全文获取类型
收费全文 | 27940篇 |
免费 | 2635篇 |
国内免费 | 46篇 |
出版年
2021年 | 384篇 |
2020年 | 261篇 |
2019年 | 317篇 |
2018年 | 380篇 |
2017年 | 370篇 |
2016年 | 564篇 |
2015年 | 890篇 |
2014年 | 1003篇 |
2013年 | 1286篇 |
2012年 | 1481篇 |
2011年 | 1480篇 |
2010年 | 1091篇 |
2009年 | 861篇 |
2008年 | 1120篇 |
2007年 | 1121篇 |
2006年 | 1014篇 |
2005年 | 1007篇 |
2004年 | 1096篇 |
2003年 | 994篇 |
2002年 | 932篇 |
2001年 | 787篇 |
2000年 | 772篇 |
1999年 | 709篇 |
1998年 | 368篇 |
1997年 | 372篇 |
1996年 | 365篇 |
1995年 | 315篇 |
1994年 | 258篇 |
1993年 | 294篇 |
1992年 | 623篇 |
1991年 | 508篇 |
1990年 | 547篇 |
1989年 | 540篇 |
1988年 | 442篇 |
1987年 | 443篇 |
1986年 | 387篇 |
1985年 | 394篇 |
1984年 | 411篇 |
1983年 | 317篇 |
1982年 | 294篇 |
1981年 | 281篇 |
1980年 | 248篇 |
1979年 | 340篇 |
1978年 | 274篇 |
1977年 | 287篇 |
1976年 | 221篇 |
1975年 | 241篇 |
1974年 | 245篇 |
1973年 | 219篇 |
1971年 | 185篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Carcinogen aflatoxin B1 is located preferentially in internucleosomal deoxyribonucleic acid following exposure in vivo in rainbow trout 总被引:7,自引:0,他引:7
The purpose of this work was to investigate the distribution in chromatin of deoxyribonucleic acid (DNA) adducts of aflatoxin B1, following exposure in vivo. Rainbow trout were injected intraperitoneally with radiolabeled aflatoxin B1, a potent procarcinogen known to readily induced hepatocellular carcinomas in these fish. After maximum incorporation, liver nuclei were prepared and digested with micrococcal nuclease. Mono-, di-, and trinucleosomal fractions were purified from several stages of nuclease digestion, and the lengths and specific activities of their DNA were determined. The results indicate that aflatoxin B1 is approximately 5 times as likely on a per nucleotide basis to localize on internucleosomal (linker) DNA as on nucleosomal core DNA in this system. 相似文献
993.
Rabbit liver aryl sulfatase A (aryl sulfate sulfohydrolase, EC 3.1.6.1) is inactivated during the hydrolysis of nitrocatechol sulfate and the rate of formation of turnover-modified aryl sulfatase A depends on the initial velocity of the enzymatic reaction. Organic solvents such as ethanol and dioxane favor the anomalous kinetic behavior. The turnover-modified enzyme can apparently be reactivated by arsenate, phosphate, pyrophosphate, and sulfate in the presence of nitrocatechol sulfate. The apparent dissociation constants of these ions in the reactivation of the enzyme are similar to their Ki values. Sulfite, which is a competitive inhibitor, does not reactivate the turnover-modified enzyme. Thus, all known activators are competitive inhibitors but not all competitive inhibitors are effective as activators. Inactivation of aryl sulfatase A during hydrolysis of 35S-labeled substrate at pH values near the pH optimum (pH 5–6) is accompanied by the incorporation of radioactivity into the protein molecule and the turnover-modified enzyme is thereby covalently labeled. The stoichiometry of the incorporation of radioactivity corresponds to 2 g atom of sulfur per mole of enzyme monomer, or 1 g atom of sulfur per equivalent peptide chain. It is also shown that isolated turnover-modified rabbit liver aryl sulfatase A has lost approximately 76% of its secondary structure as compared to the native enzyme. The specific activity of the inactive enzyme is also decreased by 82%. Turnover-modified rabbit liver aryl sulfatase A is partially reactivated by sulfate ions in the presence of nitrocatechol sulfate. However, circular dichroism measurements and fluorescence spectra of the isolated “reactivated” turnover-modified enzyme indicate only a further loss of secondary structure. The specific activity of this “reactivated” enzyme is in fact decreased. The loss in secondary structure and the enzyme activity of the “reactivated” aryl sulfatase A is prevented in the presence of sulfate ions. Turnover-modified rabbit liver aryl sulfatase A behaves as a very fragile molecule. 相似文献
994.
M. Podojil M. Blumauerová V. Přikrylová Z. Vaněk G. F. Gauze T. S. Maksimova 《Folia microbiologica》1980,25(6):464-466
The strainStreptomyces griseoruber 4620 produces, besides the anthracycline antibiotics beromycins, some other anthracyclines of the rhodomycin type. Twelve
isolates exhibiting a higher antibiotic activity (up to 2.5×), as compared to the parent strain, were obtained after a spontaneous
selection. The following species were isolated from the hydrolysate of mycelial extract: β-rhodomycinone, β-isorhodomycinone,
α2-rhodomycinone and 10-deoxy-β-rhodomycinone, which has not yet been described. 相似文献
995.
Nematodes of three genera (Acrobeloides sp., Aphelenchus avenae, and Scutellonema brachyurum) were induced to coil and enter anhydrobiosis in drying soil of two types: sandy loam and loamy sand. Coiling was studied in relationship to soil moisture characteristics. Coiling and the physiological state of anhydrobiosis occurred before the water in sandy soils reached a water potential of -15 bars. Coiling was maximum at 3-6 bars, depending on the soil type and nematode species. It appeared that induction of coiling and anhydrohiosis were determined by the physical forces exerted by the water film surrounding the nematode, which, for these three species, was 6-9 monomolecular layers of water, rather than the % moisture and relative humidity of the soil per se. 相似文献
996.
After priming with oestradiol, ovariectomized rats were given 6 days of progesterone treatment in which two doses of 50 ng oestradiol were given on Days 3 and 6. This basic treatment allows the oestradiol-induced (1st injection) disappearance of uterine sensitivity to decidual stimuli to occur. Cycloheximide could not mimic oestrogen action in the production of the uterine refractory state. However, a high dose (500 micrograms per animal) of this inhibitor given with the first injection of oestradiol allowed the uterus to remain in a neutral state and to respond to decidual induction after the second dose of oestradiol. By delaying the injection of cycloheximide after the first oestrogen treatment, protein synthesis requisite to the occurrence of uterine refractoriness would not take place within 12 h after the 'nidatory' oestrogen injection. 相似文献
997.
W E Borrias M Hagenaar R Van Den Brekel C Kühlemeijer P J Weisbeek 《Journal of virology》1979,31(2):288-298
Mutants of bacteriophage G4 were isolated and characterized, and their mutations were mapped. They constitute six different genes, namely, A, B, E, F, G, and H. The functional relationship with bacteriophage phi X174 was determined by complementation experiments using amber mutants of phi X and amber mutants of G4. Bacteriophage phi X was able to use the products of G4 genes E, F, G, and H. In bacteriophage G4, however, only the phi X gene H product was functional. 相似文献
998.
While differing in numerous physiological and biochemical parameters, mammalian cardiac and skeletal muscles exhibit many common ultrastructural characteristics. General subcellular organization is similar with longitudinal disposition and organization of the myofibrils as well as subcellular organelles such as mitochondria, sarcoplasmic reticulum and transverse tubules. Significant differences are more readily discerned in terms of degree, not only with respect to relative amounts of various organelles, but also in regard to membrane composition. It is these macromolecular variations in membrane components which may, at least in part, provide the basis for differences in overall functional characteristics in the muscles.In cardiac, as well as skeletal muscle, the concentration of Ca2+ ions at specific intracellular sites regulates the contractile state of the muscle. The differences in mechanism and sources of Ca2+ for contraction in cardiac and skeletal muscle are but a few of the unsolved areas which are now being addressed. We shall focus primarily on research advances involving cardiac and skeletal SR emphasizing the contrasting features related to their functional roles in control of contraction and metabolic events. 相似文献
999.
H Maisin A Van Evercoren M A Anckaert B M de Coster 《Comptes rendus des séances de la Société de biologie et de ses filiales》1979,173(4):832-834
We have studied the recovery for mice bone marrow cells after fractionated irradiation of the whole body. The additional dose (Dr) to obtain a given biological effect if the irradiation is split in two equal subfractions (2 Di) separated by a short interval of time (i) is 40 rad per day when the interval of time between the two irradiations is lengthened of one day. 相似文献
1000.
16S ribosomal RNA of Escherichia coli contains a N2-methylguanosine at 27 nucleotides from the 3'' end 总被引:4,自引:4,他引:0 下载免费PDF全文
The 49 nucleotides fragment derived from the 3' end of 16S rRNA by cloacin DF13, is not cleaved by ribonuclease T1 at a guanosine residue tha is present at 27 nucleotides from the 3' terminus (position 115 in 16S rRNA). Analysis of the isolated nucleotide indicates that it is a modified G residue. In vivo labeling with (3H)methionine shows that this G is methylated and co-chromatography with markers reveals that it is N2-methylguanosine. 相似文献