全文获取类型
收费全文 | 4286篇 |
免费 | 379篇 |
国内免费 | 1篇 |
专业分类
4666篇 |
出版年
2023年 | 20篇 |
2022年 | 29篇 |
2021年 | 61篇 |
2020年 | 39篇 |
2019年 | 46篇 |
2018年 | 90篇 |
2017年 | 65篇 |
2016年 | 106篇 |
2015年 | 171篇 |
2014年 | 217篇 |
2013年 | 216篇 |
2012年 | 302篇 |
2011年 | 275篇 |
2010年 | 191篇 |
2009年 | 162篇 |
2008年 | 240篇 |
2007年 | 254篇 |
2006年 | 220篇 |
2005年 | 226篇 |
2004年 | 179篇 |
2003年 | 213篇 |
2002年 | 180篇 |
2001年 | 45篇 |
2000年 | 33篇 |
1999年 | 35篇 |
1998年 | 46篇 |
1997年 | 38篇 |
1996年 | 31篇 |
1995年 | 30篇 |
1994年 | 25篇 |
1993年 | 28篇 |
1992年 | 22篇 |
1991年 | 33篇 |
1990年 | 47篇 |
1989年 | 29篇 |
1988年 | 32篇 |
1987年 | 36篇 |
1986年 | 29篇 |
1985年 | 27篇 |
1984年 | 30篇 |
1983年 | 23篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 23篇 |
1978年 | 23篇 |
1977年 | 26篇 |
1975年 | 26篇 |
1972年 | 20篇 |
1969年 | 21篇 |
1967年 | 21篇 |
排序方式: 共有4666条查询结果,搜索用时 15 毫秒
51.
N S Yadav A Wierzbicki M Aegerter C S Caster L Prez-Grau A J Kinney W D Hitz J R Booth Jr B Schweiger K L Stecca et al. 《Plant physiology》1993,103(2):467-476
Arabidopsis thaliana T-DNA transformants were screened for mutations affecting seed fatty acid composition. A mutant line was found with reduced levels of linolenic acid (18:3) due to a T-DNA insertion. Genomic DNA flanking the T-DNA insertion was used to obtain an Arabidopsis cDNA that encodes a polypeptide identified as a microsomal omega-3 fatty acid desaturase by its complementation of the mutation. Analysis of lipid content in transgenic tissues demonstrated that this enzyme is limiting for 18:3 production in Arabidopsis seeds and carrot hairy roots. This cDNA was used to isolate a related Arabidopsis cDNA, whose mRNA is accumulated to a much higher level in leaf tissue relative to root tissue. This related cDNA encodes a protein that is a homolog of the microsomal desaturase but has an N-terminal extension deduced to be a transit peptide, and its gene maps to a position consistent with that of the Arabidopsis fad D locus, which controls plastid omega-3 desaturation. These Arabidopsis cDNAs were used as hybridization probes to isolate cDNAs encoding homologous proteins from developing seeds of soybean and rapeseed. The high degree of sequence similarity between these sequences suggests that the omega-3 desaturases use a common enzyme mechanism. 相似文献
52.
53.
A sensitive and reproducible HPLC method utilizing a commercially available chiral α1-acid glycoprotein (AGP) phase has been developed to separate and quantify the enantiomers of nicotine. The method is suitable for routine use as indicated by column life. The quantification of (R/S:0.05/99.95)-nicotine or (R/S:99/1)-nicotine was possible. In addition, the separation or at least partial separation of the enantiomers of nornicotine and nornicotine-derived compounds was achieved. © 1993 Wiley-Liss, Inc. 相似文献
54.
We studied host plant preference of the common blue butterfly, Polyommatus icarus , and larval performance on two different host plants, Oxytropis campestris and Lotus corniculatus . The study species is a small lycaenid butterfly believed to be relatively sedentary. The study populations originated from two different and widely separated geographical areas. In one area both hosts are naturally occurring, with O. campestris being most abundant at the study sites, in the other area only one of the host plants, L. corniculatus , is present. There was no difference in oviposition preference or larval performance between populations from the two different areas. Hence, P. icarus from sites dominated by O. campestris has not evolved a higher preference for or better performance on this host plant. More surprisingly, P. icarus from the area were O. campestris is completely absent has retained not only good larval performance on this host plant but also high female preference for it. This conservatism at a large geographical scale is seen even though there seems to be genetic variation present in both populations, at least for preference but perhaps also for performance. We suggest that such lack of variation in resource utilization between populations may be evidence for weak selection against "preferences" for plants that are rare or absent. A combination of other constraining factors may also contribute to some degree, especially stepping-stone gene flow between populations. 相似文献
55.
Ziv Bar-Joseph Georg K Gerber David K Gifford Tommi S Jaakkola Itamar Simon 《Journal of computational biology》2003,10(3-4):341-356
We present algorithms for time-series gene expression analysis that permit the principled estimation of unobserved time points, clustering, and dataset alignment. Each expression profile is modeled as a cubic spline (piecewise polynomial) that is estimated from the observed data and every time point influences the overall smooth expression curve. We constrain the spline coefficients of genes in the same class to have similar expression patterns, while also allowing for gene specific parameters. We show that unobserved time points can be reconstructed using our method with 10-15% less error when compared to previous best methods. Our clustering algorithm operates directly on the continuous representations of gene expression profiles, and we demonstrate that this is particularly effective when applied to nonuniformly sampled data. Our continuous alignment algorithm also avoids difficulties encountered by discrete approaches. In particular, our method allows for control of the number of degrees of freedom of the warp through the specification of parameterized functions, which helps to avoid overfitting. We demonstrate that our algorithm produces stable low-error alignments on real expression data and further show a specific application to yeast knock-out data that produces biologically meaningful results. 相似文献
56.
57.
In vitro and in vivo action of antisense RNA 总被引:3,自引:0,他引:3
The transient or permanent expression of antisense RNA represents one option to apply antisense techniques in biotechnology
and medical research. Despite the increasing importance and use of antisense nucleic acids as well as their significant antisense-specific
phenotypic effects in vivo, there is an obvious lack of explanation for the mechanism of their action. By studying naturally
occurring antisense RNA and analyzing their mechanism of action we attempt to learn more about the design, the use, and the
critical parameters of artificial antisense RNA. Attempts to derive models from biochemical and structural studies for the
interactions between antisense RNAs and their targets will be discussed. 相似文献
58.
Abstract: Stimulation of chick sympathetic neurons in culture by the cholinergic agonists acetylcholine, nicotine, and 1,1-dimethyl-4-phenylpiperazinium (all at 10–1,000 µmol/L) induced concentration-dependent increases of free calcium levels measured by fura 2 fluorescence in neuronal processes. The response evoked by acetylcholine had both nicotinic and muscarinic components, whereas that induced by 1,1-dimethyl-4-phenylpiperazinium was purely nicotinic. Tetrodotoxin (0.3 µmol/L) blocked completely the increase of intraterminal free calcium level evoked by electrical stimulation. On the other hand, stimulation with 1,1-dimethyl-4-phenylpiperazinium still evoked 20–25% of the control response in the presence of tetrodotoxin. The concentration-response relationship of 1,1-dimethyl-4-phenylpiperazinium stimulation did not differ in the absence and in the presence of tetrodotoxin. The nicotinic antagonists d -tubocurarine (10 µmol/L) and mecamylamine (10 µmol/L), but not α-bungarotoxin (125 nmol/L), prevented the increase of intraterminal free calcium level evoked by 1,1-dimethyl-4-phenylpiperazinium (100 µmol/L) in the presence of tetrodotoxin. These observations indicate the presence of nicotinic receptors on neuronal processes that increase the intraterminal concentration of free calcium and probably modulate transmitter release. Their pharmacological properties are similar to those of nicotinic receptors located on neuronal cell bodies. 相似文献
59.
Prof. Dr. Wolf-Christian Dullo Dr. Marcos Gektidis Prof. Dr. Stjepko Golubic Dr. Georg A. Heiss Dipl. Biol. Heike Kampmann Dr. William Kiene Dipl. Ökol. Dieter K. Kroll Dipl. Biol. Martin L. Kuhrau Dr. Gudrun Radtke Dr. John G. Reijmer Dr. Götz B. Reinicke Prof. Dr. Dietrich Schlichter Prof. Dr. Helmut Schuhmacher Klaus Vogel 《Facies》1995,32(1):145-188
60.
Androgen Ablation Leads to an Upregulation and Intranuclear Accumulation of Deoxyribonuclease I in Rat Prostate Epithelial Cells Paralleling Their Apoptotic Elimination 总被引:1,自引:0,他引:1 下载免费PDF全文