全文获取类型
收费全文 | 29581篇 |
免费 | 81篇 |
国内免费 | 127篇 |
专业分类
系统科学 | 134篇 |
丛书文集 | 293篇 |
教育与普及 | 74篇 |
理论与方法论 | 109篇 |
现状及发展 | 12634篇 |
研究方法 | 1317篇 |
综合类 | 14773篇 |
自然研究 | 455篇 |
出版年
2013年 | 230篇 |
2012年 | 458篇 |
2011年 | 873篇 |
2010年 | 194篇 |
2008年 | 585篇 |
2007年 | 611篇 |
2006年 | 627篇 |
2005年 | 610篇 |
2004年 | 588篇 |
2003年 | 563篇 |
2002年 | 516篇 |
2001年 | 969篇 |
2000年 | 911篇 |
1999年 | 612篇 |
1994年 | 358篇 |
1992年 | 557篇 |
1991年 | 397篇 |
1990年 | 500篇 |
1989年 | 440篇 |
1988年 | 425篇 |
1987年 | 468篇 |
1986年 | 460篇 |
1985年 | 621篇 |
1984年 | 464篇 |
1983年 | 382篇 |
1982年 | 376篇 |
1981年 | 341篇 |
1980年 | 358篇 |
1979年 | 931篇 |
1978年 | 689篇 |
1977年 | 642篇 |
1976年 | 579篇 |
1975年 | 614篇 |
1974年 | 756篇 |
1973年 | 692篇 |
1972年 | 688篇 |
1971年 | 781篇 |
1970年 | 1039篇 |
1969年 | 823篇 |
1968年 | 861篇 |
1967年 | 793篇 |
1966年 | 716篇 |
1965年 | 528篇 |
1959年 | 290篇 |
1958年 | 502篇 |
1957年 | 326篇 |
1956年 | 321篇 |
1955年 | 270篇 |
1954年 | 304篇 |
1948年 | 247篇 |
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
921.
Telomere-associated chromosome fragmentation (TACF) is a new approach for chromosome mapping based on the non-targeted introduction of cloned telomeres into mammalian cells. TACF has been used to generate a panel of somatic cell hybrids with nested terminal deletions of the long arm of the human X chromosome, extending from Xq26 to the centromere. This panel has been characterized using a series of X chromosome loci. Recovery of the end clones by plasmid rescue produces a telomeric marker for each cell line and partial sequencing will allow the generation of sequence tagged sites (STSs). TACF provides a powerful and widely applicable method for genome analysis, a general way of manipulating mammalian chromosomes and a first step towards constructing artificial mammalian chromosomes. 相似文献
922.
Ornithine decarboxylase activity is critical for cell transformation. 总被引:50,自引:0,他引:50
The enzyme ornithine decarboxylase is the key regulator of the synthesis of polyamines which are essential for cell proliferation. Expression of this enzyme is transiently increased upon stimulation by growth factors, but becomes constitutively activated during cell transformation induced by carcinogens, viruses or oncogenes. To test whether ornithine decarboxylase could be a common mediator of transformation and oncogenic itself, we transfected NIH3T3 cells with expression vectors carrying the complementary DNA encoding human ornithine decarboxylase in sense and antisense orientations. The increased expression of the enzyme (50-100-times endogenous levels) induced not only cell transformation, but also anchorage-independent growth in soft agar and increased tyrosine phosphorylation of a protein of M(r) 130K. Expression of ornithine decarboxylase antisense RNA was associated with an epithelioid morphology and reduced cell proliferation. Moreover, blocking the endogenous enzyme using specific inhibitor or synthesizing antisense RNA prevented transformation of rat fibroblasts by temperature-sensitive v-src oncogene. Our results imply that the gene encoding ornithine decarboxylase is a proto-oncogene central for regulation of cell growth and transformation. 相似文献
923.
Dynamin was initially identified in calf brain tissue as a protein of relative molecular mass 100,000 which induced nucleotide-sensitive bundling of microtubules. Purified dynamin showed only trace ATPase activity. But in combination with an activating factor removed during the purification, it exhibited microtubule-activated ATPase activity and dynamin-induced bundles showed evidence of ATP-dependent force production. Dynamin is the product of the Drosophila gene shibire, which has been implicated in synaptic vesicle recycling and, more generally, in the budding of endocytic vesicles from the plasma membrane. Dynamin also shows extensive homology with proteins that participate in vacuolar protein sorting and spindle pole-body separation in yeast, and in interferon-induced viral resistance in mammals. All members of this family contain consensus sequence elements consistent with GTP binding near their amino termini, although none has been shown to have GTPase activity. We report here that dynamin is a specific GTPase which can be stimulated to very high levels of activity by microtubules. 相似文献
924.
Specific low-affinity recognition of major histocompatibility complex plus peptide by soluble T-cell receptor. 总被引:24,自引:0,他引:24
The T-cell receptor is necessary and sufficient for recognition of peptides presented by major histocompatibility complex molecules. Other adhesion molecules, like CD4 or CD8, play an auxiliary role in antigen recognition by T cells. Here we analyse T-cell receptor (TCR) binding using a soluble rather than a cell-bound receptor molecule. A TCR-immunoglobulin chimaera is constructed with the variable and the first constant regions of both the TCR alpha- and beta-chains linked to the immunoglobulin light-chain constant regions. This soluble TCR is expressed, assembled and secreted as an alpha beta heterodimer by a myeloma cell line transfected with the recombinant genes. Furthermore, the soluble TCR is biologically active: it specifically inhibits antigen-dependent activation of the relevant T-cell clones and thus discriminates between proper and irrelevant peptides presented by major histocompatibility complex molecules. 相似文献
925.
Retinoic acid alters hindbrain Hox code and induces transformation of rhombomeres 2/3 into a 4/5 identity. 总被引:11,自引:0,他引:11
It has been suggested that Hox genes play an important part in the patterning of limbs, vertebrae and craniofacial structures by providing an ordered molecular system of positional values, termed the Hox code. Little is known about the nature of the signals that govern the establishment and regulation of Hox genes, but retinoic acid can affect the expression of these genes in cell lines and in embryonic tissues. On the basis of experimental and clinical evidence, the hindbrain and branchial region of the head are particularly sensitive to the effects of retinoic acid but the phenotypes are complex and hard to interpret, and how and if they relate to Hox expression has not been clear. Here we follow the changes induced by retinoic acid to hindbrain segmentation and the branchial arches using transgenic mice which contain lacZ reporter genes that reveal the endogenous segment-restricted expression of the Hox-B1 (Hox-2.9), Hox-B2(Hox-2.8) and Krox-20 genes. Our results show that these genes rapidly respond to exposure to retinoic acid at preheadfold stages and undergo a progressive series of changes in segmental expression that are associated with specific phenotypes in hindbrain of first branchial arch. Together the molecular and anatomical alterations indicate that retinoic acid has induced changes in the hindbrain Hox code which result in the homeotic transformation of rhombomeres (r) 2/3 to an r4/5 identity. A main feature of this rhombomeric phenotype is that the trigeminal motor nerve is transformed to a facial identity. Furthermore, in support of this change in rhombomeric identity, neural crest cells derived from r2/3 also express posterior Hox markers suggesting that the retinoic acid-induced transformation extends to multiple components of the first branchial arch. 相似文献
926.
Shuttling of pre-mRNA binding proteins between nucleus and cytoplasm. 总被引:118,自引:0,他引:118
927.
928.
Effects of an Rb mutation in the mouse. 总被引:126,自引:0,他引:126
The retinoblastoma gene is mutated in several types of human cancer and is the best characterized of the tumour-suppressor genes. A mouse strain has been constructed in which one allele of Rb is disrupted. These heterozygous animals are not predisposed to retinoblastoma, but some display pituitary tumours arising from cells in which the wild-type Rb allele is absent. Embryos homozygous for the mutation die between days 14 and 15 of gestation, exhibiting neuronal cell death and defective erythropoiesis. 相似文献
929.
930.
House mice (Mus musculus domesticus) form communal nests and appear to nurse each other's pups indiscriminately. Communal nesting probably functions to reduce infanticide, but it also makes females vulnerable to exploitation if nursing partners fail to provide their fair share of care. Kinship theory predicts that females will preferentially form communal nests with relatives to minimize exploitation and further increase inclusive fitness. Here we provide evidence from seminatural populations that females prefer communal nesting partners that share allelic forms of major histocompatibility complex genes. Such behaviour would lead to the selection of close relatives as communal nesting partners. Although criteria for the demonstration of kin recognition are currently embroiled in controversy, this is the first vertebrate study to meet Grafen's restrictive requirements: discrimination is based on genetic similarity at highly polymorphic loci, incidental correlations due to relatedness are experimentally controlled, and strong reasons exist for expecting the assayed behaviour to be kin-selected. 相似文献