全文获取类型
收费全文 | 122441篇 |
免费 | 8729篇 |
国内免费 | 20篇 |
出版年
2022年 | 721篇 |
2021年 | 1625篇 |
2020年 | 1354篇 |
2019年 | 1423篇 |
2018年 | 2859篇 |
2017年 | 2566篇 |
2016年 | 3659篇 |
2015年 | 5143篇 |
2014年 | 5405篇 |
2013年 | 6987篇 |
2012年 | 8332篇 |
2011年 | 7602篇 |
2010年 | 5003篇 |
2009年 | 3985篇 |
2008年 | 6123篇 |
2007年 | 5862篇 |
2006年 | 5745篇 |
2005年 | 4930篇 |
2004年 | 4969篇 |
2003年 | 4419篇 |
2002年 | 4138篇 |
2001年 | 2729篇 |
2000年 | 2558篇 |
1999年 | 2119篇 |
1998年 | 1102篇 |
1997年 | 835篇 |
1996年 | 868篇 |
1995年 | 824篇 |
1994年 | 758篇 |
1993年 | 686篇 |
1992年 | 1351篇 |
1991年 | 1224篇 |
1990年 | 1197篇 |
1989年 | 1286篇 |
1988年 | 1125篇 |
1987年 | 1109篇 |
1986年 | 984篇 |
1985年 | 1078篇 |
1984年 | 961篇 |
1983年 | 838篇 |
1982年 | 745篇 |
1979年 | 913篇 |
1978年 | 695篇 |
1977年 | 703篇 |
1975年 | 812篇 |
1974年 | 820篇 |
1973年 | 796篇 |
1972年 | 734篇 |
1970年 | 698篇 |
1969年 | 762篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Use of fluorinated tyrosine phosphates to probe the substrate specificity of the low molecular weight phosphatase activity of calcineurin 总被引:5,自引:0,他引:5
B Martin C J Pallen J H Wang D J Graves 《The Journal of biological chemistry》1985,260(28):14932-14937
Calcineurin, a calmodulin-activated protein phosphatase, is known to dephosphorylate certain low molecular weight phosphate esters. The low molecular weight phosphatase activity of calcineurin has been studied by utilizing tyrosine phosphate derivatives. Kinetic studies suggest that the substrate specificity is dependent upon the electronic nature of the substrate in contrast to results obtained with alkaline phosphatase from Escherichia coli. Comparison of calcineurin and acid-catalyzed hydrolyses indicates a 1:1 correlation between the rate constants for the two processes. This correlation and other model studies have been utilized to provide insight into the chemical mechanism of calcineurin. Possible chemical mechanisms for calcineurin are discussed. 相似文献
992.
Methylation of type II and type I collagen genes in differentiated and dedifferentiated chondrocytes 总被引:2,自引:0,他引:2
The methyl-sensitive restriction endonucleases HpaII and HhaI as well as the methyl-insensitive enzyme MspI were used to examine the methylation status of the pro-alpha 1(II) collagen gene of cartilage. Five different cell types with varying abilities to express type II collagen were studied. Chick embryo chondrocytes express type II collagen, while 5-bromodeoxyuridine-treated chondrocytes, retinoic acid-treated chondrocytes, chick embryo fibroblasts, and erythrocytes do not synthesize type II collagen. Both cDNA and genomic probes for the pro-alpha 1(II) collagen gene were used, covering the complete 3' end of the gene and its flanking sequences. The pro-alpha 1(II) collagen DNA was undermethylated in chondrocytes, compared to either fibroblasts or erythrocytes. However, the methylation of the 5-bromodeoxyuridine-treated and retinoic acid-treated chondrocytes was identical to that of control chondrocytes. The methylation pattern of two regions of the gene of the pro-alpha 2(I) collagen chain was identical in all cell types tested, whether or not the gene was expressed. Our results indicate that genes for these collagen chains differ in their methylation pattern. The type II collagen gene shows reduced methylation in expressing cartilage, but does not acquire an increase in methylation in "dedifferentiated" chondrocytes. The changes in DNA methylation that occur during cell differentiation do not appear to be sufficient to explain gene activation and deactivation. 相似文献
993.
994.
The changes of H-D exchange rates upon protein-protein interactions are generally interpreted as a result of the changes of the dynamic properties of the proteins. The effect of trypsin binding on the H-D exchange kinetics of some trypsin inhibitor amide H's was reported (Simon et al., 1984). In this paper the electrostatic potential originating from the trypsin molecule is calculated at the positions of the studied amide H's in the trypsin-trypsin inhibitor complex. We conclude that the observed decrease of the exchange rates is mainly due to the electrostatic field of the trypsin molecule. 相似文献
995.
Differences in regulatory sequences of naturally occurring JC virus variants. 总被引:22,自引:14,他引:8
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)
The regulatory region was sequenced for DNAs representative of seven independent isolates of JC virus, the probable agent of progressive multifocal leukoencephalopathy. The isolates included an oncogenic variant (MAD-4), an antigenic variant (MAD-11), and two different isolates derived from the urine (MAD-7) and from the brain (MAD-8) of the same patient. The representative DNAs were molecularly cloned directly from diseased brain tissue and from human fetal glial cells infected with the corresponding isolated viruses. The regulatory sequences of these DNAs were compared with those of the prototype isolate, MAD-1, sequenced previously (R. J. Frisque, J. Virol. 46:170-176, 1983). We found that the regulatory region of JC viral DNA is highly variable due to complex alterations of the previously described 98-base-pair repeat of MAD-1 DNA. On the basis of these alterations, there are two general types of JC virus. There were no consistent alterations in regulatory sequences which could distinguish brain tissue DNAs from tissue culture DNAs. Furthermore, for each isolate except MAD-1 (R. J. Frisque, J. Virol. 46:170-176, 1983), the regulatory regions of brain tissue and tissue culture DNAs were not identical. The arrangement, sequence, or both of potential regulatory elements (TATA sequence, GGGXGGPuPu, tandem repeats) of JC viral DNAs are sufficiently different from those in other viral and eucaryotic systems that they may effect the unique properties of this slow virus. 相似文献
996.
Summary Monoclonal antibodies able to recognize single antigenic determinants are a powerful tool for the study of immunological heterogeneity of antigens. In this paper we have used a monoclonal antibody against the -subunit of pig brain tubulin (TU-01) to investigate the immunoreactivity of tubulins from mammals, avians, amphibia, echinodermata, plathelmints, slime moulds and protozoa. Immunoreactivity was detected using immunoblotting and indirect immunofluorescence of isolated cells. Our results show that the antigenic determinant recognized by the TU-01 antibody is present in all metazoan tubulin tested and among the eukaryotic microorganisms only in the flagellateTrichomonas vaginalis. Indirect immunofluorescence also reveals that not allTrichomonas microtubules are stained by TU-01 antibody indicating the presence of different tubulins within a single cell. This results are consistent with the multitubulin hypothesis (Fulton andSimpson 1976). 相似文献
997.
The enzymatic activities of two "key" enzymes of the glycolytic pathway, pyruvate kinase and lactic dehydrogenase, were studied in seven areas of the brain in male adult rats in states of pharmacologically induced hyper and hypothyroidism. The brain areas were: anterior cortex, adenohypophysis, hypothalamus, amygdaline nucleus, septum, hippocampus and cerebellum. In T3 treated animals, pyruvate kinase activity showed significant increase in all the areas studied while lactic dehydrogenase activity decreased. In propyl-thiouracil treated animals these enzyme activities showed no significant variations from those in animals of the control group. 相似文献
998.
J M Fernández M F Rubio-Arroyo M Soriano-García R A Toscano M C Pérez-César 《Steroids》1985,45(2):151-157
The synthesis and molecular structure of prolame, N-(3-hydroxy-1,3,5(10)-estratrien-17 beta-yl)-3-hydroxypropylamine, is described. It was characterized by ir, nmr, mass spectrometry and chemical analysis. The crystal structure of this compound was determined by single-crystal x-ray diffraction. Prolame belongs to space group P212121. Cell dimensions are: a = 8.356(2), b = 13.343(4) and c = 16.119(4) A. Z = 4; R = 4.1%. 相似文献
999.
Demonstration of a beta-casomorphin immunoreactive material in the plasma of newborn calves after milk intake 总被引:1,自引:0,他引:1
Martin Umbach Hansjörg Teschemacher Karina Praetorius Richard Hirschhäuser Hartwig Bostedt 《Regulatory peptides》1985,12(3):223-230
Blood was collected from newborn calves before and after their first milk intake after birth; extracts of plasma were assayed by radioimmunoassay for the presence of beta-casomorphin-7 immunoreactive materials. No beta-casomorphin immunoreactivity was found in samples collected before milk ingestion; however, in samples collected after milk ingestion a beta-casomorphin-7 immunoreactive material was detected. Chromatographic characterization showed that this material was not identical with beta-casomorphin-7 but might rather represent a precursor thereof. The material proved resistant to enzymatic attack during a 30-min incubation period at 37 degrees C in the plasma of newborn calves, whereas beta-casomorphin-7 was degraded under these conditions. A physiological significance of beta-casomorphin-7 eventually cleaved from such a precursor material at any site in the newborn mammal is suggested. 相似文献
1000.
Using a commercial flow cytometer (Cyto-fluorograf), narrow-forward-angle (NFA) light-scatter signals were detected for spore preparations of Bacillus anthracis Vollum, B. anthracis Sterne, B. cereus NCTC 8035, and B. subtilis var niger. In the flow immunofluorescence (FIF) analysis of spores stained with fluorescein-conjugated hyperimmune antibody to B. anthracis Vollum spores, fluorescence histograms could be acquired by selecting on NFA scatter. Fluorescence data selected on ninety degree scatter were rather noisier. Fluorescence analysis by dual parameter NFA scatter-FIF techniques was shown to have several advantages over the subtraction FIF method reported earlier. The implication from FIF analysis of spore suspensions and corresponding cell-free supernatants that the peak in the fluorescence histogram was caused by signals from fluorescing spores, was confirmed by use of the cell sorter and subsequent microscopy of the sorted samples. Although a proportion of spore aggregates was present in samples sorted from the right-hand tail of the fluorescence histogram, it was demonstrated that the majority of the observed distribution of fluorescence was not due to the formation of aggregates but was rather an expression of variation in the degree of staining of individual spores. 相似文献