全文获取类型
收费全文 | 2572篇 |
免费 | 266篇 |
国内免费 | 2篇 |
专业分类
2840篇 |
出版年
2021年 | 37篇 |
2019年 | 29篇 |
2018年 | 27篇 |
2017年 | 20篇 |
2016年 | 46篇 |
2015年 | 71篇 |
2014年 | 69篇 |
2013年 | 104篇 |
2012年 | 99篇 |
2011年 | 116篇 |
2010年 | 89篇 |
2009年 | 72篇 |
2008年 | 101篇 |
2007年 | 109篇 |
2006年 | 106篇 |
2005年 | 106篇 |
2004年 | 93篇 |
2003年 | 119篇 |
2002年 | 100篇 |
2001年 | 99篇 |
2000年 | 85篇 |
1999年 | 82篇 |
1998年 | 38篇 |
1997年 | 24篇 |
1996年 | 40篇 |
1995年 | 30篇 |
1994年 | 21篇 |
1993年 | 32篇 |
1992年 | 46篇 |
1991年 | 56篇 |
1990年 | 40篇 |
1989年 | 27篇 |
1988年 | 53篇 |
1987年 | 39篇 |
1986年 | 40篇 |
1985年 | 35篇 |
1984年 | 27篇 |
1983年 | 32篇 |
1982年 | 25篇 |
1981年 | 29篇 |
1979年 | 19篇 |
1978年 | 23篇 |
1977年 | 25篇 |
1976年 | 25篇 |
1975年 | 25篇 |
1974年 | 37篇 |
1973年 | 19篇 |
1972年 | 27篇 |
1971年 | 21篇 |
1969年 | 21篇 |
排序方式: 共有2840条查询结果,搜索用时 15 毫秒
51.
52.
Glutamate Oxidation by Soybean Cotyledon and Leaf Mitochondria 总被引:1,自引:0,他引:1
Day David A.; Salom Caroline L.; Azcon-Bieto Joaquim; Dry Ian B.; Wiskich Joseph T. 《Plant & cell physiology》1988,29(7):1193-1200
Mitochondria purified from cotyledons of soybean seedlings fiveto ten days old have the capacity to rapidly oxidize glutamate(measured as glutamate dependent oxygen consumption). This capacitywas greatest at ten days after planting but was very low priorto emergence of cotyledons from the vermiculite and during senescence.Solubilized glutamate dehydrogenase activity, on the other hand,was substantial at two days after planting, peaked at sevendays, then declined and rose again during senescence. It issuggested that mitochondrial glutamate oxidation plays a rolein reserve mobilization and amino acid metabolism during seedlinggrowth. Leaf mitochondria and those from senescing cotyledonscould not sustain rapid rates of glutamate oxidation despiteready oxidation of other substrates and high solubilized glutamatedehydrogenase activity, suggesting an alternative role for theenzyme in these tissues. Possible controlling factors are discussed.
2 Present address, Garvan Institute, Darlinghurst, N. S. W.,Australia.
3 Permanent address, Department de Biologia Vegetal, Facultatde Biologia, Universitat de Barcelona, Barcelona, Spain. (Received May 6, 1988; Accepted August 3, 1988) 相似文献
53.
Polymerization of nonfilamentous actin into microfilaments is an important process for porcine oocyte maturation and early embryo development 总被引:4,自引:0,他引:4
Actin is one of the major proteins in mammalian oocytes. Most developmental events are dependent on the normal distribution of filamentous (F-) actin. Polymerization of nonfilamentous (G-) actin into F-actin is important for both meiosis and mitosis. This study examined G- and F-actin distribution in pig oocytes and embryos by immunocytochemical staining and confocal microscopy. Actin protein was quantified by electrophoresis and immunoblotting. G-Actin was distributed in the whole cytoplasm of oocytes and embryos irrespective of their stages. F-Actin was distributed at the cortex of oocytes and embryos at all stages, at the joint of blastomeres in the embryos, in the cytoplasm around the germinal vesicle (GV), and in the perinuclear area of 2- to 4-cell-stage embryos. No differences in the amount of actin protein were found among oocytes and embryos. Oocytes cultured in medium with cytochalasin D (CD), an inhibitor of microfilament polymerization, underwent GV breakdown and reached metaphase I but did not proceed to metaphase II. Two- to 4-cell-stage embryos cultured in medium with CD did not develop to blastocysts. When GV-stage oocytes or 2- to 4-cell-stage embryos treated with CD for 6 h were re-cultured in media without CD, oocytes or embryos re-assembled actin filaments and underwent a meiotic maturation or blastocyst formation similar to that of controls. These results indicate that it is the polymerization of G-actin into F-actin, not actin protein synthesis, that is important for both meiosis and mitosis in pig oocytes and embryos. 相似文献
54.
A review of the literature indicates that intraspecific variation among the lens proteins (crystallins) is almost invariably quantitative rather than qualitative. Lens extracts were examined from inbred strains of the laboratory mouse, rat, and guinea pig and from domestic breeds of sheep. Differences in the mobilities of protein bands after electrophoresis in polyacrylamide gels were never found to be greater than the variation observed when a sample was repeatedly subjected to electrophoresis. Differences, however, were observed between widely separated populations of the bank vole (Clethrionomys glareolus). In bank voles and mice, there were variations in the intensities of bands. Electrophoresis of extracts in the presence of 6 m urea indicated that there were no major differences between six mouse strains in the subunits constituting their - and -crystallins. More detailed examination of -crystallins from two mouse strains by isoelectric focusing and amino acid analysis supported this conclusion.This work was carried out while one of the authors (T.H.D.) was supported by a Medical Research Council Research Studentship and a European Molecular Biology Organisation Fellowship. A grant from the Edinburgh University Weir Memorial Fund allowed the collection of some of the Yugoslavian bank voles. 相似文献
55.
56.
57.
Induction of alternative oxidase synthesis by herbicides inhibiting branched-chain amino acid synthesis 总被引:4,自引:0,他引:4
Serge Aubert Richard Bligny David A. Day James Whelan Roland Douce 《The Plant journal : for cell and molecular biology》1997,11(4):649-657
Sycamore suspension cells ( Acer pseudoplatanus L.) were incubated in the presence of sulfonylurea and imidazolinone herbicides. These inhibitors of acetolactate synthase (ALS), a key enzyme of branched-chain amino acid synthesis, triggered a dramatic induction of the alternative oxidase (AOX). AOX activity increased in treated cells, eventually exceeding cytochrome (cyt) pathway activity. This induction of AOX activity was correlated with the accumulation of a 35 kDa AOX protein in isolated mitochondria, detected by Western blotting with a monoclonal antibody against Sauromatum guttatum AOX. It was preceded by the accumulation of putative 1.6 kb AOX mRNA, detected using an Aox cDNA probe from soybean. The metabolic perturbations induced by the herbicides rather than the herbicide molecules themselves were responsible for this induction of AOX. However, α-oxobutyrate (one of the substrates of ALS) and its transamination product, α-aminobutyrate, which accumulated after herbicide treatment, were not involved. The inhibition of branched-chain amino acid synthesis was probably somehow responsible for the AOX induction since: (i) a mixture of those amino acids (leucine, isoleucine, valine) prevented AOX induction by ALS inhibitors; (ii) the herbicide Hoe 704, a potent inhibitor of acetolactate reducto-isomerase (the enzyme following ALS in the branched-chain amino acid pathway), also triggered AOX induction. 相似文献
58.
Probing the role of glutamic acid 144 in the EcoRI endonuclease using aspartic acid and glutamine replacements 总被引:1,自引:0,他引:1
P W Hager N O Reich J P Day T G Coche H W Boyer J M Rosenberg P J Greene 《The Journal of biological chemistry》1990,265(35):21520-21526
The x-ray structure of the EcoRI endonuclease-DNA complex (3) suggests that hydrogen bonds between amino acids, glutamic acid 144, arginine 145, and arginine 200, and major groove base moieties are the molecular determinants of specificity. We have investigated residue 144 using aspartate and glutamine substitutions introduced by site-directed mutagenesis. Substitution with glutamine results in a null phenotype (at least a 2000-fold reduction in activity). On the other hand, the aspartic acid mutant (ED144) retained in vivo activity. Substrate binding and catalytic studies were done with purified ED144 enzyme. The affinity of the ED144 enzyme for the canonical sequence 5'-GAATTC-3' is about 340-fold less than the wild-type (WT) enzyme, while its affinity for nonspecific DNA is about 50 times greater. The ED144 enzyme cleaves one strand in the EcoRI site in plasmid pBR322 with a kcat/Km similar to WT. In contrast to the WT enzyme, the ED144 enzyme dissociates after the first strand cleavage. Partitioning between cleavage and dissociation at the first and second cleavage steps for the ED144 enzyme is extremely salt-sensitive. The altered partitioning results largely from a destabilization of the enzyme-DNA complex, particularly the enzyme-nicked DNA complex, with only small changes in the respective cleavage rates. The hydrogen bonds of Glu-144 are critical, they appear to act cooperatively with other specificity contacts to stabilize the enzyme-DNA complex. 相似文献
59.
A study was conducted to identify the embryonic stage when the zygotic genome begins to direct development and to characterize protein synthesis in pig oocytes and embryos. Reproductive tracts of gilts were flushed to obtain unfertilized oocytes (UFO), zygotes (Z), 2-, 4-, and 8-cell embryos, compact morulae (M), initial blastocysts (IB), blastocysts (B), and hatched blastocysts (HB). Pig eggs and embryos were cultured in medium containing 1 microM L-[35S]methionine and evaluated for amino acid uptake, incorporation of the radiolabel into protein, and qualitative changes in protein profiles specific to each cleavage stage. Unfertilized oocytes sequestered 65.7 fmol methionine/4 h/embryo. Uptake of methionine decreased (p less than 0.05) from the Z (49.4), 2-cell (41.8), and 4-cell (37.6) embryonic stages to the M (8.97 fmol/4 h/embryo) stage. This downward trend was reversed at the IB, B, and HB stages when uptake increased to 37.3, 50.3, and 84.2 fmol/4 h/embryo, respectively. Incorporation of methionine into protein followed a similar pattern, being relatively higher in the UFO (21.0), Z (20.5), and 2-cell stages (16.0); decreased (p less than 0.05) at the 4-cell (6.67), 8-cell (6.84), and M (6.16) stages; and increased (p less than 0.05) at the IB (28.0), B (41.5), and HB (69.6 fmol/4 h/embryo) stages. Differences in protein profiles were observed for UFO, Z, 4-cell, and M stages using lysates of single embryos, one-dimensional SDS-PAGE, and fluorography.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
60.
Hamdan FF Gauthier J Araki Y Lin DT Yoshizawa Y Higashi K Park AR Spiegelman D Dobrzeniecka S Piton A Tomitori H Daoud H Massicotte C Henrion E Diallo O;SD Group Shekarabi M Marineau C Shevell M Maranda B Mitchell G Nadeau A D'Anjou G Vanasse M Srour M Lafrenière RG Drapeau P Lacaille JC Kim E Lee JR Igarashi K Huganir RL Rouleau GA Michaud JL 《American journal of human genetics》2011,(3):1427-316
Little is known about the genetics of nonsyndromic intellectual disability (NSID). We hypothesized that de novo mutations (DNMs) in synaptic genes explain an important fraction of sporadic NSID cases. In order to investigate this possibility, we sequenced 197 genes encoding glutamate receptors and a large subset of their known interacting proteins in 95 sporadic cases of NSID. We found 11 DNMs, including ten potentially deleterious mutations (three nonsense, two splicing, one frameshift, four missense) and one neutral mutation (silent) in eight different genes. Calculation of point-substitution DNM rates per functional and neutral site showed significant excess of functional DNMs compared to neutral ones. De novo truncating and/or splicing mutations in SYNGAP1, STXBP1, and SHANK3 were found in six patients and are likely to be pathogenic. De novo missense mutations were found in KIF1A, GRIN1, CACNG2, and EPB41L1. Functional studies showed that all these missense mutations affect protein function in cell culture systems, suggesting that they may be pathogenic. Sequencing these four genes in 50 additional sporadic cases of NSID identified a second DNM in GRIN1 (c.1679_1681dup/p.Ser560dup). This mutation also affects protein function, consistent with structural predictions. None of these mutations or any other DNMs were identified in these genes in 285 healthy controls. This study highlights the importance of the glutamate receptor complexes in NSID and further supports the role of DNMs in this disorder. 相似文献