全文获取类型
收费全文 | 31777篇 |
免费 | 129篇 |
国内免费 | 264篇 |
专业分类
系统科学 | 451篇 |
丛书文集 | 624篇 |
教育与普及 | 94篇 |
理论与方法论 | 124篇 |
现状及发展 | 13027篇 |
研究方法 | 1194篇 |
综合类 | 15995篇 |
自然研究 | 661篇 |
出版年
2013年 | 342篇 |
2012年 | 605篇 |
2011年 | 1288篇 |
2010年 | 412篇 |
2009年 | 393篇 |
2008年 | 686篇 |
2007年 | 797篇 |
2006年 | 814篇 |
2005年 | 749篇 |
2004年 | 684篇 |
2003年 | 514篇 |
2002年 | 513篇 |
2001年 | 840篇 |
2000年 | 873篇 |
1999年 | 625篇 |
1992年 | 504篇 |
1991年 | 425篇 |
1990年 | 437篇 |
1989年 | 420篇 |
1988年 | 378篇 |
1987年 | 398篇 |
1986年 | 417篇 |
1985年 | 525篇 |
1984年 | 410篇 |
1983年 | 350篇 |
1982年 | 330篇 |
1981年 | 340篇 |
1980年 | 377篇 |
1979年 | 930篇 |
1978年 | 739篇 |
1977年 | 725篇 |
1976年 | 553篇 |
1975年 | 666篇 |
1974年 | 926篇 |
1973年 | 755篇 |
1972年 | 770篇 |
1971年 | 906篇 |
1970年 | 1093篇 |
1969年 | 891篇 |
1968年 | 864篇 |
1967年 | 899篇 |
1966年 | 747篇 |
1965年 | 613篇 |
1964年 | 200篇 |
1959年 | 325篇 |
1958年 | 522篇 |
1957年 | 392篇 |
1956年 | 312篇 |
1955年 | 325篇 |
1954年 | 303篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Myelin-associated glycoprotein (MAG) is a quantitatively minor component in both peripheral and central myelin sheaths that is thought to have a role in cell-cell interactions within the nervous system. We show here that a mouse monoclonal antibody, HNK-1, which is directed against human natural killer cells also recognizes an antigenic determinant of human central and peripheral nervous system white matter by immunoperoxidase staining of tissue sections. Immunoblot analysis of myelin proteins and purified extracted MAG indicates that the antigen recognized is MAG. 相似文献
993.
There is now good evidence that cytoplasmic pH (pHi) may have an important role in the metabolic activation of quiescent cells. In particular, growth stimulation of mammalian fibroblasts leads to a rapid increase in pHi (refs 3-6), due to activation of a Na+/H+ exchanger in the plasma membrane, and this alkalinization is necessary for the initiation of DNA synthesis. However, the mechanism by which mitogens activate the Na+/H+ exchanger to raise pHi is not known, although an increase in cytoplasmic free Ca2+ ([Ca2+]i) has been postulated as the primary trigger. We now present data suggesting that the Na+/H+ exchanger is set in motion through protein kinase C, a phospholipid- and Ca2+-dependent enzyme normally activated by diacylglycerol produced from inositol phospholipids in response to external stimuli. Using newly developed pH microelectrodes and fluorimetric techniques, we show that a tumour promoting phorbol ester and synthetic diacylglycerol, both potent activators of kinase C (refs 12-15), mimic the action of mitogens in rapidly elevating pHi in different cell types. Furthermore, we demonstrate that, contrary to previous views, an early rise in [Ca2+]i is not essential for the activation of Na+/H+ exchange and the resultant increase in pHi. Finally, we suggest that an alkaline pHi shift, mediated by Na+/H+ exchange, may be a common signal in the action of those hormones which elicit the breakdown of inositol phospholipids. 相似文献
994.
Loss of a Harvey ras allele in sporadic Wilms' tumour 总被引:5,自引:0,他引:5
A E Reeve P J Housiaux R J Gardner W E Chewings R M Grindley L J Millow 《Nature》1984,309(5964):174-176
Genomic changes within chromosome band 11p13 appear to have a role in the initiation of Wilms' tumour. The human Harvey ras oncogene, c-Ha-ras 1, has been located by Jhanwar et al. immediately adjacent to this region at band 11p14 .1, although several groups have assigned the gene more distally at band 11p15 . We have examined tumour DNA from two cases of sporadic Wilms' tumour, and report here that in both cases one of the two constitutional c-Ha-ras 1 alleles was absent. One tumour had a reciprocal translocation between the short arm of chromosome 11 (at band 11p13), and the long arm of chromosome 12, with no visible loss of chromosomal material. The loss of a c-Ha-ras 1 allele in association with this translocation indicates that a submicroscopic deletion had occurred. The resulting hemizygosity may have had a role in tumour initiation. Our results indicate that the c-Ha-ras 1 gene and the 'Wilms' tumour locus' may be in close proximity. It would, therefore, be premature to exclude the possibility that these two sites are functionally related. 相似文献
995.
G. Planz J. Bieber H. J. Mencke G. v. Bernuth 《Cellular and molecular life sciences : CMLS》1983,39(5):497-498
Summary Plasma catecholamine concentrations in 46 children of various ages were determined by a sensitive radioenzymatic assay. Noradrenaline levels were found to be in the same range as in adults, whereas adrenaline levels in a few of the children were abnormally high. 相似文献
996.
Molecular structure of r(GCG)d(TATACGC): a DNA--RNA hybrid helix joined to double helical DNA 总被引:20,自引:0,他引:20
A H Wang S Fujii J H van Boom G A van der Marel S A van Boeckel A Rich 《Nature》1982,299(5884):601-604
The molecule r(GCG)d(TATACGC) is self-complementary and forms two DNA--RNA hybrid segments surrounding a central region of double helical DNA; its molecular structure has been solved by X-ray analysis. All three parts of the molecule adopt a conformation which is close to that seen in the 11-fold RNA double helix. The conformation of the ribonucleotides is partly determined by water molecules bridging between the ribose O2' hydroxyl group and cytosine O2. The hybrid-DNA duplex junction contains no structural discontinuities. However, the central DNA TATA sequence has some structural irregularities. 相似文献
997.
998.
999.
J. B. Gibson A. V. Wilks G. K. Chambers 《Cellular and molecular life sciences : CMLS》1982,38(6):653-654
Summary A 3rd allele at theAdh locus,Adh
FCh.D., has been found at polymorphic frequencies in natural populations ofD. melanogaster. The ADH-FChD enzyme has properties distinct from those of the 2 more common forms of ADH. TheAdh polymorphism should now be analyzed as a triallelic system. 相似文献
1000.