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   1951篇
  免费   109篇
  国内免费   4篇
  2064篇
  2023年   11篇
  2022年   11篇
  2021年   31篇
  2020年   10篇
  2019年   11篇
  2018年   27篇
  2017年   17篇
  2016年   42篇
  2015年   58篇
  2014年   73篇
  2013年   130篇
  2012年   144篇
  2011年   118篇
  2010年   84篇
  2009年   87篇
  2008年   124篇
  2007年   140篇
  2006年   140篇
  2005年   141篇
  2004年   138篇
  2003年   118篇
  2002年   126篇
  2001年   17篇
  2000年   13篇
  1999年   13篇
  1998年   21篇
  1997年   23篇
  1996年   14篇
  1995年   20篇
  1994年   23篇
  1993年   24篇
  1992年   17篇
  1991年   14篇
  1990年   9篇
  1989年   13篇
  1988年   4篇
  1987年   3篇
  1986年   10篇
  1985年   5篇
  1984年   2篇
  1983年   6篇
  1982年   4篇
  1981年   6篇
  1980年   2篇
  1979年   5篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1975年   2篇
  1966年   1篇
排序方式: 共有2064条查询结果,搜索用时 0 毫秒
81.
A method for simultaneous detection and quantification is presented to determine the presence of isoflavones and bisphenol A in a biological sample. A coulometric array detector was used with reversed-phase high-performance liquid chromatography (HPLC). Daidzein (1), glycitein (2), genistein (3) and their glucoside conjugates, daidzin (4), glycitin (5) and genistin (6), were measured as phytochemicals. Also assayed here was equol (7), a metabolite from compound 1, and bisphenol A (8), an industrial chemical that acts as an endocrine disrupter. All chemicals were simultaneously detected by using a 600-mV single detection voltage with high efficacy. A mixture of 1, 3 and 8 was orally administered to rats, and the levels of these three chemicals in the serum were clearly increased after a 4 kU beta-glucuronidase treatment. The levels of compounds 1 and 3 in the serum were detected at 1665 and 2040 ng/ml, while 8 was at a low level of 417 ng/ml. Compound 7 in the serum was not detected until after enzymatic hydrolysis (72 ng/ml). These results suggest that this analytical method would be useful for metabolic and pharmacokinetic studies on isoflavones and bisphenol A.  相似文献   
82.
83.
Proteolysis by sequence-specific proteases is the key step for positive sequencing in proteomic studies integrated with mass spectrometry (MS). The conventional method of in-solution digestion of protein is a time-consuming procedure and has limited sensitivity. In this study, we report a simple and rapid system for the analysis of protein sequence and protein posttranslational modification by multienzymatic reaction in a continuous flow using the enzyme (trypsin, chymotrypsin, or alkaline phosphatase)-immobilized microreactor. The feasibility and performance of the single microreactor and tandem microreactors that were connected by the different microreactors were determined by the digestion of nonphosphoprotein (cytochrome c) and phosphoproteins (β-casein and pepsin A). The single microreactor showed rapid digestion compared with that of in-solution digestions. Multiple digestion by the tandem microreactors showed higher sequence coverage compared with that by in-solution or the single microreactor. Moreover, the tandem microreactor that was made by using the combination of protease-immobilized microreactor and phosphatase-immobilized microreactor showed the capability for phosphorylation site analysis in phosphoproteins without the use of any enrichment strategies or radioisotope labeling techniques. This approach provides a strategy that can be applied to various types of linking microreactor-based multienzymatic reaction systems for proteomic analysis.  相似文献   
84.
Eukaryotic initiation factor 5A (eIF-5A) was originally isolated as a translation initiation factor. However, this function has since been reconsidered, with recent studies pointing to roles for eIF-5A in mRNA metabolism and trafficking [Microbiol. Mol. Biol. Rev. 66 (2002) 460; Eur. Mol. Biol. Org. J. 17 (1998) 2914]. The Caenorhabditis elegans genome contains two eIF-5A homologues, iff-1 and iff-2, whose functions in vivo were examined in this study. The iff-2 mutation causes somatic defects that include slow larval growth and disorganized somatic gonadal structures in hermaphrodites. iff-2 males show disorganized tail sensory rays and spicules. On the other hand, iff-1 mRNA is expressed in the gonad, and the lack of iff-1 activity causes sterility with an underproliferated germline resulting from impaired mitotic proliferation in both hermaphrodites and males. In spite of underproliferation, meiotic nuclei are observed, as revealed by presence of immunoreactivity to the anti-HIM-3 antibody; however, no gametogenesis occurs in the iff-1 gonads. These phenotypes are in part similar to the mutants affected in the components of P granules, which are the C. elegans counterparts of germ granules [Curr. Top Dev. Biol. 50 (2000) 155]. We found that localization of the P-granule component PGL-1 to P granules is disrupted in the iff-1 mutant. In summary, the two C. elegans homologues of eIF-5A act in different tissues: IFF-2 is required in the soma, and IFF-1 is required in the germline for germ cell proliferation, for gametogenesis after entry into meiosis, and for proper PGL-1 localization on P granules.  相似文献   
85.
Estrogen related receptor beta (ERR-beta) is an orphan nuclear receptor specifically expressed in a subset of extra-embryonic ectoderm of post-implantation embryos. ERR-beta is essential for placental development since the ERR-beta null mutants die at 10.5dpc due to the placenta abnormality. Here, we show that the ERR-beta is specifically expressed in primordial germ cells (PGC), obviously another important cell type for reproduction. Expression of the ERR-beta mRNA in embryonic germ cells started at E11.5 as soon as PGC reached genital ridges, and persisted until E15-E16 in both sexes. Immunostaining with anti-ERR-beta antibody revealed that the ERR-beta protein is exclusively expressed in germ cells in both male and female gonads from E11.5 to E16. 5. To study function of the ERR-beta in PGC, we complemented placental defects of the ERR-beta null mutants with wild-type tetraploid embryos, and analyzed germ cell development in the rescued embryos. It was found that development of gonad and PGC was not apparently affected, but number of germ cells was significantly reduced in male and female gonads, suggesting that the ERR-beta appears to be involved in proliferation of gonadal germ cells. The rescued embryos could develop to term and grow up to adulthood. The rescued ERR-beta null male were found to be fertile, but both male and female null mutants exhibited behavioural abnormalities, implying that the ERR-beta plays important roles in wider biological processes than previously thought.  相似文献   
86.
Yan  Xing-Hong  Fujita  Yuji  Yusho Aruga 《Hydrobiologia》2004,512(1-3):133-140
Hydrobiologia - Two high monospore-producing pigmentation mutants were obtained by treatment with MNNG in Porphyra yezoensis Ueda. The mutants produced many monospores in young gametophytic blades...  相似文献   
87.
The genes encoding the catalytic domains (CD) of the three endoglucanases (EG I; Cel7B, EG II; Cel5A, and EG III; Cel12A) from Trichoderma reesei QM9414 were expressed in Escherichia coli strains Rosetta-gami B (DE3) pLacI or Origami B (DE3) pLacI and were found to produce functional intracellular proteins. Protein production by the three endoglucanase transformants was evaluated as a function of growth temperature. Maximal productivity of EG I-CD at 15°C, EG II-CD at 20°C and EG III at 37°C resulted in yields of 6.9, 72, and 50 mg/l, respectively. The endoglucanases were purified using a simple purification method based on removing E. coli proteins by isoelectric point precipitation. Specific activity toward carboxymethyl cellulose was found to be 65, 49, and 15 U/mg for EG I-CD, EG II-CD, and EG III, respectively. EG II-CD was able to cleave 1,3–1,4-β-d-glucan and soluble cellulose derivatives. EG III was found to be active against cellulose, 1,3–1,4-β-d-glucan and xyloglucan, while EG I-CD was active against cellulose, 1,3–1,4-β-d-glucan, xyloglucan, xylan, and mannan.  相似文献   
88.
89.
Human immunodeficiency virus type 1 (HIV-1) replication in macaque cells is restricted mainly by antiviral cellular APOBEC3, TRIM5α/TRIM5CypA, and tetherin proteins. For basic and clinical HIV-1/AIDS studies, efforts to construct macaque-tropic HIV-1 (HIV-1mt) have been made by us and others. Although rhesus macaques are commonly and successfully used as infection models, no HIV-1 derivatives suitable for in vivo rhesus research are available to date. In this study, to obtain novel HIV-1mt clones that are resistant to major restriction factors, we altered Gag and Vpu of our best HIV-1mt clone described previously. First, by sequence- and structure-guided mutagenesis, three amino acid residues in Gag-capsid (CA) (M94L/R98S/G114Q) were found to be responsible for viral growth enhancement in a macaque cell line. Results of in vitro TRIM5α susceptibility testing of HIV-1mt carrying these substitutions correlated well with the increased viral replication potential in macaque peripheral blood mononuclear cells (PBMCs) with different TRIM5 alleles, suggesting that the three amino acids in HIV-1mt CA are involved in the interaction with TRIM5α. Second, we replaced the transmembrane domain of Vpu of this clone with the corresponding region of simian immunodeficiency virus SIVgsn166 Vpu. The resultant clone, MN4/LSDQgtu, was able to antagonize macaque but not human tetherin, and its Vpu effectively functioned during viral replication in a macaque cell line. Notably, MN4/LSDQgtu grew comparably to SIVmac239 and much better than any of our other HIV-1mt clones in rhesus macaque PBMCs. In sum, MN4/LSDQgtu is the first HIV-1 derivative that exhibits resistance to the major restriction factors in rhesus macaque cells.  相似文献   
90.
This paper describes recent material gathered during the second fieldwork at Ma U'Oi in November 2002 by a Vietnamese–French–Japanese team. The Ma U'Oi cave, located in the province of Hoà Binh (60 km SW from Hanoi), northern Vietnam, belongs to a karstic network developed in Triassic dark-grey limestones.

The cave is filled with coarse-grained breccias containing numerous fossil remains, partially preserved at several loci inside the cave (wall, vault and ground). We describe new teeth which confirm the occurrence of mammal taxa already mentioned at Ma U'Oi (Bacon et al., 2004)[Bacon, A-M., Demeter, F., Schuster, M., Long, V.T., Thuy, N.K., Antoine, P-O., Sen, S., Nga, H.H., Huong, N.T.M., 2004. The Pleistocene Ma U'Oi cave, northern Vietnam: palaeontology, sedimentology and palaeoenvironments. Geobios 37, 305–314], while others, mainly microvertebrates, emphasize the occurrence of new species for the Pleistocene of Vietnam. We report here, for the first time, the occurrence of these microvertebrates of different groups (primates, rodents, insectivores, small reptiles and amphibians) in the faunal assemblage. Among mammal taxa, the presence of one more hominid affiliated to archaic Homo is also attested by our findings. U/Th dating carried out on 2 samples extracted from breccia speleothems confirms the biochronological estimate, with fossiliferous fillings ranging from late Middle Pleistocene to Late Pleistocene.  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