全文获取类型
收费全文 | 12792篇 |
免费 | 41篇 |
国内免费 | 50篇 |
专业分类
系统科学 | 60篇 |
丛书文集 | 52篇 |
教育与普及 | 54篇 |
理论与方法论 | 39篇 |
现状及发展 | 5312篇 |
研究方法 | 718篇 |
综合类 | 6446篇 |
自然研究 | 202篇 |
出版年
2012年 | 224篇 |
2011年 | 422篇 |
2010年 | 106篇 |
2009年 | 103篇 |
2008年 | 274篇 |
2007年 | 310篇 |
2006年 | 288篇 |
2005年 | 296篇 |
2004年 | 299篇 |
2003年 | 274篇 |
2002年 | 275篇 |
2001年 | 514篇 |
2000年 | 514篇 |
1999年 | 367篇 |
1996年 | 69篇 |
1994年 | 295篇 |
1992年 | 279篇 |
1991年 | 222篇 |
1990年 | 247篇 |
1989年 | 213篇 |
1988年 | 210篇 |
1987年 | 209篇 |
1986年 | 218篇 |
1985年 | 293篇 |
1984年 | 223篇 |
1983年 | 179篇 |
1982年 | 157篇 |
1981年 | 159篇 |
1980年 | 188篇 |
1979年 | 360篇 |
1978年 | 298篇 |
1977年 | 245篇 |
1976年 | 229篇 |
1975年 | 241篇 |
1974年 | 234篇 |
1973年 | 226篇 |
1972年 | 245篇 |
1971年 | 324篇 |
1970年 | 361篇 |
1969年 | 284篇 |
1968年 | 282篇 |
1967年 | 247篇 |
1966年 | 274篇 |
1965年 | 192篇 |
1959年 | 95篇 |
1958年 | 151篇 |
1957年 | 108篇 |
1956年 | 82篇 |
1954年 | 78篇 |
1948年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Nras loss induces metastatic conversion of Rb1-deficient neuroendocrine thyroid tumor 总被引:2,自引:0,他引:2
Takahashi C Contreras B Iwanaga T Takegami Y Bakker A Bronson RT Noda M Loda M Hunt JL Ewen ME 《Nature genetics》2006,38(1):118-123
Mutations in the gene encoding the retinoblastoma tumor suppressor predispose humans and mice to tumor development. Here we have assessed the effect of Nras loss on tumor development in Rb1 heterozygous mice. Loss of one or two Nras alleles is shown to significantly reduce the severity of pituitary tumors arising in Rb1(+/-) animals by enhancing their differentiation. By contrast, C-cell thyroid adenomas occurring in Rb1(+/-) mice progress to metastatic medullary carcinomas after loss of Nras. In Rb1(+/-)Nras(+/-) animals, distant medullary thyroid carcinoma metastases are associated with loss of the remaining wild-type Nras allele. Loss of Nras in Rb1-deficient C cells results in elevated Ras homolog family A (RhoA) activity, and this is causally linked to the invasiveness and metastatic behavior of these cells. These findings suggest that the loss of the proto-oncogene Nras in certain cellular contexts can promote malignant tumor progression. 相似文献
42.
提出了一个关于氧化物半导瓷晶界势垒起源的新观点,认为晶界势垒起源于烧结过程中外界氧在晶界中的扩散,与材料的结构、化学缺陷、掺杂、外界气氛、烧结工艺、组成状态等有密切关系,并用此理论解释了许多实验现象。 相似文献
43.
论述了交织编码的工作原理,给出了一种交织扩展Golay码的改进型软件设计方案,用TMS320C25实现了其编译码器并给出了模拟实验结果。实验结果表明,它能准确地纠正不大于72比特位的差错;处理128×128×8比特位的一幅图像数据的运行时间为0.412s. 相似文献
44.
45.
史济群 《华中科技大学学报(自然科学版)》1994,(3)
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。 相似文献
46.
Development and function of T cells in mice rendered interleukin-2 deficient by gene targeting. 总被引:69,自引:0,他引:69
Interleukin-2 (IL-2) is a lymphocytotropic hormone which is thought to have a key role in the immune response of mammalian cells. It is produced by a subpopulation of activated T-lymphocytes and acts in vitro as the principal auto- and paracrine T-cell growth factor (for reviews see refs 1-3). IL-2 is, however, not the sole T-cell growth factor, nor does it act exclusively on T cells, also promoting growth of NK cells and differentiation of B cells. A role for IL-2 in T-cell development has been postulated but remains controversial. Here we test the requirement for IL-2 in vivo using IL-2-deficient mice generated by targeted recombination. We find that mice homozygous for the IL-2 gene mutation are normal with regard to thymocyte and peripheral T-cell subset composition, but that a dysregulation of the immune system is manifested by reduced polyclonal in vitro T-cell responses and by dramatic changes in the isotype levels of serum immunoglobulins. 相似文献
47.
M. Ishibashi M. Tsuda Y. Ohizumi T. Sasaki J. Kobayashi 《Cellular and molecular life sciences : CMLS》1991,47(3):299-300
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data. 相似文献
48.
Low-molecular-weight GTP-binding proteins are strong candidates for regulators of membrane traffic. In yeast, mutations in the sec4 or ypt1 genes encoding small GTP-binding proteins inhibit constitutive membrane flow at the plasma membrane or Golgi complex, respectively. It has been suggested that membrane fusion-fission events are regulated by cycling of small GTP-binding proteins between a membrane-bound and free state, but although most of these small proteins are found in both soluble and tightly membrane-bound forms, there is no direct evidence to support such cycling. In rat brain a small GTP-binding protein, rab3A, is exclusively associated with synaptic vesicles, the secretory organelles of nerve terminals. Here we use isolated nerve terminals to study the fate of rab3A during synaptic vesicle exocytosis. We find that rab3A dissociates quantitatively from the vesicle membrane after Ca2(+)-dependent exocytosis and that this dissociation is partially reversible during recovery after stimulation. These results are direct evidence for an association-dissociation cycle of a small GTP-binding protein during traffic of its host membrane. 相似文献
49.
对目前的UBBE模型进行了适当的推广,使在解决估计问题时可以考虑那些可能是误差上界的数值,并在此基础上提出了一种方法,能够通过对估计精度和可靠性进行合理的权衡确定所需估计值,实际案例研究结果表明所提方法能够较好地解决估计精度和可靠性之间的矛盾。 相似文献
50.
Scorpio A Blank TE Day WA Chabot DJ 《Cellular and molecular life sciences : CMLS》2006,63(19-20):2237-2248
Anthrax has been a major cause of death in grazing animals and an occasional cause of death in humans for thousands of years. Since the late 1800s there has been an exceptional international history of anthrax vaccine development. Due to animal vaccinations, the rate of infection has dropped dramatically. Anthrax vaccines have progressed from uncharacterized whole-cell vaccines in 1881, to pXO2-negative spores in the 1930s, to culture filtrates absorbed to aluminum hydroxide in 1970, and likely to recombinant protective antigen in the near future. Each of these refinements has increased safety without significant loss of efficacy. The threat of genetically engineered, antibiotic and vaccine resistant strains of Bacillus anthracis is fueling hypothesis-driven research and global techniques--including genomics, proteomics and transposon site hybridization--to facilitate the discovery of novel vaccine targets. This review highlights historical achievements and new developments in anthrax vaccine research. 相似文献