全文获取类型
收费全文 | 10279篇 |
免费 | 536篇 |
国内免费 | 4篇 |
专业分类
10819篇 |
出版年
2022年 | 53篇 |
2021年 | 110篇 |
2020年 | 53篇 |
2019年 | 79篇 |
2018年 | 115篇 |
2017年 | 105篇 |
2016年 | 150篇 |
2015年 | 230篇 |
2014年 | 280篇 |
2013年 | 609篇 |
2012年 | 500篇 |
2011年 | 455篇 |
2010年 | 297篇 |
2009年 | 294篇 |
2008年 | 478篇 |
2007年 | 447篇 |
2006年 | 460篇 |
2005年 | 413篇 |
2004年 | 419篇 |
2003年 | 443篇 |
2002年 | 393篇 |
2001年 | 416篇 |
2000年 | 408篇 |
1999年 | 339篇 |
1998年 | 139篇 |
1997年 | 109篇 |
1996年 | 85篇 |
1995年 | 83篇 |
1994年 | 78篇 |
1993年 | 87篇 |
1992年 | 244篇 |
1991年 | 249篇 |
1990年 | 199篇 |
1989年 | 182篇 |
1988年 | 183篇 |
1987年 | 161篇 |
1986年 | 149篇 |
1985年 | 147篇 |
1984年 | 122篇 |
1983年 | 111篇 |
1982年 | 90篇 |
1981年 | 59篇 |
1980年 | 57篇 |
1979年 | 91篇 |
1978年 | 84篇 |
1977年 | 61篇 |
1976年 | 57篇 |
1975年 | 49篇 |
1973年 | 58篇 |
1970年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Yuichi Fujita Yasuhiro Takahashi Takayuki Kohchi Haruo Ozeki Kanji Ohyama Hiroshi Matsubara 《Plant molecular biology》1989,13(5):551-561
The frxC gene, one of the unidentified open reading frames present in liverwort chloroplast DNA, shows significant homology with the nifH genes coding for the Fe protein, a component of the nitrogenase complex (Ohyama et al., 1986, Nature 322: 572–574). A truncated form of the frxC gene was designed to be over-expressed in Escherichia coli and an antibody against this protein was prepared using the purified product as an antigen. This antibody reacted with a protein in the soluble fraction of liverwort chloroplasts, which had an apparent molecular weight of 31 000, as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in good agreement with a putative molecular weight of 31945 deduced from the DNA sequence of the frxC gene. In a competitive inhibition experiment, the antigenicity of this protein was indicated to be similar to that of the over-expressed protein in E. coli. Therefore, we concluded that the frxC gene was expressed in liverwort chloroplasts and that its product existed in a soluble form. The molecular weight of the frxC protein was approximately 67 000, as estimated by gel filtration chromatography, indicating that the frxC protein may exist as a dimer of two identical polypeptides analogous to the Fe protein of nitrogenase. The results obtained from affinity chromatography supported the possibility that the frxC protein, which possesses a ATP-binding sequence in its N-terminal region that is conserved among various other ATP-binding proteins, has the ability to bind ATP. 相似文献
42.
Reverse gyrase binding to DNA alters the double helix structure and produces single-strand cleavage in the absence of ATP. 总被引:5,自引:0,他引:5 下载免费PDF全文
Stoichiometric amounts of pure reverse gyrase, a type I topoisomerase from the archaebacterium Sulfolobus acidocaldarius were incubated at 75 degrees C with circular DNA containing a single-chain scission. After covalent closure by a thermophilic ligase and removal of bound protein molecules, negatively supercoiled DNA was produced. This finding, obtained in the absence of ATP, contrasts with the ATP-dependent positive supercoiling catalyzed by reverse gyrase and is interpreted as the result of enzyme binding to DNA at high temperature. Another consequence of reverse gyrase stoichiometric binding to DNA is the formation of a cleavable complex which results in the production of single-strand breaks in the presence of detergent. Like eubacterial type I topoisomerase (protein omega), reverse gyrase is tightly attached to the 5' termini of the cleaved DNA. In the light of these results, a comparison is tentatively made between reverse gyrase and the eubacterial type I (omega) and type II (gyrase) topoisomerases. 相似文献
43.
Purification of GTP-binding proteins from bovine brain membranes. Identification of heterogeneity of the alpha-subunit of Go proteins 总被引:3,自引:0,他引:3
Using high-resolution Mono Q column chromatography, we purified 6 distinct peaks of GTP-binding proteins from bovine brain membranes. Five of them consisted of 3 polypeptides with alpha beta gamma-subunits and served as the substrate of islet-activating protein (IAP), pertussis toxin. The other one was purified as alpha-subunit alone and was also ADP-ribosylated by IAP in the presence of beta gamma-subunits. When each alpha-subunit was characterized by immunoblot analysis using various antibodies with defined specificity, the two of them were identified as Gi-1 and Gi-2, and other 4 appeared to be Go or Go-like G proteins. The alpha-subunits of immunologically Go-like proteins were apparently distinguishable from one another on elution profiles from the Mono Q column. Thus, there was a heterogeneity of the alpha-subunit of Go in the brain membranes. 相似文献
44.
Amino acid sequence and relative biological activity of eel atrial natriuretic peptide 总被引:1,自引:0,他引:1
Y Takei A Takahashi T X Watanabe K Nakajima S Sakakibara 《Biochemical and biophysical research communications》1989,164(1):537-543
A peptide exhibiting vasodepressor and natriuretic activities in rats was isolated from eel atria, and its primary structure was determined as H-Ser-Lys-Ser-Ser-Ser-Pro-Cys-Phe-Gly-Gly-Lys-Leu-Asp-Arg-Ile-Gly-Ser-Tyr-Ser- Gly-Leu-Gly-Cys-Asn-Ser-Arg-Lys-OH. This peptide, termed eel atrial natriuretic peptide (ANP), has sequence homology of 59% to mammalian (human or rat) ANP, 52% to fowl ANP, and 46% to frog ANP. When the biological activity of synthetic eel ANP was compared with that of human ANP, the eel peptide was 110 times more potent for the vasodepressor activity in eels, nearly equipotent for the vasodepressor activity in quails, and 20 times less potent for the vasodepressor and natriuretic activity in rats. 相似文献
45.
46.
S Tanaka S Shiojiri Y Takahashi N Kitaguchi H Ito M Kameyama J Kimura S Nakamura K Ueda 《Biochemical and biophysical research communications》1989,165(3):1406-1414
Expression of three types of mRNA encoding amyloid beta-protein precursor (APP) in various tissues was analysed, using a ribonuclease protection assay, with special reference to Alzheimer's disease (AD). The total content and the proportion of APP mRNAs were specific to each tissue. Among eight tissues examined, the brain was distinct in that the expression level was highest and APP695 mRNA was expressed in abundance. The ratio of APP770/APP751/APP695 mRNAs was approximately 1:10:20 in the cerebral cortex of control brain. The proportions of APP770 mRNA and APP770-plus-APP751 mRNAs increased up to 2.6- and 1.4-fold, respectively, in various regions of AD brain compared with control. The enhanced expression of protease inhibitor-harboring types (APP770 and APP751) may disturb the balance between biosynthesis and degradation of APPs and ultimately lead to accumulation of beta-protein as amyloid. 相似文献
47.
M Kozuka T Ito S Hirose K Takahashi H Hagiwara 《Biochemical and biophysical research communications》1989,159(1):317-323
Effects of endothelin on nonvascular smooth muscle have been examined using rat uterine horns and two modes of endothelin action have been revealed. Endothelin (0.3 nM) caused rhythmic contractions of isolated uterus in the presence of extracellular calcium. The rhythmic contractions were completely inhibited by calcium channel antagonists. These characteristics of endothelin-induced contractions were very similar to those induced by oxytocin. Binding assays using 125I-endothelin showed that endothelin and the calcium channel blockers did not compete for the binding sites. However, endothelin was unique in that it caused, in addition to rhythmic contractions, a slowly developing monophasic contraction that was insensitive to calcium channel blockers. This developing contraction became dominant at higher concentrations of endothelin and was also calcium dependent. 相似文献
48.
Molecular cloning and sequence analysis of cDNA for human hepatocyte growth factor 总被引:62,自引:0,他引:62
K Miyazawa H Tsubouchi D Naka K Takahashi M Okigaki N Arakaki H Nakayama S Hirono O Sakiyama K Takahashi 《Biochemical and biophysical research communications》1989,163(2):967-973
Amino acid sequences of four peptide fragments of human hepatocyte growth factor purified from the plasma of patients with fulminant hepatic failure were determined. Based on the amino acid sequence of one of the fragments, two oligodeoxyribonucleotide mixtures were synthesized and used to screen a human placenta cDNA library. On the screening, two overlapping cDNA clones for human hepatocyte growth factor were isolated and the nucleotide sequence of the cDNA was determined. The entire primary structure of the protein was deduced from the sequence. The protein consists of 728 amino acid residues, including a possible signal peptide at the N-terminus. The sequence revealed that the heavy and light chains which comprise the protein are encoded by the same mRNA and are produced from a common translation product by proteolytic processing. 相似文献
49.
Mechanism of membrane damage induced by the amphipathic peptides gramicidin S and melittin 总被引:4,自引:0,他引:4
T Katsu M Kuroko T Morikawa K Sanchika Y Fujita H Yamamura M Uda 《Biochimica et biophysica acta》1989,983(2):135-141
The action of gramicidin S and melittin on human erythrocytes, Staphylococcus aureus and Escherichia coli was studied as an extension of the previous study (Katsu, T., Ninomiya, C., Kuroko, M., Kobayashi, H., Hirota, T. and Fujita, Y. (1988) Biochim. Biophys. Acta 939, 57-63). These amphipathic peptides stimulated the release of membrane phospholipids outside cells in a concentration range causing permeability change. The shape change of erythrocytes from normal discoid to spiculate form was observed just prior to the release of membrane components. We have proposed the following action mechanism of gramicidin S and melittin. The peptide molecules were predominantly accumulated in the outer half of the bilayer, deforming the erythrocyte cell into crenature. A large accumulation made the membrane structure unstable, resulting in the release of membrane fragments and the simultaneous enhancement of permeability. The action mechanism of these peptides was compared with that of simple surfactants. 相似文献
50.
Myocardial extracellular matrix is organized into a complex arrangement of intercellular and pericellular fibres and fibrils that serves as a supporting framework for contracting cells. Recent evidence suggests that changes in ventricular shape and function occurring after ischaemic injury may be related to alterations of this matrix. In this report we describe the rapid and extensive loss of collagen in myocardial infarction produced by ligating the left anterior descending coronary artery of the rat for 1-3 h. The total collagen content in the myocardial infarct zones after 1, 2 and 3 h of ligation was 75 +/- 8%, 65 +/- 7% and 50 +/- 10% respectively (mean +/- S.D.) of that of either the non-infarcted tissue controls or of the same regions in sex- and age-matched normal left ventricles. A marked decrease also occurred in the residual collagens which were not extractable with 6 M-guanidine hydrochloride, suggesting that rapid degradation of insoluble collagen fibres may also occur. The decreased collagen content in the 3 h myocardial infarct coincided with the appearance of several enzyme activities. Collagenase, other neutral proteinase and presumed lysosomal serine proteinase activities were increased by 3, 3 and 2 times the control values respectively. These results suggest that the increased activities of collagenase and other neutral proteinases may be responsible for the rapid degradation of extracellular matrix collagen in myocardial infarct. 相似文献