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   278篇
  免费   53篇
  国内免费   1篇
  2021年   4篇
  2020年   6篇
  2019年   8篇
  2017年   3篇
  2016年   9篇
  2015年   7篇
  2014年   7篇
  2013年   10篇
  2012年   14篇
  2011年   17篇
  2010年   10篇
  2009年   11篇
  2008年   12篇
  2007年   16篇
  2006年   15篇
  2005年   8篇
  2004年   10篇
  2003年   9篇
  2002年   9篇
  2001年   6篇
  2000年   10篇
  1999年   4篇
  1998年   4篇
  1997年   3篇
  1996年   4篇
  1995年   4篇
  1994年   3篇
  1992年   3篇
  1991年   5篇
  1990年   4篇
  1989年   6篇
  1988年   4篇
  1987年   4篇
  1985年   7篇
  1983年   4篇
  1980年   4篇
  1979年   11篇
  1978年   3篇
  1977年   2篇
  1976年   2篇
  1975年   3篇
  1974年   4篇
  1973年   5篇
  1972年   7篇
  1971年   7篇
  1970年   2篇
  1969年   2篇
  1968年   5篇
  1967年   2篇
  1965年   2篇
排序方式: 共有332条查询结果,搜索用时 15 毫秒
71.
Research laboratories studying the genetics of companion animals have no database tools specifically designed to aid in the management of the many kinds of data that are generated, stored and analyzed. We have developed a relational database, "DOG-SPOT," to provide such a tool. Implemented in MS-Access, the database is easy to extend or customize to suit a lab's particular needs. With DOG-SPOT a lab can manage data relating to dogs, breeds, samples, biomaterials, phenotypes, owners, communications, amplicons, sequences, markers, genotypes and personnel. Such an integrated data structure helps ensure high quality data entry and makes it easy to track physical stocks of biomaterials and oligonucleotides.  相似文献   
72.
Recent findings indicate that proteins in the SNARE superfamily are essential for cell signaling, in addition to facilitating vesicle traffic in plant cell homeostasis, growth, and development. We previously identified SNAREs SYP121/Syr1 from tobacco (Nicotiana tabacum) and the Arabidopsis thaliana homolog SYP121 associated with abscisic acid and drought stress. Disrupting tobacco SYP121 function by expressing a dominant-negative Sp2 fragment had severe effects on growth, development, and traffic to the plasma membrane, and it blocked K(+) and Cl(-) channel responses to abscisic acid in guard cells. These observations raise questions about SNARE control in exocytosis and endocytosis of ion channel proteins and their organization within the plane of the membrane. We have used a dual, in vivo tagging strategy with a photoactivatable green fluorescent protein and externally exposed hemagglutinin epitopes to monitor the distribution and trafficking dynamics of the KAT1 K(+) channel transiently expressed in tobacco leaves. KAT1 is localized to the plasma membrane within positionally stable microdomains of approximately 0.5 microm in diameter; delivery of the K(+) channel, but not of the PMA2 H(+)-ATPase, to the plasma membrane is suppressed by Sp2 fragments of tobacco and Arabidopsis SYP121, and Sp2 expression leads to profound changes in KAT1 distribution and mobility within the plane of the plasma membrane. These results offer direct evidence for SNARE-mediated traffic of the K(+) channel and a role in its distribution within subdomains of the plasma membrane, and they implicate a role for SNAREs in positional anchoring of the K(+) channel protein.  相似文献   
73.
The Balanopaceae, whose flowers were poorly known, have, in the past, been variously allocated to the Fagales, Euphorbiaceae, Salicales or other hamamelids and rosids (these groups being in Fagales, Malpighiales and Saxifragales, according to the Angiosperm Phylogeny Group). This paper attempts a clarification based on flower morphology. Female flowers and cupules were studied in Balanops vieillardii, young fruits in B. australiana. The cupules are simple involucres of bracts which are spirally arranged (according to a Fibonacci pattern) on the floral axis preceding the flower. They contrast with the complicated cupules of Fagaceae which consist of a condensed cymose ramification system of axes of several orders around the flower. Flowers appear later than most of the cupular bracts, in contrast to Fagaceae. In addition to a terminal flower there may be several smaller lateral flowers in the axil of cupular bracts, each surrounded by its own small cupule. The female flowers do not have a perianth. They consist of two to three large carpels. At anthesis, the ovary is completely septate; the syncarpous part (ovary and lower style) is completely symplicate. The carpels are free for most of their length, with the free parts once, twice or three times bifurcate, in contrast to simple in Fagales. The stigmatic surface covers the ventral side of each stigmatic branch and at the margins also spreads to the dorsal side. The stigma is wet and secretion appears holocrine. The two ovules per carpel are collateral and axile in early development. However, at anthesis they appear one above the other, because in one ovule the funicle greatly elongates. As the ovary elongates only above the placenta, the ovules appear basal at anthesis. The ovules are (weakly) crassinucellar, bitegmic (not unitegmic), anatropous, and intermediate between apotropous and epitropous (not apotropous). The ovules are mature at anthesis, in contrast to Fagales. In mature ovules the upper part of the nucellus disintegrates, and a weakly differentiated endothelium is present in the inner integument. The morphological results of this study support a position of Balanopaceae in Malpighiales, and not Fagales or other orders, and are thus in accordance with recent molecular results based on chloroplast rbcL sequences data. However, within Malpighiales, as opposed to molecular results, Balanopaceae agree more with Euphorbiaceae s.l. than with Dichapetalaceae/Trigoniaceae and Chrysobalanaceae/Euphroniaceae.  相似文献   
74.
The heterodimeric peptide transporter TAP belongs to the ABC transporter family. Sequence comparisons with the P-glycoprotein and cystic fibrosis transmembrane conductance regulator and the functional properties of selective amino acids in these ABC transporters postulated that the glutamic acid at position 263 and the phenylalanine at position 265 of the TAP1 subunit could affect peptide transporter function. To define the role of both amino acids, TAP1 mutants containing a deletion or a substitution to alanine at position 263 or 265 were generated and stably expressed in murine and human TAP1(-/-) cells. The different TAP1 mutants were characterized in terms of expression and function of TAP, MHC class I surface expression, immune recognition, and species-specific differences. The phenotype of murine and human cells expressing human TAP1 mutants with a deletion or substitution of Glu(263) was comparable to that of TAP1(-/-) cells. In contrast, murine and human TAP1 mutant cells containing a deletion or mutation of Phe(265) of the TAP1 subunit exhibit wild-type TAP function. This was associated with high levels of MHC class I surface expression and recognition by specific CTL, which was comparable to that of wild-type TAP1-transfected control cells. Thus, biochemical and functional evidence is presented that the Glu(263) of the TAP1 protein, but not the Phe(265), is critical for proper TAP function.  相似文献   
75.
Juvenile polyposis syndrome (JPS) is an autosomal dominant predisposition to multiple juvenile polyps in the gastrointestinal tract. Germline mutations in the MADH4 or BMPR1A genes have been found to be causative of the disease in a subset of JPS patients. So far, no genotype-phenotype correlation has been reported. We examined 29 patients with the clinical diagnosis of JPS for germline mutations in the MADH4 or BMPR1A genes and identified MADH4 mutations in seven (24%) and BMPR1A mutations in five patients (17%). A remarkable prevalence of massive gastric polyposis was observed in patients with MADH4 mutations when compared with patients with BMPR1A mutations or without identified mutations. This is the first genotype-phenotype correlation observed in JPS.  相似文献   
76.
Recent evidence suggests that a CD8-mediated cytotoxic T-cell response against the regulatory proteins of human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) may control infection after pathogenic virus challenge. Here, we evaluated whether vaccination with Tat or Tat and Rev could significantly reduce viral load in nonhuman primates. Rhesus macaques were primed with Semliki forest Virus (SFV) expressing HIV-1 tat (SFV-tat) and HIV-1 rev (SFV-rev) and boosted with modified vaccinia virus Ankara (MVA) expressing tat and rev. A second group of monkey was primed with SFV-tat only and boosted with MVA-tat. A third group received a tat and rev DNA/MVA prime-boost vaccine regimen. Monitoring of anti-Tat and anti-Rev antibody responses or antigen-specific IFN-gamma production, as measured by enzyme-linked immunospot assays revealed no clear differences between the three groups. These results suggest that priming with either DNA or SFV seemed to be equivalent, but the additive or synergistic effect of a rev vaccine could not be clearly established. The animals were challenged by the rectal route 9 weeks after the last booster immunization, using 10 MID(50) of a SHIV-BX08 stock. Postchallenge follow-up of the monkeys included testing seroconversion to Gag and Env antigens, measuring virus infectivity in PBMC by cocultivation with noninfected human cells, and monitoring of plasma viral load. None of the animals was protected from infection as assessed by PCR, but peak viremia was reduced more than 200-fold compared to sham controls in one third (6/18) of vaccinated macaques, whatever the vaccine regimen they received. Interestingly, among these six protected animals four did not seroconvert. Altogether, these results clearly indicated that the addition of early HIV proteins like Tat and Rev in a multicomponent preventive vaccine including structural proteins like Env or Gag may be beneficial in preventive vaccinal strategies.  相似文献   
77.
Two yeast artificial chromosomes containing the entire human nerve growth factor gene were isolated and mapped. By homologous recombination a luciferase gene was precisely engineered into the coding portion of the NGF gene and a neomycin selection marker was placed adjacent to one of the YAC telomeres. Expression of the YAC-based NGF reporter gene and a plasmid-based NGF reporter gene were compared with the regulation of endogenous mouse NGF protein in mouse L929 fibroblasts. In contrast to the plasmid-based reporter gene, expression and regulation of the YAC-based reporter gene was independent of the site of integration of the transgene. Basic fibroblast growth factor and okadaic acid stimulated expression of the YAC transgene, whereas transforming growth factor-beta and dexamethasone inhibited it. Although cyclic AMP strongly stimulated production of the endogenous mouse NGF, no effect was seen on the human NGF reporter genes. Downregulation of the secretion of endogenous mouse NGF already occurred at an EC50 of 1-2 nM dexamethasone, but downregulation of the expression of NGF reporter genes occurred only at EC50 of 10 nM. This higher concentration was also required for upregulation of luciferase genes driven by the dexamethasone-inducible promoter of the mouse mammary tumor virus in L929 fibroblasts.  相似文献   
78.
Rates of microsomal 17β-estradiol (E2) hydroxylation at the C-2, -4, -6, and -15 positions are each induced greater than 10-fold by treating MCF-7 breast cancer cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The TCDD-induced activities at the C-2, -6 and -15 positions have been attributed to cytochrome P450 1A1 (CYP1A1); however, the low Km 4-hydroxylase induced by TCDD appears to be a distinct enzyme. We report here that antibodies to cytochrome P450-EF (mouse CYP1B1) selectivity inhibited the C-4 hydroxylation of E2 catalyzed by microsomes from TCDD-treated MCF-7 cells. Western blots probed with anti-CYP1B antibodies showed the induction of a 52 kDa microsomal protein in response to treatment with TCDD in MCF-7 cells. Western blots of microsomes from HepG2 cells did not show the TCDD-induced 52 kDa protein, and microsomes from TCDD-treated HepG2 cells did not catalyze a low Km hydroxylation of E2 at C-4. Cellular metabolism experiments also showed induction of both the C-2 and -4 hydroxylation pathways in TCDD-treated MCF-7 cells as evidenced by elevated 2- and 4-methoxyestradiol (MeOE2) formation. In contrast, TCDD-treated HepG2 cells showed 2-MeOE2 formation predominantly over 4-MeOE2. Northern blots of RNA isolated from untreated and TCDD-treated cells, when probed with the human CYP1B1 cDNA, showed induction of a 5.2 kb RNA in MCF-7 cells but not in HepG2 cells in response to treatment with TCDD. These results provide additional evidence for the induction by TCDD of a novel E2 4-hydroxylase in MCF-7 cells but not in HepG2 cells and indicate possible endocrine regulatory roles for the newly discovered group of enzymes of the CYP1B subfamily.  相似文献   
79.
The in vivo metabolism of 6-keto PGF1 alpha was investigated in rats. Following continuous intravenous infusion for 14 days the urinary metabolites were isolated and identified. A substantial amount of unchanged 6-keto PGF1 alpha was recovered in the urine. The metabolic pattern very closely resembles that of PGI2 in rats. Metabolites were found which represented 15-dehydrogenation, beta-oxidation, omega and omega-1-hydroxylation and oxidation. Previous work showed that 6-keto PGF1 alpha is very poorly oxidized by 15-PGDH. We administered 15-[H3]-PGI2 and 15-[H3]-6-keto PGF1 alpha to rats and measured urinary tritiated water as an index for in vivo 15-PGDH activity. The results showed that PGI2 and 6-keto PGF1 alpha were both oxidized to the 15-keto product, although the rate of oxidation of PGI2 was greater than that of 6-keto PGF1 alpha. We concluded that the administered PGI2 was oxidized by 15-PGDH before hydrolysis to 6-keto PGF1 alpha. A portion of the dose is probably hydrolzyed before 15-dehydrogenation.  相似文献   
80.
Of 37 strains of Eubacterium lentum and phenotypically similar organisms, 26 (70%) synthesized a corticoid 21-dehydroxylase and/or a 3 alpha-hydroxysteroid dehydrogenase. It appeared that the corticoid 3 alpha-hydroxysteroid dehydrogenase was identical to the bile acid 3 alpha-hydroxysteroid dehydrogenase. Steroid-metabolizing enzymes were found both in E. lentum and in phenotypically similar organisms. E. lentum is characterized by nitrate reduction and enhanced growth in the presence of arginine. Many phenotypically similar organisms possess either one or the other of the two markers. In contrast, using the steroid-metabolizing enzymes as markers, a "steroid-active" and a "steroid-inactive" group were established with minimal overlapping of metabolic characteristics. Synthesis of the steroid enzymes was positively correlated with production of gas from H2O2 and formation of H2S. A simple method for the detection of corticoid 21-dehydroxylase and 3 alpha-hydroxysteroid dehydrogenase, one or both of which were present in 92% of the steroid-active group, is described.  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