全文获取类型
收费全文 | 1153篇 |
免费 | 35篇 |
国内免费 | 1篇 |
专业分类
林业 | 121篇 |
农学 | 51篇 |
基础科学 | 1篇 |
190篇 | |
综合类 | 96篇 |
农作物 | 41篇 |
水产渔业 | 179篇 |
畜牧兽医 | 392篇 |
园艺 | 21篇 |
植物保护 | 97篇 |
出版年
2023年 | 7篇 |
2022年 | 16篇 |
2021年 | 18篇 |
2020年 | 11篇 |
2019年 | 17篇 |
2018年 | 16篇 |
2017年 | 23篇 |
2016年 | 29篇 |
2015年 | 17篇 |
2014年 | 43篇 |
2013年 | 99篇 |
2012年 | 63篇 |
2011年 | 63篇 |
2010年 | 40篇 |
2009年 | 54篇 |
2008年 | 64篇 |
2007年 | 59篇 |
2006年 | 63篇 |
2005年 | 63篇 |
2004年 | 48篇 |
2003年 | 46篇 |
2002年 | 30篇 |
2001年 | 46篇 |
2000年 | 48篇 |
1999年 | 26篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 13篇 |
1990年 | 15篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1974年 | 4篇 |
1972年 | 3篇 |
1968年 | 4篇 |
排序方式: 共有1189条查询结果,搜索用时 15 毫秒
91.
Multiple principal sigma factor homologs in eubacteria: identification of the "rpoD box" 总被引:23,自引:0,他引:23
Genes for the principal sigma factor (rpoD genes) of various eubacteria were identified with a synthetic oligonucleotide probe corresponding to a conserved sequence in rpoD gene products of Escherichia coli and Bacillus subtilis. Multiple rpoD homologs were found in the strains of Micrococcus, Pseudomonas, and Streptomyces, whereas single genes were detected in E. coli, B. subtilis, and Staphylococcus aureus. The four rpoD homologs of Streptomyces coelicolor A3(2) were cloned and sequenced. A homologous portion with 13 amino acids was found in the rpoD genes of S. coelicolor A3(2), E. coli, and B. subtilis and was named the "rpoD box." 相似文献
92.
Rats maintained on a low phosphorus diet supplemented with 25-hydroxyvitamin D(3) show high intestinal calcium transport activity as compared to rats similarly treated but fed a diet containing adequate phosphorus. This increased transport activity is correlated with an increased biosynthesis of 1,25-dihydroxyvitamin D(3), the probable metabolically active form of the vitamin in the intestine. 相似文献
93.
Wahlen M Tanaka N Henry R Deck B Zeglen J Vogel JS Southon J Shemesh A Fairbanks R Broecker W 《Science (New York, N.Y.)》1989,245(4915):286-290
Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4). 相似文献
94.
Reversal of oncogenesis by the expression of a major histocompatibility complex class I gene 总被引:26,自引:0,他引:26
The classical transplantation antigens (the major histocompatibility complex class I antigens) play a key role in host defense against cells expressing foreign antigens. Several naturally occurring tumors and virally transformed cells show an overall suppression of these surface antigens. Since the class I molecules are required in the presentation of neoantigens on tumor cells to the cytotoxic T lymphocytes, their absence from the cell surface may lead to the escape of these tumors from immunosurveillance. To test this possibility, a functional class I gene was transfected into human adenovirus 12-transformed mouse cells that do not express detectable levels of class I antigens; the transformants were tested for expression of the transfected gene and for changes in oncogenicity. The expression of a single class I gene, introduced by DNA-mediated gene transfer into highly tumorigenic adenovirus 12-transformed cells, was sufficient to abrogate the oncogenicity of these cells. This finding has important implications for the regulation of the malignant phenotype in certain tumors and for the potential modulation of oncogenicity through derepression of the endogenous class I genes. 相似文献
95.
Earth tides can trigger shallow thrust fault earthquakes 总被引:6,自引:0,他引:6
We show a correlation between the occurrence of shallow thrust earthquakes and the occurrence of the strongest tides. The rate of earthquakes varies from the background rate by a factor of 3 with the tidal stress. The highest correlation is found when we assume a coefficient of friction of mu = 0.4 for the crust, although we see good correlation for mu between 0.2 and 0.6. Our results quantify the effect of applied stress on earthquake triggering, a key factor in understanding earthquake nucleation and cascades whereby one earthquake triggers others. 相似文献
96.
Hanayama R Tanaka M Miyasaka K Aozasa K Koike M Uchiyama Y Nagata S 《Science (New York, N.Y.)》2004,304(5674):1147-1150
Apoptotic cells expose phosphatidylserine and are swiftly engulfed by macrophages. Milk fat globule epidermal growth factor (EGF) factor 8 (MFG-E8) is a protein that binds to apoptotic cells by recognizing phosphatidylserine and that enhances the engulfment of apoptotic cells by macrophages. We report that tingible body macrophages in the germinal centers of the spleen and lymph nodes strongly express MFG-E8. Many apoptotic lymphocytes were found on the MFG-E8-/- tingible body macrophages, but they were not efficiently engulfed. The MFG-E8-/- mice developed splenomegaly, with the formation of numerous germinal centers, and suffered from glomerulonephritis as a result of autoantibody production. These data demonstrate that MFG-E8 has a critical role in removing apoptotic B cells in the germinal centers and that its failure can lead to autoimmune diseases. 相似文献
97.
Oda E Ohki R Murasawa H Nemoto J Shibue T Yamashita T Tokino T Taniguchi T Tanaka N 《Science (New York, N.Y.)》2000,288(5468):1053-1058
A critical function of tumor suppressor p53 is the induction of apoptosis in cells exposed to noxious stresses. We report a previously unidentified pro-apoptotic gene, Noxa. Expression of Noxa induction in primary mouse cells exposed to x-ray irradiation was dependent on p53. Noxa encodes a Bcl-2 homology 3 (BH3)-only member of the Bcl-2 family of proteins; this member contains the BH3 region but not other BH domains. When ectopically expressed, Noxa underwent BH3 motif-dependent localization to mitochondria and interacted with anti-apoptotic Bcl-2 family members, resulting in the activation of caspase-9. We also demonstrate that blocking the endogenous Noxa induction results in the suppression of apoptosis. Noxa may thus represent a mediator of p53-dependent apoptosis. 相似文献
98.
Periodic signaling controlled by an oscillatory circuit that includes protein kinases ERK2 and PKA 总被引:1,自引:0,他引:1
Maeda M Lu S Shaulsky G Miyazaki Y Kuwayama H Tanaka Y Kuspa A Loomis WF 《Science (New York, N.Y.)》2004,304(5672):875-878
Self-regulating systems often use robust oscillatory circuits. One such system controls the chemotactic signaling mechanism of Dictyostelium, where pulses of adenosine 3',5'-monophosphate (cAMP) are generated with a periodicity of 7 minutes. We have observed spontaneous oscillations in activation of the mitogen-activated protein (MAP) kinase ERK2 that occur in phase with peaks of cAMP, and we show that ERK2 modulates cAMP levels through the phosphodiesterase RegA. Computer modeling and simulations of the underlying circuit faithfully account for the ability of the cells to spontaneously generate periodic pulses during specific stages of development. Similar oscillatory processes may occur in cells of many different species. 相似文献
99.
Matsuishi S Toda Y Miyakawa M Hayashi K Kamiya T Hirano M Tanaka I Hosono H 《Science (New York, N.Y.)》2003,301(5633):626-629
We removed approximately 100% of clathrated oxygen ions from the crystallographic cages in a single crystal of 12CaO.7Al2O3, leading to the formation of high-density (approximately 2 x 10(21) cm-3) electrons highly localized in the cages. The resulting electron forms a structure that we interpret as an F+ center and migrates throughout the crystal by hopping to a neighboring cage with conductivity approximately 100 siemens per centimeter, demonstrating that the encaged electron behaves as an anion. The electron anions couple antiferromagnetically with each other, forming a diamagnetic pair or singlet bipolaron. The resulting [Ca24Al28O64]4+(4e-) may be regarded as a thermally and chemically stable single crystalline "electride." 相似文献
100.
Chloride and iodide ions exhibit noncompetitive inhibition of carbon dioxide hydration catalyzed by bovine carbonic anhydrase and competitive inhibition of bicarbonate dehydration. Consideration of the net velocity at equilibrium leads to a satisfactory resolution of the dilemma concerning the different concentrations of such anions required to halve the initial reaction rate for each substrate. 相似文献