全文获取类型
收费全文 | 3474篇 |
免费 | 244篇 |
国内免费 | 7篇 |
专业分类
3725篇 |
出版年
2024年 | 6篇 |
2023年 | 33篇 |
2022年 | 87篇 |
2021年 | 146篇 |
2020年 | 95篇 |
2019年 | 129篇 |
2018年 | 124篇 |
2017年 | 103篇 |
2016年 | 142篇 |
2015年 | 206篇 |
2014年 | 203篇 |
2013年 | 264篇 |
2012年 | 301篇 |
2011年 | 276篇 |
2010年 | 155篇 |
2009年 | 130篇 |
2008年 | 159篇 |
2007年 | 156篇 |
2006年 | 168篇 |
2005年 | 124篇 |
2004年 | 117篇 |
2003年 | 85篇 |
2002年 | 73篇 |
2001年 | 31篇 |
2000年 | 28篇 |
1999年 | 33篇 |
1998年 | 23篇 |
1997年 | 20篇 |
1996年 | 18篇 |
1995年 | 9篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1991年 | 14篇 |
1990年 | 12篇 |
1989年 | 17篇 |
1988年 | 24篇 |
1987年 | 15篇 |
1985年 | 9篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1969年 | 10篇 |
1968年 | 6篇 |
1967年 | 6篇 |
排序方式: 共有3725条查询结果,搜索用时 15 毫秒
981.
Manuel Feria-Ortiz Uri Omar García-Vázquez Carlos Joaquín Pavón-Vázquez Adrián Nieto-Montes de Oca 《Acta zoologica》2023,104(4):608-620
Reproductive traits are critically important for understanding how organisms adapt to their respective environments. In this study, we provide information on relative litter mass (RLM) and other litter and neonate related characters of nine female Plestiodon lynxe captured in the field. We also recorded seven body dimensions in 16 neonates and 15 two-month juveniles, and on the basis of these dimensions we compared the body shape of these two age classes to detect changes in the proportions of body parts. The average litter size (4.55) is larger than that found in other viviparous species of Plestiodon, but smaller than those of congeneric oviparous species of similar size. However, the average body size of newborns (25.49 mm) is similar to that of other oviparous and viviparous species of Plestiodon. The average RLM was relatively high (0.36). The relative size of the head and limbs becomes proportionally smaller, whereas the axilla-groin length becomes proportionally larger in the first 2 months of life. We suggest that these changes are related to changes in locomotion and microhabitat use. 相似文献
982.
Barbara Wroblewska Jarda T. Wroblewski Omar H. Saab Joseph H. Neale 《Journal of neurochemistry》1993,61(3):943-948
Abstract: The neuronal dipeptide N -acetylaspartylglutamate (NAAG) fulfills several of the criteria for classification as a neurotransmitter including localization in synaptic vesicles, calcium-dependent release after neuronal depolarization, and low potency activation of N -methyl- d -aspartate receptors. In the present study, the influence of NAAG on metabotropic receptor activation in cerebellar granule cells was examined in cell culture. Stimulation of granule cell adenylate cyclase with forskolin increased cyclic AMP (cAMP) several hundredfold above basal levels within 10 min in a concentration-dependent manner. Although gluta-mate, NAAG, and the metabotropic receptor agonist frans-1-amino-1, 3-cyclopentanedicarboxylic acid did not alter the low basal cAMP levels, the application of 300 μ M glutamate or NAAG or trans-1-amino-1, 3-cyclopentanedicarboxylic acid reduced forskolin-stimulated cAMP in granule cells by 30–50% in the absence or presence of inhibitors of ionotropic acidic amino acid receptors, as well as 2-amino-4-phosphonobutyrate. No additivity in the inhibition of cAMP was found when 300 μ M NAAG and trans -1-amino-1, 3-cyclopentanedicarboxylic acid were coapplied. The β-analogue of NAAG failed to reduce cAMP levels. Similar effects of NAAG and glutamate were obtained under conditions of inhibition of phosphodiesterase activity and were prevented by pretreatment of the cells with pertussis toxin. These data are consistent with the activation by NAAG of a metabotropic acidic amino acid receptor coupled to an inhibitory G protein. In contrast, the metabotropic acidic amino acid receptor coupled to phosphoinositol turnover in these cells was not activated by NAAG. Granule cells in culture expressed very low levels of extracellular peptidase activity against NAAG, converting to glutamate <0.1% of the 10 μ M through 1 m M NAAG applied to these cells during 15-min in vitro assays. 相似文献
983.
Clara P. Rubinstein Omar A. Coso Sandra Ruzal Carmen Sanchez-Rivas 《Archives of microbiology》1993,160(6):486-491
The presence of the mutant prophage 105cts23 in Bacillus subtilis strains strongly affected several biological parameters including the viability of protoplasts and the establishment of plasmid pC194. A defective inducibility of the prophage after treatments that de-repress the SOS-like response were also observed. Although these alterations suggested a Rec-deficient phenotype, homologous recombination was not impaired in these lysogenic derivatives. In fact, chromosomal DNA transformation in these competent cells was more efficient than in cells carrying the wild type prophage: cell death due to prophage induction upon competence development was lower than expected. Alterations in the response to SOS-inducing agents and to osmotic stress correlated with the presence of this particular mutant prophage or the cloned thermosensitive repressor at the permissive temperature. The induction of an anti-SOS effect is discussed. 相似文献
984.
Amin A. Nanji Shamsuddin Khwaja S. M. Hossein Sadrzadeh 《Molecular and cellular biochemistry》1994,140(1):85-89
We investigated the association between vitamin E, lipid peroxidation and eicosanoid production in experimental alcoholic liver injury. We used the intragastric feeding rat model in which animals were fed corn oil and ethanol (CO+E) and corn oil and dextrose (CO+D) for 2 and 4 week periods. At sacrifice, we measured plasma levels of alpha-tocopherol, 8-isoprostane, thromboxane B2(TXB2) and 6-ketoprostaglandin F1(6-KetoPGF1). Animals fed CO+E had significantly lower concentrations of -tocopherol and higher concentrations of 8 isoprostane at both 2 and 4 weeks. a significant inverse correlation was seen between -tocopherol concentrations and the TXB2: PGF1 ratio (r=0.72, p<0.01). A positive correlation was seen between the TXB2: PGF1 ratio and 8 isoprostane levels (r=0.84, p<0.001). These results suggest that vitamin E depletion and enhanced lipid peroxidation may affect eicosanoid metabolism in experimental alcoholic liver disease in such a way so as to increase the thromboxane to prostacyclin ration.Abbreviations PGH2
Prostaglandin H2
- PGL2
Prostacyclin
- CO+E
Corn Oil and Ethanol
- CO+D
Corn Oil and Dextrose
- TXB2
Thromboxane B2
- TXA2
Thromboxane A2 相似文献
985.
S. G. Tan M. Y. Omar K. W. Mahani M. Rahani O. S. Selvaraj 《Biochemical genetics》1994,32(11-12):415-422
Nine populations of three species ofNephotettix (Insecta: Hemiptera) from Peninsular Malaysia were analysed for nine enzymes comprising 11 loci. Nei's (Genetics 89, 583, 1978) genetic distance,D, betweenN. virescens andN. malayanus was 0.181, that betweenN. virescens andN. nigropictus was 0.283, and that betweenN. malayanus andN. nigropictus was 0.203. The genetic distance betweenN. nigropictus from rice plant and from the weed-grassL. hexandra at Universiti Pertanian Malaysia was 0.004 and their genetic identity was 0.996, thus indicating that this insect species feeds on both host plants. The proportion of polymorphic loci and the observed heterozygosities were higher inN. nigropictus, with a wider range of host plants, than inN. virescens andN. malayanus, restricted to rice andL. hexandra, respectively. 相似文献
986.
Omar Valentini Giuseppe Biamonti Giorgio Mastromei Silvano Riva 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1984,782(2):147-155
A new purification technique for ‘single-stranded DNA-binding proteins’ from calf thymus permits the demonstration of a considerable heterogeneity within these proteins. Several molecular species are obtained with Mr between 24·103 and 30·103 and pI values between 6 and 8, showing significant differences with regard to the following functional properties: (1) strength of binding to single-stranded DNA; (2) lowering of melting temperature of poly[d(A-T)]; (3) stimulation of DNA polymerase α on a poly[d(A-T)] template. Analysis of trypsin digestion products demonstrates that the different molecular species share extensive primary sequence homology. Experiments with antibodies show that the different molecular species are antigenically related and that a 31 kDa protein present in low amounts in our preparations is very cross-reactive. 相似文献
987.
988.
The estrogen of the olive kernel and of commercial oils has been investigated. A crystalline estrone has been isolated from olive kernel. An estrogen ester has been assayed in olive oil and a free estrone in corn oil. 相似文献
989.
Two separate regions of the extrachromosomal ribosomal deoxyribonucleic acid of Tetrahymena thermophila enable autonomous replication of plasmids in Saccharomyces cerevisiae. 总被引:31,自引:8,他引:23 下载免费PDF全文
Plasmids containing the nontranscribed central and terminal, but not the coding, regions of the extrachromosomal ribosomal deoxyribonucleic acid (rDNA) of Tetrahymena thermophila are capable of autonomous replication in Saccharomyces cerevisiae. These plasmids transform S. cerevisiae at high frequency; transformants are unstable in the absence of selection, and plasmids identical to those used for transformation were isolated from the transformed yeast cells. One plasmid contains a 1.85-kilobase Tetrahymena DNA fragment which includes the origin of bidirectional replication of the extrachromosomal rDNA. The other region of Tetrahymena rDNA allowing autonomous replication of plasmids in S. cerevisiae is a 650-base pair, adenine plus thymine-rich segment from the rDNA terminus. Neither of these Tetrahymena fragments shares obvious sequence homology with the origin of replication of the S. cerevisiae 2-microns circle plasmid or with ars1, an S. cerevisiae chromosomal replicator. 相似文献
990.
Human histidyl-tRNA synthetase: recognition of amino acid signature regions in class 2a aminoacyl-tRNA synthetases. 总被引:2,自引:0,他引:2 下载免费PDF全文
We have determined the sequence of cDNA for the human histidyl-tRNA synthetase (HRS) in a hepatoma cell line and confirmed it in fetal myoblast and fibroblast cell lines. The newly determined sequence differs in 48 places, including insertions and deletions, from a previously published sequence. By sequence specific probing and by direct sequencing, we have established that only the newly determined sequence is present in genomic DNA and we have sequenced 500 hundred bases upstream of the translation start site. The predicted amino acid sequence now clearly demonstrates all three motifs recognized in class 2 aminoacyl-tRNA synthetases. Alignment of E. coli, yeast, and when available, mammalian predicted amino acid sequences for three of the four members of the class 2a subgroup (his, pro, ser, and thr) shows strong preservation of amino acid specific signature regions proximal to motif 2 and proximal to motif 3. These probably represent the active site binding regions for the proximal acceptor stem and for the amino acid. The first two exons of human HRS contain a 32 amino acid helical motif, first described in human QRS, a class 1 synthetase, which is found also in a yeast RNA polymerase, a rabbit termination factor, and both bovine and human WRS, suggesting that it may be an RNA binding motif. 相似文献