全文获取类型
收费全文 | 2083篇 |
免费 | 249篇 |
出版年
2021年 | 20篇 |
2019年 | 20篇 |
2018年 | 33篇 |
2017年 | 19篇 |
2016年 | 31篇 |
2015年 | 69篇 |
2014年 | 70篇 |
2013年 | 91篇 |
2012年 | 97篇 |
2011年 | 126篇 |
2010年 | 64篇 |
2009年 | 48篇 |
2008年 | 87篇 |
2007年 | 77篇 |
2006年 | 78篇 |
2005年 | 60篇 |
2004年 | 73篇 |
2003年 | 71篇 |
2002年 | 67篇 |
2001年 | 56篇 |
2000年 | 63篇 |
1999年 | 54篇 |
1998年 | 23篇 |
1997年 | 21篇 |
1996年 | 35篇 |
1995年 | 19篇 |
1994年 | 23篇 |
1993年 | 20篇 |
1992年 | 46篇 |
1991年 | 44篇 |
1990年 | 43篇 |
1989年 | 34篇 |
1988年 | 46篇 |
1987年 | 49篇 |
1986年 | 28篇 |
1985年 | 35篇 |
1984年 | 25篇 |
1983年 | 28篇 |
1982年 | 21篇 |
1981年 | 18篇 |
1980年 | 19篇 |
1979年 | 31篇 |
1978年 | 25篇 |
1977年 | 21篇 |
1975年 | 17篇 |
1974年 | 28篇 |
1972年 | 19篇 |
1970年 | 16篇 |
1968年 | 16篇 |
1967年 | 17篇 |
排序方式: 共有2332条查询结果,搜索用时 31 毫秒
171.
Changes in mitochondrial electron partitioning in response to herbicides inhibiting branched-chain amino acid biosynthesis in soybean 总被引:3,自引:0,他引:3
Gaston S Ribas-Carbo M Busquets S Berry JA Zabalza A Royuela M 《Plant physiology》2003,133(3):1351-1359
The adaptation of the respiratory metabolism in roots of soybean (Glycine max L. Merr. cv Ransom) treated with herbicides that inhibit the enzyme acetolactate synthase (ALS) was analyzed. A new gas phase dual-inlet mass spectrometry system for simultaneous measurement of 34O2 to 32O2 and O2 to N2 ratios has been developed. This system is more accurate than previously described systems, allows measurements of much smaller oxygen gradients, and, as a consequence, works with tissues that have lower respiration rates. ALS inhibition caused an increase of the alternative oxidase (AOX) protein and an accumulation of pyruvate. The combination of these two effects is likely to induce the activation of the alternative pathway and its participation in the total respiration. Moreover, the start of the alternative pathway activation and the increase of AOX protein were before the decline in the activity of cytochrome pathway. The possible role of AOX under ALS inhibition is discussed. 相似文献
172.
173.
N-acetylneuraminate synthase (NeuAc-synthase; E.C. 4.1.3.19) is one of the two enzymes responsible for sialic acid (N-acetylneuraminic acid) synthesis in bacteria. Potential genes encoding NeuAc synthase in Streptococcus agalactiae and Bacillus subtilis were identified from a BLAST search of the EMBL/GenBank/DDBJ database using the E. coli neuB gene sequence as a probe and the genes cloned and expressed at high level in Escherichia coli. The neuB gene of S. agalactiae was shown to encode an active NeuAc synthase, whereas the spsE gene product from B. subtilis did not have this activity. Expression of the native S. agalactiae neuB gene product enzyme in E. coli resulted in a product that was prone to proteolysis during purification so the protein was tagged with a hexa-histidine tag at its N-terminus and the enzyme was rapidly purified to homogeneity by ammonium sulphate fractionation and Ni-chelating affinity chromatography in two steps. Measurement of the subunit molecular mass by electrospray ionisation mass spectrometry (M(r) = 38, 987 +/- 3) and of the native molecular mass by gel filtration chromatography (M(r) = 78,000) clearly demonstrated that the enzyme is dimeric. The effects of EDTA, temperature, and pH on the activity of the S. agalactiae NeuAc synthase were examined. Enzyme activity was maximal at pH 7 and was dependent on the presence of metal ions such as Mg(2+), Mn(2+) or Co(2+). The purified enzyme was inhibited by the reagent phenylglyoxal and the substrates N-acetyl mannosamine or phosphoenol pyruvate afforded protection against this inhibition, suggesting that one or more arginine residues are involved in substrate recognition and binding. The ease of expression and the properties of the enzyme should now permit a thorough study of the specificity of the enzyme and provide the prerequisites for attempts to alter this specificity by directed evolution for the production of novel sialic acid analogues. 相似文献
174.
Plant genome archaeology: evidence for conserved ancestral chromosome segments in dicotyledonous plant species 总被引:3,自引:0,他引:3
Dominguez I Graziano E Gebhardt C Barakat A Berry S Arús P Delseny M Barnes S 《Plant biotechnology journal》2003,1(2):91-99
We have developed genetic maps, based on expressed sequence tags (ESTs) that are homologous to Arabidopsis genes, in four dicotyledonous crop plant species from different families. A comparison of these maps with the physical map of Arabidopsis reveals common genome segments that appear to have been conserved throughout the evolution of the dicots. In the four crop species analysed these segments comprise between 16 and 33% of the Arabidopsis genome. Our findings extend the synteny patterns previously observed only within plant families, and indicate that structural and functional information from the model species will be, at least in part, applicable in crop plants with large genomes. 相似文献
175.
As whole genome sequences continue to expand in number and complexity, effective methods for comparing and categorizing both genes and species represented within extremely large datasets are required. Methods introduced to date have generally utilized incomplete and likely insufficient subsets of the available data. We have developed an accurate and efficient method for producing robust gene and species phylogenies using very large whole genome protein datasets. This method relies on multidimensional protein vector definitions supplied by the singular value decomposition (SVD) of a large sparse data matrix in which each protein is uniquely represented as a vector of overlapping tetrapeptide frequencies. Quantitative pairwise estimates of species similarity were obtained by summing the protein vectors to form species vectors, then determining the cosines of the angles between species vectors. Evolutionary trees produced using this method confirmed many accepted prokaryotic relationships. However, several unconventional relationships were also noted. In addition, we demonstrate that many of the SVD-derived right basis vectors represent particular conserved protein families, while many of the corresponding left basis vectors describe conserved motifs within these families as sets of correlated peptides (copeps). This analysis represents the most detailed simultaneous comparison of prokaryotic genes and species available to date. 相似文献
176.
Lakes in Taylor Valley, southern Victoria Land,Antarctica, are unusual in that they areperennially covered by a 3–5 m thick ice layer.Previous work on gas concentrations in theselakes has shown that the surface waters aresupersaturated with respect to O2,N2O, as well as the noble gases. Our datashow that the dissolved CO2 (CO2(aq))concentrations, calculated from pH andCO2, can be highly undersaturatedat shallow depths of the lakes. CO2partial pressure values (pCO2) are as lowas 10–4.3 atm and 10–4.2 atm in theeast and west lobes of Lake Bonney,respectively, and 10–3.8 atm in LakeHoare. CO2(aq) depletion occurred only inthe uppermost part of the water column, inassociation with elevated primary productivity(PPR). The upward diffusion of CO2(aq)from the aphotic zone, and the annual input ofCO2 via glacial meltwater can notreplenish the amount of CO2(aq) annuallylost to primary productivity in the uppermostmeters of the water column. Calcification is alimited source of CO2(aq), since the lakesare undersaturated with respect to calcitethrough portions of the austral summer.Preliminary respiration rates have been used toobtain an annual inorganic carbon balance.Further down in the water column, at the sitesof the deep-water maximum in primary production(PPRmax), which in Lakes Bonney andFryxell is associated with nutrient gradients,CO2(aq) is not undersaturated. A largeupward flux from CO2-supersaturatedaphotic waters provides a surplus ofCO2(aq) at the PPRmax. Lake Fryxell,unlike the other lakes, is supersaturated withCO2(aq) throughout the entire water column. 相似文献
177.
Selective inhibition of selenocysteine tRNA maturation and selenoprotein synthesis in transgenic mice expressing isopentenyladenosine-deficient selenocysteine tRNA 下载免费PDF全文
Moustafa ME Carlson BA El-Saadani MA Kryukov GV Sun QA Harney JW Hill KE Combs GF Feigenbaum L Mansur DB Burk RF Berry MJ Diamond AM Lee BJ Gladyshev VN Hatfield DL 《Molecular and cellular biology》2001,21(11):3840-3852
Selenocysteine (Sec) tRNA (tRNA([Ser]Sec)) serves as both the site of Sec biosynthesis and the adapter molecule for donation of this amino acid to protein. The consequences on selenoprotein biosynthesis of overexpressing either the wild type or a mutant tRNA([Ser]Sec) lacking the modified base, isopentenyladenosine, in its anticodon loop were examined by introducing multiple copies of the corresponding tRNA([Ser]Sec) genes into the mouse genome. Overexpression of wild-type tRNA([Ser]Sec) did not affect selenoprotein synthesis. In contrast, the levels of numerous selenoproteins decreased in mice expressing isopentenyladenosine-deficient (i(6)A(-)) tRNA([Ser]Sec) in a protein- and tissue-specific manner. Cytosolic glutathione peroxidase and mitochondrial thioredoxin reductase 3 were the most and least affected selenoproteins, while selenoprotein expression was most and least affected in the liver and testes, respectively. The defect in selenoprotein expression occurred at translation, since selenoprotein mRNA levels were largely unaffected. Analysis of the tRNA([Ser]Sec) population showed that expression of i(6)A(-) tRNA([Ser]Sec) altered the distribution of the two major isoforms, whereby the maturation of tRNA([Ser]Sec) by methylation of the nucleoside in the wobble position was repressed. The data suggest that the levels of i(6)A(-) tRNA([Ser]Sec) and wild-type tRNA([Ser]Sec) are regulated independently and that the amount of wild-type tRNA([Ser]Sec) is determined, at least in part, by a feedback mechanism governed by the level of the tRNA([Ser]Sec) population. This study marks the first example of transgenic mice engineered to contain functional tRNA transgenes and suggests that i(6)A(-) tRNA([Ser]Sec) transgenic mice will be useful in assessing the biological roles of selenoproteins. 相似文献
178.
Kalderon B Mayorek N Berry E Zevit N Bar-Tana J 《American journal of physiology. Endocrinology and metabolism》2000,279(1):E221-E227
Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands. 相似文献
179.
Phosphorylation status of the SCR homeodomain determines its functional activity: essential role for protein phosphatase 2A,B' 下载免费PDF全文
Sex combs reduced (SCR) is a DROSOPHILA: Hox protein that determines the identity of the labial and prothoracic segments. In search of factors that might associate with SCR to control its activity and/or specificity, we performed a yeast two-hybrid screen. A DROSOPHILA: homologue of the regulatory subunit (B'/PR61) of serine-threonine protein phosphatase 2A (dPP2A,B') specifically interacted with the SCR homeodomain. The N-terminal arm within the SCR homeodomain was shown to be a target of phosphorylation/dephosphorylation by cAMP-dependent protein kinase A and protein phosphatase 2A, respectively. In vivo analyses revealed that mutant forms of SCR mimicking constitutively dephosphorylated or phosphorylated states of the homeodomain were active or inactive, respectively. Inactivity of the phosphorylated mimic form was attributed to impaired DNA binding. Specific ablation of dPP2A,B' gene activity by double-stranded RNA-mediated genetic interference resulted in embryos without salivary glands, an SCR null phenotype. Our data demonstrate an essential role for DROSOPHILA: PP2A,B' in positively modulating SCR function. 相似文献
180.