitumor efficacy and delayed progression of MmB16 melanoma. The strengthened antitumor effect of combination therapy including M-CSF, TNF and IFN was most probably due to the increased release of monocytes from the bone marrow, their recruitment into the site of tumor growth and subsequent local stimulation of their antitumor activity. 相似文献