全文获取类型
收费全文 | 327730篇 |
免费 | 27494篇 |
国内免费 | 459篇 |
专业分类
355683篇 |
出版年
2018年 | 3968篇 |
2017年 | 3878篇 |
2016年 | 4751篇 |
2015年 | 4639篇 |
2014年 | 6240篇 |
2013年 | 8579篇 |
2012年 | 10138篇 |
2011年 | 10646篇 |
2010年 | 7453篇 |
2009年 | 6591篇 |
2008年 | 9392篇 |
2007年 | 9655篇 |
2006年 | 9352篇 |
2005年 | 8691篇 |
2004年 | 8757篇 |
2003年 | 8506篇 |
2002年 | 8452篇 |
2001年 | 18331篇 |
2000年 | 17860篇 |
1999年 | 13729篇 |
1998年 | 3635篇 |
1997年 | 3640篇 |
1996年 | 3307篇 |
1995年 | 2995篇 |
1994年 | 2856篇 |
1993年 | 2724篇 |
1992年 | 10014篇 |
1991年 | 9979篇 |
1990年 | 9497篇 |
1989年 | 9445篇 |
1988年 | 8658篇 |
1987年 | 7823篇 |
1986年 | 7115篇 |
1985年 | 7240篇 |
1984年 | 5585篇 |
1983年 | 4688篇 |
1982年 | 3187篇 |
1981年 | 2660篇 |
1979年 | 5013篇 |
1978年 | 3878篇 |
1977年 | 3512篇 |
1976年 | 3314篇 |
1975年 | 3931篇 |
1974年 | 4204篇 |
1973年 | 4199篇 |
1972年 | 3727篇 |
1971年 | 3506篇 |
1970年 | 3001篇 |
1969年 | 2877篇 |
1968年 | 2684篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
M V Sauer 《The Western journal of medicine》1994,161(2):164-165
62.
L-myo-Inositol-1-phosphate synthase has been found to have at least a 5-fold preference for the beta-anomer of its natural substrate D-Glc-6-P. The alpha-anomer appears to be an inhibitor of the reaction and may be converted to product as well. As well as showing an enzymatic preference for the equatorial C-1 hydroxyl of D-Glc-6-P, our results suggest that it is the pyranose form of D-Glc-6-P that binds to the enzyme and that ring-opening is an enzymatic step. We have also found D-2-dGlc-6-P, D-2-F-2-dGlc-6-P, and D-Man-6-P each to be both competitive inhibitors and substrates that are converted to inositol phosphates by the synthase. D-Allose-6-P is a weak inhibitor of the enzyme, but not a substrate. D-Gal-6-P is neither substrate nor inhibitor. Thus the specificity of the synthase with respect to single position epimers of D-Glc-6-P increases in the order C1 less than C2 much less than C3 less than C4. 相似文献
63.
64.
F. Kato T. Hino A. Nakaji M. Tanaka Y. Koyama 《Molecular & general genetics : MGG》1995,247(3):387-390
In many species of actinomycetes, carotenogenesis can be photoinduced. The capacity to respond to photoinduction is, however unstable and, in various strains of Streptomyces, is lost at a relatively high frequency. In Streptomyces setonii ISP5395, which normally produces no carotenoids, carotenoid-producing mutants can be obtained following protoplast regeneration. We report here the characterization of a gene, crtS, which was isolated from one such mutant and can confer on wild-type S. setonii ISP5395 cells the capacity to synthesize carotenoids. Sequence analysis of crtS reveals an open reading frame, which shows homology to genes that encode alternative sigma factors in Bacillus subtilis. We propose that crtS encodes a sigma factor which is necessary for the expression of a cryptic gene(s) for carotenoid biosynthesis in S. setonii ISP5395. 相似文献
65.
66.
67.
Convenient extraction and radioimmunoassay methods for measurement of leukotrienes C4 and D4 (LTC4 and LTD4) in biological fluids are described. LTC4 or LTD4 in plasma was extracted with acetonitrile, and the extract was washed with dichloromethane then adjusted to pH 3.5 or 6.0, respectively. Each leukotriene was partially purified by using a C18-bonded silica cartridge and quantitated by radioimmunoassay. Amounts of LTC4 and LTD4 in the range of 0.025-1.6 ng could be assayed in plasma. This procedure was employed to examine the increase in plasma LTC4 (0.249 +/- 0.036 ng/ml) and LTD4 (1.399 +/- 0.235 ng/ml) of guinea pigs during intravenous challenge-induced anaphylactic bronchoconstriction, and the suppression of the increase of bronchoconstriction and leukotrienes by the administration of 5-lipoxygenase inhibitors such as E6080 (6-hydroxy-2-(4-sulfamoylbenzyl-amino)- 4,5,7-trimethylbenzothiazole hydrochloride), AA861 (2,3,5-trimethyl-6-(12-hydroxy-5,10-dodecadiynyl)-1,4-benzoquinone ) and phenidone. On the other hand, LTC4 and LTD4 were not detected in plasma after an inhaled challenge, though significant bronchoconstriction was provoked. It was concluded that the present study validates a new technique for quantitating plasma leukotrienes on the basis of pH and a suitable method for evaluating the pharmacological efficacy of 5-lipoxygenase inhibitors. 相似文献
68.
Hugo R. Permingeat Maria V. Romagnoli Juliana I. Sesma Ruben H. Vallejos 《Plant Molecular Biology Reporter》1998,16(1):89-89
An easy, reproducible and fast procedure to isolate DNA from cotton leaves is described. The addition of 0.5 M glucose in the extraction buffer avoids browning by polyphenolic compounds and improves the quality of DNA for molecular analysis. The DNA yield ranged between 150–400 mg per gram of fresh tissue. The DNA was suitable for digestion by restriction enzymes and amplificatiion by Taq DNA polymerase. 相似文献
69.
E A Rybnikova V V Rakitskaia V G Shaliapina 《Rossi?skii fiziologicheski? zhurnal imeni I.M. Sechenova / Rossi?skaia akademiia nauk》1999,85(4):594-597
Intrastriatal corticotrophin-releasing hormone (CRH) was shown to induce a decrease in the plasma tectosterone concentration. Besides, the 6-OHDA pre-treatment completely prevented the suppression of plasma testosterone in response to intrastriatal CRH administration. The findings suggest that the striatum is involved in the extrahypothalamic regulation of the gonadal endocrine function. 相似文献
70.
V. G. Borkhvardt 《Russian Journal of Developmental Biology》2000,31(3):154-161
The development of the fin and limb buds involves a balance of centrifugal (active) and centripetal (passive) mechanical forces,
the first of which acts to move the walls of these structures away from each other and the second of which holds them together.
When the volume of the mesodermal core increases, the generated force meets with the resistance of the basal membrane, and
as a result, the limb bud has a tendency to acquire a cylindrical shape. Collagen fibers, individual mesenchymal cells, and
their groups hold together the dorsal and the ventral wall of the limb bud, prevent the movement of these walls away from
each other, and in this way direct bud growth along the proximodistal and the anteroposterior axes. The balance of the forces
which stretch the ectodermal layer and those which constrain it has also been observed in the development of other body parts. 相似文献