全文获取类型
收费全文 | 1925篇 |
免费 | 263篇 |
出版年
2023年 | 9篇 |
2022年 | 8篇 |
2021年 | 21篇 |
2020年 | 16篇 |
2019年 | 18篇 |
2018年 | 30篇 |
2017年 | 26篇 |
2016年 | 37篇 |
2015年 | 82篇 |
2014年 | 75篇 |
2013年 | 93篇 |
2012年 | 116篇 |
2011年 | 136篇 |
2010年 | 96篇 |
2009年 | 75篇 |
2008年 | 121篇 |
2007年 | 128篇 |
2006年 | 116篇 |
2005年 | 118篇 |
2004年 | 122篇 |
2003年 | 117篇 |
2002年 | 124篇 |
2001年 | 48篇 |
2000年 | 32篇 |
1999年 | 32篇 |
1998年 | 21篇 |
1997年 | 20篇 |
1996年 | 15篇 |
1995年 | 19篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 21篇 |
1990年 | 20篇 |
1989年 | 19篇 |
1988年 | 9篇 |
1987年 | 19篇 |
1986年 | 19篇 |
1985年 | 12篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1975年 | 9篇 |
1974年 | 6篇 |
1973年 | 8篇 |
1972年 | 15篇 |
排序方式: 共有2188条查询结果,搜索用时 15 毫秒
1.
Construction of improved bacteriophage phi 105 vectors for cloning by transfection in Bacillus subtilis 总被引:5,自引:0,他引:5
A series of improved phage vectors have been constructed, based on Bacillus subtilis bacteriophage phi 105, which can be used to clone genes in B. subtilis by direct transfection of protoplasts. The new vectors, designated phi 105J23, phi 105J24, phi 105J27 and phi 105J28, show frequencies of plaque formation that are equal to those of wild-type phi 105. This represents at least a 10-fold improvement over phi 105J9, the vector used in previous cloning experiments. Two of the new vectors phi 105J27 and phi 105J28 incorporate a mutation, cts-52, that renders the prophage temperature inducible. This has made it possible to devise a rapid small-scale procedure for screening progeny phage for the presence of inserted DNA. The usefulness of the new vectors is illustrated in the accompanying paper by cloning more than 20 B. subtilis sporulation genes. 相似文献
2.
3.
Characteristics of alcohol/polyol dehydrogenases. The zinc-containing long-chain alcohol dehydrogenases 总被引:24,自引:0,他引:24
Sixteen characterized alcohol dehydrogenases and one sorbitol dehydrogenase have been aligned. The proteins represent two formally different enzyme activities (EC 1.1.1.1 and EC 1.1.1.14), three different types of molecule (dimeric alcohol dehydrogenase, tetrameric alcohol dehydrogenase, tetrameric sorbitol dehydrogenase), metalloproteins with different zinc contents (1 or 2 atoms per subunit), and polypeptide chains from different kingdoms and orders (mammals, higher plants, fungus, yeasts). Present comparisons utilizing all 17 forms reveal extensive variations in alcohol dehydrogenase, but with evolutionary changes that are of the same order in different branches and at different times. They emphasize the general importance of particular residues, suggesting related overall functional constraints in the molecules. The comparisons also define a few coincidences between intron positions in the genes and gap positions in the gene products. Only 22 residues are strictly conserved; half of these are Gly, and most of the remaining ones are Pro or acidic residues. No basic residue, no straight-chain hydrophobic residues, no aromatic residues, and essentially no branched-chain or polar neutral residues are invariable. Tentative consensus sequences were calculated, defining 13 additional residues likely to be typical of but not invariant among the alcohol dehydrogenases. These show a predominance of Val, charged residues, and Gly. Combined, the comparisons, which are particularly relevant to the data base for protein engineering, illustrate the requirements for functionally important binding interactions, and the extent of space restrictions in proteins with related overall conformations and functions. 相似文献
4.
Frederick Errington is Aaaociate Professor of Anthropology, at Keene State College, New Hampshire. Deborah Gewertz is Professor of Anthropology, at Amherst College, Massachusetts. 相似文献
5.
一、前言哈巴德布鲁克实验林(Hubbard Brook Experimental Forest)位于美国东北部新罕布什尔州中部的白山国家森林中。该地区位于典型温带湿润气候区内,年平均降水量为129.5cm,全年月平均降水量变化不大,冬雪夏雨。蒸发蒸腾量以每年6—9月为最大(Likens等,1977;Bormann等,1979)。该实验林为北美温带落叶阔叶林,属红果云杉(Picea rubens)-阔叶林。Hubbard Brook Ecosystem Study(HBES)是开始最 相似文献
6.
7.
8.
The soluble form of human brain catechol-O-methyltransferase (EC 2.1.1.6, COMT) has been purified approximately 4,000-fold from a 250,000 X g supernatant solution. The purified enzyme exhibits a molecular weight near 27,500 and a pI value equal to approximately pH 5.0. Initial velocity and product inhibition studies are consistent with an ordered reaction mechanism for soluble COMT. Tropolone, a dead-end inhibitor, exhibited a competitive pattern of inhibition when dopamine (DA) was the varied substrate and an uncompetitive pattern when S-adenosyl-L-methionine (SAM) was the varied substrate. These observations strongly suggest that the soluble form of COMT from human brain catalyzes the O-methylation of catecholamines via an ordered reaction mechanism in which SAM is the leading substrate. Since the membrane-bound form of COMT catalyzes the O-methylation of catecholamines through an identical reaction mechanism, these data provide further evidence that two forms of COMT, while being localized in distinct subcellular compartments, are quite similar in their molecular structure. 相似文献
9.
Characterization of Membrane-Bound and Soluble Catechol-O-Methyltransferase from Human Frontal Cortex 总被引:4,自引:3,他引:1
Abstract: Catechol- O -methyltransferase (COMT; E.C. 2.1.1.6) from human frontal cortex occurs in both a soluble and membrane-bound form. Attempts to solubilize the membrane-bound transferase by repeated washing or by extraction into solutions of high ionic strength were unsuccessful. The finding that Triton X-100 was capable of solubilizing membrane-bound COMT suggested that the membrane-bound transferase is an integral membrane protein. The membrane-bound and soluble enzymes did not differ in their requirements for magnesium ions or in their pH-activity profiles; both enzymes showed an optimum near pH 8.0 when assayed in phosphate buffer. In addition, the two enzymes did not differ in the degree of inhibition caused by CaCl2 , both enzymes displaying 65% inhibition at 2.5 m M CaCl2 . The competitive inhibitors tropolone and nordihydroguaiaretic acid displayed K i values for the membrane-bound transferase five- to 10-fold lower than those observed for the soluble transferase. Solubilization of membrane-bound COMT in Triton X-100 resulted in an increase in the apparent K m value of the membrane-bound transferase for dopamine. The increase in K m appeared to be due to apparent competitive inhibition by Triton X-100 and reached a limiting value of approximately 80 μM. These results confirm that membrane-bound COMT is an integral membrane protein that may be structurally distinct from soluble COMT. 相似文献
10.
Efficient preparation of spheroplasts fromCandida utilis, Saccharomyces cerevisiae, andSchizosaccharomyces pombe, using a purified mixture of enzymes fromTrichoderma harzianum, is described. Limitations of other methods, and differences between yeasts are demonstrated. 相似文献