全文获取类型
收费全文 | 334000篇 |
免费 | 35508篇 |
国内免费 | 293篇 |
出版年
2018年 | 2962篇 |
2016年 | 4197篇 |
2015年 | 6165篇 |
2014年 | 7015篇 |
2013年 | 10227篇 |
2012年 | 11565篇 |
2011年 | 11843篇 |
2010年 | 7849篇 |
2009年 | 7234篇 |
2008年 | 10577篇 |
2007年 | 10906篇 |
2006年 | 10453篇 |
2005年 | 9958篇 |
2004年 | 9887篇 |
2003年 | 9594篇 |
2002年 | 9169篇 |
2001年 | 12360篇 |
2000年 | 12200篇 |
1999年 | 9995篇 |
1998年 | 4204篇 |
1997年 | 4214篇 |
1996年 | 4065篇 |
1995年 | 3671篇 |
1994年 | 3625篇 |
1993年 | 3666篇 |
1992年 | 8374篇 |
1991年 | 8292篇 |
1990年 | 7930篇 |
1989年 | 7897篇 |
1988年 | 7267篇 |
1987年 | 7018篇 |
1986年 | 6455篇 |
1985年 | 6686篇 |
1984年 | 5717篇 |
1983年 | 4860篇 |
1982年 | 4023篇 |
1981年 | 3817篇 |
1980年 | 3586篇 |
1979年 | 5337篇 |
1978年 | 4244篇 |
1977年 | 4151篇 |
1976年 | 3796篇 |
1975年 | 4107篇 |
1974年 | 4559篇 |
1973年 | 4413篇 |
1972年 | 3889篇 |
1971年 | 3640篇 |
1970年 | 3310篇 |
1969年 | 3204篇 |
1968年 | 2933篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
Robert S. Steneck 《Current biology : CB》2009,19(3):R117-R119
54.
55.
Abstract Brassica rupestris Raf. is a chasmophyte species that includes two subspecies, both endemic to Central-Western Sicily (Italy). Inter-Simple Sequence Repeat (ISSR) markers were used to detect genetic diversity within and among eight populations representative of the species' distribution range. High levels of genetic diversity were revealed both at the population (PPB = 53.88%, H S = 0.212, Sh = 0.309) and at the species level (PPB = 96.55%, H T = 0.307, Sh = 0.464). The correlation between genetic and geographical distances was negative (Mantel test, r = ?0.06, P < 0.95). The two subspecies of B. rupestris, subsp. rupestris and subsp. hispida, showed remarkable genetic similarity and molecular data did not unequivocally support their distinctness. The pattern of genetic variation revealed by our study bears important consequences for conservation management: It is desirable to preserve B. rupestris populations in situ with a “dynamic” strategy, while, ex situ conservation programmes might be improved to safeguard maximum genetic diversity. 相似文献
56.
Background
Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation. 相似文献57.
The cellular long-chain component patterns of 33 strains of Eubacterium lentum were determined by gas chromatography. Two main types of long-chain component patterns were distinguished. The first (26 strains) was characterized by saturated branched-chain fatty acids (br14:0, br15:0, br16:0 and br17:0). The second (7 strains) did not contain branched-chain fatty acids and was characterized by saturated straight-chain fatty acids (11:0, 12:0, 14:0 and 16:0). Both types contained fatty aldehydes and their respective dimethyl acetals (14ald and 14dma, 16ald and 16dma). br16dma was only found in the first type. The G + C content of the DNA (Tm) of the 33 strains varied between 63.7 and 69.1 mol %. Canonical correlation analysis distinguished three subtypes within the first main type. 相似文献
58.
59.
Complementarity of regulation for the two glutamine synthetases from Bacillus caldolyticus, an extreme thermophile 总被引:1,自引:0,他引:1
F C Wedler D S Shreve K E Fisher D J Merkler 《Archives of biochemistry and biophysics》1981,211(1):276-287
Chromatophores from Rhodopseudomonas capsulata cells grown semiaerobically in the dark oxidize NADH, succinate, and dichlorophenolindophenol. In the presence of N3? these activities are inhibited, but light induces oxidation of dichlorophenolindophenol with O2 as a terminal electron acceptor. Cyanide also inhibits electron transport but much higher concentrations are required to inhibit the photooxidation than the dark oxidation. The photooxidation was studied in a mutant strain of Rhodopseudomonas capsulata (YIV) which cannot grow anaerobically in the light, but similarly to the wild type, grows in the presence of oxygen. Chromatophores from YIV mutant catalyze photophosphorylation and dark oxidation activities with the same properties as those of the wild type. However, the rate of photooxidation in the mutant is only one-third that of the wild type. Based on the differential inhibitor sensitivity and on the mutation it is suggested that the photooxidase is different from the two respiratory oxidases and that this photooxidation activity might be essential for growth of the cells under anaerobic conditions in the light. 相似文献
60.
This study evaluates dopaminergic regulation of aldosterone secretion in 6 patients with high spinal cord transections. Administration of the dopamine antagonist metoclopramide resulted in a marked rise in plasma aldosterone and 18-hydroxycorticosterone levels in 12 normal individuals, but no change in plasma levels of these zona glomerulosa corticosteroid products in spinal cord patients. Spinal cord transected patients also did not have the rise in plasma renin activity that was observed in normals following metoclopramide administration. Basal levels of aldosterone, 18 hydroxycorticosterone, corticosterone and renin activity as well as the aldosterone responses to graded dose infusion of adrenocorticotropin were similar in the spinal cord patients and the normals. These data suggest that dopaminergic regulation of adrenal zona glomerulosa corticosteroid and renal renin secretion is absent in patients with high spinal cord transections, suggesting that intact neural pathways from the central nervous system are necessary for metoclopramide stimulation of aldosterone and renin secretion in men. Since basal plasma aldosterone levels were normal in spinal cord transected patients, it appears that the absence of dopaminergic control does not result in elevated secretion. 相似文献