全文获取类型
收费全文 | 9540篇 |
免费 | 831篇 |
国内免费 | 2篇 |
专业分类
10373篇 |
出版年
2024年 | 12篇 |
2023年 | 39篇 |
2022年 | 112篇 |
2021年 | 233篇 |
2020年 | 117篇 |
2019年 | 154篇 |
2018年 | 191篇 |
2017年 | 167篇 |
2016年 | 299篇 |
2015年 | 488篇 |
2014年 | 529篇 |
2013年 | 570篇 |
2012年 | 802篇 |
2011年 | 757篇 |
2010年 | 480篇 |
2009年 | 441篇 |
2008年 | 587篇 |
2007年 | 597篇 |
2006年 | 549篇 |
2005年 | 556篇 |
2004年 | 508篇 |
2003年 | 470篇 |
2002年 | 445篇 |
2001年 | 81篇 |
2000年 | 59篇 |
1999年 | 128篇 |
1998年 | 121篇 |
1997年 | 80篇 |
1996年 | 64篇 |
1995年 | 66篇 |
1994年 | 54篇 |
1993年 | 58篇 |
1992年 | 58篇 |
1991年 | 50篇 |
1990年 | 59篇 |
1989年 | 43篇 |
1988年 | 41篇 |
1987年 | 36篇 |
1986年 | 30篇 |
1985年 | 28篇 |
1984年 | 32篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1976年 | 11篇 |
1974年 | 11篇 |
1973年 | 8篇 |
1971年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Abstract Detection of plant viruses by ELISA using different reagent strips
A simplified immunoassay for detection of plant viruses under field conditions was developed on the basis of the direct double antibody sandwich ELISA using dry reagent carriers immobilized on PVC-supports which are arranged in a fan-like manner. The fan consists of a first reagent carrier with antivirus IgG covalently bound to cyanuric chloride-activated (CCA) paper while the second and third are filter papers impregnated with alkaline phosphatase labelled antivirus IgG and the fluorogenic subsrate, 4-methylumbelliferylphosphate, respectively. Performing the test, the first carrier is contacted with a liquid sample containing the virus to be identified, the second and third carriers is contacted with a liquid sample containing the virus to be identified the second and third carriers are sequentially put one upon the other and the reactions carried out. The virus is detected by the reacted substrate fluorescing under a UV-light. The applicability of the test is demonstrated with cucumber mosaic virus and potato virus X. 相似文献
A simplified immunoassay for detection of plant viruses under field conditions was developed on the basis of the direct double antibody sandwich ELISA using dry reagent carriers immobilized on PVC-supports which are arranged in a fan-like manner. The fan consists of a first reagent carrier with antivirus IgG covalently bound to cyanuric chloride-activated (CCA) paper while the second and third are filter papers impregnated with alkaline phosphatase labelled antivirus IgG and the fluorogenic subsrate, 4-methylumbelliferylphosphate, respectively. Performing the test, the first carrier is contacted with a liquid sample containing the virus to be identified, the second and third carriers is contacted with a liquid sample containing the virus to be identified the second and third carriers are sequentially put one upon the other and the reactions carried out. The virus is detected by the reacted substrate fluorescing under a UV-light. The applicability of the test is demonstrated with cucumber mosaic virus and potato virus X. 相似文献
32.
Sallie W. Chisholm Sheila L. Frankel Ralf Goericke Robert J. Olson Brian Palenik John B. Waterbury Lisa West-Johnsrud Erik R. Zettler 《Archives of microbiology》1992,157(3):297-300
Several years ago, prochlorophyte picoplankton were discovered in the N. Atlantic. They have since been found to be abundant within the euphotic zone of the world's tropical and temperate oceans. The cells are extremely small, lack phycobiliproteins, and contain divinyl chlorophyll a and b as their primary photosynthetic pigments. Phylogenies constructed from DNA sequence data indicate that these cells are more closely related to a cluster of marine cyanobacteria than to their prochlorophyte relatives Prochlorothrix and Prochloron. Several strains of this organism have recently been brought into culture, and herewith are given the name Prochlorococcus marinus. 相似文献
33.
Juliane Plieth Frauke Rininsland Manfred Schlösser David N. Cooper Jochen Reiss 《Human genetics》1992,88(3):283-287
Summary In Central Europe, the F508 deletion accounts for approximately 75% of mutations in the cystic fibrosis transmembrane conductance regulator gene causing cystic fibrosis. The remainder comprise a large number of individually infrequent mutations whose detection requires a disproportionately large effort. However, a sizeable proportion of non-F508 mutations have been found to cluster within exon 11. We have taken advantage of this clustering to detect a total of five previously described point mutations present on 26/72 (36%) non-F508 chromosomes by polymerase chain reaction/direct sequencing of exon 11. These exon 11 mutations were then subjected to single-strand conformation polymorphism (SSCP) analysis, which was shown (i) to discriminate reliably between mutant and wildtype alleles and (ii) to generate reproducible mutation-specific band patterns. This analysis thus represents the first attempt to assess SSCP analysis retrospectively, and serves to illustrate the potential of this screening technique in diagnostic medicine. 相似文献
34.
Seasonal carbon isotope discrimination in a grassland community 总被引:11,自引:0,他引:11
Mark P. Smedley Todd E. Dawson Jonathan P. Comstock Lisa A. Donovan Dorothy E. Sherrill Craig S. Cook James R. Ehleringer 《Oecologia》1991,85(3):314-320
Summary Grassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture. 相似文献
35.
Twila Jackson Michael F. Allard Catherine M. Sreenan Lisa K. Doss Sanford P. Bishop Judith L. Swain 《Molecular and cellular biochemistry》1991,104(1-2):15-19
Transgenic animals provide a model system to elucidate the role of specific proteins in development. This model is now being used increasingly in the cardiovascular system to study cardiac growth and differentiation. During cardiac myocyte development a transition occurs from hyperplastic to hypertrophic growth. In the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc functions to regulate myocyte proliferation and/or differentiation, we examined the in vivo effect of increasing c-myc expression during fetal development and of preventing the decrease in c-myc mRNA expression that normally occurs during myocyte development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. Increased c-myc mRNA expression is associated with both atrial and ventricular enlargement in the transgenic mice. This increase in cardiac mass is secondary to myocyte hyperplasia, with the transgenic hearts containing greater than twice as many myocytes as nontransgenic hearts. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth, and suggest a regulatory role for the c-myc protooncogene in cardiac myogenesis. 相似文献
36.
Lisa G. Shaffer Colleen K. Jackson-Cook Joanne M. Meyer Judith A. Brown J. Edward Spence 《Human genetics》1991,86(4):375-382
Summary The largest class of de novo chromosomal rearrangements in Down syndrome are rea(21q21q). Classically, these rearrangements have been termed Robertsonian translocations, implying an attachment of two different chromosome 21 homologues. Additionally, a Robertsonian translocation between two chromosomes 21 cannot be distinguished from an isochromosome composed of genetically identical arms by cytogenetic analyses. Therefore, we have used molecular techniques to differentiate between true Robertsonian translocations and isochromosomes. Samples were obtained from 12 probands, ascertained for de novo rearrangements between homologous chromosomes 21 [11 rea(21q21q) and 1 rea (21;21)(q22;q22)], their parents (n = 24) and available siblings (n = 7). The parental origins of the de novo rearrangements were assigned using molecular and cytogenetic analyses. Although not statistically significant, there was a two-fold increase in the number of paternally derived de novo rearrangements (n = 8) as compared with maternally derived rearrangements (n = 4). To distinguish between rob(21q21q) and i(21q), we used restriction fragment length polymorphisms (RFLPs) spanning the length of chromosome 21. Using all informative and partially informative RFLPs, we used the method of maximum likelihood to assign the most likely rearrangement definition (i or rob) and parental origin in each family. The maximum likelihood estimates indicated that all rearrangements tested (n = 8) were isochromosomes. C-banding revealed two centromeres in three cases indicating that a U-type exchange occurred between sister chromatids in these rearrangements. Our results suggest that the majority of de novo rea(21q21q) are isochromosomes derived from a single parental chromosome 21. 相似文献
37.
38.
The release of endogenous acetylcholine and dopamine and the appearance of their metabolites, choline and dihydroxyphenylacetic acid (DOPAC), from neostriatal slices prepared from Fischer 344 rats was examined under various experimental conditions. There was a dose-dependent increase in the amount of neurotransmitter or metabolite as the medium potassium concentration was increased from 5 to 50 mM. Over an eight minute period in Krebs Ringer bicarbonate buffer containing 25 mM potassium, the rate of release of acetylcholine was 6 to 13 times greater than that of dopamine. The dopamine endogenous to the slice preparation appeared to have little effect on the release of endogenous acetylcholine since manipulations that significantly altered dopamine release (depletion with 6-hydroxydopamine or uptake inhibition with nomifensine) had minimal effects on the cholinergic neurons. In contrast, increasing the endogenous acetylcholine in the preparation by inhibiting acetylcholinesterase resulted in a 1.2 to 12 fold increase in dopamine release depending upon the incubation time and the potassium concentration. These studies indicate that within the neostriatal slices there is minimal influence of the endogenous dopamine on the cholinergic neurons, whereas the extracellular acetylcholine can influence dopamine release when its concentration is increased by inhibition of acetylcholinesterase. 相似文献
39.
Zoospores of the marine chytrid Rhizophydium littoreum are attracted to a variety of substances common to their environment. In general, carbohydrates and polysaccharides elicited strong concentration-dependent positive responses. There was no direct correlation between all substances used as foods and those stimulating positive responses. The chemotactic activities of this organism should, however, tend to bring it toward concentrated food sources. 相似文献
40.
The human genetic material is packed hierarchically within the metaphase chromosome: the DNA moleculet together with histone proteins form 11 nm diameter nucleosomes, which are then ordered into the 30 nm thick chromatin fiber. Little is known about the packing of this fiber within the chromosome. We have developed a tracking algorithm with which we followed its path within a three-dimensional reconstruction of a human chromosome computed from a series of electron micrographie projections. Fiber segments were seen to form loops of 100–350 nm diameter. Our observations indicate that these loops — which themselves show no preferred orientation — are organised into regions of roughly 200 nm axial extent. 相似文献