全文获取类型
收费全文 | 3849篇 |
免费 | 362篇 |
出版年
2021年 | 53篇 |
2019年 | 41篇 |
2018年 | 46篇 |
2017年 | 44篇 |
2016年 | 62篇 |
2015年 | 123篇 |
2014年 | 107篇 |
2013年 | 143篇 |
2012年 | 233篇 |
2011年 | 244篇 |
2010年 | 139篇 |
2009年 | 138篇 |
2008年 | 204篇 |
2007年 | 231篇 |
2006年 | 242篇 |
2005年 | 200篇 |
2004年 | 229篇 |
2003年 | 197篇 |
2002年 | 200篇 |
2001年 | 46篇 |
2000年 | 26篇 |
1999年 | 39篇 |
1998年 | 50篇 |
1997年 | 43篇 |
1996年 | 46篇 |
1995年 | 32篇 |
1994年 | 44篇 |
1993年 | 28篇 |
1992年 | 33篇 |
1991年 | 29篇 |
1990年 | 25篇 |
1988年 | 24篇 |
1987年 | 21篇 |
1986年 | 35篇 |
1985年 | 34篇 |
1984年 | 47篇 |
1983年 | 35篇 |
1982年 | 51篇 |
1981年 | 40篇 |
1980年 | 36篇 |
1979年 | 33篇 |
1978年 | 26篇 |
1977年 | 28篇 |
1976年 | 32篇 |
1975年 | 29篇 |
1974年 | 26篇 |
1973年 | 31篇 |
1972年 | 20篇 |
1971年 | 20篇 |
1969年 | 20篇 |
排序方式: 共有4211条查询结果,搜索用时 15 毫秒
31.
32.
Martin Yanofsky Brenda Lowe Alice Montoya Robert Rubin William Krul Milton Gordon Eugene Nester 《Molecular & general genetics : MGG》1985,201(2):237-246
Summary We have investigated the factors which contribute to the host specificity of a tumor inducing plasmid of Agrobacterium, pTiAg162, which confers a narrow host range. Determinants both within the T-DNA and virulence regions contribute to host specificity. Within the T-DNA a defective cytokinin biosynthetic gene limits host range. Nucleotide sequence analysis revealed a large deletion in the 5 coding region of this gene when compared with the homologous gene from the wide host range tumor inducing plasmid, pTiA6. Introduction of the wide host range cytokinin biosynthesis gene into the T-DNA of the limited host range strain expanded the host range and suppressed the rooty morphology of tumors incited by the limited host range strain. Two genes from the virulence region of the wide host range plasmid, designated virA and virC, must also be introduced into the limited host range strain in order to restore a wide host range phenotype. The wide host range strain is avirulent on some cultivars of Vitis plants on which the limited host range strain induces tumors. This avirulence is apparently due to a hypersensitive response in which infected plant cells are killed at the site of inoculation. Mutations within the virC locus of the wide host range plasmid prevented the hypersensitive response and allowed the formation of tumors by the wide host range strain. 相似文献
33.
34.
The rate constants for the reversible addition of protons and sulfite to the 5,6 double bond of cytidine and 3-methylcytidine have been spectrophotometrically measured under conditions (25°C, μ = 1.0
) where the deamination of 5,6-dihydrocytidine-6-sulfonate is minimal. Both the addition and the elimination of sulfite from the ring system are subject to general catalysis of proton transfer. For the reaction in either direction, plots of the pseudo-firstorder rate constants against increasing buffer concentration are biphasic and indicative of at least a two-step reaction pathway with both steps being subject to general acid-base catalysis. Kinetic hydrogen-deuterium isotope effects were measured for both buffer-catalyzed steps of sulfite elimination from 3-methyl-5,6-dihydrocytidine-6-sulfonate and sulfite addition to 3-methylcytidine. Both H2O and D2O were used as solvent. For both the addition and the elimination of SO32− values of k2H/k2D were 6.3–7.1 and 2.3–2.6 at low and high imidazole buffer concentration, respectively. The large isotope effects values in the range of 6–7 can be attributed to rate-determining proton transfer to carbon-5 of the cytidine ring system. The smaller values are more likely caused by proton transfer to a electronegative atom such as the oxygen on carbon-2 of the cytidine ring. The equilibrium constants for bisulfite buffer addition to 3-methylcytidine and cytidine at 25°C, μ = 1.0
, pH 7.2, are 10.2 and 1.3
−1, respectively. 相似文献
35.
Eugene D Robin 《BMJ (Clinical research ed.)》1985,290(6477):1259-1260
36.
A salt storage facility has been located adjacent to Fonda Lake since 1953. In February 1981 a core was taken from the profundal sediments of the lake and analyzed to determine the effects of salt perturbation on the diatom community over a 32-year period. Diatom assemblages from different levels were compared using multivariate techniques including cluster analysis and principal component analysis. Shifts in diatom composition related to salinification were revealed most clearly by subdominant taxa. Five distinct groups of diatom taxa were found to correspond with 5 depth intervals. The diatom component of the lake up to 1960 included two groups of taxa which were alkaliphilous and chloride indifferent. A reduction in species diversity beginning in 1960 may indicate a salt effect. By 1968, when diversity reached a minimum, a variety of halophilic taxa (including Diatoma tenue, Navicula gregaria and Synedra fasciculata) attained their highest relative abundances. At the top of the core, diversity increased slightly and some halophilic taxa decreased in relative abundance, which suggests a possible decrease in salt loading to the lake. 相似文献
37.
Barbara Stay Lynda S. Ostedgaard Stephen S. Tobe Alain Strambi Eugene Spaziani 《Journal of insect physiology》1984,30(8):643-651
The free (non-conjugated) ecdysteroid in the ovaries during the first gonadotrophic cycle of Diploptera punctata was identified as 20-hydroxyecdysone. The hormone, quantified by radioimmunoassay and by ultraviolet absorbance, was detectable in the ovary toward the end of vitellogenesis; the quantity increased rapidly during chorion formation. Ovaries with chorionated eggs contained 67 μg of 20-hydroxyecdysone per g fresh weight. The haemolymph free-ecdysteroid, not identified physicochemically, was quantified by radioimmunoassays. The highest concentration was observed at adult emergence; the titre declined between days 1–3 and then remained at a relatively constant level through oviposition (which occurs between day 7 and 8); titres in pregnant females were higher. Ovariectomized females exhibited the same pattern of ecdysteroid titres in the haemolymph as the sham operated controls throughout the period corresponding to the first gonadotrophic cycle. Thus the ovary may not be the only source of haemolymph ecdysteroid related to reproduction in adult females. 相似文献
38.
Barton L. Bergquist Ellen Wyttenbach Eugene C. Bovee 《Biological trace element research》1986,9(2):125-138
Changes in the locomotor rate of the ciliateTetrahymena pyriformis were used to quantitatively evaluate chemical interactions produced by: cadmium in combination with varying amounts of calcium, andcis-dichlorodiammineplatinum (II) (cisplatin) with varying amounts of sodium chloride. Cadmium (as CdCl2) produces a measurable decline in the locomotor rate of the cells. Cadmium's detrimental effect can be reduced by the addition of calcium (as CaCl2) in combination with cadmium. At a ratio of 30∶1 (calcium: cadmium), cadmium's negative effect upon motility is essentially nullified. It is suggested that the “protective” action afforded by calcium stems from the chemical similarity of the two cations and their involvement/competition for molecular sites responsible for the energy release and/or delivery of ciliary activity. Cisplatin will also effect a reduction in ciliary activity. However, the interaction between cisplatin, sodium chloride, and the cell appears more complex than that found with cadmium-calcium. At the lower range of chloride (as NaCl) used in this study, increased chloride concentration produces an increase in cisplatin's action against ciliary activity. At the higher levels, the chloride reduced cisplatin's negative effects. It is suggested that the increases in cisplatin's effects are caused by mass chemical action of increased chloride, which increases the concentration of the nonpolar cisplatin. The reduced effects found with the higher concentrations of sodium chloride may be because of the presence and action of elevated NaCl in/on the cell. This study clearly demonstrates differences in biologically relevant chemical interactions occurring with the two sets: cadmium-calcium and cisplatin-chloride. 相似文献
39.
Clastogen-induced chromosomal breakage as a marker for first trimester prenatal diagnosis of Fanconi anemia 总被引:5,自引:0,他引:5
Arleen D. Auerbach Zhang Min Rita Ghosh Eugene Pergament Yuri Verlinsky Henriette Nicolas Joëlle Boué 《Human genetics》1986,73(1):86-88
Summary Using cultured trophoblast cells obtained by chorionic villus biopsy, we diagnosed Fanconi anemia (FA) in two pregnancies and excluded it in eight pregnancies at risk for the syndrome. Baseline chromosomal breakage and breakage induced by diepoxybutane (DEB) were analyzed. Increased breakage was used as a marker for the syndrome. Our results were unambiguous and provide a reliable method for prenatal detection of FA in the first trimester of pregnancy. 相似文献
40.