全文获取类型
收费全文 | 12378篇 |
免费 | 210篇 |
国内免费 | 136篇 |
专业分类
系统科学 | 742篇 |
丛书文集 | 807篇 |
教育与普及 | 546篇 |
理论与方法论 | 35篇 |
现状及发展 | 998篇 |
研究方法 | 1323篇 |
综合类 | 8262篇 |
自然研究 | 11篇 |
出版年
2022年 | 22篇 |
2017年 | 106篇 |
2016年 | 124篇 |
2015年 | 117篇 |
2014年 | 184篇 |
2013年 | 54篇 |
2012年 | 667篇 |
2011年 | 840篇 |
2010年 | 250篇 |
2009年 | 58篇 |
2008年 | 755篇 |
2007年 | 825篇 |
2006年 | 990篇 |
2005年 | 986篇 |
2004年 | 766篇 |
2003年 | 730篇 |
2002年 | 659篇 |
2001年 | 541篇 |
2000年 | 775篇 |
1999年 | 267篇 |
1998年 | 70篇 |
1997年 | 39篇 |
1996年 | 41篇 |
1995年 | 54篇 |
1994年 | 59篇 |
1993年 | 88篇 |
1992年 | 51篇 |
1991年 | 72篇 |
1990年 | 48篇 |
1989年 | 63篇 |
1988年 | 45篇 |
1987年 | 54篇 |
1986年 | 70篇 |
1985年 | 68篇 |
1984年 | 61篇 |
1983年 | 58篇 |
1982年 | 65篇 |
1981年 | 46篇 |
1980年 | 26篇 |
1979年 | 32篇 |
1971年 | 34篇 |
1970年 | 61篇 |
1966年 | 21篇 |
1959年 | 206篇 |
1958年 | 369篇 |
1957年 | 272篇 |
1956年 | 221篇 |
1955年 | 220篇 |
1954年 | 250篇 |
1948年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 671 毫秒
991.
The RNA component of telomerase is mutated in autosomal dominant dyskeratosis congenita 总被引:55,自引:0,他引:55
Dyskeratosis congenita is a progressive bone-marrow failure syndrome that is characterized by abnormal skin pigmentation, leukoplakia and nail dystrophy. X-linked, autosomal recessive and autosomal dominant inheritance have been found in different pedigrees. The X-linked form of the disease is due to mutations in the gene DKC1 in band 2, sub-band 8 of the long arm of the X chromosome (ref. 3). The affected protein, dyskerin, is a nucleolar protein that is found associated with the H/ACA class of small nucleolar RNAs and is involved in pseudo-uridylation of specific residues of ribosomal RNA. Dyskerin is also associated with telomerase RNA (hTR), which contains a H/ACA consensus sequence. Here we map the gene responsible for dyskeratosis congenita in a large pedigree with autosomal dominant inheritance. Affected members of this family have an 821-base-pair deletion on chromosome 3q that removes the 3' 74 bases of hTR. Mutations in hTR were found in two other families with autosomal dominant dyskeratosis congenita. 相似文献
992.
Calcitic microlenses as part of the photoreceptor system in brittlestars 总被引:12,自引:0,他引:12
Photosensitivity in most echinoderms has been attributed to 'diffuse' dermal receptors. Here we report that certain single calcite crystals used by brittlestars for skeletal construction are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma showed that in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design. These structures are absent in light-indifferent species. Photolithographic experiments in which a photoresist film was illuminated through the lens array showed selective exposure of the photoresist under the lens centres. These results provide experimental evidence that the microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4-7 micrometer below the lenses) coincides with the location of nerve bundles-the presumed primary photoreceptors. The lens array is designed to minimize spherical aberration and birefringence and to detect light from a particular direction. The optical performance is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical functions. 相似文献
993.
Myosins constitute a superfamily of at least 18 known classes of molecular motors that move along actin filaments. Myosins move towards the plus end of F-actin filaments; however, it was shown recently that a certain class of myosin, class VI myosin, moves towards the opposite end of F-actin, that is, in the minus direction. As there is a large, unique insertion in the myosin VI head domain between the motor domain and the light-chain-binding domain (the lever arm), it was thought that this insertion alters the angle of the lever-arm switch movement, thereby changing the direction of motility. Here we determine the direction of motility of chimaeric myosins that comprise the motor domain and the lever-arm domain (containing an insert) from myosins that have movement in the opposite direction. The results show that the motor core domain, but neither the large insert nor the converter domain, determines the direction of myosin motility. 相似文献
994.
995.
The third Conference of the Parties in Kyoto set the target of reducing greenhouse-gas emissions by an average of 5.3% with respect to 1990 values by 2008-2012. One of the main objections to the protocol's ratification is that compliance would pose an unbearable economic burden on the countries involved. But we show here that this is not the case if costs apart from the direct costs of energy production are also considered. Costs are also incurred in rectifying damage to human health, material goods, agriculture and the environment related to greenhouse-gas emissions. 相似文献
996.
Bentley DR Deloukas P Dunham A French L Gregory SG Humphray SJ Mungall AJ Ross MT Carter NP Dunham I Scott CE Ashcroft KJ Atkinson AL Aubin K Beare DM Bethel G Brady N Brook JC Burford DC Burrill WD Burrows C Butler AP Carder C Catanese JJ Clee CM Clegg SM Cobley V Coffey AJ Cole CG Collins JE Conquer JS Cooper RA Culley KM Dawson E Dearden FL Durbin RM de Jong PJ Dhami PD Earthrowl ME Edwards CA Evans RS Gillson CJ Ghori J Green L Gwilliam R Halls KS Hammond S Harper GL Heathcott RW Holden JL 《Nature》2001,409(6822):942-943
We constructed maps for eight chromosomes (1, 6, 9, 10, 13, 20, X and (previously) 22), representing one-third of the genome, by building landmark maps, isolating bacterial clones and assembling contigs. By this approach, we could establish the long-range organization of the maps early in the project, and all contig extension, gap closure and problem-solving was simplified by containment within local regions. The maps currently represent more than 94% of the euchromatic (gene-containing) regions of these chromosomes in 176 contigs, and contain 96% of the chromosome-specific markers in the human gene map. By measuring the remaining gaps, we can assess chromosome length and coverage in sequenced clones. 相似文献
997.
998.
999.
1000.