全文获取类型
收费全文 | 4772篇 |
免费 | 442篇 |
国内免费 | 5篇 |
出版年
2022年 | 23篇 |
2021年 | 75篇 |
2020年 | 26篇 |
2019年 | 39篇 |
2018年 | 64篇 |
2017年 | 49篇 |
2016年 | 115篇 |
2015年 | 244篇 |
2014年 | 251篇 |
2013年 | 315篇 |
2012年 | 432篇 |
2011年 | 379篇 |
2010年 | 201篇 |
2009年 | 177篇 |
2008年 | 237篇 |
2007年 | 247篇 |
2006年 | 232篇 |
2005年 | 214篇 |
2004年 | 200篇 |
2003年 | 172篇 |
2002年 | 163篇 |
2001年 | 148篇 |
2000年 | 136篇 |
1999年 | 108篇 |
1998年 | 51篇 |
1997年 | 39篇 |
1996年 | 40篇 |
1995年 | 31篇 |
1994年 | 30篇 |
1993年 | 31篇 |
1992年 | 70篇 |
1991年 | 70篇 |
1990年 | 59篇 |
1989年 | 61篇 |
1988年 | 58篇 |
1987年 | 60篇 |
1986年 | 37篇 |
1985年 | 20篇 |
1984年 | 31篇 |
1983年 | 22篇 |
1982年 | 14篇 |
1981年 | 17篇 |
1980年 | 12篇 |
1979年 | 26篇 |
1978年 | 24篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 18篇 |
1974年 | 23篇 |
1973年 | 22篇 |
排序方式: 共有5219条查询结果,搜索用时 46 毫秒
31.
Translational alterations in maize leaves responding to pathogen infection, paraquat treatment, or heat shock : polysome dissociation and accumulation of a 57 kilodalton protein 下载免费PDF全文
Translational alterations occur in maize (Zea mays L.) leaves stressed by pathogen infection or herbicide paraquat treatment. These translational changes include: (a) dissociation of large polysomes to small polysomes, monosomes, and subunits; (b) a decreased rate of total protein synthesis; and (c) a reduced synthesis of several proteins by polysomes in vitro. The polysome dissociation was neither due to an extraction artifact nor to degradation of RNA by RNase. The protein patterns of polysomes isolated from leaves inoculated with Bipolaris maydis at 6 to 48 hours showed an increase in the intensity of a 57 kilodalton protein. When inoculated with less virulent pathogens, such as B. zeicola, Exserohilum turcicum, or Colletotrichum graminicola, the protein was accumulated in polysomes of leaves at 24 to 48 hours after inoculation. The 57 kilodalton protein was also accumulated in polysomes of maize leaves responding to heat shock or herbicide paraquat treatments. The purified 57 kilodalton protein reassociated with polysomes isolated from healthy leaves and inhibited polysomal translation in vitro. Since the 57 kilodalton protein is rapidly accumulated in maize polysomes in response to various biological and environmental stresses and may affect protein synthesis, it may be involved in translational regulation of maize leaves during stress response. 相似文献
32.
Rapid method for direct extraction of DNA from soil and sediments. 总被引:23,自引:0,他引:23
A rapid method for the direct extraction of DNA from soil and sediments was developed. The indigenous microorganisms in the soil and sediments were lysed by using lysozyme and a freeze-thaw procedure. The lysate was extracted with sodium dodecyl sulfate and phenol-chloroform. In addition to a high recovery efficiency (greater than 90%), the yields of DNA were high (38 and 12 micrograms/g [wet weight] from sediments and soil, respectively). This method generated minimal shearing of the extracted DNA. The crude DNA could be further purified with an Elutip-d column if necessary. An additional advantage of this method is that only 1 g of sample is required, which allows for the analysis of small samples and the processing of many samples in a relatively short (7 h) period. 相似文献
33.
Yang YB Lin CS Tseng CP Wang YJ Tsai YC 《Applied and environmental microbiology》1991,57(4):1259-1260
A d-aminoacylase from Alcaligenes faecalis DA1 has been purified to homogeneity by a simple purification procedure with two columns, Fractogel DEAE-650 and HW-50. The specific activity of the purified enzyme was found to be 580 U/mg of protein with N-acetyl-dl-methionine as the reaction substrate. The apparent molecular weight and isoelectric point of this enzyme were determined to be 55,000 and 5.4, respectively. 相似文献
34.
Y J Tsai H Abe H Maruta T Hatano H Nishina H Sakagami T Okuda S Tanuma 《Biochemistry international》1991,24(5):889-897
Three classes of chemically defined tannins, gallotannins, ellagitannins and condensed tannins were examined for their inhibitory activities against purified poly (ADP-ribose) glycohydrolase. Ellagitannins showed higher inhibitory activities than gallotannins. In contrast, condensed tannins, which consist of an epicathechin gallate (ECG) oligomer without a glucose core were not appreciably inhibitory. Kinetic analysis revealed that the inhibition of ellagitannins was competitive with respect to the substrate poly(ADP-ribose), whereas gallotannins exhibited mixed-type inhibition. These results suggest that conjugation with glucose of hexahydroxy-diphenoyl (HHDP) group, which is a unique component of ellagitannins, potentiated the inhibitory activity, and that the structure of ellagitannins may have a functional domain which competes with poly(ADP-ribose) on the poly(ADP-ribose) glycohydrolase molecule. 相似文献
35.
Crystal structure of the Y52F/Y73F double mutant of phospholipase A2: increased hydrophobic interactions of the phenyl groups compensate for the disrupted hydrogen bonds of the tyrosines. 总被引:3,自引:1,他引:2 下载免费PDF全文
C. Sekharudu B. Ramakrishnan B. Huang R. T. Jiang C. M. Dupureur M. D. Tsai M. Sundaralingam 《Protein science : a publication of the Protein Society》1992,1(12):1585-1594
The enzyme phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 ester bond of membrane phospholipids. The highly conserved Tyr residues 52 and 73 in the enzyme form hydrogen bonds to the carboxylate group of the catalytic Asp-99. These hydrogen bonds were initially regarded as essential for the interfacial recognition and the stability of the overall catalytic network. The elimination of the hydrogen bonds involving the phenolic hydroxyl groups of the Tyr-52 and -73 by changing them to Phe lowered the stability but did not significantly affect the catalytic activity of the enzyme. The X-ray crystal structure of the double mutant Y52F/Y73F has been determined at 1.93 A resolution to study the effect of the mutation on the structure. The crystals are trigonal, space group P3(1)21, with cell parameters a = b = 46.3 A and c = 102.95 A. Intensity data were collected on a Siemens area detector, 8,024 reflections were unique with an R(sym) of 4.5% out of a total of 27,203. The structure was refined using all the unique reflections by XPLOR to a final R-factor of 18.6% for 955 protein atoms, 91 water molecules, and 1 calcium ion. The root mean square deviation for the alpha-carbon atoms between the double mutant and wild type was 0.56 A. The crystal structure revealed that four hydrogen bonds were lost in the catalytic network; three involving the tyrosines and one involving Pro-68. However, the hydrogen bonds of the catalytic triad, His-48, Asp-99, and the catalytic water, are retained. There is no additional solvent molecule at the active site to replace the missing hydroxyl groups; instead, the replacement of the phenolic OH groups by H atoms draws the Phe residues closer to the neighboring residues compared to wild type; Phe-52 moves toward His-48 and Asp-99 of the catalytic diad, and Phe-73 moves toward Met-8, both by about 0.5 A. The closing of the voids left by the OH groups increases the hydrophobic interactions compensating for the lost hydrogen bonds. The conservation of the triad hydrogen bonds and the stabilization of the active site by the increased hydrophobic interactions could explain why the double mutant has activity similar to wild type. The results indicate that the aspartyl carboxylate group of the catalytic triad can function alone without additional support from the hydrogen bonds of the two Tyr residues. 相似文献
36.
Construction and expression of nonsense suppressor tRNAs which function in plant cells 总被引:3,自引:0,他引:3
Scott Franklin Tsai Yun Lin William R. Folk 《The Plant journal : for cell and molecular biology》1992,2(4):583-588
An Arabidopsis thaliana L. DNA containing the tRNA(TrpUGG) gene was isolated and altered to encode the amber suppressor tRNA(TrpUAG) or the ochre suppressor tRNA(TrpUAA). These DNAs were electroporated into carrot protoplasts and tRNA expression was demonstrated by the translational suppression of amber and ochre nonsense mutations in the chloramphenicol acetyltransferase (CAT) reporter gene. DNAs encoding tRNA(TrpUAG) and tRNA(TrpUAA) nonsense suppressor tRNAs caused suppression of their cognate nonsense codons in CAT mRNAs, with the tRNA(TrpUAG) gene exhibiting the greater suppression under optimal conditions for expression of CAT. The development of these translational suppressors which function in plant cells facilitates the study of plant tRNA gene expression and will make possible the manipulation of plant protein structure and function. 相似文献
37.
38.
The GTPase stimulatory activities of the neurofibromatosis type 1 and the yeast IRA2 proteins are inhibited by arachidonic acid. 总被引:3,自引:1,他引:2 下载免费PDF全文
M Golubi K Tanaka S Dobrowolski D Wood M H Tsai M Marshall F Tamanoi D W Stacey 《The EMBO journal》1991,10(10):2897-2903
Three proteins, GTPase activating protein (GAP), neurofibromatosis 1 (NF1) and the yeast inhibitory regulator of the RAS-cAMP pathway (IRA2), have the ability to stimulate the GTPase activity of Ras proteins from higher animals or yeast. Previous studies indicate that certain lipids are able to inhibit this activity associated with the mammalian GAP protein. Inhibition of GAP would be expected to biologically activate Ras protein. In these studies arachidonic acid is shown also to inhibit the activity of the catalytic fragments of the other two proteins, mammalian NF1 and the yeast IRA2 proteins. In addition, phosphatidic acid (containing arachidonic and stearic acid) was inhibitory for the catalytic fragment of NF1 protein, but did not inhibit the catalytic fragments of GAP or IRA2 proteins. These observations emphasize the biochemical similarity of these proteins and provide support for the suggestion that lipids might play an important role in their biological control, and therefore also in the control of Ras activity and cellular proliferation. 相似文献
39.
40.
Kenneth H. Astrin Cecilia A. Warner Han-Wook Yoo Paul J. Goodfellow Shih-Feng Tsai Robert J. Desnick 《Human genetics》1991,87(1):18-22
Summary Uroporphyrinogen III synthase [UROS; hydroxymethylbilane hydro-lyase (cyclizing), EC 4.2.1.75] is the fourth enzyme in the human heme biosynthetic pathway. The recent isolation of the cDNA encoding human UROS facilitated its chromosomal localization. Human UROS sequences were specifically amplified by the polymerase chain reaction (PCR) from genomic DNA of two independent panels of human-rodent somatic cell hybrids. There was 100% concordance for the presence of the human UROS PCR product and human chromosome 10. For each of the other chromosomes, there was 19%–53% discordance with human UROS. The chromosomal assignment was confirmed by Southern hybridization analysis of DNA from somatic cell hybrids with the full-length UROS cDNA. Using human-rodent hybrids containing different portions of human chromosome 10, we assigned the UROS gene to the region 10q25.2 q26.3. 相似文献