全文获取类型
收费全文 | 105235篇 |
免费 | 7205篇 |
国内免费 | 5121篇 |
专业分类
117561篇 |
出版年
2024年 | 236篇 |
2023年 | 1680篇 |
2022年 | 2562篇 |
2021年 | 3468篇 |
2020年 | 3263篇 |
2019年 | 4512篇 |
2018年 | 3913篇 |
2017年 | 2832篇 |
2016年 | 2927篇 |
2015年 | 3677篇 |
2014年 | 6562篇 |
2013年 | 8176篇 |
2012年 | 4931篇 |
2011年 | 6474篇 |
2010年 | 4904篇 |
2009年 | 5396篇 |
2008年 | 5640篇 |
2007年 | 5742篇 |
2006年 | 5203篇 |
2005年 | 4536篇 |
2004年 | 4027篇 |
2003年 | 3307篇 |
2002年 | 3059篇 |
2001年 | 1970篇 |
2000年 | 1657篇 |
1999年 | 1656篇 |
1998年 | 1703篇 |
1997年 | 1338篇 |
1996年 | 1219篇 |
1995年 | 1176篇 |
1994年 | 1045篇 |
1993年 | 852篇 |
1992年 | 806篇 |
1991年 | 691篇 |
1990年 | 568篇 |
1989年 | 482篇 |
1988年 | 424篇 |
1987年 | 394篇 |
1986年 | 309篇 |
1985年 | 462篇 |
1984年 | 708篇 |
1983年 | 549篇 |
1982年 | 569篇 |
1981年 | 403篇 |
1980年 | 314篇 |
1979年 | 296篇 |
1978年 | 224篇 |
1977年 | 175篇 |
1976年 | 137篇 |
1975年 | 122篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
F. Kato T. Hino A. Nakaji M. Tanaka Y. Koyama 《Molecular & general genetics : MGG》1995,247(3):387-390
In many species of actinomycetes, carotenogenesis can be photoinduced. The capacity to respond to photoinduction is, however unstable and, in various strains of Streptomyces, is lost at a relatively high frequency. In Streptomyces setonii ISP5395, which normally produces no carotenoids, carotenoid-producing mutants can be obtained following protoplast regeneration. We report here the characterization of a gene, crtS, which was isolated from one such mutant and can confer on wild-type S. setonii ISP5395 cells the capacity to synthesize carotenoids. Sequence analysis of crtS reveals an open reading frame, which shows homology to genes that encode alternative sigma factors in Bacillus subtilis. We propose that crtS encodes a sigma factor which is necessary for the expression of a cryptic gene(s) for carotenoid biosynthesis in S. setonii ISP5395. 相似文献
12.
13.
14.
15.
Concentration factor and biological half-life of 54Mn were determined in three species representing an ecologically and economically important food chain. Green algae (Chlorella spp.), Daphnia magna and yellow perch (Perca flavescens) were exposed to 54Mn in water and assayed for 54Mn uptake. Steady state concentration factors computed from the laboratory data for algae, Daphnia and perch were 4230, 17 000 and 11, respectively. Respective biological half-lives were 1.6, 1.2 and 8.3 days. 相似文献
16.
《European journal of cell biology》2019,98(5-8):151044
Cripto-1 is a protein participating in tissue orientation during embryogenesis but has also been implicated in a wide variety of cancers, such as colon, lung and breast cancer. Cripto-1 plays a role in the regulation of different pathways, including TGF-β/Smad and Wnt/β-catenin, which are highly associated with cell migration both during embryonal development and cancer progression. Little is known about the detailed subcellular localization of cripto-1 and how it participates in the directional movement of cells. In this study, the subcellular localization of cripto-1 in glioblastoma cells was investigated in vitro with high-resolution microscopy techniques. Cripto-1 was found to be localized to dynamic and shed filopodia and transported between cells through tunneling nanotubes. Our results connect the refined subcellular localization of cripto-1 to its functions in cellular orientation and migration. 相似文献
17.
18.
Pedro J. I. Salas Dora E. Vega-Salas Enrique Rodriguez-Boulan 《The Journal of membrane biology》1987,98(3):223-236
Summary Madin-Darby canine kidney (MDCK) cells kept in suspension culture for 12–15 hr displayed high-affinity binding sites for125I-lathyritic (soluble) collagen (120,000/cell,K
D
=30nm) and preferred collagens types I and IV over laminin or fibronectin as substrates during the first hour of attachment. On the other hand, after 4 hr, attachment to all four substrates was equally efficient. Upon challenge with a collagen substrate, the high-affinity sites were rapidly recruited on it (T1/2=6 min). Their occupancy by soluble collagen triggered the exocytosis of a second large population of low-affinity collagen binding sites that included laminin and seems to be involved in a second cell-attachment mechanism. These results are compatible with a twostep model of MDCK cell attachment to the substrate: first, via high-affinity collagen binding sites, and second, via laminin of cellular origin. 相似文献
19.
Serotonin Inhibits Acetylcholine Release from Rat Striatum Slices: Evidence for a Presynaptic Receptor-Mediated Effect 总被引:4,自引:2,他引:2
Rat brain striatum slices were incubated with [3H]choline, perfused with a physiological buffer, and stimulated by perfusion with a K+-enriched buffer for 2 min. The tritium overflow evoked by K+ was decreased by 5-hydroxytryptamine (serotonin, 5-HT) (maximal inhibition 10(-6) M). This effect of 5-HT was mimicked by several agonists (5-methoxytryptamine, N,N-dimethyl-tryptamine, bufotenin) and blocked by serotonergic antagonists (methiothepin, methysergide, cinanserin) but not by haloperidol; methiothepin and methysergide alone slightly increased the K+-evoked overflow of tritium (3H). Inhibition of the tritium release by 5-HT was not suppressed in the presence of tetrodotoxin (TTX) (10(-6) M). These results suggest that 5-HT tonically inhibits acetylcholine (ACh) release from striatal cholinergic neurons by acting on a presynaptic receptor localized on cholinergic terminals. 相似文献
20.
Several unit-length minicircles from the kinetoplast DNA of Leishmania tarentolae were cloned into pBR322 and into M13 phage vectors. The complete nucleotide sequences of three different partially homologous minicircles were obtained. The molecules contained a region of approx. 80% sequence homology extending for 160–270 bp and a region unique to each minicircle. A 14-mer was found to be conserved in all kinetoplast minicircle sequences reported to date. The frequency distributions of various minicircle sequence classes in L. tarentolae were obtained by quantitative gel electrophoresis and by examination of the “T ladder” patterns of minicircles randomly cloned into M13 at several sites. By these methods we could assign approx. 50% of the total minicircle DNA into a minimum of five sequence classes. A sequence-dependent polyacrylamide gel migration abnormality was observed with several minicircle fragments both cloned and uncloned. The abnormality was dependent on the presence of a portion of the conserved region of the minicircle. 相似文献