全文获取类型
收费全文 | 14033篇 |
免费 | 1074篇 |
国内免费 | 7篇 |
专业分类
15114篇 |
出版年
2023年 | 121篇 |
2022年 | 239篇 |
2021年 | 440篇 |
2020年 | 319篇 |
2019年 | 327篇 |
2018年 | 511篇 |
2017年 | 378篇 |
2016年 | 605篇 |
2015年 | 763篇 |
2014年 | 805篇 |
2013年 | 993篇 |
2012年 | 1188篇 |
2011年 | 1129篇 |
2010年 | 692篇 |
2009年 | 621篇 |
2008年 | 781篇 |
2007年 | 809篇 |
2006年 | 727篇 |
2005年 | 567篇 |
2004年 | 514篇 |
2003年 | 474篇 |
2002年 | 426篇 |
2001年 | 202篇 |
2000年 | 180篇 |
1999年 | 160篇 |
1998年 | 107篇 |
1997年 | 109篇 |
1996年 | 76篇 |
1995年 | 72篇 |
1994年 | 69篇 |
1993年 | 66篇 |
1992年 | 72篇 |
1991年 | 51篇 |
1990年 | 31篇 |
1989年 | 59篇 |
1988年 | 38篇 |
1987年 | 31篇 |
1986年 | 20篇 |
1985年 | 26篇 |
1984年 | 49篇 |
1983年 | 36篇 |
1982年 | 29篇 |
1981年 | 22篇 |
1980年 | 19篇 |
1979年 | 13篇 |
1978年 | 18篇 |
1977年 | 12篇 |
1975年 | 16篇 |
1974年 | 15篇 |
1971年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Summary Three patients with mental retardation and multiple congenital abnormalities are described.Although their clinical appearance was not suggestive of Down's syndrome, chromosome studies showed a non-disjunctional trisomy 21 in two of the patients. The third case had an unsuspected XXY karyotype. 相似文献
122.
Marden A. de Alvarenga Raimundo Braz Fo Otto R. Gottlieb João P. de P. Dias Aderbal F. Magalhães Eva G. Magalhães Gouvan C. de Magalhães Mauro T. Magalhães José G.S. Maia Raquel Marques Anita J. Marsaioli Antônio A.L. Mesquita Anselmo A. de Moraes Alaide B. de Oliveira Geovane G. de Oliveira Gentil Pedreira Sebastião K. Pereira Sonildes L.V. Pinho Celira C. Santos 《Phytochemistry》1978,17(3):511-516
Wood samples, infested by fungi during storage, were shown to contain, besides the known 5-methyl-mellein, additional (3R)-8-hydroxy-3-methyl-3,4-dihydroisocoumarins substituted by 7-methyl, 5-formyl, 5-carboxy, 5-hydroxy, 5-methoxy, 6-methoxy-5-methyl and 6,7-dimethoxy-5-methyl groups, as well as 6-formyl-7-hydroxy-5-methoxy-4-methylphthalide. Several 2-methylchromanones were synthesized in order to show that this class of compounds can be distinguished from 3-methyl-3,4-dihydroisocoumarins by MS. 相似文献
123.
Using the C-banding technique, the morphology and meiotic behaviour of four different types of B-chromosomes present in several populations of Eyprepocnemis plorans have been studied. The possible relationship between these four types is discussed. The analysis of chiasma frequency in A and B-chromosomes suggests the existence of interchromosomal interference and demonstrates that the chiasma frequency of B-s depends on the genetic background of the individual carrying them. 相似文献
124.
NMR redox studies of Desulfovibrio vulgaris Cytochrome c3. Electron transfer mechanisms 总被引:1,自引:0,他引:1
J J Moura H Santos I Moura J LeGall G R Moore R J Williams A V Xavier 《European journal of biochemistry》1982,127(1):151-155
The 300-MHz proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio vulgaris were examined while varying the pH and the redox potential. The analysis of the complete NMR reoxidation pattern was done taking into account all the 16 redox states that can be present in the redox titration of a tetra-redox-center molecule. A network of saturation transfer experiments performed at different oxidation stages, between the fully reduced and the fully oxidized states, allowed the observation of different resonances for some of the haem methyl groups. In the present experimental conditions, some of the haems show a fast intramolecular electron exchange rate, but the intermolecular electron exchange is always slow. In intermediate reoxidation stages, large shifts of the resonances of some haem methyl groups were observed upon changing the pH. These shifts are discussed in terms of a pH dependence of the haem midpoint redox potentials. The physiological relevance of this pH dependence is discussed. 相似文献
125.
Exposure of human KC to IFN-gamma increases their susceptibility to lysis by CTL. The mechanism of this enhanced lysis was investigated by analyzing interactions of IFN-gamma-treated and nontreated cultured KC with allogeneic class I-specific CTL clones. rIFN-gamma treatment augmented KC lysis in a time- and dose-dependent manner. Increased lysis of IFN-KC was detected after only 2 h of IFN-gamma treatment and was maximal by 12 h. Enhanced lysis of IFN-KC was Ag-specific, inasmuch as nonantigenic IFN-KC were not lysed either directly or as bystanders during the lysis of antigenic KC. Parallel immunofluorescence and cytotoxicity assays of KC treated with IFN-gamma for various intervals revealed a direct correlation between the degree of increased KC lysis and levels of cell surface ICAM-1 (CD54), but not of specific alloantigen or beta 2-microglobulin. Lysis of nontreated KC was blocked by mAb against class I or CD3, but not by mAb against ICAM-1 or LFA-1. In contrast, lysis of IFN-KC was partially inhibited by anti-ICAM-1 or anti-LFA-1 mAb, but resisted inhibition by anti-class I mAb except in the presence of anti-ICAM-1. These results indicate that both ICAM-1/LFA-1 and Ag/CD3-TcR interactions are important for Ag-specific lysis of IFN-KC, whereas lysis of nontreated KC depends on Ag/CD3-TcR but not ICAM-1/LFA-1 interactions. Equivalent inhibition of IFN-KC lysis by mAb against ICAM-1 or LFA-1 suggests that ICAM-1 is the only LFA-1 ligand involved in enhanced IFN-KC lysis. Furthermore, enhanced CTL lysis of KC after short-term IFN-gamma treatment can be explained solely on the basis of ICAM-1 induction, because all of the increase in specific lysis associated with IFN-gamma treatment could be blocked by mAb that block ICAM-1/LFA-1 interactions. 相似文献
126.
Y Wang C de los Santos X O Gao K Greene D Live D J Patel 《Journal of molecular biology》1991,222(3):819-832
There has been much recent interest in the self-association of short deoxyguanosine-rich motifs within single-stranded DNAs to generate monovalent cation modulated four-stranded helical segments called G-quadruplexes stabilized by hydrogen-bonded G-tetrad alignments. We have addressed structural aspects of this novel alignment and report on multinuclear 1H, 31P and 13C nuclear magnetic resonance studies on the d(G2T4CG2) deoxynonanucleotide with Na cation as counterion in aqueous solution at low temperature. This sequence forms stable structures even though it cannot align by Watson-Crick hydrogen bond formation (see the paper on d(G2T5G2) describing optical and calorimetric measurements by Jin, R., Breslauer, K. J., Jones, R. A. & Gaffney, B. L. (1990), Science, 250, 543-546). The four narrow exchangeable protons detected between 11.5 and 12.0 parts per million (p.p.m.), which are common to the d(G2T4CG2) deoxynonanucleotide and the d(G2TCG2) deoxyhexanucleotide sequences, are assigned to deoxyguanosine imino protons hydrogen-bonded to carbonyl acceptor groups. These narrow imino protons are not detected for d(IGN5IG) and d(I2N5G2), where two deoxyguanosine residues are replaced by two deoxyinosine residues in the deoxynonanucleotide sequences. This implies that the 2-amino protons of deoxyguanosine must also participate in hydrogen bond formation and stabilize the structured conformation of d(G2T4CG2) in Na cation-containing solution. We have completely assigned the base and sugar H1', H2',2', H3', and H4' protons of the d(G2T4CG2) oligomer following analysis of two-dimensional nuclear Overhauser enhancement spectroscopy and two-dimensional correlated spectroscopy data sets in 0.1 M-NaCl, 10 mM-sodium phosphate, 2H2O solution at 0 degree C. The relative magnitude of the nuclear Overhauser enhancements (NOEs) between the base H8 and its own sugar H1' protons of individual deoxyguanosine residues establishes that G1 and G8 adopt syn orientations while G2 and G9 adopt anti orientations about the glycosidic bond in the d(G1-G2-T3-T4-T5-T6-C7-G8-G9) sequence in both Na and K cation-containing aqueous solution. Consequently, any structure proposed for the tetramolecular complex of d(G2T4CG2) must exhibit alternating G(syn) and G(anti) glycosidic torsion angles within each strand. The directionality and magnitude of the observed NOEs are consistent with the G(syn)-G(anti) steps adopting right-handed helical conformations in solution. We also note that the H8 protons of G1 and G8 (7.35 to 7.45 p.p.m.) in a syn alignment are shifted significantly upfield from the H8 protons of G2 and G9 (8.0 to 8.3 p.p.m.) in an anti alignment.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
127.
A Porras A R Nebreda M Benito E Santos 《The Journal of biological chemistry》1992,267(29):21124-21131
Insulin-induced differentiation of 3T3 L1 cells to adipocytes can be mimicked by the expression of transfected ras oncogenes but not of the tyrosine-kinase oncogenes src and trk. Expression of two different transfected, dominant inhibitory ras mutants resulted in significant inhibition of insulin-induced differentiation, suggesting that endogenous Ras proteins are mediators of insulin signaling in these cells. Exposure of untransfected 3T3 L1 cells to insulin resulted in significant formation of the active Ras.GTP complex, at levels comparable with those resulting from exposure to platelet-derived growth factor. However, whereas exposure of the same cells to platelet-derived growth factor resulted in significant tyrosine phosphorylation of the p21ras GTPase-activating protein (GAP), insulin-treated cells did not show any detectable levels of de novo GAP tyrosine phosphorylation. Interestingly, insulin caused tyrosine phosphorylation of the p62 polypeptide coprecipitated with GAP by anti-GAP antibodies. Insulin-induced activation of cytosolic MAP kinase activity in untransfected 3T3 L1 cells was also mimicked by Ras expression (in the absence of insulin) in the same cells transfected with an inducible ras construct. These results confirm that Ras proteins participate in insulin signaling pathways in these mammalian cells and indicate that activation of cytosolic MAP kinases is an early event occurring downstream from Ras activation. However, tyrosine phosphorylation of GAP appears not to be a significant upstream regulatory event in the activation of Ras by insulin. 相似文献
128.
Walter Borzani Haroldo Hiss Teresinha W. de Santos Marina L. R. Vairo 《Biotechnology letters》1992,14(10):981-984
Summary A mathematical model is proposed to explain the influence of the volume fraction of inoculum on the fermentation time and ethanol productivity in semicontinuous ethanol fermentation of sugar cane blackstrap molasses by pressed yeast.Nomenclature a, b, c, d
constants, see equation (5)
- Eo
initial ethanol concentration
- Ef
final ethanol concentration
- K1, K2, K3
constants, see equation (1)
- P
ethanol productivity
- Pc
calculated values of P
- Pe
experimental values of P
- r
correlation coefficient
- So
initial TRS concentration
- Sm
TRS concentration of the feeding mash
- T
fermentation time (average of the experimental values)
- Tc
calculated value of T
- Te
experimental value of T
- TRS
total reducing sugars calculated as glucose
- Uo
initial urea concentration
- Um
urea concentration of the feeding mash
- V
reactor working volume
- Vi
volume of the inoculum
-
volume fraction of inoculum=Vi/V 相似文献
129.
F M Salzano F L Black S M Callegari-Jacques S E Santos T A Weimer M A Mestriner R R Kubo J P Pandey M H Hutz 《American journal of physical anthropology》1991,85(1):51-60
Data on 31 genetic systems were obtained for 421 individuals belonging to the Arara, Araweté, Mundurucu, and Jamamadi tribes of northern Brazil. The Jamamadi depart farthest, and the Mundurucu least, from South American Indian averages. These data are analyzed together with those of 24 other Amazonian groups. Genetic distances and corresponding dendrograms indicate a cluster of 14 related tribes living north of the Amazon river. These genetic results show only a modest correlation with linguistic and geographic relationships among these groups. 相似文献
130.
G J Liu D E Santos H Takeuchi Y Kamatani H Minakata K Nomoto I Kubota T Ikeda Y Muneoka 《Biochemical and biophysical research communications》1991,177(1):27-33
APGWamide (L-Ala-L-Pro-Gly-L-Trp-NH2) was purified from the ganglia of an African giant snail (Achatina fulica Ferussac). This peptide inhibited (hyperpolarized) more than half of the Achatina neurone types tested. This produced an outward current with the membrane conductance increase of RAPN (right anterior pallial neurone) under voltage clamp. The ED50 of the peptide was 6.2 x 10(-6) M (95% confidence limit: 5.0-7.8 x 10(-6) M) and the Emax was 3.9 +/- 0.2 nA. The effects were due to a membrane permeability increase to K+. The peptide is proposed as an inhibitory neurotransmitter of the Achatina neurones. 相似文献