全文获取类型
收费全文 | 1509篇 |
免费 | 92篇 |
国内免费 | 2篇 |
专业分类
1603篇 |
出版年
2023年 | 4篇 |
2022年 | 21篇 |
2021年 | 24篇 |
2020年 | 12篇 |
2019年 | 27篇 |
2018年 | 34篇 |
2017年 | 24篇 |
2016年 | 41篇 |
2015年 | 59篇 |
2014年 | 62篇 |
2013年 | 82篇 |
2012年 | 110篇 |
2011年 | 121篇 |
2010年 | 87篇 |
2009年 | 82篇 |
2008年 | 78篇 |
2007年 | 106篇 |
2006年 | 82篇 |
2005年 | 84篇 |
2004年 | 90篇 |
2003年 | 85篇 |
2002年 | 72篇 |
2001年 | 17篇 |
2000年 | 14篇 |
1999年 | 15篇 |
1998年 | 24篇 |
1997年 | 19篇 |
1996年 | 11篇 |
1995年 | 18篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1971年 | 2篇 |
1967年 | 2篇 |
1935年 | 1篇 |
排序方式: 共有1603条查询结果,搜索用时 15 毫秒
21.
Thyroid hormone-regulated enhancer blocking: cooperation of CTCF and thyroid hormone receptor 下载免费PDF全文
Lutz M Burke LJ LeFevre P Myers FA Thorne AW Crane-Robinson C Bonifer C Filippova GN Lobanenkov V Renkawitz R 《The EMBO journal》2003,22(7):1579-1587
The highly conserved, ubiquitously expressed, zinc finger protein CTCF is involved in enhancer blocking, a mechanism crucial for shielding genes from illegitimate enhancer effects. Interestingly, CTCF-binding sites are often flanked by thyroid hormone response elements (TREs), as at the chicken lysozyme upstream silencer. Here we identify a similar composite site positioned upstream of the human c-myc gene. For both elements, we demonstrate that thyroid hormone abrogates enhancer blocking. Relief of enhancer blocking occurs even though CTCF remains bound to the lysozyme chromatin. Furthermore, chromatin immunoprecipitation analysis of the lysozyme upstream region revealed that histone H4 is acetylated at the CTCF-binding site. Loss of enhancer blocking by the addition of T3 led to increased histone acetylation, not only at the CTCF site, but also at the enhancer and the promoter. Thus, when TREs are adjacent to CTCF-binding sites, thyroid hormone can regulate enhancer blocking, thereby providing a new property for what was previously thought to be constitutive enhancer shielding by CTCF. 相似文献
22.
23.
Kreslavski V Tatarinzev N Shabnova N Semenova G Kosobryukhov A 《Journal of plant physiology》2008,165(15):1592-1600
The nature of photosynthetic recovery was investigated in 10-d-old wheat (Triticum aestivum L., cv. Moskovskaya-35) seedlings exposed to temperatures of 40 and 42 °C for 20 min and to temperature 42 °C for 40 min in the dark. The aftereffect of heat treatment was monitored by growing the heat-treated plants in low/moderate/high light at 20 °C for 72 h. The net photosynthetic rates (PN) and the fluorescence ratios Fv/Fm were evaluated in intact primary leaves and the rates of cyclic and non-cyclic photophosphorylation were measured in the isolated thylakoids. At least two temporally separated steps were identified in the path of recovery from heat stress at 40 and 42 °C in the plants growing in high and moderate/high light, respectively. Both photochemical activity of the photosystem II (PSII) and the activity of CO2 assimilation system were lowered during the first step in comparison with the corresponding activities immediately after heat treatment. During the second step, the photosynthetic activities completely or partly recovered. Recovery from heat stress at 40 °C was accompanied by an appreciably higher rate of cyclic photophosphorylation in comparison with control non-heated seedlings. In pre-heated seedlings, the tolerance of the PSII to photoinhibition was higher than in non-treated ones. The mode of acclimation to different light intensities after heat exposures is analyzed. 相似文献
24.
25.
Galina A. Korshunova Eugene Y. Krynetski Manana T. Mtchedlidze Denis V. Agapkin William E. Evans Natalia F. Krynetskaia 《Nucleosides, nucleotides & nucleic acids》2013,32(6-7):1747-1748
Abstract Synthesis of a number of photoactive thiopurine-containing nucleosides was described. S-methylation of the synthesized compounds in the course of the reaction catalyzed by recombinant human thiopurine S-methyltransferase was studied by UV-spectroscopy. 相似文献
26.
Sud'ina GF Pushkareva MA Galkina SI Surkov SA Mehl M Ullrich V 《Bioscience reports》1999,19(6):547-558
Human polymorphonuclear leukocytes (PMN) were found to tightly adhere on endothelial (lines EAhy926 and ECV304) and collagen surfaces under the influence of the chemotherapeutic drug suramin. This was observed by scanning electron microscopy and quantitated by myeloperoxidase assays. Suramin also inhibited Ca2+ ionophore A23187-stimulated leukotriene (LT) synthesis in PMN interaction with endothelial cells or with collagen surface. Suramin decreased the release of radiolabeled arachidonic acid (AA) and 5-lip-oxygenase (5-LO) metabolites by prelabeled PMN stimulated with A23187. Using agents releasing the suramin-stimulated adhesion namely jasplakonolide and dextran sulfate, we observed a reversal of the suramin effect on leukotriene synthesis. Jasplakonolide released the adhesion of PMN on endothelial and collagen-coated surfaces and restored 5-LO activity. Dextran-sulfate released adhesion on collagen-coated surfaces and abolished suramin inhibition. Arachidonate could also overcome adhesion and inhibition of 5-LO. We conclude that suramin-induced tight attachment of PMN on to solid surfaces lead to decreased leukotriene synthesis during subsequent A23187 stimulation in the absence of exogenous substrates. 相似文献
27.
Mona Vetter Christian Wirth Hannes Böttcher Galina Churkina Ernst-Detlef Schulze Thomas Wutzler Georg Weber† 《Global Change Biology》2005,11(5):810-827
Temperate forest ecosystems have recently been identified as an important net sink in the global carbon budget. The factors responsible for the strength of the sinks and their permanence, however, are less evident. In this paper, we quantify the present carbon sequestration in Thuringian managed coniferous forests. We quantify the effects of indirect human‐induced environmental changes (increasing temperature, increasing atmospheric CO2 concentration and nitrogen fertilization), during the last century using BIOME‐BGC, as well as the legacy effect of the current age‐class distribution (forest inventories and BIOME‐BGC). We focused on coniferous forests because these forests represent a large area of central European forests and detailed forest inventories were available. The model indicates that environmental changes induced an increase in biomass C accumulation for all age classes during the last 20 years (1982–2001). Young and old stands had the highest changes in the biomass C accumulation during this period. During the last century mature stands (older than 80 years) turned from being almost carbon neutral to carbon sinks. In high elevations nitrogen deposition explained most of the increase of net ecosystem production (NEP) of forests. CO2 fertilization was the main factor increasing NEP of forests in the middle and low elevations. According to the model, at present, total biomass C accumulation in coniferous forests of Thuringia was estimated at 1.51 t C ha?1 yr?1 with an averaged annual NEP of 1.42 t C ha?1 yr?1 and total net biome production of 1.03 t C ha?1 yr?1 (accounting for harvest). The annual averaged biomass carbon balance (BCB: biomass accumulation rate‐harvest) was 1.12 t C ha?1 yr?1 (not including soil respiration), and was close to BCB from forest inventories (1.15 t C ha?1 yr?1). Indirect human impact resulted in 33% increase in modeled biomass carbon accumulation in coniferous forests in Thuringia during the last century. From the forest inventory data we estimated the legacy effect of the age‐class distribution to account for 17% of the inventory‐based sink. Isolating the environmental change effects showed that these effects can be large in a long‐term, managed conifer forest. 相似文献
28.
Vitaly N. Nikandrov Oleg N. Murashko Galina V. Vorobyova Nelly S. Pyzhova Natalie V. Kvyatkovskaya Oksana A. Bartalevich 《International journal of peptide research and therapeutics》1997,4(4-6):497-502
Summary The formation of stable equimolar complexes of streptokinase or plasminogen with muscle lactate dehydrogenase or pyruvate
kinase, heart mitochondrial malate dehydrogenase and hepatic catalase at pH 7.4, 3.0 and 10.0 was first detected by differential
spectroscopy methods. All complexes, except those of plasminogen with dehydrogenases, were resistant to 6 M urea. Judging
from circular dichroism spectra, tertiary and secondary structures were considerably changed in the complexes. These changes
were significantly dependent upon the nature of interacting proteins; in some cases their structures were more ordered. NAD
(but not NADH) hampered the formation of streptokinase complexes with dehydrogenases. The plasminogen-activating function
of streptokinase and the ability of plasminogen to be activated by streptokinase in the complexes with oxidoreductases were
essentially unchanged. Pyruvate kinase induced a moderate (by 35%) increase in the streptokinase activating function. It is
assumed that the formation of complexes of streptokinase or plasminogen with enzymes may serve as a link in metabolic regulation
and/or intercellular interactions. 相似文献
29.
Gladyshev MI Sushchik NN Anishchenko OV Makhutova ON Kolmakov VI Kalachova GS Kolmakova AA Dubovskaya OP 《Oecologia》2011,165(2):521-531
One of the central paradigms of ecology is that only about 10% of organic carbon production of one trophic level is incorporated
into new biomass of organisms of the next trophic level. Many of energy-yielding compounds of carbon are designated as ‘essential’,
because they cannot be synthesized de novo by consumers and must be obtained with food, while they play important structural
and regulatory functions. The question arises: are the essential compounds transferred through trophic chains with the same
efficiency as bulk carbon? To answer this question, we measured gross primary production of phytoplankton and secondary production
of zooplankton and content of organic carbon and essential polyunsaturated fatty acids of ω-3 family with 18–22 carbon atoms
(PUFA) in the biomass of phytoplankton and zooplankton in a small eutrophic reservoir during two summers. Transfer efficiency
between the two trophic levels, phytoplankton (producers) and zooplankton (consumers), was calculated as ratio of the primary
production versus the secondary (zooplankton) production for both carbon and PUFA. We found that the essential PUFA were transferred
from the producers to the primary consumers with about twice higher efficiency than bulk carbon. In contrast, polyunsaturated
fatty acids with 16 carbon atoms, which are synthesized exclusively by phytoplankton, but are not essential for animals, had
significantly lower transfer efficiency than both bulk carbon, and essential PUFA. Thus, the trophic pyramid concept, which
implicitly implies that all the energy-yielding compounds of carbon are transferred from one trophic level to the next with
the same efficiency of about on average 10%, should be specified for different carbon compounds. 相似文献
30.
Rat liver chromatin activated by partial hepatectomy is more susceptible to the action of DNAse I than control chromatin isolated from intact liver. The study on the transfer of chromatin material to the acid-soluble fraction reveals a higher rate of activated chromatin degradation. Activated chromatin shows also an increased capacity for ethidium bromide (EB) binding as estimated from the isotherms of adsorption. The difference in EB binding between activated and control chromatin is abolished after DNAse I treatment. Conditions of mild digestion with DNAse I have been found under which the number of binding sites for EB per nucleotide decreases to almost the same level in activated and non-activated chromatin. The results suggest a preferential degradation of those DNA sequences in activated chromatin that are responsible for the increase in the ligand binding. 相似文献