全文获取类型
收费全文 | 3714篇 |
免费 | 339篇 |
国内免费 | 4篇 |
专业分类
4057篇 |
出版年
2023年 | 25篇 |
2022年 | 51篇 |
2021年 | 75篇 |
2020年 | 54篇 |
2019年 | 56篇 |
2018年 | 81篇 |
2017年 | 85篇 |
2016年 | 129篇 |
2015年 | 184篇 |
2014年 | 218篇 |
2013年 | 218篇 |
2012年 | 273篇 |
2011年 | 273篇 |
2010年 | 175篇 |
2009年 | 135篇 |
2008年 | 186篇 |
2007年 | 180篇 |
2006年 | 182篇 |
2005年 | 171篇 |
2004年 | 157篇 |
2003年 | 143篇 |
2002年 | 103篇 |
2001年 | 61篇 |
2000年 | 38篇 |
1999年 | 46篇 |
1998年 | 39篇 |
1997年 | 29篇 |
1996年 | 26篇 |
1995年 | 23篇 |
1994年 | 32篇 |
1993年 | 14篇 |
1992年 | 49篇 |
1991年 | 25篇 |
1990年 | 39篇 |
1989年 | 35篇 |
1988年 | 25篇 |
1987年 | 37篇 |
1986年 | 25篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 16篇 |
1982年 | 27篇 |
1981年 | 15篇 |
1979年 | 24篇 |
1978年 | 23篇 |
1977年 | 18篇 |
1975年 | 15篇 |
1974年 | 20篇 |
1973年 | 19篇 |
1969年 | 14篇 |
排序方式: 共有4057条查询结果,搜索用时 10 毫秒
11.
No catalase activity was detected in four strains of glucose-grown Mycoplasma pneumoniae at any time during the replication of the organism. Exogenous catalase dramatically increased the O(2) uptake with glycerol, presumably by releasing inhibition caused by hydrogen peroxide. The effect of added catalase on the O(2) uptake of washed organisms with glucose as substrate was moderate and variable in degree. The production of hydrogen peroxide was demonstrated by the quantitative enzymatic assay for inorganic peroxide and by the fact that added pyruvate, which is non-enzymatically oxidized by H(2)O(2) to acetic acid and CO(2) could mimic the action of catalase. 相似文献
12.
Analysis of nonmethylated GATC sites in the Escherichia coli chromosome and identification of sites that are differentially methylated in response to environmental stimuli. 总被引:3,自引:1,他引:2 下载免费PDF全文
Seven GATC sites that are nonmethylated in logarithmic growth phase cells using glycerol as a carbon source were isolated from the Escherichia coli chromosome. Three of these GATC sites are located upstream of the operons gut, mtl, and ppiA, whereas DNA sequences adjacent to three other nonmethylated GATC sites are not homologous to previously identified genes. The seventh nonmethylated GATC site is located downstream of uspA. The protection of this site from DNA methylation requires leucine-responsive regulatory protein and is leucine responsive. The carbon source and the growth phase influenced the protection of the GATC site 5' of the ppiA gene. The other five sites were protected under all the environmental conditions examined. 相似文献
13.
James Kang William Low Thomas Norberg Jill Meisenhelder Karin Hansson Johan Stenflo Guo-Ping Zhou Julita Imperial Baldomero M Olivera Alan C Rigby A Grey Craig 《European journal of biochemistry》2004,271(23-24):4939-4949
The 13-amino acid glycopeptide tx5a (Gla-Cys-Cys-Gla-Asp-Gly-Trp*-Cys-Cys-Thr*-Ala-Ala-Hyp-OH, where Trp* = 6-bromotryptophan and Thr* = Gal-GalNAc-threonine), isolated from Conus textile, causes hyperactivity and spasticity when injected intracerebral ventricularly into mice. It contains nine post-translationally modified residues: four cysteine residues, two gamma-carboxyglutamic acid residues, and one residue each of 6-bromotryptophan, 4-trans-hydroxyproline and glycosylated threonine. The chemical nature of each of these has been determined with the exception of the glycan linkage pattern on threonine and the stereochemistry of the 6-bromotryptophan residue. Previous investigations have demonstrated that tx5a contains a disaccharide composed of N-acetylgalactosamine (GalNAc) and galactose (Gal), but the interresidue linkage was not characterized. We hypothesized that tx5a contained the T-antigen, beta-D-Gal-(1-->3)-alpha-D-GalNAc, one of the most common O-linked glycan structures, identified previously in another Conus glycopeptide, contalukin-G. We therefore utilized the peracetylated form of this glycan attached to Fmoc-threonine in an attempted synthesis. While the result-ing synthetic peptide (Gla-Cys-Cys-Gla-Asp-Gly-Trp*-Cys-Cys-Thr*-Ala-Ala-Hyp-OH, where Trp* =6-bromotryptophan and Thr* = beta-D-Gal-(1-->3)-alpha-D-GalNAc-threonine) and the native peptide had almost identical mass spectra, a comparison of their RP-HPLC chromatograms suggested that the two forms were not identical. Two-dimensional 1H homonuclear and 13C-1H heteronuclear NMR spectroscopy of native tx5a isolated from Conus textile was then used to determine that the glycan present on tx5a indeed is not the aforementioned T-antigen, but rather alpha-D-Gal-(1-->3)-alpha-D-GalNAc. 相似文献
14.
Observation of the sac-spawning euphausiid Nematoscelis difficilisHansen during shipboard laboratory incubations showed that itsembryos usually hatch as pseudometanauplius (PMN) or metanauplius(MN). The eggs, which have a minute perivitelline space, arespherical at spawning and become elliptical after the nauplius1 develops. When ready to hatch, the PMN or MN embryos extendand contract their first and second antennae in a swimming movement,breaking the chorion into almost equal halves joined by onesmall section in the anterior part of the chorion. The mandiblesplay a secondary role in cutting the chorion. Then the embryopushes itself backwards with the first and second antennae toescape from the chorion. This is known as push-offhatching. The embryos always hatch progressively from the distalend towards the proximal end of the ovigerous sac. The timebetween hatching of the first and last embryo may reflect thetime the females require to lay a clutch of eggs (<2.1 h).Development time to the PMN stage at 10°C was 相似文献
15.
Despite the likely requirement for a DNA topoisomerase II activity during synthesis of mitochondrial DNA in mammals, this activity has been very difficult to identify convincingly. The only DNA topoisomerase II activity conclusively demonstrated to be mitochondrial in origin is that of a type II activity found associated with the mitochondrial, kinetoplast DNA network in trypanosomatid protozoa [Melendy, T., Sheline, C., and Ray, D.S. (1988) Cell 55, 1083-1088; Shapiro, T.A., Klein, V.A., and Englund, P.A. (1989) J. Biol. Chem.264, 4173-4178]. In the present study, we report the discovery of a type DNA topoisomerase II activity in bovine mitochondria. Identified among mtDNA replicative proteins recovered from complexes of mtDNA and protein, the DNA topoisomerase relaxes a negatively, supercoiled DNA template in vitro, in a reaction that requires Mg2+ and ATP. The relaxation activity is inhibited by etoposide and other inhibitors of eucaryotic type II enzymes. The DNA topoisomerase II copurifies with mitochondria and directly associates with mtDNA, as indicated by sensitivity of some mtDNA circles in the isolated complex of mtDNA and protein to cleavage by etoposide. The purified activity can be assigned to a approximately 150-kDa protein, which is recognized by a polyclonal antibody made against the trypanosomal mitochondrial topo II enzyme. Mass spectrometry performed on peptides prepared from the approximately 150-kDa protein demonstrate that this bovine mitochondrial activity is a truncated version of DNA topoisomerase IIbeta, one of two DNA topoisomerase II activities known to exist in mammalian nuclei. 相似文献
16.
The effect of the plant growth retardants ancymidol. mefluidide and uniconazole on germination of two melon accessions differing in their ability to germinate at 14°C was examined. The accessions were the cold sensitive Noy Yizre'el and the cold tolerant Persia 202. The three growth retardants were able to delay the germination of intact Noy Yizre'el seeds, but did not affect that of intact Persia 202 seeds. On the other hand germination of decoated seeds of both accessions was unaffected by these inhibitors at normal oxygen concentration, but was inhibited at 5% oxygen. When gibberellin-like activity was measured by a dwarf rice biological assay following HPLC fractionation, it was found that seeds of Persia 202 contained much more gibberellin-like activity than Noy Yizre'el seeds. Among the extracted compounds several endogenous gibberellins were identified by combined gas chromatography-mass spectrometry (GC-MS). They included GA4 , GA20 , GA1 and GA3 in Noy Yizre'el and GA34 , GA20 , GA1 and GA8 in Persia 202. It is suggested that the better germination of intact Persia 202 seeds, compared to Noy Yizre'el seeds at low temperature and low oxygen concentration, is due to a higher endogenous level of GA and a better seed coat permeability to oxygen. 相似文献
17.
Most metazoan cells are 'polarized'. A crucial aspect of this polarization is that the plasma membrane is divided into two or more domains with different protein and lipid compositions or example, the apical and basolateral domains of epithelial cells or the axonal and somatodendritic domains of neurons. This polarity is established and maintained by highly specific vesicular membrane transport in the biosynthetic, endocytic and transcytotic pathways. Two important concepts, the 'SNARE' and the 'raft' hypotheses, have been developed that together promise at least a partial understanding of the underlying general mechanisms that ensure the necessary specificity of these pathways. 相似文献
18.
Dirofilariasis is now recognized as a zoonosis. Infection of humans occurs when a mosquito that has obtained larvae-containing blood from an infected animal transmits the larvae, after they have developed to the infective stage, to a human. Dirofilarial infections in humans have been reported from around the world. In this paper a case is reported in which subcutaneous dirofilariasis in a human presented as a lump in the breast. The epidemiologic and pathogenetic features of this disease are discussed. 相似文献
19.
This report identifies a component of normal human fibroblasts that forms a covalent linkage with thrombin and urokinase (urinary plasminogen activator) and mediates most of the specific cellular binding of these proteases. This component, here named protease-nexin (PN), is both associated with the cell surface and released into the culture medium. In several ways PN resembles antithrombin III (AT3), a prominent inhibitor of thrombin in serum: PN links thrombin, probably via an ester bond; PN does not link thrombin blocked at its catalytic site serine; PN has a high-affinity heparin-binding site; and heparin greatly accelerates the rate of linkage between soluble PN and thrombin. Despite these similarities, PN and AT3 are distinct; they differ in size and are not immunologically cross-reactive. Whereas AT3 regulates the proteolytic activity of thrombin in serum, PN may regulate the activity of serine proteases at and near the cell surface. 相似文献
20.
Jaime Milstein 《Mathematical biosciences》1978,40(3-4):175-184
The iterative scheme described by Milstein [6] is used to estimate the rate constants of a model describing an inducible enzyme system. The model was formulated by Heinmets [5], and was mathematically analyzed by Roth and Roth [9] using Bellman's quasilinearization technique. The method used in this article makes use of multiple trajectories of the system to obtain a set of parameters which fit the data of all trajectories simultaneously. A comparison of the numerical results is presented. The data used in the calculations were generated in a similar way to that of Roth and Roth. However, we introduce into each datum 5% noise from a random number generator uniformly distributed [0,1]. 相似文献