全文获取类型
收费全文 | 5314篇 |
免费 | 15篇 |
国内免费 | 32篇 |
专业分类
系统科学 | 31篇 |
丛书文集 | 57篇 |
教育与普及 | 4篇 |
理论与方法论 | 19篇 |
现状及发展 | 2617篇 |
研究方法 | 264篇 |
综合类 | 2300篇 |
自然研究 | 69篇 |
出版年
2017年 | 34篇 |
2012年 | 77篇 |
2011年 | 151篇 |
2008年 | 110篇 |
2007年 | 131篇 |
2006年 | 116篇 |
2005年 | 105篇 |
2004年 | 109篇 |
2003年 | 92篇 |
2002年 | 112篇 |
2001年 | 156篇 |
2000年 | 173篇 |
1999年 | 104篇 |
1992年 | 91篇 |
1991年 | 71篇 |
1990年 | 70篇 |
1989年 | 64篇 |
1988年 | 51篇 |
1987年 | 70篇 |
1986年 | 69篇 |
1985年 | 127篇 |
1984年 | 76篇 |
1983年 | 68篇 |
1982年 | 60篇 |
1981年 | 61篇 |
1980年 | 69篇 |
1979年 | 131篇 |
1978年 | 124篇 |
1977年 | 133篇 |
1976年 | 121篇 |
1975年 | 134篇 |
1974年 | 145篇 |
1973年 | 110篇 |
1972年 | 102篇 |
1971年 | 149篇 |
1970年 | 229篇 |
1969年 | 151篇 |
1968年 | 138篇 |
1967年 | 163篇 |
1966年 | 124篇 |
1965年 | 82篇 |
1964年 | 48篇 |
1962年 | 37篇 |
1959年 | 67篇 |
1958年 | 82篇 |
1957年 | 81篇 |
1956年 | 52篇 |
1955年 | 48篇 |
1954年 | 54篇 |
1948年 | 41篇 |
排序方式: 共有5361条查询结果,搜索用时 15 毫秒
171.
V Timmerman E Nelis W Van Hul B W Nieuwenhuijsen K L Chen S Wang K Ben Othman B Cullen R J Leach C O Hanemann 《Nature genetics》1992,1(3):171-175
Charcot-Marie-Tooth disease (CMT1) is the most common form of inherited peripheral neuropathy. Although the disease is genetically heterogeneous, it has been demonstrated that the gene defect is the most frequent type (CMT1A) is the result of a partial duplication of band 17p11.2. Recent studies suggested that the peripheral hypomyelination syndrome in the trembler (Tr) mouse, a possible animal model for CMT1 disease, is associated with a point mutation in the peripheral myelin protein-22 gene (pmp-22). Expression of pmp-22 is particularly high in Schwann cells, and the protein is found in peripheral myelin. We now report that the human PMP-22 gene is contained within the CMT1A duplication. We therefore, suggest that increased dosage of the PMP-22 gene may be the cause of CMT1A neuropathy. 相似文献
172.
173.
Human aminopeptidase N is a receptor for human coronavirus 229E. 总被引:62,自引:0,他引:62
C L Yeager R A Ashmun R K Williams C B Cardellichio L H Shapiro A T Look K V Holmes 《Nature》1992,357(6377):420-422
Human coronaviruses (HCV) in two serogroups represented by HCV-229E and HCV-OC43 are an important cause of upper respiratory tract infections. Here we report that human aminopeptidase N, a cell-surface metalloprotease on intestinal, lung and kidney epithelial cells, is a receptor for human coronavirus strain HCV-229E, but not for HCV-OC43. A monoclonal antibody, RBS, blocked HCV-229E virus infection of human lung fibroblasts, immunoprecipitated aminopeptidase N and inhibited its enzymatic activity. HCV-229E-resistant murine fibroblasts became susceptible after transfection with complementary DNA encoding human aminopeptidase N. By contrast, infection of human cells with HCV-OC43 was not inhibited by antibody RBS and expression of aminopeptidase N did not enhance HCV-OC43 replication in mouse cells. A mutant aminopeptidase lacking the catalytic site of the enzyme did not bind HCV-229E or RBS and did not render murine cells susceptible to HCV-229E infection, suggesting that the virus-binding site may lie at or near the active site of the human aminopeptidase molecule. 相似文献
174.
175.
Ehrlich carcinoma and EL-4 thymoma ascites cells were subjected in vitro to heat shock, ATP depletion, oxidative stress, Ca2+ overlading and iodoacetamide treatment. After the transient stresses, Triton (X-100)-insoluble TIS) fractions were isolated from the cells and analysed by electrophoresis and immunoblotting. All stresses used caused rapid aggregation of cell proteins. This was manifested in a signficant rise in protein content in the TIS fractions. The protein increase was mostly due to and increase in the insolubility of actin, 57 kDa protein of intermediate filaments, 70 kDa heat shock protein (HSP 70), and some specific proteins whose insolubilization was a characteristic sign for each type of cell injury. Different survival rates in the cell lines after either stress corrlated well with differences in their TIS protein accretion. Possible mechanisms for stress-induced protein aggregation and its relationship with cell viability are suggested. 相似文献
176.
Three polyhydroxylated sterol hemiacetals, pectinoacetals A-C (1–3) have been isolated as their acetyl derivatives (4–6) from the acetic anhydride treated organic extract of the Indo-Pacific gorgonianCtenocella pectinata. These natural products were found to undergo very rapid epimerization at the C-18 chiral center and thus exist only as an equilibrium mixture of two diastereomers. The structure assignments are based on spectral studies and chemical modifications of the natural products. 相似文献
177.
It is usually assumed that, after construction of basic network architecture in embryos, immature networks undergo progressive maturation to acquire their adult properties. We examine this assumption in the context of the lobster stomatogastric nervous system. In the lobster, the neuronal population that will form this system is at first orgnanized into a single embryonic network that generates a single rhythmic pattern. The system then splits into different functional adult networks controlled by central descending systems; these adult networks produce multiple motor programmes, distinctively different from the single output of the embryonic network. We show here that the single embryonic network can produce multiple adult-like programmes. This occurs after the embryonic network is silenced by removal of central inputs, then pharmacologically stimulated to restore rhythmicity. Furthermore, restoration of the flow of descending information reversed the adult-like pattern to an embryonic pattern. This indicates that the embryonic network possesses the ability to express adult-like network characteristics, but descending information prevents it from doing so. Functional adult networks may therefore not necessarily be derived from progressive ontogenetic changes in networks themselves, but may result from maturation of descending systems that unmask preexisting adult networks in an embryonic system. 相似文献
178.
179.
Sequence and analysis of chromosome 2 of the plant Arabidopsis thaliana 总被引:21,自引:0,他引:21
Lin X Kaul S Rounsley S Shea TP Benito MI Town CD Fujii CY Mason T Bowman CL Barnstead M Feldblyum TV Buell CR Ketchum KA Lee J Ronning CM Koo HL Moffat KS Cronin LA Shen M Pai G Van Aken S Umayam L Tallon LJ Gill JE Adams MD Carrera AJ Creasy TH Goodman HM Somerville CR Copenhaver GP Preuss D Nierman WC White O Eisen JA Salzberg SL Fraser CM Venter JC 《Nature》1999,402(6763):761-768
Arabidopsis thaliana (Arabidopsis) is unique among plant model organisms in having a small genome (130-140 Mb), excellent physical and genetic maps, and little repetitive DNA. Here we report the sequence of chromosome 2 from the Columbia ecotype in two gap-free assemblies (contigs) of 3.6 and 16 megabases (Mb). The latter represents the longest published stretch of uninterrupted DNA sequence assembled from any organism to date. Chromosome 2 represents 15% of the genome and encodes 4,037 genes, 49% of which have no predicted function. Roughly 250 tandem gene duplications were found in addition to large-scale duplications of about 0.5 and 4.5 Mb between chromosomes 2 and 1 and between chromosomes 2 and 4, respectively. Sequencing of nearly 2 Mb within the genetically defined centromere revealed a low density of recognizable genes, and a high density and diverse range of vestigial and presumably inactive mobile elements. More unexpected is what appears to be a recent insertion of a continuous stretch of 75% of the mitochondrial genome into chromosome 2. 相似文献
180.
Prion diseases can be infectious, sporadic and genetic. The infectious forms of these diseases, including bovine spongiform encephalopathy and Creutzfeldt-Jakob disease, are usually characterized by the accumulation in the brain of the transmissible pathogen, an abnormally folded isoform of the prion protein (PrP) termed PrPSc. However, certain inherited PrP mutations appear to cause neurodegeneration in the absence of PrPSc, working instead by favoured synthesis of CtmPrP, a transmembrane form of PrP. The relationship between the neurodegeneration seen in transmissible prion diseases involving PrPSc and that associated with ctmPrP has remained unclear. Here we find that the effectiveness of accumulated PrPSc in causing neurodegenerative disease depends upon the predilection of host-encoded PrP to be made in the ctmPrP form. Furthermore, the time course of PrPSc accumulation in transmissible prion disease is followed closely by increased generation of CtmPrP. Thus, the accumulation of PrPSc appears to modulate in trans the events involved in generating or metabolising CtmPrP. Together, these data suggest that the events of CtmPrP-mediated neurodegeneration may represent a common step in the pathogenesis of genetic and infectious prion diseases. 相似文献