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   791篇
  免费   61篇
  2022年   3篇
  2021年   14篇
  2020年   4篇
  2019年   17篇
  2018年   11篇
  2017年   10篇
  2016年   18篇
  2015年   40篇
  2014年   39篇
  2013年   44篇
  2012年   64篇
  2011年   53篇
  2010年   56篇
  2009年   32篇
  2008年   53篇
  2007年   55篇
  2006年   45篇
  2005年   57篇
  2004年   42篇
  2003年   43篇
  2002年   33篇
  2001年   5篇
  2000年   2篇
  1999年   7篇
  1998年   13篇
  1997年   10篇
  1996年   20篇
  1995年   10篇
  1994年   4篇
  1992年   3篇
  1991年   5篇
  1989年   8篇
  1988年   3篇
  1987年   2篇
  1986年   2篇
  1985年   3篇
  1984年   3篇
  1982年   1篇
  1981年   2篇
  1978年   1篇
  1974年   1篇
  1972年   1篇
  1971年   1篇
  1970年   2篇
  1967年   1篇
  1956年   1篇
  1955年   1篇
  1928年   2篇
  1927年   2篇
  1925年   1篇
排序方式: 共有852条查询结果,搜索用时 15 毫秒
1.
Cyclizations of alkylhydrazines with N-acyl-S-methylisothioureas, readily synthesized from acyl chlorides, sodium thioisocyanate, dialkylamines then methyl iodide in a one-pot reaction, gave 1-alkyl-3-dialkylamino-5-phenyltriazoles 7 as major products. The regioisomers were assigned through the use of NOE NMR experiments. While bearing a N-bis(cyclopropyl)methyl-N-propylamino group, this series of compounds shows very good binding affinity on the human CRF1 receptor. Among them, 1-methyl-3-[N-bis(cyclopropyl)methyl-N-propylamino]-5-(2,4-dichlorophenyl)-1H-[1,2,4]triazole 7a had the best binding affinity for the CRF1 receptor (Ki=9 nM).  相似文献   
2.
Summary A localized region of low DNA sequence homology was revealed in two strains of Bacillus subtilis by a specific 100-fold reduction in transformation by W23 DNA of the tag1 locus, a teichoic acid marker of strain 168. Fifty nine rare recombinants, hybrid at this locus, had all acquired donor-specific phage resistance characters, while losing those specific to the 168 recipient. Chemical analysis of isolated cell walls showed that these modifications are associated with major changes in the wall teichoic acids. Genetic analysis demonstrated that determinants for the ribitol phosphate polymer of strain W23 had been transferred to 168, replacing those for the glycerol phosphate polymer in the recipient. All W23 genes coding for poly(ribitol phosphate) in the hybrids and those specifying anionic wall polymers in strain 168 are clustered near hisA. In addition to tag1, the region exchanged extends just beyond gtaA in some hybrids, whereas in others it may include the more distant gtaB marker, encompassing a region sufficient to contain at least 20 average-sized genes. Surface growth, flagellation, transformability and sporulation all appeared normal in hybrids examined. Recombinants without a major wall teichoic acid from either strain were not found, suggesting that an integral transfer of genes for poly(ribitol phosphate) from W23 had occurred in all hybrids isolated. We interpret these results as indicating an essential role for anionic wall polymers in the growth of B. subtillis.  相似文献   
3.
The genomic distribution of elements of the copia, 412, B 104, mdg 1, mdg 4 and 1731 transposon families was compared by the Southern technique in DNA preparations extracted from brains, salivary glands and adult flies of two related Drosophila lines. The copia, 412 and mdg 1 sequences were also probed in DNA from sperm, embryos, and 1st and 2nd instar larvae. The homogeneity of the patterns observed shows that somatic transposition is unlikely to occur frequently. A correlation between mobility and the euchromatic or heterochromatic location of transposable elements is discussed. In addition, an explanation of the variable band intensities of transposable elements in Southern autoradiographs is proposed.  相似文献   
4.
P. Dimitri  C. Pisano 《Genetics》1989,122(4):793-800
Position effect variegation results from chromosome rearrangements which translocate euchromatic genes close to the heterochromatin. The euchromatin-heterochromatin association is responsible for the inactivation of these genes in some cell clones. In Drosophila melanogaster the Y chromosome, which is entirely heterochromatic, is known to suppress variegation of euchromatic genes. In the present work we have investigated the genetic nature of the variegation suppressing property of the D. melanogaster Y chromosome. We have determined the extent to which different cytologically characterized Y chromosome deficiencies and Y fragments suppress three V-type position effects: the Y-suppressed lethality, the white mottled and the brown dominant variegated phenotypes. We find that: (1) chromosomes which are cytologically different and yet retain similar amounts of heterochromatin are equally effective suppressors, and (2) suppression effect is positively related to the size of the Y chromosome deficiencies and fragments that we tested. It increases with increasing amounts of Y heterochromatin up to 60-80% of the entire Y, after which the effect reaches a plateau. These findings suggest suppression is a function of the amount of Y heterochromatin present in the genome and is not attributable to any discrete Y region.  相似文献   
5.
6.
Summary Administration of interferon as a single therapeutic regimen in cancer patients with various neoplasias has had only limited efficacy in ameliorating the negative clinical course of their disease. In the present study, we have evaluated the effect of recombinant human fibroblast (IFN) and immune (IFN) interferon, alone and in combination, on growth, differentiation and the expression of class I and II histocompatibility locus antigens (HLA) and melanoma-associated antigens on the human melanoma cell line H0-1. The effect of combinations of interferons on the antigenic profile of human melanoma cells displaying different organ colonization and spontaneous metastatic potential in athymic nude mice was also determined. H0-1 cells were more sensitive to the antiproliferative activity of IFN than to IFN and the combination of interferons resulted in a potentiation of growth suppression. The antiproliferative effect of both interferons was greater in later-passage than in earlier-passage H0-1 cells, possibly reflecting alterations in the evolving tumor cell population as a result of long-term in vitro propagation and/or the selective outgrowth of cells with an increased growth rate. The enhanced growth suppression observed in H0-1 cells treated with the combination of IFN plus IFN was not associated with a significant increase in the level of melanin, a marker of melanoma differentiation, above that observed with either interferon used alone. IFN and IFN differentially modulated the expression of class I and II HLA and melanoma-associated antigens in H0-1 cells and a series of melanoma cells with different organ colonization and metastatic potential, including MeWo, MeM 50-10, MeM 50-17, 3S5 and 70W. No consistent potentiation or antagonism in the expression of any specific antigen was observed in any of the melanoma cell lines exposed to the combination of interferons. The present study demonstrates that the combination of IFN plus IFN can potentiate growth suppression in H0-1 human melanoma cells and that this effect is not associated with an increase in differentiation or a potentiation in antigenic modulation. In addition, no direct correlation between the expression of any specific antigen or its modulation by IFN or IFN, alone or in combination, and organ colonization and metastatic potential in nude mice was observed in the different melanoma cell lines.  相似文献   
7.
Abstract: Hippocampal slices were prepared under three conditions: (1) in medium containing glucose and oxygen at 4°C; (2) as in (1), but at 37°C; (3) in medium devoid of glucose and oxygen at 37°C. The rates of recovery to roughly steady-state levels and through 8 h of incubation were monitored for energy metabolite levels and related parameters. In vitro stable values are compared with in situ hippocampal levels. Regardless of the conditions under which slices were prepared, metabolite levels required up to 3 h to stabilize, and these levels were maintained or improved through 8 h of incubation. Further, the maximal concentrations of metabolites were independent of the conditions of slice preparation. Total adenylates and total creatine levels reached 55% of those in vivo. Lactate decreased from the decapitation-induced high levels, but stabilized at concentrations about twice those in rapidly frozen brain. Cyclic AMP and cyclic GMP exhibited peak levels at 30 min of incubation, and cyclic GMP remained elevated for 3 h. Although all three methods of slice preparation resulted in similar metabolite profiles on incubation, the initial decreases in high energy phosphates were delayed by chilling. Most striking, the slices prepared in the absence of glucose and oxygen exhibited much smaller orthodromic evoked potentials in the dentate gyrus. The presence of glucose and oxygen during preparation of the slices appears to be critical to the electrophysiological response of the tissue.  相似文献   
8.
A panel of biotinylated (neo)glycoproteins was used for specific detection of endogenous sugar receptors, especially lectins, in formaldehyde-fixed, paraffin-embedded muscle biopsy specimens from human deltoid, quadriceps, and biceps muscles, tibial and quadriceps muscles of rat, and bovine masseter muscle. The glycohistochemical probes used consisted of conjugates of a labeled, histochemically inert carrier protein and various covalently linked, histochemically crucial sugar moieties. Specific binding of alpha-L-fucoside, beta-D-galactoside, beta-D-xyloside, and alpha-D-mannoside to muscle sections was detected, showing no species-specific differences. The presence of receptors for the N-acetylated sugars in natural glycoconjugates, and for sugars with a phosphate group, i.e., mannose-6-phosphate and galactose-6-phosphate, was demonstrated glycohistochemically. However, these binding specificities revealed species-specific differences, e.g., the absence of N-acetyl-D-galactosamine-specific receptors or galactose-6-phosphate-specific receptors in rat muscle. Other charged sugars included glucuronic acid and sialic acid, which bound only to ox and rat muscle or failed to reveal their respective receptors in all types of muscle investigated. This different extent of staining with anionic probes served as a further control to ascertain carbohydrate binding specificity. Positive glycohistochemical reaction developed within sarcomeres only at the level of A-bands. Granular staining was observed in the sarcoplasm among the myofibrils and also in the subsarcolemmal regions. Differences in expression of glycohistochemically detectable sugar receptors were noted between type 1, type 2A, and type 2B fibers. The molecular properties of one type of glycohistochemically detectable sugar receptor were inferred both immunohistochemically and biochemically. An antiserum against an endogenous beta-galactoside-specific lectin from muscle tissue localized this lectin within sections consistently similar to (neo)glycoproteins, detecting beta-galactoside-specific receptor(s). This similarity of binding patterns strongly supports the assumption that (neo)glycoproteins with beta-galactoside termini indeed bind to the respective endogenous lectin. The lectin-specific antiserum enabled us to ascertain that glycohistochemical fiber typing corresponds to enzyme histochemical typing. Moreover, biochemical purification using affinity chromatography and subsequent affinity elution revealed only the immunohistochemically detectable beta-galactoside-specific lectin. Consequently, use of a panel of neoglycoproteins, when frozen sections for histochemical analysis are not available, co  相似文献   
9.
As conventional treatments are unsuccessful, the survival rate of stage D3 prostate cancer patients is poor. Reports have suggested the existence of humoral and cell-mediated immunity (CMI) against prostate cancer tumour-associated antigens (TAA). These observations prompted us to treat stage D3 prostate cancer patients with an in vitro produced transfer factor (TF) able to transfer, in vitro and in vivo, CMI against bladder and prostate TAA. Fifty patients entered this study and received one intramuscular injection of 2–5 units of specific TF monthly. Follow-up, ranging from 1 to 9 years, showed that complete remission was achieved in 2 patients, partial remission in 6, and no progression of metastatic disease in 14. The median survival was 126 weeks, higher than the survival rates reported in the literature for patients of the same stage.  相似文献   
10.
Recurrent ocular herpes is an insoluble problem for the clinician. As cellular immunity plays an important role in controlling herpes relapses, and other studies have shown the efficacy of HSV-specific transfer factor (TF) for the treatment of herpes patients, an open clinical trial was undertaken in 134 patients (71 keratitis, 29 kerato-uveitis, 34 uveitis) suffering from recurrent ocular herpetic infections. The mean duration of the treatment was 358 days, and the entire follow-up period 189121 before, and 64062 days after TF treatment. The cell-mediated immune response to the viral antigens, evaluated by the lymphocyte stimulation test (LST) and the leucocyte migration test (LMT) (P<0.001), was significantly increased by the TF treatment. The total number of relapses was decreased significantly during/after TF treatment, dropping from 832 before, to 89 after treatment, whereas the cumulative relapse index (RI) dropped, during the same period, from 13.2 to 4.17 (P<0.0001). No side effects were observed. It is concluded that patients with relapsing ocular herpes can benefit from treatment with HSV-specific TF.  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