全文获取类型
收费全文 | 34848篇 |
免费 | 2537篇 |
国内免费 | 1595篇 |
出版年
2024年 | 45篇 |
2023年 | 328篇 |
2022年 | 920篇 |
2021年 | 1523篇 |
2020年 | 1056篇 |
2019年 | 1309篇 |
2018年 | 1360篇 |
2017年 | 988篇 |
2016年 | 1480篇 |
2015年 | 2285篇 |
2014年 | 2518篇 |
2013年 | 2718篇 |
2012年 | 3182篇 |
2011年 | 2791篇 |
2010年 | 1741篇 |
2009年 | 1502篇 |
2008年 | 1870篇 |
2007年 | 1680篇 |
2006年 | 1453篇 |
2005年 | 1248篇 |
2004年 | 1040篇 |
2003年 | 914篇 |
2002年 | 692篇 |
2001年 | 573篇 |
2000年 | 460篇 |
1999年 | 478篇 |
1998年 | 270篇 |
1997年 | 289篇 |
1996年 | 266篇 |
1995年 | 227篇 |
1994年 | 223篇 |
1993年 | 155篇 |
1992年 | 224篇 |
1991年 | 191篇 |
1990年 | 134篇 |
1989年 | 110篇 |
1988年 | 82篇 |
1987年 | 111篇 |
1986年 | 83篇 |
1985年 | 71篇 |
1984年 | 54篇 |
1983年 | 38篇 |
1982年 | 38篇 |
1981年 | 27篇 |
1980年 | 24篇 |
1979年 | 27篇 |
1978年 | 17篇 |
1975年 | 22篇 |
1974年 | 18篇 |
1972年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
61.
62.
I. L. Sun W. Toole-Simms F. L. Crane D. J. Morré H. Löw J. Y. Chou 《Journal of bioenergetics and biomembranes》1988,20(3):383-391
Retinoic acid inhibits the reduction of diferric transferrin through the transplasma membrane electron transport system on fetal rat liver cells infected with a temperature-sensitive SV40 virus when the cells are in the nontransformed state cultured at 40°C. When the cells are in the transformed state (grown at the permissive 33°C temperature), retinoic acid does not inhibit the diferric transferrin reduction. Inhibition of activity of nontransformed cells is specific for retinoic acid with only slight inhibition by retinol and retinyl acetate at higher concentrations. Isolated rat liver plasma membrane NADH diferric transferrin reductase is also inhibited by retinoic acid. The effect of transformation with SV40 virus to decrease susceptibility to retinoic acid inhibition stands in contrast to much greater adriamycin inhibition of diferric transferrin reduction in the transformed cells than in nontransformed cells. 相似文献
63.
Structure and expression of the cAMP cell-surface receptor 总被引:3,自引:0,他引:3
Using antibodies specific for the 3',5'-cyclic AMP (cAMP) cell surface receptor of Dictyostelium discoideum, we have screened lambda gtll expression libraries and isolated a series of cDNAs derived from cAMP receptor mRNA during early development. The identity of the cDNA clones was verified by multiple criteria: 1) beta-galactosidase fusion proteins synthesized by isolated cDNA clones stain intensely with cAMP receptor directed antiserum, 2) these fusion proteins affinity purify antibodies specific for the cAMP receptor, 3) the cDNA probes hybridize to a 2 kb mRNA whose change in relative level of abundance during development parallels that of receptor mRNA as assayed by in vitro translation, 4) the 2 kb mRNA size equals that of receptor mRNA as determined by in vitro translation of size fractionated poly (A)+ RNA, and 5) RNA transcribed in vitro from cDNAs containing the entire protein-coding region produces a polypeptide by in vitro translation with an apparent molecular weight in close agreement with that of nascent cAMP receptor protein produced by in vitro translation of cellular RNA. The DNA sequence predicts an open reading frame of 392 amino acids. The deduced amino acid sequence contains seven domains enriched in hydrophobic residues. A model is proposed in which the cAMP cell-surface receptor traverses the lipid bilayer seven times in a pattern similar to that of other receptors, such as rhodopsin, which interact with G-proteins. The structural similarities suggest a gene family of related surface receptors from such evolutionarily diverse species as Dictyostelium, yeast, and mammals. 相似文献
64.
Incubation of rat brain synaptosomes prelabeled with [2-3H]inositol resulted in a time-dependent release of labeled inositol 1-phosphate. This process was Ca2+ dependent, and ATP (1 mM) enhanced the inositol 1-phosphate formation three- to fivefold. Using [1-14C]arachidonoyl-phosphatidylinositol which was introduced into saponin-permeabilized synaptosomes, ATP (1 mM) and free Ca2+ (approximately 20 microM) enhanced the phospholipase C hydrolysis of this substrate to form labeled diacylglycerol. When the same permeabilized synaptosomal preparation was incubated with [2-3H]inositol-phosphatidylinositol, ATP not only enhanced the formation of labeled inositol 1-phosphate, but also inhibited the conversion of inositol 1-phosphate to inositol. Furthermore, ATP appeared to reduce the Ca2+ requirement of the phosphatidylinositol-phospholipase C. Inhibition of the conversion of inositol 1-phosphate to inositol could not be overcome by increasing the Mg2+ concentration in the incubation medium. Although the ATP effect is not viewed as a receptor-mediated event, it is possible that such an event may occur in synaptosomes under conditions in which intrasynaptic Ca2+ concentration becomes elevated. 相似文献
65.
66.
Strain-dependent expression of VH gene families 总被引:12,自引:0,他引:12
H D Jeong J L Komisar E Kraig J M Teale 《Journal of immunology (Baltimore, Md. : 1950)》1988,140(7):2436-2441
The tremendous diversity of the antibody specificity repertoire stems from the ability of each developing B cell to select one out of many possible variable, diversity, and joining gene segments by specific rearrangement of the DNA. The mechanism by which V region gene segments is selected is not known. Moreover, evidence for both random and nonrandom expression of VH genes in mature B cells has been presented previously. In this report, the technique of in situ hybridization is used to accurately measure at the single cell level VH gene family expression in LPS-induced cells from several strains. In this way, at least one-third of the B cells are stimulated and a large sampling of activated splenocytes from each strain analyzed. The use of in situ hybridization eliminates any potential biases resulting from transformation protocols. In addition, because all populations of cells are analyzed by both in situ hybridization and immunocytochemical staining with anti-IgM, the proportion of cells detected by in situ hybridization could be compared with the proportion of B cells, blasts, and plasma cells in the population. It was concluded from these comparisons that the cells being detected by in situ hybridization under the conditions described are plasmablasts and plasma cells. Therefore, an accurate measure of the functional and expressed VH gene repertoire could be made. The results clearly demonstrate strain-dependent variation in VH gene family expression, particularly VH 7183 and VH J558 with up to three-fold differences observed. Thus, either there is considerable strain variation in the number of functional VH gene family segments or the expression of VH genes is not entirely random. 相似文献
67.
68.
A comprehensive classification of nucleic acid structural families based on strand direction and base pairing. 下载免费PDF全文
We propose a classification of DNA structures formed from 1 to 4 strands, based only on relative strand directions, base to strand orientation and base pairing geometries. This classification and its associated notation enable all nucleic acids to be grouped into structural families and bring to light possible structures which have not yet been observed experimentally. It also helps in understanding transitions between families and can assist in the design of multistrand structures. 相似文献
69.
By means of technique of cell culture, 3H-thymidine incorporation and dot blot, it was demonstrated that angiotensin II (AGT II) stimulated proliferation and c-fos oncogene expression in cultured SHR vascular smooth muscle cells (VSMC) in a dose-dependent manner. This effect of AGT II was significantly inhibited by co-incubation with ANP. The results suggest that proliferation of VSMC is regulated by some interaction between AGT II and ANP. 相似文献
70.