全文获取类型
收费全文 | 15232篇 |
免费 | 1345篇 |
国内免费 | 224篇 |
专业分类
16801篇 |
出版年
2023年 | 97篇 |
2022年 | 189篇 |
2021年 | 382篇 |
2020年 | 240篇 |
2019年 | 263篇 |
2018年 | 333篇 |
2017年 | 275篇 |
2016年 | 416篇 |
2015年 | 735篇 |
2014年 | 689篇 |
2013年 | 924篇 |
2012年 | 1062篇 |
2011年 | 1007篇 |
2010年 | 617篇 |
2009年 | 592篇 |
2008年 | 801篇 |
2007年 | 772篇 |
2006年 | 662篇 |
2005年 | 608篇 |
2004年 | 628篇 |
2003年 | 566篇 |
2002年 | 526篇 |
2001年 | 419篇 |
2000年 | 345篇 |
1999年 | 332篇 |
1998年 | 176篇 |
1997年 | 115篇 |
1996年 | 123篇 |
1995年 | 144篇 |
1994年 | 119篇 |
1993年 | 106篇 |
1992年 | 177篇 |
1991年 | 206篇 |
1990年 | 149篇 |
1989年 | 168篇 |
1988年 | 144篇 |
1987年 | 136篇 |
1986年 | 117篇 |
1985年 | 139篇 |
1984年 | 117篇 |
1983年 | 92篇 |
1982年 | 65篇 |
1981年 | 70篇 |
1980年 | 67篇 |
1979年 | 77篇 |
1978年 | 80篇 |
1977年 | 68篇 |
1975年 | 61篇 |
1974年 | 54篇 |
1973年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Christian Chervaux Nathalie Sauvonnet Annick Le Clainche Brendan Kenny A. Lesley Hunt Jenny K. Broome-Smith I. Barry Holland 《Molecular & general genetics : MGG》1995,249(2):237-245
An in frame gene fusion containing the coding region for mature -lactamase and the 3-end of hylA encoding the haemolysin secretion signal, was constructed under the control of a lac promoter. The resulting 53 kDa hybrid protein was specifically secreted to the external medium in the presence of the haemolysin translocator proteins, HlyB and HlyD. The specific activity of the -lactamase portion of the secreted protein (measured by the hydrolysis of penicillin G), approximately 1 U/g protein, was close to that of authentic, purified TEM--lactamase. This is an important example of a hybrid protein that is enzymatically active, and secreted via the haemolysin pathway. Previous studies have indicated that haemolysin is secreted directly into the medium, bypassing the periplasm, to which -lactamase is normally targeted. This study indicated, therefore, that normal folding of an active -lactamase, can occur, at least when fused to the HlyA C-terminus, without the necessity of entering the periplasm. Despite the secretion of approximately 5 g/ml levels of the active -lactamase fusion into the medium, there was maximally only a 50% detectable increase in the LD50 for resistance to ampicillin at the individual cell level. This result suggests that, normally, resistance to ampicillin requires a high concentration of the enzyme close to killing targets, i.e. in the periplasm, in order to achieve significant levels of protection.These authors made an equal contribution to this work 相似文献
52.
53.
Direct ex vivo simian immunodeficiency virus (SIV)-specific cytotoxic activity detected from small intestine intraepithelial lymphocytes of SIV-infected macaques at an advanced stage of infection. 总被引:2,自引:1,他引:1 下载免费PDF全文
A Couëdel-Courteille R Le Grand M Tulliez J G Guillet A Venet 《Journal of virology》1997,71(2):1052-1057
Human immunodeficiency virus (HIV) induces a profound disorganization of the lymphoid tissues with marked abnormalities of the immune system at the terminal stage of infection. Since the digestive mucosal immune system is by far the largest lymphoid organ of the body, we attempted to evaluate its functional activity in advanced stages of simian immunodeficiency virus (SIV) infection in the SIV-macaque model of HIV infection. Two chronically intravenously SIV-infected macaques, including one at the AIDS stage, were studied. Intestinal intraepithelial lymphocytes (IEL) were isolated, analyzed, and compared to lymphocytes obtained from blood, spleen, and different lymph nodes: IEL were predominantly CD8+ T lymphocytes expressing the alphaE beta7 integrin and lacking the CD28 coactivatory molecule. A direct ex vivo SIV-specific cytotoxic activity was prominently found in the IEL of both macaques and was weaker or absent in the other sites. To our knowledge, this is the first report of SIV-specific cytotoxic activity from small intestine IEL in SIV-infected macaques. Considering the high similitude of the SIV-macaque model with the HIV infection in humans, these results may be highly important for the pathogenesis of HIV infection and more generally important for the characterization and function of digestive CD8+ IEL population. 相似文献
54.
The decomposition of ochratoxin A (OTA) was examined, under different temperature and moisture conditions. The calculated half-lives, corresponding to 50% values, were 707, 201, 12, and 6 min, respectively, at 100, 150, 200, and 250 degrees C for dry wheat and 145, 60, and 19 min, respectively, at 100, 150, and 200 degrees C for wheat heated under wet conditions. The presence of water (50%) increased the decomposition of OTA at 100 and 150 degrees C; the opposite was observed at 200 degrees C. Complete destruction of OTA within the limits of this study (100 to 250 degrees C) was not obtained. 相似文献
55.
56.
Decline in photopositive tendencies with age inDrosophila melanogaster (Diptera: Drosophilidae) 总被引:1,自引:0,他引:1
Phototaxis was measured in young, middle-aged, and oldDrosophila melanogaster flies of both sexes. The apparatus allowed us to measure the tendency to go toward light, independently of the time needed to do so; under such conditions, phototaxis is dissociated from locomotor activity. The percentage of photopositive flies decreased slightly with age (93.96, 80.17, and 78.97%, respectively, in young, middle-aged, and old flies). Results are discussed in connection with previous data for which the tendency to go toward light and the time to do so were not dissociated. 相似文献
57.
Intestinal mucosal damage was produced in rats by the s.c. administration of indomethacin (10 mg/kg). The number and severity of the small intestinal mucosal lesions was recorded. Different doses of prostacyclin (PGI2), 7-oxo-PGI2 and 17-aza-PGF2 alpha (0.25-0.5-1.00 mg/kg) were given i.p. at the time of administration of indomethacin. The effects of these compounds were studied on the number and severity of the small intestinal mucosal lesions. It was shown that (1) all tested compounds inhibited the number and severity of the intestinal mucosal lesions, however, to different extent; (2) the inhibition of the development of small intestinal mucosal damage displayed a dose-response relationship; (3) 17-aza-PGF2 alpha was found to have the most potent effect on the development of the intestinal lesions as well as on the development of gastric mucosal damage produced by ethanol. 相似文献
58.
N I Ghali E J Kattelman S C Hung K E Schnorf G C Le Breton D L Venton 《Prostaglandins》1984,27(6):865-876
Because of its highly unstable nature, TXA2, produced by platelet metabolism of arachidonic acid, does not lend itself to use as a receptor probe for its own receptor. As such, the stable TXA2/PGH2 antagonist, trans-13-azaprostanoic acid (trans-13-APA, 12b), was prepared as the [17, 18 3H] derivative [( 3H] trans-13-APA, 12c) to study this receptor and to better evaluate the mechanism of action of these azaprostanoids. Tritiated trans-13-APA, 12c, was prepared in nearly theoretical specific activity (57 Ci/mmole) from (17Z)-trans-13-azaprost-17-enoic acid (11b) by catalytic tritiation. The unsaturated 11b was prepared by condensation of cis-7-amino-3-heptene (8) with 2-(6-carboxyhexyl) cyclopentanone (9), NaBH4 reduction, chromatography, and hydrolysis of the trans isomer so isolated. The olefins 11a and b were also of biochemical interest because of the unsaturation in the lower side chain. The presence of similar unsaturation in PGH3(4) and TXA3 (3) renders these prostaglandins inactive as proaggregatory agents. Evaluation of the antiaggregatory activity of 11a and b indicated it to be about the same potency in inhibiting human platelet aggregation as the parent cis and trans-13-APAs, suggesting that introduction of a double bond at the 17 position in platelet prostaglandin antagonists is unlikely to result in enhanced antiplatelet activity. 相似文献
59.
60.
We surveyed a French male population for the incidence of missing or reduced upper lateral incisors (ULI). In 5,738 subjects, we observed an incidence of 1.59% with one or two reduced ULI (the other normal) and 1.90% with one or two missing ULI (the other normal or reduced), altogether, 3.49% affected subjects. Furthermore, 250 random controls were observed. Agenesis of other teeth is more frequent in propositi. Missing third molars were 12.4% in controls, 24.0% in propositi with reduced ULI and 39.6% in propositi with two missing ULI. Furthermore, agenesis of incisors, canines and premolars ranges from 0.4% in controls to 1.3% in propositi having reduced ULI and 5.0% in propositi with two missing ULI. So, propositi with reduced ULI are intermediate between the controls and the propositi with missing ULI with respect to the frequency of agenesis of other teeth. On the other hand, a different ranking is observed with respect to the teeth measurements: reduction of tooth size is more marked in propositi with reduced ULI than in propositi with missing ULI. The reduction mainly affects canines, incisors and to a lesser degree, premolars. Arch length and interpremolar diameters are smaller in propositi with missing ULI, compared with controls. 相似文献