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   1672篇
  免费   112篇
  1784篇
  2022年   10篇
  2021年   16篇
  2020年   8篇
  2018年   17篇
  2017年   12篇
  2016年   15篇
  2015年   35篇
  2014年   49篇
  2013年   131篇
  2012年   75篇
  2011年   76篇
  2010年   46篇
  2009年   52篇
  2008年   76篇
  2007年   89篇
  2006年   91篇
  2005年   75篇
  2004年   69篇
  2003年   82篇
  2002年   58篇
  2001年   62篇
  2000年   51篇
  1999年   54篇
  1998年   21篇
  1997年   23篇
  1996年   17篇
  1995年   7篇
  1994年   18篇
  1993年   13篇
  1992年   36篇
  1991年   51篇
  1990年   33篇
  1989年   28篇
  1988年   36篇
  1987年   25篇
  1986年   26篇
  1985年   34篇
  1984年   15篇
  1983年   17篇
  1982年   13篇
  1981年   10篇
  1980年   13篇
  1979年   8篇
  1978年   8篇
  1977年   15篇
  1975年   16篇
  1974年   7篇
  1973年   16篇
  1972年   8篇
  1971年   6篇
排序方式: 共有1784条查询结果,搜索用时 15 毫秒
111.
Polyurethane (PU) foams were prepared using a palm oil-based polyol (PO-p). At the first stage, palm oil was converted to monoglycerides as a new type of polyol by glycerolysis. A yield of the product reached 70% at reaction temperature of 90 degrees C by using an alkali catalyst and a solvent. At the second stage, PU foams were prepared from mixtures of the polyol and polyethylene glycol (PEG) or diethylene glycol (DEG) and an isocyanate compound. Characterization of the foams was carried out by thermal and mechanical analyses. The analyses showed that the chain motion of polyurethane becomes more flexible at the higher PO-p content in the whole polymer, which indicates that the monoglyceride molecules work as soft segments. The study here may lead to a development of a new type of polyurethane foams using palm oil as a raw material.  相似文献   
112.
AimWe performed a replication study in a Japanese population to evaluate the association between type 2 diabetes and 7 susceptibility loci originally identified by European genome-wide association study (GWAS) in 2012: ZMIZ1, KLHDC5, TLE1, ANKRD55, CILP2, MC4R, and BCAR1. We also examined the association of 3 additional loci: CCND2 and GIPR, identified in sex-differentiated analyses, and LAMA1, which was shown to be associated with non-obese European type 2 diabetes.MethodsWe genotyped 6,972 Japanese participants (4,280 type 2 diabetes patients and 2,692 controls) for each of the 10 single nucleotide polymorphisms (SNPs): rs12571751 in ZMIZ1, rs10842994 near KLHDC5, rs2796441 near TLE1, rs459193 near ANKRD55, rs10401969 in CILP2, rs12970134 near MC4R, rs7202877 near BCAR1, rs11063069 near CCND2, rs8108269 near GIPR, and rs8090011 in LAMA1 using a multiplex polymerase chain reaction invader assay. The association of each SNP locus with the disease was evaluated using a logistic regression analysis.ResultsAll SNPs examined in this study had the same direction of effect (odds ratio > 1.0, p = 9.77 × 10-4, binomial test), as in the original reports. Among them, rs12571751 in ZMIZ1 was significantly associated with type 2 diabetes [p = 0.0041, odds ratio = 1.123, 95% confidence interval 1.037–1.215, adjusted for sex, age and body mass index (BMI)], but we did not observe significant association of the remaining 9 SNP loci with type 2 diabetes in the present Japanese population (p ≥ 0.005). A genetic risk score, constructed from the sum of risk alleles for the 7 SNP loci identified by un-stratified analyses in the European GWAS meta-analysis were associated with type 2 diabetes in the present Japanese population (p = 2.3 × 10-4, adjusted for sex, age and BMI).ConclusionsZMIZ1 locus has a significant effect on conferring susceptibility to type 2 diabetes also in the Japanese population.  相似文献   
113.
114.
The mechanism of translational initiation differs between prokaryotes and eukaryotes. Prokaryotic mRNAs generally contain within their 5′-untranslated region (5′-UTR) a Shine-Dalgarno (SD) sequence that serves as a ribosome-binding site. Chloroplasts possess prokaryotic-like translation machinery, and many chloroplast mRNAs have an SD-like sequence, but its position is variable. Tobacco chloroplast atpB mRNAs contain no SD-like sequence and are U-rich in the 5′-UTR (−20 to −1 with respect to the start codon). In vitro translation assays with mutated mRNAs revealed that an unstructured sequence encompassing the start codon, the AUG codon and its context are required for translation. UV crosslinking experiments showed that a 50 kDa protein (p50) binds to the 5′-UTR. Insertion of an additional initiation region (SD-sequence and AUG) in the 5′-UTR, but not downstream, arrested translation from the authentic site; however, no inhibition was observed by inserting only an AUG triplet. We hypothesize for translational initiation of the atpB mRNA that the ribosome enters an upstream region, slides to the start codon and forms an initiation complex with p50 and other components.  相似文献   
115.
A method that makes use of polyacrylamide gel electrophoresis was developed for the analysis of intramolecular disulfide bonds in proteins. Proteins with different numbers of cleaved disulfide bonds are alkylated with iodoacetic acid or iodoacetamide as the first step. The disulfide bonds remaining were reduced by excess dithiothreitol, and the newly generated free sulfhydryl groups were alkylated with the reagent not yet used (iodoacetamide, iodoacetic acid, or vinyl-pyridine) as the second step. This treatment made it possible for lysozyme (Mr, 14,000; 4 disulfides), the N-terminal half-molecule of conalbumin (Mr, 36,000; 6 disulfides), the C-terminal half-molecule of conalbumin (Mr, 40,000; 9 disulfides), and whole conalbumin (Mr, 78,000; 15 disulfides) to be separated by acid-urea polyacrylamide gel electrophoresis into distinct bands depending on the number of disulfide bonds cleaved. The method allowed us to determine the total number of disulfide bonds in native proteins and to assess the cleaved levels of disulfide bonds in partially reduced proteins. Two-step alkylation used in combination with radioautography was especially useful for the analysis of disulfide bonds in proteins synthesized in complex biological systems.  相似文献   
116.
We have developed a novel double-layer zymographic method for the detection of specific processing proteases of a target proprotease using a specific fluorescent substrate. The target processing proteases were separated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the gel was subsequently incubated with the target proenzyme used as the substrate. A cellulose acetate membrane was immersed in 10% glycerol and then soaked in the fluorescent substrate solution. The slab gel of the processing protease was covered with the fluorescent substrate membrane, making a double layer. The double layer was incubated at 37 degrees C, and the released fluorescent band, in which the processing protease was located, was detected using UV light. The advantages of the double-layer fluorescent zymographic method are as follows: (i) the specific detection of target proprotease using a specific substrate, (ii) a relatively rapid and sensitive method, (iii) effective detection using small amounts of crude material, and (iv) wide applications that include the detection of processing proteases and activators for target proteases. Typical examples used for the detection of the processing proteases, such as plasminogen activator, chymotrypsinogen activator, procaspase-3 processing protease and caspase-3 activators, using this new method are described in this article.  相似文献   
117.
Both NADPH- and ascorbic acid-dependent lipid peroxidations were inhibited by spermine, the degree of inhibition being greater with the former peroxidation. The effective concentration of spermine required for inhibition was higher when larger amounts of microsomes were used. However, the activities of NADPH-cytochrome c reductase and NADPH-peroxidase were not influenced by spermine. These results suggest that spermine inhibits lipid peroxidation by binding to phospholipids in the microsomes.  相似文献   
118.
Fresh and dry body weights (FW, DW) were greater for adult southern green stink bug,Nezara viridula (L.) than for the brown stink bug,Euschistus heros F. throughout the year in southern Brazil. FemalesN. viridula significantly increased FW and DW in late summer-early autumn, and during mid-spring; femaleE. heros did not show the same rates of increase in FW and DW. FemaleN. viridula were heavier than males, particularly during summer; however, female and maleE. heros were generally similar in weight.E. heros contained significantly greater amounts of lipid thanN. viridula, during mid-autumn to early-spring (April–September). Survivorship (%) and total longevity ofE. heros adults provided water only was greater (34.6–24.6 days, for females and males) than that forN. viridula (14.8–13.0 days); without water and food, longevity was drastically reduced (<7 days) for both species.  相似文献   
119.
cDNA clones for the P-450(M-1) mRNA, which exhibits a male-specific expression in rat livers, were isolated by using synthetic oligonucleotides as the probes. Sequence analysis of the cDNAs showed that P-450(M-1) mRNA contains 1,853 nucleotides in addition to a poly(A) chain, and a single open reading frame of 1,500 nucleotides encodes a polypeptide of 500 amino acids with a Mr = 57,187. The predicted NH2-terminal sequence of 30 amino acids agrees well with that of the purified protein determined by Edman degradation, and the predicted primary structure included all the partial sequences of six internal peptides of P-450(M-1) obtained by the proteolytic digestion and a conserved amino acid sequence containing a putative heme-binding cysteine, proximate to the COOH terminus of the molecules. P-450(M-1) showed relatively high sequence similarity with P-450b (Fujii-Kuriyama, Y., Mizukami, Y., Kawajiri, K., Sogawa, K., and Muramatsu, M. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2793-2797) (52% similarity), P-450-3b (Ozols, J., Heinemann, F. S., and Johnson, E. F. (1985) J. Biol. Chem. 260, 5427-5434) (64%), P-450-1 (Tukey, R. H., Okino, S., Barnes, H., Griffin, K. J., and Johnson, E. F. (1985) J. Biol. Chem. 260, 13347-13354) (74%), P-450PBc1 (Leighton, J. K., DeBrunner-Vossbrinck, B. A., and Kemper, B. (1984) Biochemistry 23, 4598-4603) (71%), while its sequence similarity with 3-methylcholanthrene-inducible P-450c and P-450d is rather low. Consequently, P-450(M-1) could be structurally classified into the phenobarbital-inducible type of P-450 gene family. RNA blot analysis using a synthetic oligonucleotide specific for P-450(M-1) revealed that P-450(M-1) mRNA was expressed exclusively in the livers of mature male rats in a sex-specific manner, but not in other tissues so far examined.  相似文献   
120.
Abstract. We studied canopy structure, shoot architecture and light harvesting efficiencies of the species (photon flux captured per unit above‐ground plant mass) in a series of exclosures of different age (up to 4.5 yr) in originally heavily grazed grassland in N Japan.Vegetation height and Leaf Area Index (LAI) increased in the series and Zoysia japonica, the dominant in the beginning, was replaced by the much taller Miscanthus sinensis. We showed how this displacement in dominance can be explained by inherent constraints on the above‐ground architecture of these two species. In all stands light capture of plants increased with their above‐ground biomass but taller species were not necessarily more efficient in light harvesting. Some subordinate species grew disproportionally large leaf areas and persisted in the shady undergrowth. Some other species first grew taller and managed to stay in the better‐lit parts of the canopy, but ultimately failed to match the height growth of their neighbours in this early successional series. Their light harvesting efficiencies declined and this probably led to their exclusion. By contrast, species that maintained their position high in the canopy managed to persist in the vegetation despite their relatively low light harvesting efficiencies. In the tallest stands ‘later successional’ species had higher light harvesting efficiencies for the same plant height than ‘early successional’ species which was mostly the result of the greater area to mass ratio (specific leaf area, SLA) of their leaves. This shows how plant stature, plasticity in above‐ground biomass partitioning, and architectural constraints determine the ability of plants to efficiently capture light, which helps to explain species replacement in this early successional series.  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