全文获取类型
收费全文 | 224596篇 |
免费 | 15029篇 |
国内免费 | 50篇 |
专业分类
239675篇 |
出版年
2021年 | 1778篇 |
2018年 | 3141篇 |
2017年 | 3039篇 |
2016年 | 3683篇 |
2015年 | 3222篇 |
2014年 | 4470篇 |
2013年 | 6354篇 |
2012年 | 7579篇 |
2011年 | 8082篇 |
2010年 | 5744篇 |
2009年 | 4892篇 |
2008年 | 7220篇 |
2007年 | 7502篇 |
2006年 | 7196篇 |
2005年 | 6722篇 |
2004年 | 6872篇 |
2003年 | 6579篇 |
2002年 | 6547篇 |
2001年 | 9294篇 |
2000年 | 8970篇 |
1999年 | 7001篇 |
1998年 | 2129篇 |
1997年 | 1960篇 |
1996年 | 1831篇 |
1992年 | 5484篇 |
1991年 | 5687篇 |
1990年 | 5659篇 |
1989年 | 5631篇 |
1988年 | 5224篇 |
1987年 | 4874篇 |
1986年 | 4454篇 |
1985年 | 4687篇 |
1984年 | 3706篇 |
1983年 | 3031篇 |
1982年 | 2014篇 |
1981年 | 1725篇 |
1980年 | 1728篇 |
1979年 | 3342篇 |
1978年 | 2581篇 |
1977年 | 2382篇 |
1976年 | 2331篇 |
1975年 | 2827篇 |
1974年 | 3118篇 |
1973年 | 3086篇 |
1972年 | 2746篇 |
1971年 | 2605篇 |
1970年 | 2212篇 |
1969年 | 2160篇 |
1968年 | 2033篇 |
1967年 | 1797篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Using two-dimensional thin-layer chromatography, the effect of serotonin on the yield of thymine dimers and on cleavage of the N-glycosidic bond in the DNA irradiated with ultraviolet (UV) light and X-ray was studied. Bound serotonin was shown to reduce the synthesis of UV-induced thymine dimers but had no effect on the number of X-ray-induced breaks in the N-glycoside bonds in thymidine residues. The data obtained are discussed in terms of the mechanisms of serotonin involvement in the photoprotection of yeast cells from the lethal action of UV and X-ray irradiations. 相似文献
122.
V E Kagan E A Serbinova A A Minin V M Savov K N Novikov 《Biokhimii?a (Moscow, Russia)》1985,50(6):986-991
The localization and mechanism of generation of active oxygen species in the enzymatic NADPH-dependent lipid peroxidation system in liver microsomes were studied. Using the spin-trapping method, the key role of active oxygen species in the initiation of NADPH-dependent enzymatic lipid peroxidation was confirmed. It was shown that active oxygen species are generated via consecutive one-electron reduction of the oxygen molecule by NADPH-cytochrome P-450 reductase. 相似文献
123.
Occurrence of multiple-antibiotic-resistant enteric bacteria in domestic sewage and oxidation lagoons 总被引:1,自引:0,他引:1
The coliform bacterial population in the Grand Forks, N.Dak. sewage system was examined for multiple-antibiotic-resistant organisms over a 1-year period. Multiple-antibiotic-resistant coliforms were found to be common in the sewage, and their numbers remained fairly constant relative to the total coliform population throughout the year. Resistance to kanamycin, tetracycline, and ampicillin was found to be transferable at variable rates. Transfer rates were found to be temperature sensitive and were optimal at 35 degrees C. Although 75% of the multiple-antibiotic-resistant coliforms were capable of transferring resistance at some level, only 25% were capable of transferring resistance at rates greater than 10(-3) transconjugants per initial donor. 相似文献
124.
Progesterone metabolism in T47Dco human breast cancer cells--II. Intracellular metabolic path of progesterone and synthetic progestins 总被引:1,自引:0,他引:1
K B Horwitz A W Pike C Gonzalez-Aller P V Fennessey 《Journal of steroid biochemistry》1986,25(6):911-916
We show here that progesterone added to the medium of proliferating T47Dco human breast cancer cells is metabolized with a half life of 2-4h. The final metabolic product, 5 alpha-pregnan-3 beta,6 alpha-diol-20-one, (P-metabolite) is released into the medium. This structure suggested that the intracellular metabolism of progesterone involves the enzymes 5 alpha-reductase, 3 beta-hydroxysteroid dehydrogenase, and 6 alpha-hydroxylase. To investigate this pathway, the cells were incubated with a variety of potential substrates. In addition to progesterone, only precursors with the 5 alpha-configuration served as substrates for the enzymes leading to P-metabolite formation. Some precursors with a 5 beta-configuration were also metabolized by T47Dco cells. This metabolism reflected activity by either 3 beta-hydroxysteroid dehydrogenase and/or 6 alpha-hydroxylase but, in contrast to progesterone metabolism, the rates were different and the products were often mixtures. In T47Dco and MCF-7 human breast tumor cells, the reduction at C-3 followed by 6 alpha-hydroxylation, appear to be the major, and possibly only, route of progesterone metabolism. In contrast, preliminary data suggest that in normal human breast epithelial cells, this is not an exclusive route. Androgens are partially subject to the same metabolic enzymes, but synthetic progestins are not metabolized by T47Dco during an 18 h incubation. 相似文献
125.
D G Astaf'ev I V Maianskaia A K Akatov A N Maianski? 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1986,(7):48-51
In investigations based on the use of a highly sensitive test system permitting the detection of normal human antibodies to S. aureus peptidoglycan, the antigenic relationships between the peptidoglycans of S. aureus and other representatives of the genus Staphylococcus have been studied. Among other staphylococcal species, S. simulans, S. xylosus, S. hyicus, S. cohnii, S. hyicus s. s. chromogenes have been found to possess peptidoglycans most closely related to S. aureus peptidoglycans, while S. warneri and S. epidermidis peptidoglycans have proved to be least closely related to it. 相似文献
126.
127.
E. B. Burova I. S. Smirnova A. N. Shatrova I. V. Gonchar N. N. Nikolskii 《Cell and Tissue Biology》2011,5(1):9-14
Interferon gamma (IFNγ) is known to inhibit the proliferation of some transformed cell lines. Recently, we demonstrated the
transactivation of the epidermal growth factor receptor (EGFR) in response to IFNγ (Burova et al., 2007) and provided direct
evidence for the dependence of IFNγ-induced EGFR transactivation on the EGFR expression level in epithelial cells (Gonchar
et al., 2008). This study examines an antiproliferative effect of IFNγ on human epithelial cell lines—A431 and HeLa that express
high levels of EGFR, as well as HEK293 that expresses low levels of EGFR. To characterize the IFNγ-induced changes in these
cells, we studied cell growth, the cell cycle, and induction of apoptosis. The response to IFNγ differed in the compared cell
lines; cell growth was inhibited in both A431 and HeLa cells, but not in HEK293 cells, as was shown by the cell count and
MTT. The cell-cycle phases analyzed by flow cytometry were disturbed in A431 and HeLa cells in response to IFNγ. On the contrary,
in HEK293 cells, the IFNγ treatment did not alter distribution by cell cycle phases. Our results indicate that IFNγ produces
an antiproliferative effect that depends on the increased expression of EGFR in A431 and HeLa cells. Furthermore, it was demonstrated
that IFNγ induced the caspase 3 activation in A431 cells, which suggests the involvement of active caspase 3 in the IFNγ-induced
apoptosis. 相似文献
128.
129.
130.