全文获取类型
收费全文 | 20041篇 |
免费 | 1731篇 |
国内免费 | 9篇 |
出版年
2023年 | 67篇 |
2022年 | 64篇 |
2021年 | 343篇 |
2020年 | 207篇 |
2019年 | 269篇 |
2018年 | 298篇 |
2017年 | 297篇 |
2016年 | 489篇 |
2015年 | 813篇 |
2014年 | 889篇 |
2013年 | 1106篇 |
2012年 | 1547篇 |
2011年 | 1603篇 |
2010年 | 991篇 |
2009年 | 848篇 |
2008年 | 1257篇 |
2007年 | 1310篇 |
2006年 | 1227篇 |
2005年 | 1204篇 |
2004年 | 1186篇 |
2003年 | 1102篇 |
2002年 | 1019篇 |
2001年 | 207篇 |
2000年 | 182篇 |
1999年 | 212篇 |
1998年 | 278篇 |
1997年 | 186篇 |
1996年 | 153篇 |
1995年 | 189篇 |
1994年 | 183篇 |
1993年 | 152篇 |
1992年 | 128篇 |
1991年 | 128篇 |
1990年 | 120篇 |
1989年 | 107篇 |
1988年 | 105篇 |
1987年 | 116篇 |
1986年 | 99篇 |
1985年 | 99篇 |
1984年 | 101篇 |
1983年 | 119篇 |
1982年 | 113篇 |
1981年 | 116篇 |
1980年 | 94篇 |
1979年 | 62篇 |
1978年 | 58篇 |
1977年 | 62篇 |
1976年 | 53篇 |
1975年 | 37篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Marion S. R?der Mark E. Sorrells Steven D. Tanksley 《Molecular & general genetics : MGG》1992,232(2):215-220
Summary The long-range structure of 5S rRNA gene clusters has been investigated in wheat (Triticum aestivum L.) by means of pulsed field gel electrophoresis. Using aneuploid stocks, 5S rRNA gene clusters were assigned to sites on chromosomes 1B, 1D, 513 and 5D. Cluster sizes were evaluated and the copy number of 5S DNA repeats was estimated at 4700-5200 copies for the short repeating unit (410 bp) and about 3100 copies for the long repeat (500 bp) per haploid genome. A comparison of wheat cultivars revealed extremely high levels of polymorphism in the 5S rRNA gene clusters. With one restriction enzyme digest all varieties tested gave unique banding patterns and, on a per fragment basis, 21-fold more polymorphism was detected among cultivars for 5S DNA compared to standard restriction fragment length polymorphisms (RFLPs) detected with single copy clones. Experiments with aneuploid stocks suggest that the 5S rRNA gene clusters at several chromosomal sites contribute to this polymorphism. A number of previous reports have shown that wheat cultivars are not easily distinguished by isozymes or RFLPs. The high level of variation detected in 5S rRNA gene clusters therefore offers the possibility of a sensitive fingerprinting method for wheat. 5S DNA and other macro-satellite sequences may also serve as hypervariable Mendelian markers for genetic and breeding experiments in wheat. 相似文献
52.
C. Steven McDaniel Taghi Manshouri M. Zouhair Atassi 《Journal of Protein Chemistry》1987,6(5):455-461
A peptide corresponding to residues 26–41 of α-bungarotoxin, and closed by a disulfide bond between two cysteine residues
at the amino and C terminal ends of the peptide, was synthesized and the monomeric form was purified. The peptide, which represents
the exposed part of the long central loop of the toxin molecule, was examined for binding to acetylcholine receptor. The peptide
was shown by radiometric titrations to bind radiolabeled receptor, and radiolabeled peptide was bound by receptor. The specificity
of the binding was confirmed by inhibition with the parent toxin. A synthetic analog of the peptide in which Trp-28 was replaced
by glycine had very little (10%) of the original activity. Succinylation of the amino groups of the peptide resulted in virtually
complete (98%) loss of the binding activity. These results indicate that a shortened loop peptide corresponding to the region
26–41 of α-bungarotoxin exhibits binding activities mimicking those of the parent molecule. In this region, Trp-28, and one
or both of Lys-26 and Lys-38, are essential contact residues in the binding to receptor. 相似文献
53.
54.
Anton Pauw Sunshine A. Van Bael Halton A. Peters Steven D. Allison José L. C. Camargo Miguel Cifuentes-Jara Aurlstela Conserva Teresa Garcia Restom Tamara Heartsill-Scalley Scott A. Mangan Gabriela Nunez-lturri Elsie Rivera-Ocasio Mark Rountree Susanne Vetter Carolina Volkmer de Castllho 《Biotropica》2004,36(3):410-413
55.
Cross-comparisons of nectar production data are complicated because different workers use bags made of various materials to exclude animal visitors. Using clonal populations of Asclepias syriaca and A. exaltata in northern Virginia, we carefully measured the effects of four bagging treatments (bridal veil, pellon, paper, plastic) on microenvironment (temperature, relative humidity) and nectar production (volume, concentration, sucrose amount) over the course of a day. In general, bridal veil bags changed the microenvironment least relative to unbagged controls. Plastic bags resulted in higher temperatures and constantly higher relative humidities. Temperature and relative humidity were also elevated, though less dramatically, in paper and pellon bags. Under more humid conditions, flowers contained larger volumes of more dilute nectar. Therefore, researchers who wish to obtain nectar production data that reflect natural field conditions should use bridal veil, or a material with similar properties, to bag inflorescences. We also performed a watering experiment, involving the addition of the equivalent of a 10-cm rain to the A. syriaca plot. After watering, nectar volumes and sucrose amounts were increased approximately twofold. 相似文献
56.
Hanafi H. Russell Richard J. Jackson David P. Spath Steven A. Book 《The Western journal of medicine》1987,147(5):615-622
Drinking water contamination by toxic chemicals has become widely recognized as a public health concern since the discovery of 1,2-dibromo-3-chloropropane in California''s Central Valley in 1979. Increased monitoring since then has shown that other pesticides and industrial chemicals are present in drinking water. Contaminants of drinking water also include naturally occurring substances such as asbestos and even the by-products of water chlorination. Public water systems, commercially bottled and vended water and mineral water are regulated, and California is also taking measures to prevent water pollution by chemicals through various new laws and programs. 相似文献
57.
Robert A. Goldstein Steven A. Gherini Charles T. Driscoll Richard April Carl L. Schofield Carl W. Chen 《Biogeochemistry》1987,3(1-3):5-20
An integrated analysis of a terrestrial-aquatic ecosystem, the North Branch of the Moose River in the Adirondack region of New York, was conducted. This basin contains a large number of interconnected surface waters that exhibit marked gradients in pH and acid neutralizing capacity (ANC). As a result, the basin has been the focus of research activity, including the Regional Integrated Lake-Watershed Acidification Study (RILWAS). The objective of the current analysis was to use the North Branch of the Moose River as a case study to:
- Evaluate processes regulating the acid-base chemistry of surface waters.
- To assess the effects of surface water acidification on fish populations.
58.
Big Moose Basin: simulation of response to acidic deposition 总被引:2,自引:1,他引:1
George F. Davis John J. Whipple Steven A. Gherini Carl W. Chen Robert A. Goldstein Arland H. Johannes Peter W.H Chan Ronald K. Munson 《Biogeochemistry》1987,3(1-3):141-161
The ILWAS model has been enhanced for application to multiple-lake hydrologic basins. This version of the model has been applied to the Big Moose basin, which includes Big Moose Lake and its tributary streams, lakes, and watersheds. The basin, as defined, includes an area of 96 km2, with over 20 lakes and ponds, and 70 km of streams. Hydrologic and chemical calibrations have been made using data from seven sampling stations. When total atmospheric sulfur loading to the basin is halved, the model predicts, after four years of simulation, a decreasing sulfate concentration and to a lesser extent a rising alkalinity at Big Moose Lake outlet. At the end of four years, the results show an increase in pH of 0.1 to 0.5 pH units depending upon season. 相似文献
59.
When detergent-derived photosystem II (PSII) membranes are treated with CaCl2 to remove the three extrinsic proteins associated with the O2-evolving complex, the resulting membranes (CaPSII) can still catalyze water oxidation if sufficient Ca2+ and Cl- are present. When CaPSII membranes are exposed to single turnover flashes on an O2 rate electrode, anomalous O2 is produced by the first two flashes. The addition of catalase to the membrane suspension completely inhibits O2 produced by the first two flashes, but not by subsequent flashes. Exogenous H2O2 stimulates anomalous O2 production by the first few flashes in CaPSII membranes, but not in control PSII membranes. Diuron (DCMU) does not inhibit H2O2-stimulated O2 production by the first flash. However, it does inhibit the O2 yield of all subsequent flashes, indicating that all flash-induced O2 signals in CaPSII membranes are dependent on photosystem II electron transport. H2O2 stimulation of O2 yields is inhibited in Tris-, heat-, and EDTA-(ethylenediaminetetraacetic acid)-treated CaPSII. In the presence of high salt, H2O2 (but not EDTA) treatment of CaPSII, extracts Mn functional in normal photosynthetic O2 evolution. The addition of exogenous Mn2+ reconstitutes anomalous O2 production in Tris-and H2O2/EDTA-treated CaPSII preparations but only in the presence of H2O2. Anomalous H2O2-stimulated O2 production can be observed both with a Clark electrode (steady state) and an O2 rate electrode (flash sequence). The mechanism involves electron donation from H2O2, mediated by free Mn2+, to PSII, and the 33-kDa extrinsic protein under some conditions can block this process. Since H2O2 can remove functional Mn from CaPSII membranes, its presence can convert functional Mn to the Mn2+ mediator state required for anomalous O2 production. EDTA binds Mn in CaPSII disrupted by H2O2 and prevents anomalous O2 evolution.Abbreviations CaPSII
a PSII preparation washed with approximately 1M CaCl2
- Chl
chlorophyll
- DCBQ
2,6-dichloro-p-benzoquinone
- DCMU (diuron)
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- EDTA
ethylenediaminetetraacetic acid
- MES
2-[N-morpholino]-ethanesulfonic acid
- PSII
a detergent-derived photosystem II membrane preparation
- RC
reaction center
- Tris
tris(hydroxymethyl)-aminomethane
- Yn
oxygen rate electrode flash yield resulting from the nth flash of a sequence of single turnover flashes of light
Operated by the Midwest Research Institute for the U.S. Department of Energy under contract DE-AC02-83CH10093. 相似文献
60.
We have cloned and characterized three distinct alpha-globin haplotypes obtained from inbred strains of the mouse, Mus domesticus. We report here the complete nucleotide sequence of the six alpha-globin genes that the haplotypes contain. Our analysis of these genes and those from one other previously described haplotype indicates that recurrent gene conversion events have played a major role in their history. The pattern of nucleotide substitutions suggests that conversions have occurred both within and between haplotypes. Limited segments of coding and noncoding DNA have been involved in these gene conversion events. In two of the haplotypes, the nonallelic genes of each maintain DNA sequence identity over discrete intervals and encode the same alpha-globin polypeptide. On the other hand, the coding regions of some genes have accumulated replacement changes that result in distinct alpha-globins. In one instance, these changes appear to reflect positive selection of advantageous mutations. 相似文献