全文获取类型
收费全文 | 515篇 |
免费 | 0篇 |
国内免费 | 4篇 |
专业分类
系统科学 | 3篇 |
教育与普及 | 4篇 |
现状及发展 | 109篇 |
研究方法 | 56篇 |
综合类 | 317篇 |
自然研究 | 30篇 |
出版年
2018年 | 2篇 |
2014年 | 2篇 |
2013年 | 3篇 |
2012年 | 21篇 |
2011年 | 64篇 |
2010年 | 8篇 |
2008年 | 27篇 |
2007年 | 23篇 |
2006年 | 19篇 |
2005年 | 23篇 |
2004年 | 9篇 |
2003年 | 11篇 |
2002年 | 17篇 |
2001年 | 18篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 12篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 17篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 15篇 |
1971年 | 20篇 |
1970年 | 14篇 |
1969年 | 4篇 |
1968年 | 11篇 |
1967年 | 3篇 |
1966年 | 8篇 |
1965年 | 8篇 |
1964年 | 1篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有519条查询结果,搜索用时 15 毫秒
211.
An African primate lentivirus (SIVsm) closely related to HIV-2 总被引:60,自引:0,他引:60
The ancestors of the human immunodeficiency viruses (HIV-1 and HIV-2) may have evolved from a reservoir of African nonhuman primate lentiviruses, termed simian immunodeficiency viruses (SIV). None of the SIV strains characterized so far are closely related to HIV-1. HIV-2, however, is closely related to SIV (SIVmac) isolated from captive rhesus macaques (Macaca mulatta). SIV infection of feral Asian macaques has not been demonstrated by serological surveys. Thus, macaques may have acquired SIV in captivity by cross-species transmission from an SIV-infected African primate. Sooty mangabeys (Cercocebus atys), an African primate species indigenous to West Africa, however, are infected with SIV (SIVsm) both in captivity and in the wild (P. Fultz, personal communication). We have molecularly cloned and sequenced SIVsm and report here that it is closely related to SIVmac and HIV-2. These results suggest that SIVsm has infected macaques in captivity and humans in West Africa and evolved as SIVmac and HIV-2, respectively. 相似文献
212.
Loss of genes coding for ribosomal RNA in ageing brain cells 总被引:4,自引:0,他引:4
213.
C. H. Hassall S. W. Holmes W. H. Johnson A. Kröhn C. E. Smithen W. A. Thomas 《Cellular and molecular life sciences : CMLS》1977,33(11):1492-1493
Summary It has been shown that cleavage of the N-terminal L-amino acids of a novel series of dipeptide derivatives of 2-aminobenzophenones occurs readily in vivo to give benzo-1,4-diazepines. Such compounds may serve as useful pro-drug forms of minor tranquillizers such as Valium®. 相似文献
214.
215.
Tracing the use of torture: electrically induced calcification of collagen in pig skin 总被引:1,自引:0,他引:1
T Karlsmark L Danielsen H K Thomsen O Aalund O Nielsen K G Nielsen E Johnson I K Genefke 《Nature》1983,301(5895):75-78
Reports of the use of electrical torture are usually denied, so that there is an urgent need of diagnostic methods to distinguish the consequences of electrical torture from other superficial injuries. The 'electrical group' of Anti-Torture Research (ATR) has accordingly studied the possibility of distinguishing between the sequelae of electrical and heat injury. Among the reported differences of the two types of injuries is the occurrence in epidermis, vessel walls and sweat glands of vesicular nuclei exclusively in electrically injured skin. On the basis of experiments with anaesthetized pigs, we now report that the late sequelae of electrical injury appear to include the deposition of calcium salts beneath the area of an electrical cathode. 相似文献
216.
217.
High resolution structure of the RNA duplex [U(U-A)6A]2 总被引:4,自引:0,他引:4
A C Dock-Bregeon B Chevrier A Podjarny D Moras J S deBear G R Gough P T Gilham J E Johnson 《Nature》1988,335(6188):375-378
RNA is involved in many biological functions, ranging from information storage and transfer to the catalysis of reactions involving both nucleic acids and proteins. Previous crystallographic studies on RNA oligomeric chains provide only averaged structures or information limited in resolution. The oligomer [U(U-A)6A]2 was chosen for the study of protein-RNA interactions in viruses. Its size and base composition mimic portions of the genomic RNA in alfalfa mosaic virus that bind to the amino terminus of the viral subunit. The actual sequence was designed to guarantee the formation of a single species of duplex and to facilitate the production of the pure oligomer in large quantities. The molecular structure, derived from the 2.25 A resolution X-ray diffraction data, allows the most detailed analysis of an A-RNA helix reported to date. Two kinks are observed that divide the duplex into three blocks, each close to a canonical A-helical conformation. A few intermolecular hydrogen bonds involving 2'-hydroxyl groups stabilize this peculiar conformation of the RNA, which may be related to the temperature used for the crystallization (35 degrees C). The structure demonstrates both the plasticity of the RNA molecule and the role of the 2'-hydroxyl groups in intermolecular interactions. 相似文献
218.
It is well established that murine multipotential and committed erythroid progenitor cells require the presence of a glycoprotein, termed multi-CSF (multi-colony-stimulating factor, IL-3) for clonal proliferation and differentiation in vitro. The initial proliferation of these cells can also be stimulated by two other glycoproteins, granulocyte-macrophage CSF (GM-CSF) and granulocyte CSF (G-CSF), although continued proliferation and differentiation requires the subsequent presence of multi-CSF. Here we report the stimulation of multipotential, erythroid and other haematopoietic progenitor cells by a number of adherent cell lines including a cloned bone marrow cell line (B.Ad). The positive cell lines, as feeder layers, exhibit colony-stimulating, erythropoietin-like and burst-promoting (BPA) activities. Optimal erythropoietic stimulation by the B.Ad line requires close cell-cell contact. The cell lines also support the in vitro clonal growth of multipotential colony-forming cells and progenitors of six other haematopoietic lineages. The biological activities observed seem not to be mediated by known multipotential or erythroid colony-stimulating factors (multi-CSF, IL-3, MCGF, HCGF, PSF, BPA). 相似文献
219.
Experimental manipulation of a contact guidance system in amphibian gastrulation by mechanical tension 总被引:3,自引:0,他引:3
Contact guidance has been implied in various morphogenetic movements including neural crest cell migration, primordial germ cell migration and guidance of axonal growth cone. In urodele gastrulae, we reported the presence of an aligned network of extracellular fibrils on the inside of the ectodermal layer and suggested that it guides the migration of the presumptive mesodermal cells from the blastopore towards the animal pole. We also reported in vitro experiments in which the fibril network of the ectodermal layer was transferred onto the surface of a coverslip. Dissociated mesodermal cells attach to and locomote actively on such conditioned surfaces in an oriented fashion along the blastopore-animal pole axis (bp-ap axis) of the ectodermal layer that conditioned the surface. Recent reports suggest that these fibrils contain fibronectin. We now report that the fibril network on the conditioned surface can be artificially aligned in any orientation by exerting mechanical tension on the ectodermal layer during the conditioning. Such prepared surfaces cause cell movements aligned along the tension axis, even when the tension axis is perpendicular to the natural axis of alignment along the bp-ap axis. These results suggest that the extracellular matrix fibrils aligned by the mechanical stress that arises in embryos during development can orient cell migration by the contact guidance, in a similar manner to that reported in the collagen gel and fibroblasts system. 相似文献
220.
D. E. Johnson 《Cellular and molecular life sciences : CMLS》1984,40(3):274-275
Summary A cell line of the spruce budworm (FPMI-CF1) consists of a mixed population of cells that possess variable sensitivity to -endotoxin from crystals ofBacillus thuringiensis. A cell strain was selected from FPMI-CF1 which was resistant to the entomocidal protein extracted fromB. thuringiensis crystals. The resistant character was unstable, however, and could not be maintained in the absence of toxin during growth.The author wishes to thank S.S. Sohi (Forest Pest Management Institute, Canadian Forestry Service, Sault Ste. Marie, Ontario, Canada) for theC.fumiferana cell line, and L.I. Davidson (USDA) for his able technical assistance. 相似文献