全文获取类型
收费全文 | 94799篇 |
免费 | 366篇 |
国内免费 | 884篇 |
专业分类
96049篇 |
出版年
2023年 | 17篇 |
2022年 | 66篇 |
2021年 | 103篇 |
2020年 | 59篇 |
2019年 | 86篇 |
2018年 | 11885篇 |
2017年 | 10721篇 |
2016年 | 7541篇 |
2015年 | 714篇 |
2014年 | 406篇 |
2013年 | 480篇 |
2012年 | 4343篇 |
2011年 | 12933篇 |
2010年 | 12057篇 |
2009年 | 8265篇 |
2008年 | 9834篇 |
2007年 | 11399篇 |
2006年 | 301篇 |
2005年 | 544篇 |
2004年 | 995篇 |
2003年 | 1032篇 |
2002年 | 788篇 |
2001年 | 274篇 |
2000年 | 174篇 |
1999年 | 44篇 |
1998年 | 12篇 |
1997年 | 28篇 |
1996年 | 20篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 42篇 |
1992年 | 27篇 |
1991年 | 57篇 |
1990年 | 16篇 |
1989年 | 21篇 |
1988年 | 23篇 |
1987年 | 19篇 |
1984年 | 14篇 |
1983年 | 19篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1975年 | 7篇 |
1973年 | 7篇 |
1972年 | 251篇 |
1971年 | 277篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
301.
302.
In many species, including wheat, crossing over is distal, and the proximal regions of chromosome arms contribute little to
genetic maps. This was thought to be a consequence of terminal initiation of synapsis favoring distal crossing over. However,
in an inverted rye chromosome arm, the pattern of metaphase I chiasmata was also inverted, suggesting that crossover frequencies
were specific to chromosome segments. Here, wheat chromosome arms 2BS and 4AL, with essentially entire arms inverted in reverse
tandem duplications (rtd), were studied in the MI of meiosis. Inversion–duplication placed the recombining segments in the
middle of the arms. While the overall pairing frequencies of the inverted–duplicated arms were considerably reduced relative
to normal arms, chiasmata, if present, were always located in the same regions as in structurally normal arms, and relative
chiasma frequencies remained the same. The frequencies of fragment or fragment + bridge configurations in AI and AII indicated
that of the two tandemly arranged copies of segments in rtds, the more distal inverted segments were more likely to cross
over than the segments in their original orientations. These observations show that also in wheat, relative crossover frequencies
along chromosome arms are predetermined and independent of the segment location. The segments normally not licensed to cross
over do not do so even when placed in seemingly most favorable positions for it. 相似文献
303.
Yasuro Fuse Takahiro Okamoto Kazuhide Hayakawa Hajime Karatani Etsu Yamada 《Limnology》2016,17(3):207-221
Humic substances extracted and purified from bottom sediments of northern Lake Biwa, Japan, in November 2012 and 2013 were characterized using elemental analysis, Fourier-transform infrared spectroscopy, hydrogen-1 nuclear magnetic resonance (1H NMR) analysis, and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The infrared (IR) bands in the spectra of humic acids showed the presence of amide linkages and polysaccharides. Results of 1H NMR analysis showed that the humic acids contained approximately twice the number of aliphatic protons as those in the Japanese soil standards used for comparison. Results of the Py-GC/MS analysis, which evaluates pyrolysis temperature dependency of the amount of pyrolysis products, showed that the generation of pyrolysis products in humic acids also differed from that in Japanese soil standards but was similar to that of phytoplankton in Lake Biwa. This analysis method is the first to provide extensive information about the chemical structure of humic substances; conventional Py-GC/MS provides limited information for a single temperature. Data suggest that humic acids in lake sediments are related to chemical characteristics of phytoplankton. Results shed new light on the origins of humic substances in deep-water-lake sediments and provide insights into material recycling in such sediments. 相似文献
304.
Hamed Zakikhani Mohd. Khanif Yusop Mohamed Musa Hanafi Radziah Othman Amin Soltangheisi 《Chemical Speciation and Bioavailability》2016,28(1-4):170-181
Intermittently submergence and drainage status of paddy fields can cause alterations in morphological and chemical characteristics of soils. We conducted a sequential fractionation study to provide an insight into solubility of Sulfur (S) and Molybdenum (Mo) in flooded alluvial paddy soils. The samples (0–15 and 15–30 cm) were taken from marine and riverine alluvial soils in Kedah and Kelantan areas, respectively, and were sequentially extracted with NaHCO3, NaOH, HCl, and HClO4–HNO3. Total S in upper and lower layers of Kedah and Kelantan ranged between 273 and 1121 mg kg?1, and 177 to 1509 mg kg?1, respectively. In upper layers and subsoil of Kedah, average total Mo were 0.34 and 0.27 mg kg?1, respectively. Average total Mo in Kelantan were 0.25 mg kg?1 (surface layer) and 0.28 mg kg?1 (subsoil). Cation exchange capacity (CEC) was positively correlated with plant available amounts of Mo in upper layers of Kedah area. Also, total and medium-term plant-available S was correlated with total carbon (C) at lower layers of Kelantan soil series. But in surface layers of Kelantan soil series, CEC was strongly correlated with total and medium-term plant-available S. Our results indicates that the influence of flooding conditions on soil S and Mo contents in paddy fields may cause long-term changes in S and Mo chemical reactivities. 相似文献
305.
306.
307.
Erwinia amylovora causes fire blight of apple, pear, and other members of the Rosaceae family. The enzyme LuxS catalyzes the last step in the
production of autoinducer-2 (AI-2), a molecule implicated with quorum sensing in many bacterial species. It is now well recognized
that LuxS also plays a central role in sulfur metabolism and in the activated methyl cycle, which is responsible for the generation
of S-adenosyl-l-methionine. A research paper has reported that luxS is not involved with quorum sensing in Er. amylovora, but in our study, Er. amylovora strain NCPPB1665 (Ea1665) produced luxS-dependent extracellular AI-2 activity. Additionally, the maximal AI-2 activity occurred during late-exponential and early-stationary
growth phases and diminished during the stationary phase. The luxS mutant of Ea1665 was constructed, and the phenotypes of a defined luxS mutant have been characterized. Inactivation of luxS in Ea1665 impaired motility, extracellular polysaccharide (EPS) production, and tolerance for hydrogen peroxide, and reduced virulence
on pear leaves.
Yan Gao and Junxian Song contributed equally to this research. 相似文献
308.
Eric Letouzé Yves Allory Marc A Bollet François Radvanyi Frédéric Guyon 《Genome biology》2010,11(7):R76
We present a computational method, TuMult, for reconstructing the sequence of copy number changes driving carcinogenesis,
based on the analysis of several tumor samples from the same patient. We demonstrate the reliability of the method with simulated
data, and describe applications to three different cancers, showing that TuMult is a valuable tool for the establishment of
clonal relationships between tumor samples and the identification of chromosome aberrations occurring at crucial steps in
cancer progression. 相似文献
309.
K. M. Bhaskara Reddy Dokka Mallikarjunasarma Kamana Bulliraju Vanjivaka Sreelatha Y. Bharathi Kumari Ramesh Dandala Kuppanna Ananda 《International journal of peptide research and therapeutics》2011,17(2):113-121
Abstract
An efficient stepwise synthesis of homo-oligomers and mixed oligomers of gabapentin and pregabalin on solid support using Fmoc-protected derivatives and HBTU/HOBt/DIEA as coupling agent is described. The synthesis was also carried out using solution phase methodology. The Gpn/Pgn homo oligomers and mixed oligomers forms C9 helix in solution as determined by NMR study. Chiral as well as achiral gamma amino acids were used for the synthesis of oligomers in order to investigate the secondary structural preferences. 相似文献310.
Chiara Beltramo Nadia Valentini Ezio Portis Daniela Torello Marinoni Paolo Boccacci Maria Angelica Sandoval Prando Roberto Botta 《Molecular breeding : new strategies in plant improvement》2016,36(3):27
The European hazelnut (Corylus avellana L.) is the most economically important nut species in the Betulaceae family. Despite the need for new improved hazelnut cultivars, few breeding programs are carried out because of the large plant size, the long lifecycle of the plant, and the expense and time required. To date, there are no reports of maps with quantitative trait loci (QTL) in hazelnut. Our objective in the present study was to identify QTL associated with vegetative traits to allow marker-assisted selection (MAS). An F1 progeny (275 plants) of Tonda Gentile delle Langhe × Merveille de Bollwiller obtained in 2009 was used to develop a QTL linkage map for vigour, sucker habit, and time of bud burst, after three years of observations. A set of 163 plants were analysed with 152 microsatellite markers. A map of 11 linkage groups was obtained, covering 663.1 cM, and 15 QTLs were identified and mapped for the traits examined. Of them, 10 were ‘major’ QTL, including a stably expressed region on LG_02 for leaf bud burst. At least one major QTL for each year underlies the variation in each trait and a clustering of QTL for trunk circumference and suckers/trunk circumference ratio with high inter-trait correlations was observed on LG_05, suggesting a single pleiotropic locus. This research represents an initial step in the future identification of chromosomal regions carrying genes of interest, important for breeding programs and MAS. 相似文献