全文获取类型
收费全文 | 1728篇 |
免费 | 213篇 |
国内免费 | 9篇 |
出版年
2022年 | 12篇 |
2021年 | 23篇 |
2020年 | 14篇 |
2019年 | 15篇 |
2018年 | 20篇 |
2017年 | 15篇 |
2016年 | 35篇 |
2015年 | 91篇 |
2014年 | 86篇 |
2013年 | 92篇 |
2012年 | 125篇 |
2011年 | 111篇 |
2010年 | 93篇 |
2009年 | 68篇 |
2008年 | 88篇 |
2007年 | 70篇 |
2006年 | 82篇 |
2005年 | 75篇 |
2004年 | 64篇 |
2003年 | 70篇 |
2002年 | 57篇 |
2001年 | 61篇 |
2000年 | 57篇 |
1999年 | 56篇 |
1998年 | 25篇 |
1997年 | 35篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 26篇 |
1993年 | 15篇 |
1992年 | 36篇 |
1991年 | 31篇 |
1990年 | 32篇 |
1989年 | 20篇 |
1988年 | 21篇 |
1987年 | 26篇 |
1986年 | 10篇 |
1985年 | 21篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1973年 | 5篇 |
1972年 | 8篇 |
1968年 | 6篇 |
排序方式: 共有1950条查询结果,搜索用时 15 毫秒
31.
A combination of "rational" and "irrational" strategies for the creation of enzymes with novel properties is proving to be a powerful concept in the field of enzyme engineering. Guided by principles of physical organic chemistry, rational design strategies are used to identify suitable target enzymes and to choose appropriate molecular biological methods for engineering purposes. In contrast, irrational (or random) strategies are centered around the biological paradigm of stochastic molecular evolution. As illustrated in this review, such a hybrid approach is particularly useful for the design of new modular enzymes. (c) 1996 John Wiley & Sons, Inc. 相似文献
32.
Tracking of single fluorescent particles in three dimensions: use of cylindrical optics to encode particle position. 总被引:7,自引:3,他引:4 下载免费PDF全文
We present a novel optical technique for three-dimensional tracking of single fluorescent particles using a modified epifluorescence microscope containing a weak cylindrical lens in the detection optics and a microstepper-controlled fine focus. Images of small, fluorescent particles were circular in focus but ellipsoidal above and below focus; the major axis of the ellipsoid shifted by 90 degrees in going through focus. Particle z position was determined from the image shape and orientation by applying a peak detection algorithm to image projections along the x and y axes; x, y position was determined from the centroid of the particle image. Typical spatial resolution was 12 nm along the optical axis and 5 nm in the image plane with a maximum sampling rate of 3-4 Hz. The method was applied to track fluorescent particles in artificial solutions and living cells. In a solution of viscosity 30 cP, the mean squared distance (MSD) traveled by a 264 nm diameter rhodamine-labeled bead was linear with time to 20 s. The measured diffusion coefficient, 0.0558 +/- 0.001 micron2/s (SE, n = 4), agreed with the theoretical value of 0.0556 micron2/s. Statistical variability of MSD curves for a freely diffusing bead was in quantitative agreement with Monte Carlo simulations of three-dimensional random walks. In a porous glass matrix, the MSD data was curvilinear and showed reduced bead diffusion. In cytoplasm of Swiss 3T3 fibroblasts, bead diffusion was restricted. The water permeability in individual Chinese Hamster Ovary cells was measured from the z movement of a fluorescent bead fixed at the cell surface in response osmotic gradients; water permeability was increased by > threefold in cells expressing CHIP28 water channels. The simplicity and precision of this tracking method may be useful to quantify the complex trajectories of fluorescent particles in living cells. 相似文献
33.
Abstract: The Na+ /Ca2+ exchanger is an important element in the maintenance of calcium homeostasis in bovine chromaffin cells. The Na+ /Ca2+ exchanger from other cell types has been extensively studied, but little is known about its regulation in the cell. We have investigated the role of reversible protein phosphorylation in the activity of the Na+ /Ca2+ exchanger of these cells. Cells treated with 1 m M dibutyryl cyclic AMP (dbcAMP), 1 µ M phorbol 12,13-dibutyrate, 1 µ M okadaic acid, or 100 n M calyculin A showed lowered Na+ /Ca2+ exchange activity and prolonged cytosolic Ca2+ transients caused by depolarization. A combination of 10 n M okadaic acid and 1 µ M dbcAMP synergistically inhibited Na+ /Ca2+ exchange activity. Conversely, 50 µ M 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a protein kinase inhibitor, enhanced Na+ /Ca2+ exchange activity. Moreover, we used cyclic AMP-dependent protein kinase and calcium phospholipid-dependent protein kinase catalytic subunits to phosphorylate isolated membrane vesicles and found that the Na+ /Ca2+ exchange activity was inhibited by this treatment. These results indicate that reversible protein phosphorylation modulates the activity of the Na+ /Ca2+ exchanger and suggest that modulation of the exchanger may play a role in the regulation of secretion. 相似文献
34.
Fa-Ten Kao Jingwei Yu Suhong Tong Jianxin Qi Sankhavaram R. Patanjali Sherman M. Weissman David Patterson 《Genomics》1994,23(3)
To increase candidate genes from human chromosome 21 for the analysis of Down syndrome and other genetic diseases localized on this chromosome, we have isolated and studied 9 cDNA clones encoded by chromosome 21. For isolating cDNAs, single-copy microclones from a chromosome 21 microdissection library were used in direct screening of various cDNA libraries. Seven of the cDNA clones have been regionally mapped on chromosome 21 using a comprehensive hybrid mapping panel comprising 24 cell hybrids that divide the chromosome into 33 subregions. These cDNA clones with refined mapping positions should be useful for identification and cloning of genes responsible for the specific component phenotypes of Down syndrome and other diseases on chromosome 21, including progressive myoclonus epilepsy in 21q22.3. 相似文献
35.
The effects of salicylic acid (SA) on ethylene biosynthesis in detached rice leaves were investigated. SA at pH 3.5 effectively inhibited ethylene production within 2 h of its application. It inhibited the conversion of ACC to ethylene, but did not affect the levels of ACC and conjugated ACC. Thus, the inhibitory effect of SA resulted from the inhibition of both synthesis of ACC and the conversion of ACC to ethylene.Abbreviations ACC
1-aminocyclopropane-1-carboxylic acid
- EFE
ethylene-forming enzyme
- SA
salicylic acid 相似文献
36.
Carbohydrate moiety of the Petunia inflata S3 protein is not required for self-incompatibility interactions between pollen and pistil. 总被引:7,自引:3,他引:4 下载免费PDF全文
For Petunia inflata and Nicotiana alata, which display gametophytic self-incompatibility, S proteins (the products of the multiallelic S gene in the pistil) have been shown to control the pistil's ability to recognize and reject self-pollen. The biochemical mechanism for rejection of self-pollen by S proteins has been shown to involve their ribonuclease activity; however, the molecular basis for self/non-self recognition by S proteins is not yet understood. Here, we addressed whether the glycan chain of the S3 protein of P. inflata is involved in self/non-self recognition by producing a nonglycosylated S3 protein in transgenic plants and examining the effect of deglycosylation on the ability of the S3 protein to reject S3 pollen. The S3 gene was mutagenized by replacing the codon for Asn-29, which is the only potential N-glycosylation site of the S3 protein, with a codon for Asp, and the mutant S3 gene was introduced into P. inflata plants of the S1S2 genotype. Six transgenic plants that produced a normal level of the nonglycosylated S3 protein acquired the ability to reject S3 pollen completely. These results suggest that the carbohydrate moiety of the S3 protein does not play a role in recognition or rejection of self-pollen and that the S allele specificity determinant of the S3 protein and those S proteins that contain a single glycan chain at the same site as the S3 protein must reside in the amino acid sequence itself. 相似文献
37.
Does the use of DM-nitrophen,nitr-5, or diazo-2 interfere with the measurement of indo-1 fluorescence? 总被引:1,自引:0,他引:1 下载免费PDF全文
Emission spectra of the photolabile Ca2+ chelators DM-nitrophen, nitr-5, and diazo-2 were studied alone, and in the presence of indo-1, to investigate potential interactions that would make the simultaneous manipulation and ratiometric measurement of the intracellular Ca2+ concentration difficult. Neither diazo-2 nor its photoproduct were found to be significantly fluorescent, and consequently concentrations of diazo-2 up to 20 times that of indo-1 did not distort the emission spectra of indo-1. DM-nitrophen was scarcely fluorescent, but its fluorescence did increase upon photolysis. In contrast to diazo-2 and DM-nitrophen, nitr-5 itself was found to be quite fluorescent, and this fluorescence was significantly increased upon photolysis. Thus, combined use of nitr-5 and indo-1 poses the most difficulty. The emission spectra of all the investigated compounds were used to define experimental conditions and calibration procedures that make possible simultaneous measurement and manipulation of the intracellular Ca2+ concentration. 相似文献
38.
Piotr Berman Paul Bertone Bhaskar Dasgupta Mark Gerstein Ming-Yang Kao Michael Snyder 《Journal of computational biology》2004,11(4):766-785
In this paper, we consider several variations of the following basic tiling problem: given a sequence of real numbers with two size-bound parameters, we want to find a set of tiles of maximum total weight such that each tiles satisfies the size bounds. A solution to this problem is important to a number of computational biology applications such as selecting genomic DNA fragments for PCR-based amplicon microarrays and performing homology searches with long sequence queries. Our goal is to design efficient algorithms with linear or near-linear time and space in the normal range of parameter values for these problems. For this purpose, we first discuss the solution to a basic online interval maximum problem via a sliding-window approach and show how to use this solution in a nontrivial manner for many of the tiling problems introduced. We also discuss NP-hardness results and approximation algorithms for generalizing our basic tiling problem to higher dimensions. Finally, computational results from applying our tiling algorithms to genomic sequences of five model eukaryotes are reported. 相似文献
39.
Localization and interaction of the cis-acting elements for abscisic acid, VIVIPAROUS1, and light activation of the C1 gene of maize. 总被引:12,自引:3,他引:9 下载免费PDF全文
The C1 regulatory gene of the maize anthocyanin pathway is regulated by a combination of developmental and environmental signals that include the Viviparous1 (Vp1) gene, abscisic acid (ABA), and light. Using protoplast electroporation and particle bombardment assays, we have defined c/s-acting elements that are necessary and sufficient for the activation of C1 by ABA, VP1, and light, respectively. The sequence from positions -142 to -132 (CGTCCATGCAT) is essential for VP1 activation, whereas a larger overlapping element from -147 to -132 (CGTGTCGTCCATGCAT) is necessary and sufficient for activation by ABA. A separate light (blue and red)-responsive c/s element is located between positions -116 and -59. Light interacts synergistically with the ABA and VP1 responses in transient expression assays, suggesting that combinatorial interaction between modules plays a role in integrating these signals in the developing seed. 相似文献
40.