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
461.
Actin-based motility of vaccinia virus mimics receptor tyrosine kinase signalling. 总被引:32,自引:0,他引:32
F Frischknecht V Moreau S R?ttger S Gonfloni I Reckmann G Superti-Furga M Way 《Nature》1999,401(6756):926-929
Studies of the actin-based motility of the intracellular pathogens Listeria monocytogenes and Shigella flexneri have provided important insight into the events occurring at the leading edges of motile cells. Like the bacteria Listeria and Shigella, vaccinia virus, a relative of the causative agent of smallpox, uses actin-based motility to spread between cells. In contrast to Listeria or Shigella, the actin-based motility of vaccinia is dependent on an unknown phosphotyrosine protein, but the underlying mechanism remains obscure. Here we show that phosphorylation of tyrosine 112 in the viral protein A36R by Src-family kinases is essential for the actin-based motility of vaccinia. Tyrosine phosphorylation of A36R results in a direct interaction with the adaptor protein Nck and the recruitment of the Ena/VASP family member N-WASP to the site of actin assembly. We also show that Nck and N-WASP are essential for the actin-based motility of vaccinia virus. We suggest that vaccinia virus spreads by mimicking the signalling pathways that are normally involved in actin polymerization at the plasma membrane. 相似文献
462.
463.
B S Mankoo N S Collins P Ashby E Grigorieva L H Pevny A Candia C V Wright P W Rigby V Pachnis 《Nature》1999,400(6739):69-73
The skeletal muscles of the limbs develop from myogenic progenitors that originate in the paraxial mesoderm and migrate into the limb-bud mesenchyme. Among the genes known to be important for muscle development in mammalian embryos are those encoding the basic helix-loop-helix (bHLH) myogenic regulatory factors (MRFs; MyoD, Myf5, myogenin and MRF4) and Pax3, a paired-type homeobox gene that is critical for the development of limb musculature. Mox1 and Mox2 are closely related homeobox genes that are expressed in overlapping patterns in the paraxial mesoderm and its derivatives. Here we show that mice homozygous for a null mutation of Mox2 have a developmental defect of the limb musculature, characterized by an overall reduction in muscle mass and elimination of specific muscles. Mox2 is not needed for the migration of myogenic precursors into the limb bud, but it is essential for normal appendicular muscle formation and for the normal regulation of myogenic genes, as demonstrated by the downregulation of Pax3 and Myf5 but not MyoD in Mox2-deficient limb buds. Our findings show that the MOX2 homeoprotein is an important regulator of vertebrate limb myogenesis. 相似文献
464.
Sister chromatid exchange was studied in lymphocyte and fibroblast cultures. Alcohol caused no disturbance under normal conditions but an acetaldehyde level above 40 muM inhibited cell multiplication and elevated SCE considerably. A high acetaldehyde level is thought to elicit the fetal alcohol syndrome, a view supported by clinical and experimental observations. 相似文献
465.
计算了激光产生铜等离子体蒸气中的物理和动力学参数,通过数值分析近似地求解玻尔兹曼动力学方程,发现存在一些具有各向同性和固定值物理量。在气体放电系统中观察了HBr混合物对其物理参数和电子能量分配函数的影响,利用交叉区电子碰撞激发过程计算了反应速度系数,表明具有HBr混合物的3d^94s^22D能级的共振激发更好。 相似文献
466.
A single EFEMP1 mutation associated with both Malattia Leventinese and Doyne honeycomb retinal dystrophy. 总被引:14,自引:0,他引:14
E M Stone A J Lotery F L Munier E Héon B Piguet R H Guymer K Vandenburgh P Cousin D Nishimura R E Swiderski G Silvestri D A Mackey G S Hageman A C Bird V C Sheffield D F Schorderet 《Nature genetics》1999,22(2):199-202
Malattia Leventinese (ML) and Doyne honeycomb retinal dystrophy (DHRD) refer to two autosomal dominant diseases characterized by yellow-white deposits known as drusen that accumulate beneath the retinal pigment epithelium (RPE). Both loci were mapped to chromosome 2p16-21 (refs 5,6) and this genetic interval has been subsequently narrowed. The importance of these diseases is due in large part to their close phenotypic similarity to age-related macular degeneration (AMD), a disorder with a strong genetic component that accounts for approximately 50% of registered blindness in the Western world. Just as in ML and DHRD, the early hallmark of AMD is the presence of drusen. Here we use a combination of positional and candidate gene methods to identify a single non-conservative mutation (Arg345Trp) in the gene EFEMP1 (for EGF-containing fibrillin-like extracellular matrix protein 1) in all families studied. This change was not present in 477 control individuals or in 494 patients with age-related macular degeneration. Identification of this mutation may aid in the development of an animal model for drusen, as well as in the identification of other genes involved in human macular degeneration. 相似文献
467.
In a group of 84 pairs of 11-year-old children of both sexes, the level of the alpha1-antitrypsin (alpha1-AT) were ascertained in the autumn and spring. Although the mean levels of alpha1-AT in the two seasons hardly differed, the highly significant seasonal changes in the distribution curves of alpha1-AT values were noted in boys, whereas the levels showed higher stability in girls. 相似文献
468.
3 experimentally distinct transverse relaxation components of the water in frog sciatic nerve are obtained by Carr-Purcell-Meiboom-Gill technique. The relative weights of these components: approximately 29%; approximately 50%; approximately 21% fit well with water compartments in this tissue as revealed by previous methods. 相似文献
469.
Sebastian Vogel Thorsten Trapp Verena Börger Corinna Peters Dalila Lakbir Dagmar Dilloo Rüdiger V. Sorg 《Cellular and molecular life sciences : CMLS》2010,67(2):295-303
Human bone marrow-derived mesenchymal stem cells (MSC) home to injured tissues and have regenerative capacity. In this study,
we have investigated in vitro the influence of apoptotic and necrotic cell death, thus distinct types of tissue damage, on
MSC migration. Concordant with an increased overall motility, MSC migrated towards apoptotic, but not vital or necrotic neuronal
and cardiac cells. Hepatocyte growth factor (HGF) was expressed by the apoptotic cells only. MSC, in contrast, revealed expression
of the HGF-receptor, c-Met. Blocking HGF bioactivity resulted in significant reduction of MSC migration. Moreover, recombinant
HGF attracted MSC in a dose-dependent manner. Thus, apoptosis initiates chemoattraction of MSC via the HGF/c-Met axis, thereby
linking tissue damage to the recruitment of cells with regenerative potential. 相似文献
470.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} In this taxonomic revision of the Nearctic flea genus Anomiopsyllus Baker morphological characters, phylogeny, affinities with other genera in the subfamily Anomiopsyllinae, evolutionary adaptations, geographic distribution, ecological parameters, and host preferences are discussed. Characters primarily associated with the male genitalia, in particular the number and placement of spiniforms on the movable process (finger) of the clasper, are used for species discrimination. A key to the males of the 15 species and subspecies is provided, including illustrations of both males and females. 相似文献