全文获取类型
收费全文 | 3817篇 |
免费 | 42篇 |
国内免费 | 57篇 |
专业分类
系统科学 | 62篇 |
丛书文集 | 26篇 |
教育与普及 | 130篇 |
理论与方法论 | 4篇 |
现状及发展 | 311篇 |
研究方法 | 691篇 |
综合类 | 2688篇 |
自然研究 | 4篇 |
出版年
2018年 | 7篇 |
2017年 | 10篇 |
2016年 | 11篇 |
2015年 | 10篇 |
2014年 | 27篇 |
2013年 | 10篇 |
2012年 | 308篇 |
2011年 | 352篇 |
2010年 | 79篇 |
2009年 | 24篇 |
2008年 | 300篇 |
2007年 | 326篇 |
2006年 | 324篇 |
2005年 | 310篇 |
2004年 | 266篇 |
2003年 | 264篇 |
2002年 | 273篇 |
2001年 | 170篇 |
2000年 | 270篇 |
1999年 | 78篇 |
1998年 | 22篇 |
1997年 | 16篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 12篇 |
1993年 | 12篇 |
1992年 | 21篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 20篇 |
1983年 | 11篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1972年 | 8篇 |
1971年 | 7篇 |
1970年 | 12篇 |
1966年 | 7篇 |
1959年 | 27篇 |
1958年 | 61篇 |
1957年 | 31篇 |
1956年 | 27篇 |
1955年 | 16篇 |
1954年 | 21篇 |
1948年 | 12篇 |
排序方式: 共有3916条查询结果,搜索用时 31 毫秒
881.
Antibacterial peptide from H. pylori 总被引:6,自引:0,他引:6
882.
Tbx5 and Tbx4 genes determine the wing/leg identity of limb buds 总被引:4,自引:0,他引:4
Takeuchi JK Koshiba-Takeuchi K Matsumoto K Vogel-Höpker A Naitoh-Matsuo M Ogura K Takahashi N Yasuda K Ogura T 《Nature》1999,398(6730):810-814
883.
Silencing of DNA repair genes plays a critical role in the development of the cancer because these genes, functioning normally, would prevent the accumulation of mutations leading to carcinogenesis. Epigenetic gene silencing is an alternative mechanism to genetic gene aberration, inactivating those genes in cancer. DNA methylation and histone modification are the major factors for epigenetic regulation of gene expression. Here, we describe recent advances in understanding of epigenetic silencing of DNA repair genes and their epigenetic mechanisms involving DNA methylation and histone modification. 相似文献
884.
Inoue K Khajavi M Ohyama T Hirabayashi S Wilson J Reggin JD Mancias P Butler IJ Wilkinson MF Wegner M Lupski JR 《Nature genetics》2004,36(4):361-369
The molecular mechanisms by which different mutations in the same gene can result in distinct disease phenotypes remain largely unknown. Truncating mutations of SOX10 cause either a complex neurocristopathy designated PCWH or a more restricted phenotype known as Waardenburg-Shah syndrome (WS4; OMIM 277580). Here we report that although all nonsense and frameshift mutations that cause premature termination of translation generate truncated SOX10 proteins with potent dominant-negative activity, the more severe disease phenotype, PCWH, is realized only when the mutant mRNAs escape the nonsense-mediated decay (NMD) pathway. We observe similar results for truncating mutations of MPZ that convey distinct myelinopathies. Our experiments show that triggering NMD and escaping NMD may cause distinct neurological phenotypes. 相似文献
885.
Oncogenic protein tyrosine kinases 总被引:6,自引:0,他引:6
Pulford K Lamant L Espinos E Jiang Q Xue L Turturro F Delsol G Morris SW 《Cellular and molecular life sciences : CMLS》2004,61(23):2939-2953
Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase, the normal role of which remains to be completely elucidated. Although work carried out in mammals suggests a function in neural development, results from studies in Drosophila indicate an additional role in visceral muscle differentiation. The aberrant expression of full-length ALK receptor proteins has been reported in neuroblastomas and glioblastomas, while the occurrence of ALK fusion proteins in anaplastic large cell lymphoma (ALCL) has resulted in the identification of the new tumor entity, ALK-positive ALCL. ALK represents one of few examples of a receptor tyrosine kinase implicated in oncogenesis in both haematopoietic and non-haematopoietic tumors, given that ALK fusions also occur in the mesenchymal tumor known as inflammatory myofibroblastic tumor (IMT). The study of ALK fusion proteins, besides demonstrating their importance in tumor development, has also raised the possibility of new therapeutic treatments for patients with ALK-positive malignancies. 相似文献
886.
Primitive chondritic meteorites contain material (presolar grains), at the level of a few parts per million, that predates the formation of our Solar System. Astronomical observations and the chemical composition of the Sun both suggest that silicates must have been the dominant solids in the protoplanetary disk from which the planets of the Solar System formed, but no presolar silicates have been identified in chondrites. Here we report the in situ discovery of presolar silicate grains 0.1-1 microm in size in the matrices of two primitive carbonaceous chondrites. These grains are highly enriched in 17O (delta17O(SMOW) > 100-400 per thousand ), but have solar silicon isotopic compositions within analytical uncertainties, suggesting an origin in an oxygen-rich red giant or an asymptotic giant branch star. The estimated abundance of these presolar silicates (3-30 parts per million) is higher than reported for other types of presolar grains in meteorites, consistent with their ubiquity in the early Solar System, but is about two orders of magnitude lower than their abundance in anhydrous interplanetary dust particles. This result is best explained by the destruction of silicates during high-temperature processing in the solar nebula. 相似文献
887.
Abazov VM Abbott B Abdesselam A Abolins M Abramov V Acharya BS Adams DL Adams M Ahmed SN Alexeev GD Alton A Alves GA Arnoud Y Avila C Babintsev VV Babukhadia L Bacon TC Baden A Baffioni S Baldin B Balm PW Banerjee S Barberis E Baringer P Barreto J Bartlett JF Bassler U Bauer D Bean A Beaudette F Begel M Belyaev A Beri SB Bernardi G Bertram I Besson A Beuselinck R Bezzubov VA Bhat PC Bhatnagar V Bhattacharjee M Blazey G Blekman F Blessing S Boehnlein A Bojko NI Bolton TA Borcherding F Bos K 《Nature》2004,429(6992):638-642
The standard model of particle physics contains parameters--such as particle masses--whose origins are still unknown and which cannot be predicted, but whose values are constrained through their interactions. In particular, the masses of the top quark (M(t)) and W boson (M(W)) constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement of M(t) can therefore indicate where to look for the Higgs, and indeed whether the hypothesis of a standard model Higgs is consistent with experimental data. As top quarks are produced in pairs and decay in only about 10(-24) s into various final states, reconstructing their masses from their decay products is very challenging. Here we report a technique that extracts more information from each top-quark event and yields a greatly improved precision (of +/- 5.3 GeV/c2) when compared to previous measurements. When our new result is combined with our published measurement in a complementary decay mode and with the only other measurements available, the new world average for M(t) becomes 178.0 +/- 4.3 GeV/c2. As a result, the most likely Higgs mass increases from the experimentally excluded value of 96 to 117 GeV/c2, which is beyond current experimental sensitivity. The upper limit on the Higgs mass at the 95% confidence level is raised from 219 to 251 GeV/c2. 相似文献
888.
Seafloor hydrothermal systems are known to respond to seismic and magmatic activity along mid-ocean ridges, often resulting in locally positive changes in hydrothermal discharge rate, temperature and microbial activity, and shifts in composition occurring at the time of earthquake swarms and axial crustal dike injections. Corresponding regional effects have also been observed. Here we present observations of a hydrological response to seafloor spreading activity, which resulted in a negative formation-fluid pressure transient during and after an earthquake swarm in the sediment-sealed igneous crust of the Middle Valley rift of the northernmost Juan de Fuca ridge. The observations were made with a borehole seal and hydrologic observatory originally established in 1991 to study the steady-state pressure and temperature conditions in this hydrothermally active area. The magnitude of the co-seismic response is consistent with the elastic strain that would be expected from the associated earthquakes, but the prolonged negative pressure transient after the swarm is surprising and suggests net co-seismic dilatation of the upper, permeable igneous crust. The rift valley was visited four weeks after the onset of the seismic activity, but no signature of increased hydrothermal activity was detected in the water column. It appears that water, not magma, filled the void left by this spreading episode. 相似文献
889.
Direct interaction of geminin and Six3 in eye development 总被引:1,自引:0,他引:1
890.