首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5635篇
  免费   424篇
  国内免费   363篇
  6422篇
  2024年   12篇
  2023年   66篇
  2022年   158篇
  2021年   284篇
  2020年   204篇
  2019年   229篇
  2018年   218篇
  2017年   185篇
  2016年   276篇
  2015年   354篇
  2014年   440篇
  2013年   450篇
  2012年   478篇
  2011年   426篇
  2010年   259篇
  2009年   224篇
  2008年   274篇
  2007年   231篇
  2006年   172篇
  2005年   129篇
  2004年   137篇
  2003年   124篇
  2002年   112篇
  2001年   98篇
  2000年   89篇
  1999年   92篇
  1998年   63篇
  1997年   61篇
  1996年   39篇
  1995年   51篇
  1994年   55篇
  1993年   53篇
  1992年   67篇
  1991年   48篇
  1990年   34篇
  1989年   29篇
  1988年   30篇
  1987年   16篇
  1986年   14篇
  1985年   22篇
  1984年   12篇
  1983年   11篇
  1982年   10篇
  1981年   7篇
  1980年   7篇
  1979年   18篇
  1978年   8篇
  1976年   7篇
  1975年   5篇
  1972年   5篇
排序方式: 共有6422条查询结果,搜索用时 0 毫秒
61.
In order to find novel antitumor candidate agents with high efficiency and low toxicity, 14 novel substituted 5‐anilino‐α‐glucofuranose derivatives have been designed, synthesized and evaluated for antiproliferative activities in vitro. Their structures were characterized by NMR (1H and 13C) and HR‐MS, and configuration (R/S) at C(5) was identified by two‐dimensional 1H,1H‐NOESY‐NMR spectrum. Their antiproliferative activities against human tumor cells were investigated by MTT assay. The results demonstrated that most of the synthesized compounds had antiproliferative effects comparable to the reference drugs gefitinib and lapatinib. In particular, (5R)‐5‐O‐(3‐chloro‐4‐{[5‐(4‐fluorophenyl)thiophen‐2‐yl]methyl}anilino)‐5‐deoxy‐1,2‐O‐(1‐methylethylidene)‐α‐glucofuranose ( 9da ) showed the most potent antiproliferative effects against SW480, A431 and A549 cells, with IC50 values of 8.57, 5.15 and 15.24 μm , respectively. This work suggested 5‐anilino‐α‐glucofuranose as an antitumor core structure that may open a new way to develop more potent anti‐cancer agents.  相似文献   
62.
Family-1 UDP glycosyltransferases (UGTs) from plants transfer sugar moieties from activated sugar donors to a wide range of small molecules, and control many metabolic processes during plant growth and development. Here, we report a genome-wide analysis of maize that identified 147 Family-1 glycosyltransferases based on their conserved PSPG motifs. Phylogenetic analysis of these genes with 18 Arabidopsis UGTs and two rice UGTs clustered them into 17 groups (A–Q). The patterns of intron gain/loss events, as well as their positions within UGTs from the same group, further aided elucidation of their divergence and evolutionary relationships between UGTs. Expression analysis of the maize UGT genes using both online microarray data and quantitative real-time PCR verification indicates that UGT genes are widely expressed in various tissues and likely play important roles in plant growth and development. Our study provides useful information on the Family-1 UGTs in maize, and will facilitate their further characterization to better understand their functions.  相似文献   
63.
Yin FG  Liu YL  Yin YL  Kong XF  Huang RL  Li TJ  Wu GY  Hou Y 《Amino acids》2009,37(2):263-270
Two experiments were conducted to evaluate the effects of dietary supplementation with Astragalus polysaccharide (APS) on growth performance, apparent ileal digestibilities (AID) of amino acids (AA), and their serum concentrations in early weaned piglets. In Exp. 1, 60 pigs were weaned at 21 days of age (BW 7.35 ± 0.23 kg) and allocated to three treatments (20 pigs/treatment), representing supplementing 0.0% (control), 0.02% colistin (antibiotic), or 0.1% APS to a corn- and soybean meal-based diet. Average daily gain (ADG), average daily feed intake (ADFI), and feed/gain ratio (F/G) were measured weekly. Blood samples were obtained from five pigs selected randomly from each treatment for the measurement of serum free AA concentrations on days 7, 14, and 28. In Exp. 2, 12 pigs were weaned at 21 day of age (BW 7.64 ± 0.71 kg), assigned to three treatment groups as in Exp. 1, and surgically fitted with a simple T-cannula at the terminal ileum. Ileal digesta samples were obtained for the measurement of AID of AA on days 7, 14 and 28. Dietary APS did not affect ADFI, but enhanced (P < 0.05) ADG by 11 and 4.4%, and improved F/G by 5.6 and 8.4%, respectively, compared with the control and antibiotic groups. Addition of APS to the diet increased AID and serum concentrations of most nutritionally essential and non-essential AA (including arginine, proline, glutamate, lysine, methionine, tryptophan, and threonine) on days 14 and 28. Circulating levels of total AA were affected by the age of pigs and treatment × time interaction. Collectively, these findings indicate that APS may ameliorate the digestive and absorptive function and regulate AA metabolism to beneficially increase the entry of dietary AA into the systemic circulation, which provide a mechanism to explain the growth-promoting effect of APS in early weaned piglets.  相似文献   
64.
Although 18F‐fluorodeoxyglucose (18F‐FDG) uptake can be used for the non‐invasive detection and monitoring of allograft rejection by activated leucocytes, this non‐specific accumulation is easily impaired by immunosuppressants. Our aim was to evaluate a 131I‐radiolabelled anti‐Toll‐like receptor 5 (TLR5) mAb for non‐invasive in vivo graft visualization and quantification in allogeneic transplantation mice model, compared with the non‐specific radiotracer 18F‐FDG under using of immunosuppressant. Labelling, binding, and stability studies were performed. BALB/c mice transplanted with C57BL/6 skin grafts, with or without rapamycin treatment (named as allo‐treated group or allo‐rejection group), were injected with 131I‐anti‐TLR5 mAb, 18F‐FDG, or mouse isotype 131I‐IgG, respectively. Whole‐body phosphor‐autoradiography and ex vivo biodistribution studies were obtained. Whole‐body phosphor‐autoradiography showed 131I‐anti‐TLR5 mAb uptake into organs that were well perfused with blood at 1 hr and showed clear graft images from 12 hrs onwards. The 131I‐anti‐TLR5 mAb had significantly higher graft uptake and target‐to‐non‐target ratio in the allo‐treated group, as determined by semi‐quantification of phosphor‐autoradiography images; these results were consistent with ex vivo biodistribution studies. However, high 18F‐FDG uptake was not observed in the allo‐treated group. The highest allograft‐skin‐to‐native‐skin ratio (A:N) of 131I‐anti‐TLR5 mAb uptake was significantly higher than the ratio for 18F‐FDG (7.68 versus 1.16, respectively). 131I‐anti‐TLR5 mAb uptake in the grafts significantly correlated with TLR5 expression in the allograft area. The accumulation of 131I‐IgG was comparable in both groups. We conclude that radiolabelled anti‐TLR5 mAb is capable of detecting allograft with high target specificity after treatment with the immunosuppressive drug rapamycin.  相似文献   
65.
Androgen receptor (AR) plays a critical role in prostate cancer and male sexual differentiation. We have identified AR from a primitive vertebrate with a sex reversal characteristic, the rice field eel. AR of this species (eAR) is distinct from human AR, especially in the ligand binding domain (LBD), and its expression in gonads shows an increasing tendency during gonadal transformation from ovary via ovotestis to testis. eAR has a restricted androgen-dependent transactivation function after a nuclear translocation upon dihydrotestosterone exposure. A functional nuclear localization signal was further identified in the DNA binding domain and hinge region. Although nuclear export is CRM1-independent, eAR has a novel nuclear export signal, which is negatively charged, indicating that a nuclear export pathway may be mediated by electrostatic interaction. Further, our studies have identified critical sequences for ligand binding in the C terminus. A structure of three α-helices in the LBD has been conserved from eels to humans during vertebrate evolution, despite a distinct amino acid sequence. Mutation analysis confirmed that the LBD is essential for dihydrotestosterone-induced nuclear import of eAR and following transactivation function in the nucleus. In addition, eAR interacts with both Sox9a1 and Sox9a2, and their interaction regulates transactivation of eAR. Our data suggest that the primitive species conserves and especially acquires key novel domains, the nuclear export signal and LBD, for the eAR function in spite of a rapid sequence evolution.  相似文献   
66.
Lactoferrin (Lf) is naturally produced by the mammary gland, having biological functions of antibacterial and anti-inflammatory activities. To investigate whether the Lf gene is associated with mastitis in dairy cattle, a DNA sequencing approach was used to identify single nucleotide polymorphisms (SNPs) in the gene. Three previously reported SNPs in the 5′ flanking region and one novel SNP in exon1 of Lf gene were identified. A total of 353 individuals from Holstein cattle populations were genotyped for their SNPs using Created Restriction Site PCR (CRS-PCR) and PCR-RFLP methods. Twenty-two and nineteen combinations of three SNPs (g.3440T>G, g.3879_3880insG, and g.4432T>C) and another three SNPs (g.3429G>A, g.3440T>G, g.3879_3880insG) were observed, respectively. The result of haplotype analysis of four SNPs showed that fourteen different haplotypes were identified. Two major haplotypes (GECB and GECA) occurred with a frequency of 22.5 and 18.5% in the study population, respectively. Statistical analyses revealed no significant association between one single SNP of Lf gene and SCS, whereas significant associations between their combined genotypes of three SNPs, haplotype and SCS. Combined genotype EFCDBB and GGEFDD with the lowest SCS were favorable for the mastitis resistance. They may be used as a possible candidate for marker-assisted selection in dairy cattle breeding program.  相似文献   
67.
对正常和半乳糖性白内障及给中草药的大鼠晶状体中某些吡啶核苷酸成分、糖类、非蛋白质巯基的含量进行了比较。结果表明,在白内障晶状体中,NADPH及非蛋白质巯基的含量明显低于正常晶状体的,而NADP、半乳糖及半乳糖醇的含量明显高于正常晶状体的;当注射半乳糖的同时分别用黄岑、石斛、菟丝子及玉蝴蝶四种中草药水煎剂灌胃,上述变化为基本恢复至正常晶状体的水平。表明四种中草药对晶状体中的异常生化变化具有阻止及纠正作用。  相似文献   
68.
Collapsin response mediator proteins (CRMPs) are key modulators of cytoskeletons during neurite outgrowth in response to chemorepulsive guidance molecules. However, their roles in adult injured neurons are not well understood. We previously demonstrated that CRMP3 underwent calcium-dependent N-terminal protein cleavage during excitotoxicity-induced neurite retraction and neuronal death. Here, we report findings that the full-length CRMP3 inhibits tubulin polymerization and neurite outgrowth in cultured mature cerebellar granule neurons, while the N-terminal truncated CRMP3 underwent nuclear translocation and caused a significant nuclear condensation. The N-terminal truncated CRMP3 underwent nuclear translocation through nuclear pores. Nuclear protein pull-down assay and mass spectrometry analysis showed that the N-terminal truncated CRMP3 was associated with nuclear vimentin. In fact, nuclear-localized CRMP3 co-localized with vimentin during glutamate-induced excitotoxicity. However, the association between the truncated CRMP3 and vimentin was not critical for nuclear condensation and neurite outgrowth since over-expression of truncated CRMP3 in vimentin null neurons did not alleviate nuclear condensation and neurite outgrowth inhibition. Together, these studies showed CRMP3's role in attenuating neurite outgrowth possibility through inhibiting microtubule polymerization, and also revealed its novel association with vimentin during nuclear condensation prior to neuronal death.  相似文献   
69.
The phylogenetic position of tarsiers within the primates has been a controversial subject for over a century. Despite numerous morphological and molecular studies, there has been weak support for grouping tarsiers with either strepsirrhine primates in a prosimian clade or with anthropoids in a haplorrhine clade. Here, we take advantage of the recently released whole genome assembly of the Philippine tarsier, Tarsius syrichta, in order to infer the phylogenetic relationship of Tarsius within the order Primates. We also present estimates of divergence times within the primates. Using a 1.26 million base pair multiple sequence alignment derived from 1078 orthologous genes, we provide overwhelming statistical support for the presence of a haplorrhine clade. We also present divergence date estimates using local relaxed molecular clock methods. The estimated time of the most recent common ancestor of extant Primates ranged from 64.9 Ma to 72.6 Ma, and haplorrhines were estimated to have a most recent common ancestor between 58.9 Ma and 68.6 Ma. Examination of rates of nucleotide substitution in the three major extant primate clades show that anthropoids have a slower substitution rate than either strepsirrhines or tarsiers. Our results provide the framework on which primate morphological, reproductive, and genomic features can be reconstructed in the broader context of mammalian phylogeny.  相似文献   
70.
The rhizomatous sedge Carex brevicuspis can produce clumping ramets from shortened rhizomes (phalanx) and spreading ramets from elongated rhizomes (guerrilla) to form a combined clonal growth form. In this paper, changes in clonal growth and biomass allocation pattern of C. brevicuspis in response to sedimentation were studied. Four sedimentation depths (0, 3, 6, and 9 cm) were applied to 48 ramets in a randomized block design. Plants were harvested after 20 weeks. With increasing sedimentation depth, the proportion of spreading ramets to total ramets increased from 19.6% in 0 cm to 92.9% in 9 cm sedimentation treatments, whereas that of clumping ramets decreased from 80.4% to 7.1%, indicating a change of clonal growth form from phalanx to guerrilla as a response to sedimentation. With increasing sedimentation depth, biomass allocation to shoots and roots did not change, but rhizome mass ratio increased from 2.7% in 0 cm to 7.2% in 9 cm sedimentation treatments, suggesting that production of long rhizomes changes biomass allocation pattern. The results show that plasticity of clonal growth forms, by which more spreading ramets are produced, is an effective strategy to avoid sedimentation stress under our experimental conditions.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号