全文获取类型
收费全文 | 11635篇 |
免费 | 1210篇 |
国内免费 | 3篇 |
出版年
2023年 | 68篇 |
2022年 | 158篇 |
2021年 | 285篇 |
2020年 | 162篇 |
2019年 | 178篇 |
2018年 | 236篇 |
2017年 | 183篇 |
2016年 | 313篇 |
2015年 | 546篇 |
2014年 | 606篇 |
2013年 | 720篇 |
2012年 | 970篇 |
2011年 | 828篇 |
2010年 | 506篇 |
2009年 | 453篇 |
2008年 | 624篇 |
2007年 | 619篇 |
2006年 | 571篇 |
2005年 | 493篇 |
2004年 | 478篇 |
2003年 | 417篇 |
2002年 | 367篇 |
2001年 | 219篇 |
2000年 | 203篇 |
1999年 | 183篇 |
1998年 | 126篇 |
1997年 | 99篇 |
1996年 | 77篇 |
1995年 | 78篇 |
1994年 | 87篇 |
1993年 | 57篇 |
1992年 | 144篇 |
1991年 | 111篇 |
1990年 | 103篇 |
1989年 | 102篇 |
1988年 | 98篇 |
1987年 | 93篇 |
1986年 | 97篇 |
1985年 | 69篇 |
1984年 | 70篇 |
1983年 | 60篇 |
1982年 | 76篇 |
1981年 | 59篇 |
1979年 | 69篇 |
1978年 | 70篇 |
1977年 | 47篇 |
1976年 | 55篇 |
1975年 | 60篇 |
1974年 | 66篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Methylene blue plus light mediates 8-hydroxyguanine formation in DNA 总被引:14,自引:0,他引:14
R A Floyd M S West K L Eneff J E Schneider 《Archives of biochemistry and biophysics》1989,273(1):106-111
Exposure to methylene blue (MB) plus light mediates formation of large levels of 8-hydroxyguanine in DNA. The amount of 8-hydroxy-2'-deoxyguanosine (8-OHdG) present in DNA increased as the amount of MB concentration increased throughout the 2 to 200 microM range studied and was dependent on light exposure. As the time of light exposure increased so did the 8-OHdG content to levels of about 750 8-OHdG/10(5) deoxyguanosine after 15 min of light exposure when MB was at 20 microM. Even though previous research has demonstrated that hydroxyl free radicals formed from a variety of sources mediate 8-OHdG formation in DNA, inclusion of mannitol, superoxide dismutase, catalase, and desferal in the MB plus light experiments demonstrated that these scavengers of oxygen free radical intermediates or precursors caused either no change or an increase in the 8-OHdG content of DNA exposed to MB plus light. These results appear to rule out the direct role of oxygen free radical intermediates in the primary events involved in the MB plus light mediated formation of 8-OHdG in DNA. Oxygen was essential to cause MB plus light mediated 8-OHdG formation in DNA. It was noted that when the reaction was carried out where the deuterium oxide content had been increased to 100%, the amount of 8-OHdG formed in DNA increased about threefold over that observed when comparable reactions were carried out in pure H2O. Use of the singlet oxygen scavenger 2,5-dimethylfuran has yielded variable results on the MB plus light mediated formation of 8-OHdG in DNA. The data taken collectively clearly indicate that MB plus light mediates 8-OHdG formation in DNA. The D2O data and the requirement for oxygen suggest that singlet oxygen may be an intermediate. 相似文献
52.
M A Gama Sosa J C Hall K E Schneider G C Lukaszewicz R M Ruprecht 《Biochemical and biophysical research communications》1989,161(1):134-142
Supercoiled pHXBc2 DNA (containing the genome of the human immunodeficiency virus type 1 and human sequences) migrated more slowly than linear DNA in native and ethidium bromide agarose gel electrophoresis at 4.5 volts/cm, suggesting the presence of unusual DNA structures. S1 nuclease analysis of pHXBc2 revealed two S1 hypersensitive sites. Site I was located within a 25 bp direct repeat in host DNA 0.6 kB upstream from the 5' LTR. Site II was mapped 0.2 kB upstream from the vif gene start site. Sequence analysis showed that Site I sequences could assume different unusual DNA structures, whereas sequences at Site II could assume either slipped or H-DNA forms. Unusual DNA structures in host DNA may be associated with active chromatin regions and may favor proviral integration. 相似文献
53.
Dale D. Brown Ronald G. Knowlton Joseph Hamill Teresa L. Schneider Ronald K. Hetzler 《European journal of applied physiology and occupational physiology》1990,60(3):179-182
The purpose of this study was to compare the physiological and biomechanical responses of wheelchair-dependent persons (WCD) to able-bodied persons (AB) during manual wheelchair ergometry. Five WCD and five AB performed a discontinuous wheelchair ergometer test starting at 12.8 W at 30 rev.min-1 (57 m.min-1) with increments of 7.0 W at 6-min intervals. Biomechanical data were collected 3.5 min into each stage followed by the collection of physiological data. After the fifth stage, peak oxygen consumption was determined by having the subject work against a resistance of 14.7-19.6 N at 30 rev.min-1. The WCD had significantly higher net mechanical efficiency at 26.7, 33.6 and 40.6 W in comparison to the AB. The WCD had significantly greater shoulder extension at the point of initial wheel contact as measured by the shoulder angle, while the AB had significantly greater shoulder range of motion at all work rates in comparison to the WCD. The results demonstrate that a significant physiological difference exists in the manner by which WCD and AB accomplish wheelchair ergometry. The biomechanical differences between AB and WCD were found to be a prominent factor contributing to the higher mechanical efficiency of WCD over AB. It was concluded that basic physiological and biomechanical differences exist between WCD and AB in manual wheelchair locomotion and that these differences are important considerations to the interpretation of data in wheelchair ergometry studies. 相似文献
54.
We have discovered that methylene blue plus light mediates the formation of 8-OHdG in DNA. Methylene blue is one of several thiazin dyes and we report here that the other thiazin dyes tested, in combination with white light, are effective in mediating 8-OHdG formation in DNA. The effectiveness of light plus the thiazin dyes in forming 8-OHdG in DNA were as follows: methylene blue greater than azure B greater than azure A greater than toluidine blue greater than thionin. Two other compounds tested; riboflavin and fuschin acid, in combination with light, caused formation of very little, if any, 8-OHdG in DNA. Thiazin dye mediated formation of 8-OHdG in DNA was not inhibited by the spin trap alpha-phenyl-t-butyl nitrone, which supports our previous observations that oxygen free radical scavengers did not inhibit methylene blue plus light mediated 8-OHdG formation in DNA. Ascorbate addition to methylene blue plus DNA, in the absence of light, was ineffective in mediating 8-OHdG formation in DNA. 相似文献
55.
Esmond J. Sanders Sara E. Zalik Wolfgang J. Schneider Irene M. Ledsham 《Development genes and evolution》1990,199(5):295-306
Summary Affinity purified preparations of the galactose-binding lectin from gastrulating chick blastoderms consist of three main polypeptides. Two of these have been identified as the 14 kD and 16 kD galactose-binding lectins. A third one migrates in SDS-PAGE gels with a relative molecular weight of 6,500±500 and has been identified as an apolipoprotein (Apo) of plasma very low density lipoproteins, Apo-VLDL-II. We have studied the localization of these polypeptides using immunofluorescence and ultrastructural immunocytochemistry with peroxidase and protein-A gold. The 14 kD lectin occurs in the intracellular yolk where it is mainly present within the electron lucent component. The 16 kD is also present in the intracellular yolk platelets, but tends to predominate in the electron-dense component. In addition, the 16 kD lectin is also present in pleiomorphic yolk-associated organelles and in the extracellular matrix. Apo-VLDL-II is also localized in the electron-lucent component of the yolk platelet and in the extracellular matrix. Our results suggest that the lectin(s) are associated with Apo-VLDL-II in the yolk platelet, and may subsequently become externalized. 相似文献
56.
ADP-ribosylation of histones and non-histone nuclear proteins was studied in isolated nuclei during the naturally synchronous cell cycle of Physarum polycephalum. Aside from ADP-ribosyltransferase (ADPRT) itself, histones and high mobility group-like proteins are the main acceptors for ADP-ribose. The majority of these ADP-ribose residues is NH2OH-labile. ADP-ribosylation of the nuclear proteins is periodic during the cell cycle with maximum incorporation in early to mid G2-phase. In activity gels two enzyme forms with Mr of 115,000 and 75,000 can be identified. Both enzyme forms are present at a constant ratio of 3:1 during the cell cycle. The higher molecular mass form cannot be converted in vitro to the low molecular mass form, excluding an artificial degradation during isolation of nuclei. The ADPRT forms were purified and separated by h.p.l.c. The low molecular mass form is inhibited by different ADPRT inhibitors to a stronger extent and is the main acceptor for auto-ADP-ribosylation. The high molecular mass form is only moderately auto-ADP-ribosylated. 相似文献
57.
Histochemical and microbiochemical demonstration of reduced pyruvate kinase activity in thioacetamide-induced neoplastic nodules of rat liver 总被引:1,自引:0,他引:1
A new method for the histochemical demonstration of pyruvate kinase (PK) activity was developed using a semi-permeable membrane and ATP-dependent phosphorylation of glucose coupled with tetrazolium reduction via glucose-6-phosphate dehydrogenase (G6PD) in order to investigate normal liver tissue and neoplastic hepatic nodules induced by thioacetamide (TAA). A series of control reactions and comparison with microbiochemical analysis of microdissected lyophilised material were used to determine the specificity of the reaction. In agreement with earlier reports, an activity gradient in control liver decreasing from zone 3 to zone 1 was apparent both histochemically and after biochemical analysis. Liver neoplastic nodules induced by 25 weeks dietary thioacetamide administration and characterized by increased G6PD demonstrated a clear decrease in PK activity. In contrast, epithelial cells within areas of cholangiocellular tumour development were characterized by a strong increase. Comparison of the results with immunohistochemical and biochemical data from the literature indicate that the specific histochemical method described will be of great assistance in future assessment of disease and physiological alteration in activity of this key enzyme of glycolysis. 相似文献
58.
Mitochondrial protein import: identification of processing peptidase and of PEP, a processing enhancing protein 总被引:43,自引:0,他引:43
Transport of nuclear-encoded precursor proteins into mitochondria includes proteolytic cleavage of amino-terminal targeting sequences in the mitochondrial matrix. We have isolated the processing activity from Neurospora crassa. The final preparation (enriched ca. 10,000-fold over cell extracts) consists of two proteins, the matrix processing peptidase (MPP, 57 kd) and a processing enhancing protein (PEP, 52 kd). The two components were isolated as monomers. PEP is about 15-fold more abundant in mitochondria than MPP. It is partly associated with the inner membrane, while MPP is soluble in the matrix. MPP alone has a low processing activity whereas PEP alone has no apparent activity. Upon recombining both, full processing activity is restored. Our data indicate that MPP contains the catalytic site and that PEP has an enhancing function. The mitochondrial processing enzyme appears to represent a new type of "signal peptidase," different from the bacterial leader peptidase and the signal peptidase of the endoplasmic reticulum. 相似文献
59.