全文获取类型
收费全文 | 12148篇 |
免费 | 783篇 |
国内免费 | 8篇 |
专业分类
12939篇 |
出版年
2024年 | 22篇 |
2023年 | 49篇 |
2022年 | 153篇 |
2021年 | 232篇 |
2020年 | 131篇 |
2019年 | 205篇 |
2018年 | 295篇 |
2017年 | 244篇 |
2016年 | 426篇 |
2015年 | 618篇 |
2014年 | 745篇 |
2013年 | 799篇 |
2012年 | 1068篇 |
2011年 | 1043篇 |
2010年 | 649篇 |
2009年 | 498篇 |
2008年 | 788篇 |
2007年 | 679篇 |
2006年 | 611篇 |
2005年 | 558篇 |
2004年 | 572篇 |
2003年 | 452篇 |
2002年 | 360篇 |
2001年 | 356篇 |
2000年 | 327篇 |
1999年 | 228篇 |
1998年 | 91篇 |
1997年 | 80篇 |
1996年 | 53篇 |
1995年 | 49篇 |
1994年 | 38篇 |
1993年 | 30篇 |
1992年 | 83篇 |
1991年 | 54篇 |
1990年 | 44篇 |
1989年 | 48篇 |
1988年 | 28篇 |
1987年 | 28篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 13篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 15篇 |
1977年 | 8篇 |
1975年 | 13篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Development of sucrose-utilizing Escherichia coli K-12 strain by cloning β-fructofuranosidases and its application for l-threonine production 总被引:1,自引:0,他引:1
Jeong Wook Lee Sol Choi Jin Hwan Park Claudia E. Vickers Lars K. Nielsen Sang Yup Lee 《Applied microbiology and biotechnology》2010,88(4):905-913
Sucrose is one of the most promising carbon sources for industrial fermentation. To achieve sucrose catabolism, the sucrose
utilization operons have been introduced into microorganisms that are not able to utilize sucrose. However, the rates of growth
and sucrose uptake of these engineered strains were relatively low to be successfully employed for industrial applications.
Here, we report a practical example of developing sucrose-utilizing microorganisms using Escherichia coli K-12 as a model system. The sucrose utilizing ability was acquired by introducing only β-fructofuranosidase from three different
sucrose-utilizing organisms (Mannheimia succiniciproducens, E. coli W, and Bacillus subtilis). Among them, the M. succiniciproducens β-fructofuranosidase was found to be the most effective for sucrose utilization. Analyses of the underlying mechanism revealed
that sucrose was hydrolyzed into glucose and fructose in the extracellular space and both liberated hexoses could be transported
by their respective uptake systems in E. coli K-12. To prove that this system can also be applied for the production of useful metabolites, the M. succiniciproducens β-fructofuranosidase was introduced into the engineered l-threonine production strain of E. coli K-12. This recombinant strain was able to produce 51.1 g/L l-threonine by fed-batch culture, resulting in an overall yield of 0.284 g l-threonine per g sucrose. This simple approach to make E. coli K-12 to acquire sucrose-utilizing ability and its successful biotechnological application can be employed to develop sustainable
bioprocesses using renewable biomass. 相似文献
992.
Choong Hyun Lee Jung Hoon Choi Ki-Yeon Yoo Ok Kyu Park In Koo Hwang Sang Guan You Boo-Yong Lee Il-Jun Kang Moo-Ho Won 《Cellular and molecular neurobiology》2010,30(2):255-263
Melatonin exerts many physiological functions via its G protein-coupled receptors. In the present study, we investigated age-related
changes in MT2 melatonin receptor immunoreactivity and its levels in the gerbil hippocampus during normal aging. In the postnatal
month 1 (PM 1) group, MT2 immunoreaction was well observed in neurons in all subregions of the gerbil hippocampus. In the
PM 3 and 6 groups, MT2 immunoreactivity in neurons was decreased compared to that in the PM 1 group. Thereafter, MT2 immunoreactivity
in neurons was increased. In the PM 18 and 24 groups, MT2 immunoreactivity in neurons was strong in all subregions of the
gerbil hippocampus. In addition, the number of MT2 immunoreactive cells was lowest at PM 3 and highest at PM 24. From western
blot analysis, age-dependent change pattern in MT2 level in the gerbil hippocampus was similar to the immunohistochemical
result. These results indicate that MT2 immunoreactivity and levels are altered in the gerbil hippocampus during normal aging;
lowest at young adult stage and highest at aged stage. 相似文献
993.
Eun-A Kim Hoh-Gyu Hahn Key-Sun Kim Tae Ue Kim Soo Young Choi Sung-Woo Cho 《Cellular and molecular neurobiology》2010,30(5):807-815
We have screened new drugs with a view to developing effective drugs against glutamate-induced excitotoxicity. In the present
work, we show effects of a new drug, 2-cyclopropylimino-3-methyl-1,3-thiazoline hydrochloride against glutamate-induced excitotoxicity
in primary rat glial cultures. Pretreatment of glial cells with 2-cyclopropylimino-3-methyl-1,3-thiazoline hydrochloride for
2 h significantly protected glial cells against glutamate-induced excitotoxicity in a time- and dose-dependent manner with
an optimum concentration of 100 μM. The drug significantly reduced production of proinflammatory cytokines, tumor necrosis
factor-α, and interlukin-1β in glutamate-induced excitotoxicity. The drug also prevented glutamate-induced intracellular Ca2+ influx and reduced the subsequent overproduction of nitric oxide and reactive oxygen species. Furthermore, the drug preserved
the mitochondrial potential and inhibited the overproduction of cytochrome c. In addition, the drug effectively attenuated the protein level changes of β-catenin and glycogen synthase kinase-3β. These
results suggest that 2-cyclopropylimino-3-methyl-1,3-thiazoline hydrochloride effectively protected primary cultures of rat
glial cells against glutamate-induced excitotoxicity. 相似文献
994.
Dae Won Chung Ki-Yeon Yoo In Koo Hwang Dae Won Kim Jin Young Chung Choong Hyun Lee Jung Hoon Choi Soo Young Choi Hwa Young Youn In Se Lee Moo-Ho Won 《Cellular and molecular neurobiology》2010,30(4):531-541
In this study, we observed the effects of lipopolysaccharide (LPS) on neurodegeneration and immune response in the hippocampus. LPS is a gram-negative bacterial cell surface proteoglycan and known as a bacterial endotoxin. For this, we investigated the optimal concentration of LPS influencing the ICR mouse hippocampus to measure the LPS receptor, e.g., toll-like receptor 4 (TLR4), expression in mouse hippocampal homogenates. TLR4 expression was significantly and prominently increased in the hippocampal homogenates of the LPS (1 mg/kg)-treated group. Next, we examined pro-inflammatory response in the hippocampus using cyclooxygenase-2 (COX-2, a marker for inflammatory response) immunohistochemistry after LPS treatment. COX-2 immunoreactivity was significantly increased in the endothelium of blood vessels in the hippocampus 6 h after LPS treatment, judging from double immunofluorescence study with platelet-derived endothelial cell adhesion molecule-1 (PECAM-1, a marker for endothelial cells): it decreased 12 h and disappeared 24 h after LPS treatment. In addition, the ionized calcium-binding adapter molecule 1 (Iba-1)-immunoreactive (+) microglia were morphologically activated in the mouse hippocampus after LPS treatment. At 24 h after LPS treatment, Iba-1+ microglia of activated forms were abundant in the hippocampus. However, NeuN (a neuron-specific soluble nuclear antigen)+ neurons were not significantly changed in the hippocampus after LPS treatment. Fluoro-jade B (a marker for neuronal degeneration)+ cells were not detected in the hippocampus at any time after LPS treatment. In addition, there were no significant differences in permeability of blood–brain barriers at any time points after LPS treatment. In brief, our results indicate that intraperitoneal administration of 1 mg/kg LPS effectively induces LPS receptor (TLR4) expression in the hippocampus, and the treatment increases corticosterone levels, inflammation in the blood vessels, and microglial activation in the hippocampus without any neuronal damage. 相似文献
995.
Sang Hyon Kim Mark Spensley Seung Kook Choi Cristiane P. G. Calixto Ali F. Pendle Olga Koroleva Peter J. Shaw John W. S. Brown 《Nucleic acids research》2010,38(9):3054-3067
Small nucleolar RNAs (snoRNAs) and small Cajal body-specific RNAs (scaRNAs) are non-coding RNAs whose main function in eukaryotes is to guide the modification of nucleotides in ribosomal and spliceosomal small nuclear RNAs, respectively. Full-length sequences of Arabidopsis snoRNAs and scaRNAs have been obtained from cDNA libraries of capped and uncapped small RNAs using RNA from isolated nucleoli from Arabidopsis cell cultures. We have identified 31 novel snoRNA genes (9 box C/D and 22 box H/ACA) and 15 new variants of previously described snoRNAs. Three related capped snoRNAs with a distinct gene organization and structure were identified as orthologues of animal U13snoRNAs. In addition, eight of the novel genes had no complementarity to rRNAs or snRNAs and are therefore putative orphan snoRNAs potentially reflecting wider functions for these RNAs. The nucleolar localization of a number of the snoRNAs and the localization to nuclear bodies of two putative scaRNAs was confirmed by in situ hybridization. The majority of the novel snoRNA genes were found in new gene clusters or as part of previously described clusters. These results expand the repertoire of Arabidopsis snoRNAs to 188 snoRNA genes with 294 gene variants. 相似文献
996.
Yi DK Sun IC Ryu JH Koo H Park CW Youn IC Choi K Kwon IC Kim K Ahn CH 《Bioconjugate chemistry》2010,21(12):2173-2177
Herein, we developed matrix metalloprotease (MMP) sensitive gold nanorods (MMP-AuNR) for cancer imaging and therapy. It was feasible to absorb NIR laser and convert into heat as well as visualize MMP activity. We showed the possibility of gold nanorods as a hyperthermal therapeutic agent and MMP sensitive imaging agent both in vitro and in vivo condition. The results suggested potential application of MMP-AuNR for simultaneous cancer diagnosis and therapy. 相似文献
997.
Nack-Shick Choi Dong-Min Chung Chan-Sun Park Keug-Hyun Ahn Joong Su Kim Jae Jun Song Seung-Ho Kim Byung-Dae Yoon Min-Soo Kim 《Biotechnology and Bioprocess Engineering》2010,15(3):446-452
Previously, three extracellular proteases, Vpr, PepT, and subtilisin were identified from Bacillus subtilis KCTC 3014. To confirm the activity of Vpr, two recombinant Vpr proteins, full Vpr with TTG (pGST-fTTG-Vpr) and full Vpr with
ATG (pGST-fATG-Vpr) as an initiation codon were expressed using a pGEX-2T vector encoding glutathione S-transferase (GST)
in Escherichia coli. Vpr was produced in two forms, occurring as four spots on a 2-DE gel, 68 and 75 kDa proteins with similar pI values (4.0
∼ 4.5). Activity was detected in a fibrin zymography at the expected molecular size of 68 kDa (mature form) processed from
full Vpr. However, the recombinant 75 kDa of GST-fVpr did not exhibit activity. Replacement of the TTG codon with ATG led
to 1.9-fold increased enzyme activity in 68 kDa. Interestingly, the expression of GSTVpr resulted in the proteolytic degradation
of the protein and no GST fusion Vpr protein was detected. 相似文献
998.
Choi HM Chang JY Trinh le A Padilla JE Fraser SE Pierce NA 《Nature biotechnology》2010,28(11):1208-1212
In situ hybridization methods enable the mapping of mRNA expression within intact biological samples. With current approaches, it is challenging to simultaneously map multiple target mRNAs within whole-mount vertebrate embryos, representing a significant limitation in attempting to study interacting regulatory elements in systems most relevant to human development and disease. Here, we report a multiplexed fluorescent in situ hybridization method based on orthogonal amplification with hybridization chain reactions (HCR). With this approach, RNA probes complementary to mRNA targets trigger chain reactions in which fluorophore-labeled RNA hairpins self-assemble into tethered fluorescent amplification polymers. The programmability and sequence specificity of these amplification cascades enable multiple HCR amplifiers to operate orthogonally at the same time in the same sample. Robust performance is achieved when imaging five target mRNAs simultaneously in fixed whole-mount and sectioned zebrafish embryos. HCR amplifiers exhibit deep sample penetration, high signal-to-background ratios and sharp signal localization. 相似文献
999.
Life history theory views reproduction as an outcome of resource allocation. The allocation of resources such as parental investments of time, energy and material resources involves trade-offs between number of offspring and timing of reproduction. Within the framework of mammalian parental investment, the outstanding feature of human reproduction is the high level of paternal care. Although empirical evidence suggests that human paternal investment may have evolved as a reproductive strategy to reduce infant and child mortality rates, the effects of actual paternal investment, including allocating time to child care, on female reproductive decisions have received relatively little attention. We examined the trade-off from two perspectives using a representative sample of married South Korean women aged 20–44 in 2005 (n=977). First, paternal investment in domestic labor, including child care and housework, was expected to be associated with women's preference regarding future reproduction. Second, relative paternal investment was expected to increase women's preference for future reproduction, especially among employed women. We found that increased paternal investment in child care and housework remarkably enhanced women's intention to have a second child, especially among employed women. In addition, although family members provide a low percentage of child care in South Korea, such help is likely to be a useful resource for second childbirth among employed women. Somewhat expectedly, older age and longer time since first birth had negative effects on women's second-child intention. There is growing evidence that, in the lowest fertility societies, paternal investment may be an essential resource for promoting future reproductive behavior of women, especially employed women. 相似文献
1000.