全文获取类型
收费全文 | 5988篇 |
免费 | 465篇 |
国内免费 | 2篇 |
出版年
2023年 | 15篇 |
2022年 | 31篇 |
2021年 | 71篇 |
2020年 | 50篇 |
2019年 | 52篇 |
2018年 | 81篇 |
2017年 | 72篇 |
2016年 | 122篇 |
2015年 | 190篇 |
2014年 | 271篇 |
2013年 | 356篇 |
2012年 | 361篇 |
2011年 | 393篇 |
2010年 | 263篇 |
2009年 | 254篇 |
2008年 | 351篇 |
2007年 | 375篇 |
2006年 | 350篇 |
2005年 | 351篇 |
2004年 | 313篇 |
2003年 | 294篇 |
2002年 | 310篇 |
2001年 | 79篇 |
2000年 | 74篇 |
1999年 | 89篇 |
1998年 | 112篇 |
1997年 | 86篇 |
1996年 | 76篇 |
1995年 | 61篇 |
1994年 | 65篇 |
1993年 | 69篇 |
1992年 | 52篇 |
1991年 | 61篇 |
1990年 | 50篇 |
1989年 | 59篇 |
1988年 | 47篇 |
1987年 | 35篇 |
1986年 | 44篇 |
1985年 | 36篇 |
1984年 | 32篇 |
1983年 | 26篇 |
1982年 | 52篇 |
1981年 | 33篇 |
1980年 | 36篇 |
1979年 | 25篇 |
1978年 | 17篇 |
1977年 | 24篇 |
1976年 | 16篇 |
1975年 | 21篇 |
1974年 | 15篇 |
排序方式: 共有6455条查询结果,搜索用时 15 毫秒
981.
Krzysztof J Reszka Michael L McCormick Garry R Buettner C Michael Hart Bradley E Britigan 《Nitric oxide》2006,15(2):133-141
The effect nitric oxide (NO*) on the stability of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts has been investigated using EPR spectroscopy. We report that the DMPO/HO* adduct, generated by porcine pulmonary artery endothelial cells in the presence of H2O2 and DMPO, or by a Fenton system (Fe(II)+H2O2) is degraded in the presence of the NO*-donor, 2-(N,N-diethylamino)-diazenolate-2-oxide (DEANO) or by bolus addition of an aqueous solution of NO*. A similar effect of DEANO was observed on other DMPO adducts, such as DMPO/*CH3 and DMPO/*CH(CH3)OH, generated in cell-free systems. Measurements of the loss of DMPO/HO* in the presence of DEANO in aerated and oxygen-free buffers showed that in both of these settings the process obeys first-order kinetics and proceeds with similar efficacy. This indicates that direct interaction of the nitroxide with NO*, rather than with NO2* (formed from NO* and O2 in aerated media), is responsible for destruction of the spin adduct. These results suggest that the presence of NO* may substantially affect the quantitative determination of DMPO adducts. We also show that NO2* radicals, generated by a myeloperoxidase/H2O2/nitrite system, also degrade DMPO/HO*. Because DMPO is frequently used to study generation of superoxide and hydroxyl radicals in biological systems, these observations indicate that extra caution is required when studying generation of these species in the presence of NO* or NO2* radicals. 相似文献
982.
983.
Birgit Gielen Hans J. De Boeck Catherine M. H. M. Lemmens Roland Valcke Ivan Nijs Reinhart Ceulemans 《Physiologia plantarum》2005,125(1):52-63
Model ecosystems were grown in 12 sunlit, climate-controlled chambers to gain insight into the effects of elevated (+3°C) air temperature (Tair ) on temperate grasslands. In this study, the hypothesis of delayed senescence in response to elevated Tair was tested for Rumex acetosa L. and Plantago lanceolata L. During the autumn of the first treatment year, frequent measurements were made of leaf chlorophyll a (Chl a ) fluorescence transients. Chl fluorescence images of individual leaves as well as digital colour images of these ecosystems were captured. Chl fluorescence variables, such as the maximum quantum yield of primary photochemistry (Fv /Fm ), indicated a decreasing efficiency with time. Despite no treatment effect on Fv /Fm , other variables derived from the Chl fluorescence transients showed a strong trend towards a positive effect of a 3°C temperature increase on the photosynthetic performance of R. acetosa and P. lanceolata in the first year. After mid-September, the initial positive treatment effect disappeared for R. acetosa , strongly suggesting that leaf lifespan of this species was shortened by higher Tair . One possible explanation is more intense drought stress in the elevated compared to the ambient temperature treatments. Second-year measurements were possibly too limited in time to confirm this trend. These results show that temperate grassland species may take advantage of a future increase in Tair during autumn. This will ultimately depend on the species' degree of acclimation to a temperature change and on the resistance to drought stress. 相似文献
984.
The U3 Promoter and the nef Gene of Simian Immunodeficiency Virus (SIV) smmPBj1.9 Do Not Confer Acute Pathogenicity upon SIVagm
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Stefan Wagener Matthias T. Dittmar Brigitte Beer Renate Knig Roland Plesker Stephen Norley Reinhard Kurth Klaus Cichutek 《Journal of virology》1998,72(4):3446-3450
Two chimeric proviruses comprising the U3 promoter and the nef gene of simian immunodeficiency virus (SIV) smmPBj1.9 in addition to other genomic regions of SIVagm3mc from African green monkeys (Cercopithecus aethiops) were constructed. The derived chimeric viruses (SIVagm3mc/SIVsmmPBj1.9) were both able to replicate in nonstimulated peripheral blood leukocytes from pig-tailed macaques (Macaca nemestrina), a biological property often correlated with acute pathogenicity. However, only one of the chimeric viruses was acutely pathogenic, inducing a rapid depletion of the peripheral CD4+ T cells in two infected pig-tailed macaques within 10 days after infection in a manner similar to infection with SIVsmmPBj1.9 itself. The other chimeric virus actively replicated during the first 8 weeks after experimental infection of two pig-tailed macaques but induced neither acute disease nor CD4+ T-cell depletion for 113 weeks after infection. Thus, the U3 promoter and the nef gene of SIVsmmPBj1.9 alone appear to be insufficient to confer acute pathogenicity to SIVagm3mc. 相似文献
985.
Pascale Hubert Roland Greimers Elizabeth Franzen-Detrooz Jean Doyen Pierre Delanaye Jacques Boniver Philippe Delvenne 《Cancer immunology, immunotherapy : CII》1998,47(2):81-89
Dendritic cells (DC) are the most efficient antigen presenting cells. The clinical use of DC as vectors for antitumor and
anti-infectious disease immunotherapy has been limited by their low level and accessibility in normal tissue. Substantial
numbers of DC can be generated from peripheral blood cultured in the presence of interleukin-4 (IL-4) and granulocyte/macrophage-colony-stimulating
factor (GM-CSF). We showed in this study that substantial numbers of DC can be obtained from the peripheral blood of patients
with (pre)neoplastic lesions of the uterine cervix. The procedure required relatively small blood samples (10 ml) and the
presence of 100 U/ml IL-4 and 800 U/ml GM-CSF in the culture medium. There was no significant difference in the morphology,
yield, phenotype and function of generated DC between patients with cervical (pre)neoplastic lesions and healthy individuals.
When the hematopoietic factor Flt3 ligand (Flt3L, 40 ng/ml) was added, there was an average increase in the DC population
of 26% compared to cultures with GM-CSF and IL-4 alone. Approximately 1.2 × 106 cells with the characteristics of dendritic cells could be obtained when Flt3L was included in the medium. The addition of
Flt3L did not modify the phenotypic profile of DC (HLA-DR+, CD1a+, CD4+, CD54+, CD80+, CD86+, CD40+, CD3− and CD14−). In addition, Flt3L generated functional DC capable of stimulating the proliferation of alloreactive T cells. These results
suggest that Flt3L, in association with GM-CSF and IL-4, provides an advantageous tool for the large-scale generation of DC
and that an immunotherapy based on the use of DC generated in vitro is possible in patients with (pre)neoplastic lesions of
the uterine cervix.
Received: 8 January 1998 / Accepted: 30 April 1998 相似文献
986.
The iron requirement of four strains of lactobacilli (L. acidophilus, L. delbrueckii subsp. bulgaricus, L. plantarum, and L. pentosus) was studied in a synthetic medium under aerobic or anaerobic conditions. Effects of iron salt and iron-chelated compounds
were tested on bacterial growth in manganese-free or -supplemented media. No significant growth stimulation was observed in
any condition. These results support the absolute manganese requirement for optimum growth of lactobacilli and the needless
incorporation of iron in growth media.
Received: 5 November 1997 / Accepted: 20 January 1998 相似文献
987.
Previous work has shown that translation of the encephalomyocarditis (EMC) viral ribonucleic acid (RNA) generates at least three primary products, polypeptides A, F, and C. The A and C polypeptides then undergo post-translational cleavages to complete the production of the stable viral polypeptides (delta, beta, gamma, alpha, G, I, F, H, and E). In this communication we show that A, F, and C are produced in equimolar amounts giving further support to the theory that the RNA of picornaviruses has only a single site for the initiation of protein synthesis. The biosynthesis of viral proteins in EMC virus-infected HeLa cells was studied in the presence of pactamycin at concentrations which preferentially inhibit the initiation of protein synthesis. The amount of each polypeptide formed during the residual period of protein synthesis observed after the addition of pactamycin was used as a criterion for ordering the genes on the viral RNA. The results obtained indicate that the primary gene products are ordered on the EMC viral RNA 5' --> 3' A-F-C and that the stable products are ordered delta-beta-gamma-alpha-G-I-F-H-E. Moreover, the intermediate chains B and epsilon map in the capsid region, whereas the intermediate chain D maps in the E region. This order is largely consistent with previously established relationships of the viral polypeptides and thus indicates that pactamycin is a valid tool for "genetic" mapping of polycistronic RNA molecules with single initiation sites. 相似文献
988.
Structural Studies of Avian Myeloblastosis Virus: Comparison of Polypeptides in Virion and Core Component by Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis 总被引:29,自引:25,他引:4
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Kurt Stromberg Nancy E. Hurley Nancy L. Davis Roland R. Rueckert Erwin Fleissner 《Journal of virology》1974,13(2):513-528
Two different systems of dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in separate laboratories detected analogous patterns of dye bands in virions of avian myeloblastosis virus (AMV). At least 11 of the dye bands co-migrated with the major polypeptides reported in Rous sarcoma virus. Particles with the morphology of the AMV core component, obtained after exposure of AMV to the nonionic surfactant Sterox SL, contained major polypeptides p12, p27, p60, p64, p91, and p98. The polypeptide p12 has been previously shown to be the major constituent of the inner ribonucleoprotein (RNP) of the AMV core, and has been designated p12(N). Two RNP polypeptides, p64 and p91, co-electrophoresed with purified AMV DNA polymerase and have now been designated p64(P) and p91(P). The polypeptide p27 has been identified as a probable constituent of the core shell, and has accordingly now been designated p27(C). In comparison to virions of AMV, the AMV core component contained a greatly reduced amount of polypeptide p15 and appeared to lack a major polypeptide, p19. Consequently, these polypeptides may be associated either with the exterior of the core shell or the interior of the viral envelope. Glycopeptides were not detected in AMV cores, in agreement with earlier reports that they reside in external projections from the viral envelope. 相似文献
989.
N-Acetyl-β-d-hexosaminidase component A. Different forms in human tissues and fluids 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
1. Hexosaminidase A of human serum was resolved into two components, a minor form with properties identical with those of the single hexosaminidase A component of human liver, and a major form with significantly different properties. 2. The major serum hexosaminidase A form was eluted from a DEAE-cellulose column at a lower salt concentration than that required to elute the liver form. 3. A multiple-pass technique was used to elute the major serum enzyme A from a Sephadex G-150 column before that of liver enzyme A. 4. Clostridium perfringens neuraminidase converted the major component of serum hexosaminidase A into a form that was held less tightly by DEAE-cellulose, but the minor component of the A enzyme of serum, and the A enzyme of liver were not affected. 5. The hexosaminidase A from tears was similar to the A enzyme from serum, whereas those from several human tissues and from urine and lymph were similar to the liver form. 6. The A enzyme from serum may be derived from the A enzyme from liver by glycosylation before secretion. 相似文献
990.
The concentration of cyclic adenosine 3',5'-monophosphate (c-AMP) in Escherichia coli growing on different sources of carbon was studied. Cultures utilizing a source of carbon that supported growth relatively poorly had consistently higher concentrations of c-AMP than did cultures utilizing sugars that supported rapid growth. This relationship was also observed in strains defective in c-AMP phosphodiesterase and simultaneously resistant to catabolite repression; in such strains the c-AMP concentration was slightly higher for several sources of carbon tested. Cultures continued to synthesize c-AMP and secreted it into the medium, under conditions that brought about an inhibition of the intracellular accumulation of the cyclic nucleotide. Transient repression of the synthesis of beta-galactosidase was not associated with an abrupt decrease in the cellular concentration of c-AMP. 相似文献