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   1473篇
  免费   84篇
  国内免费   2篇
  2022年   5篇
  2021年   13篇
  2020年   8篇
  2019年   8篇
  2018年   23篇
  2017年   16篇
  2016年   29篇
  2015年   65篇
  2014年   63篇
  2013年   120篇
  2012年   88篇
  2011年   80篇
  2010年   49篇
  2009年   40篇
  2008年   79篇
  2007年   99篇
  2006年   83篇
  2005年   73篇
  2004年   86篇
  2003年   105篇
  2002年   82篇
  2001年   19篇
  2000年   34篇
  1999年   22篇
  1998年   24篇
  1997年   24篇
  1996年   14篇
  1995年   15篇
  1994年   21篇
  1993年   18篇
  1992年   16篇
  1991年   18篇
  1990年   14篇
  1989年   10篇
  1988年   8篇
  1987年   10篇
  1986年   10篇
  1985年   5篇
  1984年   10篇
  1983年   3篇
  1982年   9篇
  1981年   7篇
  1979年   3篇
  1978年   2篇
  1976年   5篇
  1975年   7篇
  1974年   6篇
  1972年   2篇
  1968年   3篇
  1967年   4篇
排序方式: 共有1559条查询结果,搜索用时 31 毫秒
1.
2.
The catalase molecule in germinating pumpkin cotyledons is synthesizedas a precursor (59-kDa) form, whose relative molecular massis larger than the mature enzyme (55-kDa). Although both typesof molecules are localized in the microbodies, the 59-kDa specieshas been shown to be present predominantly in the leaf peroxisomesisolated from green cotyledons, while the 55-kDa species ispredominantly in the glyoxysomes from etiolated cotyledons [Yamaguchiet al. (1984) Proc. Natl. Acad. Sci. USA, 81: 4809]. We examinedthe distribution of the 59- and 55-kDa catalase molecules indark- and light-grown tissues of pumpkin seedlings as well asin other plant species, using the immunoblotting technique.The ratios of the 59- and 55-kDa catalase species differed inthe pumpkin tissues examined. Light interferes with the conversionof the 59-kDa precursor to the 55-kDa form, especially in thecotyledons. The effect of light was less pronounced in the rootsand hypocotyls, indicating that the light regulation of theconversion is tissue-specific. Dark- and light-grown cotyledonsfrom cucumber and watermelon seedlings showed a similar lightregulation, suggesting that cucurbitaceous plants possess similarlight-regulatory mechanism. From the analysis of catalase proteinfrom various plant tissues, a limited correlation between molecularforms of catalase and different microbody populations was observed. (Received September 6, 1986; Accepted December 4, 1986)  相似文献   
3.
The purpose of this study was to examine the possibility of separation and quantification of serum alkaline phosphatase (ALP) isozymes in rats by wheatgerm lectin affinity electrophoresis. Cellulose acetate electrophoresis of the liver and bone ALPs without lectin results in overlapping bands, but in the presence of lectin, the mobility of the band of bone enzyme was retarded and well separated from the liver enzyme band. With this affinity electrophoretic method, we determined the serum ALP isozymes in fed and fasting rats grouped by age. As a result, the absolute activity of bone isozyme showed a downward trend with age in the fed and fasting rats. The serum ALP activity was steadily higher in fed rats than in fasting rats, and the increase was due to intestinal ALP isozyme. There was low activity bordering complete absence in liver isozyme under both nutritional conditions. The affinity electrophoretic method provided a rapid, reproducible, and relatively simple technique for further clinical characterization of ALP isozyme in the rat serum.  相似文献   
4.
Root elongation of Alaska pea seedling was suppressed by higherconcentrations of growth retardants, CCC and ancymidol, thanthose required for shoot elongation. Gibberellic acid (GA3)led to recovery of ancymidol-inhibited elongation, with theconcentration (1 nM) required for roots being lower than thatfor shoots (10 µM). Ancymidol caused swelling of corticalcells in the elongating zone of the root, while GA3 completelycanceled this. These results suggest that roots require muchless gibberellin than shoots for normal elongation growth. Growth kinetics recorded by a computer-regulated rhizometerindicated that the lag periods for growth suppression by ancymidoland growth recovery by GA3 were about 10 h and 7 h, respectively. The composition of the cell wall sugars changed remarkably alongthe root axis from the tip to the base. The arabinose contentwas highest in the tip and rapidly decreased toward the base,whereas galactose complementarily increased toward the base.The thickened zone of ancymidol-treated roots had a higher galactosecontent than GA3-treated slender roots. Other neutral sugarswere not significantly influenced by ancymidol and/or GA3. Theseresults suggest that ancymidol makes cells short and thick withgalactose-rich cell walls while GA3 keeps cells extensible andslender with galactose-poor cell walls. (Received March 3, 1987; Accepted December 4, 1987)  相似文献   
5.
From dilution series in defined mineral medium, a marine iregular coccoid methanogenic bacterium (strain MTP4) was isolated that was able to grow on methanethiol as sole source of energy. The strain also grew on dimethylsulfide, mono-, di-, and trimethylamine, methanol and acetate. On formate the organism produced methane without significant growth. Optimal growth on MT, with doubling times of about 20 h, occurred at 30°C in marine medium. The isolate required p-aminobenzoate and a further not identified vitamin. Strain MTP4 had a high tolerance to hydrogen sulfide but was very sensitive to mechanical forces or addition of detergents such as Triton X-100 or sodium dodecylsulfate. Methanethiol was fermented by strain MTP4 according to the following equation:
  相似文献   
6.
The spontaneous release of [3H] gamma-aminobutyric acid ([3H]GABA) in various areas of rat brain injected with [3H]putrescine was examined using a push-pull perfusion technique. The release in a 25-min perfusate was highest in the caudate-putamen. The effect of high K+ stimulation on the release of [3H]GABA formed from [3H]putrescine was examined in the caudate-putamen. The release was enhanced by high K+ solution in a Ca2+-dependent manner.  相似文献   
7.
1. CD spectra showed that the fowl protamine, galline, has an unordered structure rich in reverse turns in neutral solution. Eight reverse turns were predicted to be present in the galline molecule on the basis of its amino acid sequence. Spectrophotometric analyses revealed that galline efficiently bound to DNA in 0.25 mM EDTA/10 mM Tricine-HCl, pH 7.4, but hardly so in 30 mM NaCl/3 mM sodium citrate, pH 7.0. Citrate ions bound specifically to the galline molecule, causing a conformational change in it. As a result, galline could not interact with DNA. 2. The concentration of unbound galline in a mixture of DNA and galline in 100 mM NaCl/50 mM Tricine-HCl, pH 7.4, at 37 C was determined by measurement of the intrinsic fluorescence of tyrosine residues of galline in the supernatant after ultracentrifugation of the mixture. The Scatchard plot showed positive co-operativity in the binding of galline to DNA and the binding parameters were determined: the co-operative binding constant (Kc) = 3.3 X 10(7)M-1, the co-operativity factor (q) = 800, and the number of nucleotides of DNA occupied by one galline molecule (n) = 28. The Kc and q values were intermediate between those for clupeine Z from herring sperm and S-methyl protamine from boar sperm. That is, the binding constants of protamine as to DNA decrease in the order of herring, fowl, and boar, while the co-operativities in binding increase in that order.  相似文献   
8.
Many eukaryotic proteins are bound to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. Its core backbone, which is conserved in different organisms, is synthesized in the endoplasmic reticulum by the sequential addition of glycan components to phosphatidylinositol. One of the human GPI synthesis genes,PIGF(phosphatidylinositol glycan complementation class F), which is involved late in the synthesis pathway, has been cloned. In this study, we isolated complementary and genomic clones ofPigf,a murine counterpart ofPIGF. Pigfencodes a 219 amino acid protein that complements a class F mutation. ThePigfgene consists of six exons spanning 30 kb and was mapped to chromosome 17 at 17E4–E5. These features are very similar toPIGF,thus demonstrating the interspecies conservation of structure, function, gene organization, and genetic locus between these GPI synthesis genes. The results also extend a region in murine distal chromosome 17 that is syntenic to human chromosome 2p16–p22.  相似文献   
9.
The complete oxidation of methylmercaptan (MSH) and dimethyl sulfide (DMS) with sulfate or nitrate as electron acceptors was observed in enrichment cultures and dilution series using thermophilic fermentor sludge as the inoculum. Three new strains of thermophilic sulfate reducers were isolated in pure culture (strains MTS5, TDS2, and SDN4). Strain MTS5 grew on MSH and strain TDS2 grew on DMS whereas strain SDN4 grew on either MSH or DMS. The cellular growth yields were 2.57 g (dry weight)/mol of MSH for strain MTS5 and 6.02 g (dry weight)/mol of DMS for strain TDS2. All strains used sulfate, sulfite, or thiosulfate as electron acceptors, but only strain SDN4 used nitrate. DMS and MSH were oxidized to CO2 and sulfide with either sulfate or nitrate as the electron acceptor. Sulfate was stoichiometrically reduced to sulfide while nitrate was reduced to ammonium. All strains were motile rods, required biotin for growth, lacked desulfoviridin, had DNA with G+C contents of 48 to 57 mol% and probably belonged to the genus Desulfotomaculum. This is the first report of the oxidation of MSH and DMS by pure cultures of sulfate-reducing bacteria.  相似文献   
10.
Cerulenin, an antifungal antibiotic produced by Cephalosporium caerulens, is a potent inhibitor of fatty acid synthase in various organisms, including Saccharomyces cerevisiae. The antibiotic inhibits the enzyme by binding covalently to the active center cysteine of the condensing enzyme domain. We isolated 12 cerulenin-resistant mutants of S. cerevisiae following treatment with ethyl methanesulfonate. The mechanism of cerulenin resistance in one of the mutants, KNCR-1, was studied. Growth of the mutant was over 20 times more resistant to cerulenin than that of the wild-type strain. Tetrad analysis suggested that all mutants mapped at the same locus, FAS2, the gene encoding the subunit of the fatty acid synthase. The isolated fatty acid synthase, purified from the mutant KNCR-1, was highly resistant to cerulenin. The cerulenin concentration causing 50% inhibition (IC50) of the enzyme activity was measured to be 400 M, whereas the IC50 value was 15 M for the enzyme isolated from the wild-type strain, indicating a 30-fold increase in resistance to cerulenin. The FAS2 gene was cloned from the mutant. Sequence replacement experiments suggested that an 0.8 kb EcoRV-HindIII fragment closely correlated with cerulenin resistance. Sequence analysis of this region revealed that the GGT codon encoding Gly-1257 of the FAS2 gene was altered to AGT in the mutant, resulting in the codon for Ser. Furthermore, a recombinant FAS2 gene, in which the 0.8 Kb EcoRV-HindIII fragment of the wild-type FAS2 gene was replaced with the same region from the mutant, when introduced into FAS2-defective S. cerevisiae complemented the FAS2 pheno-type and showed cerulenin resistance. These data indicate that one amino acid substitution (Gly Ser) in the subunit of fatty acid synthase is responsible for the cerulenin resistance of the mutant KNCR-1.  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