全文获取类型
收费全文 | 8117篇 |
免费 | 839篇 |
国内免费 | 5篇 |
出版年
2022年 | 59篇 |
2021年 | 67篇 |
2019年 | 58篇 |
2018年 | 82篇 |
2017年 | 59篇 |
2016年 | 140篇 |
2015年 | 229篇 |
2014年 | 273篇 |
2013年 | 361篇 |
2012年 | 444篇 |
2011年 | 418篇 |
2010年 | 315篇 |
2009年 | 289篇 |
2008年 | 474篇 |
2007年 | 474篇 |
2006年 | 414篇 |
2005年 | 430篇 |
2004年 | 441篇 |
2003年 | 450篇 |
2002年 | 377篇 |
2001年 | 142篇 |
2000年 | 109篇 |
1999年 | 123篇 |
1998年 | 128篇 |
1997年 | 80篇 |
1996年 | 95篇 |
1995年 | 76篇 |
1994年 | 97篇 |
1993年 | 93篇 |
1992年 | 100篇 |
1991年 | 84篇 |
1990年 | 85篇 |
1989年 | 81篇 |
1988年 | 72篇 |
1987年 | 77篇 |
1986年 | 90篇 |
1985年 | 100篇 |
1984年 | 98篇 |
1983年 | 88篇 |
1982年 | 89篇 |
1981年 | 80篇 |
1980年 | 76篇 |
1979年 | 82篇 |
1978年 | 65篇 |
1977年 | 66篇 |
1976年 | 55篇 |
1975年 | 52篇 |
1974年 | 67篇 |
1973年 | 52篇 |
1972年 | 46篇 |
排序方式: 共有8961条查询结果,搜索用时 15 毫秒
41.
The homeotic gene Ultrabithorax (Ubx) is expressed in specific parts of Drosophila embryos: in a single metamer in the visceral mesoderm and forming a complex pattern limited to a broad domain in the ectoderm and in the somatic mesoderm. Here we use a linked beta-galactosidase gene to identify cis-acting regulatory sequences. In the visceral mesoderm, correct expression of Ubx depends on localized upstream sequences. In the ectoderm, all galactosidase-positive transformants show the same characteristic pattern. The repeated elements of this basal pattern appear to be a sub-pattern of engrailed (en) expression; they depend on en function as well as on sequences in the Ubx RNA leader. We use a mutant (Haltere-mimic) to show that sequences that normally restrict segmental expression of Ubx in the ectoderm are located downstream from the RNA leader. 相似文献
42.
Sensitive, high-resolution chromatin and chromosome mapping in situ: presence and orientation of two closely integrated copies of EBV in a lymphoma line 总被引:87,自引:0,他引:87
Here we describe development and application of highly sensitive fluorescence methodology for localization of single-copy sequences in interphase nuclei and metaphase chromosomes by nonisotopic in situ hybridization. Application of this methodology to the investigation of Epstein-Barr virus integration in the Namalwa lymphoma cell line has revealed two EBV genomes closely integrated at the known site on chromosome 1. Detecting sequences as small as 5 kb, we further demonstrate resolution within interphase nuclei of two fragments of the viral genome spaced only 130 kb apart. Results indicate that the viral genomes are in opposite orientations and separated by roughly 340 kb of cellular DNA. This work demonstrates the feasibility and resolving power of interphase chromatin mapping to assess the proximity of closely spaced DNA sequences. Implications for virology, gene mapping, and investigation of nuclear organization are discussed. 相似文献
43.
44.
Application of bromodeoxyuridine incorporation measurements to the determination of cell distribution within the S phase of the cell cycle 总被引:3,自引:0,他引:3
Flow cytofluorimetric measurement of incorporated bromodeoxyuridine, using a double-stained cell population, allows the determination of the distribution of cells along the cell cycle. We have developed a simple computer program for the direct treatment of 64 x 64 channel histograms. This analysis appears to provide interesting data about the distribution of cells in the various phases of the cell cycle, namely the S phase. Two examples have been chosen to illustrate possible fields for the application of such a program. Comparison of two cell lines such as friend murine erythroleukemia cells (MELC) and fibroblasts FR3T3 cells has shown that this analysis can be used for cell-cycle characterization of a given cell line. The program also allows the differential analysis of cell distribution along the cell cycle as a function of a given parameter. This possibility has been applied to study the variation of cell-cycle parameters as a function of the time of induced differentiation of MELC and reveals changes in the distribution of the cells along the various phases of the cell cycle, namely in the S phase. 相似文献
45.
Wendell L. Combest Timothy J. Bloom Lawrence I. Gilbert 《Journal of neurochemistry》1988,51(5):1581-1591
The effects of the naturally occurring polyamines spermine and spermidine on phosphorylation promoted by cyclic AMP (cAMP)-dependent protein kinase (PK) (cAMP-PK; EC 2.7.1.37) were studied using the brain of the tobacco hornworm, Manduca sexta. Four particulate-associated peptides (280, 34, 21, and 19 kilodaltons) in day 1 pupal brains are endogenous substrates for a particulate type II cAMP-PK. These phosphoproteins are present in brain synaptosomal, as well as microsomal, particulate fractions but are not present in the cytosol. They are distributed throughout the CNS and PNS and are present in several nonneuronal tissues as well. Phosphorylation of these proteins via cAMP-PK was inhibited markedly by micromolar concentrations of spermine and spermidine. Other particulate-associated peptides phosphorylated via a Ca2+/calmodulin-PK or Ca2+ and cAMP-independent PKs were unaffected by polyamines, whereas the phosphorylation of a 260-kilodalton peptide was markedly enhanced. Spermine did not exert its inhibitory effect indirectly by enhancement of cAMP or ATP hydrolysis or via proteolysis, but its action appears to involve a substrate-directed inhibition of cAMP-PK-promoted phosphorylation as well as enhanced dephosphorylation. Although addition of spermine resulted in marked ribosome aggregation in synaptosomal and microsomal particulate fractions, this phenomenon was not involved in the inhibition of cAMP-PK-promoted phosphorylation. 相似文献
46.
Sachiko Aono Hiroshi Sato Reiji Semba Shigeo Kashiwamata Kanefusa Kato † Lawrence F. Eng† 《Journal of neurochemistry》1988,50(3):717-721
The behavior of marker proteins of glial cells [alpha-enolase, beta-S100 protein, and glial fibrillary acidic protein (GFAP)] was investigated quantitatively by using enzyme immunoassay systems during the development of cerebellar hypoplasia in jaundiced Gunn rats. A neuronal marker protein, gamma-enolase, was also measured as a reference. At postnatal day 8 corresponding to the early stage of cerebellar damage, the amount of beta-S100 on a protein basis was significantly higher in jaundiced homozygotes (jj) than in control nonjaundiced heterozygotes (j+), whereas no differences in alpha- and gamma-enolases and GFAP were observed between the two groups of rats. At days 15 and 30, which correspond, respectively, to the advanced and late stages of cerebellar damage, the three glial proteins, especially GFAP, were higher and the neuronal protein was lower in the jj rat cerebellum than in the control. These results are consistent with the reported histological observations that neuronal cells are vulnerable and damaged by bilirubin, whereas glial cells seem to be less sensitive. On the other hand, the amounts of beta-S100 and alpha-enolase per cerebellum were significantly lower in jj rats at days 15 and 30, as in the case of gamma-enolase, whereas that of GFAP remained at the same level as the control at day 15 and showed a slight but significant decrease at day 30. The possibility is suggested that beta-S100 and GFAP may be available as biochemical indicators of glial cells, especially in the early and advanced stages of cerebellar damage, respectively, but that alpha-enolase is less available. 相似文献
47.
We have investigated the effects of insulin and motor denervation on the phosphorylation of glycogen synthase in skeletal muscle. Rat epitrochlearis muscles were denervated in vivo 3 days before the contralateral and denervated muscles were incubated in vitro with 32Pi to label sites in glycogen synthase. The 32P-labeled synthase was rapidly immunoprecipitated from extracts under conditions which prevented changes in the phosphorylation state of the enzyme. When 32P-labeled synthase from contralateral muscles was cleaved with CNBr, essentially all of the 32P was recovered in two fragments, denoted CB-1 and CB-2. Incubating these muscles with insulin decreased the 32P content of each fragment by approximately 25%, indicating that the hormone stimulated dephosphorylation of at least two sites. Peptide mapping by reverse phase high performance liquid chromatography was performed to resolve phosphorylation sites more completely. The results suggest that the enzyme was phosphorylated in sites 1a, 1b, 2, 3(a+b+c), and 5. Insulin stimulated dephosphorylation of sites in peptides presumed to contain sites 1b, 2, and 3(a+b+c). Synthase from denervated muscles appeared to contain the same amount of phosphate as enzyme from contralateral muscles, and denervation did not detectably affect the distribution of 32P within the subunit. However, denervation abolished the effect of insulin on decreasing the 32P content of synthase. The results indicate that the insulin resistance induced by denervation involves a loss in the ability of insulin to stimulate dephosphorylation of glycogen synthase. 相似文献
48.
Effect of postnatal development on calcium currents and slow charge movement in mammalian skeletal muscle 总被引:16,自引:3,他引:13
下载免费PDF全文
![点击此处可从《The Journal of general physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Single- (whole-cell patch) and two-electrode voltage-clamp techniques were used to measure transient (Ifast) and sustained (Islow) calcium currents, linear capacitance, and slow, voltage-dependent charge movements in freshly dissociated fibers of the flexor digitorum brevis (FDB) muscle of rats of various postnatal ages. Peak Ifast was largest in FDB fibers of neonatal (1-5 d) rats, having a magnitude in 10 mM external Ca of 1.4 +/- 0.9 pA/pF (mean +/- SD; current normalized by linear fiber capacitance). Peak Ifast was smaller in FDB fibers of older animals, and by approximately 3 wk postnatal, it was so small as to be unmeasurable. By contrast, the magnitudes of Islow and charge movement increased substantially during postnatal development. Peak Islow was 3.6 +/- 2.5 pA/pF in FDB fibers of 1-5-d rats and increased to 16.4 +/- 6.5 pA/pF in 45-50-d-old rats; for these same two age groups, Qmax, the total mobile charge measurable as charge movement, was 6.0 +/- 1.7 and 23.8 +/- 4.0 nC/microF, respectively. As both Islow and charge movement are thought to arise in the transverse-tubular system, linear capacitance normalized by the area of fiber surface was determined as an indirect measure of the membrane area of the t-system relative to that of the fiber surface. This parameter increased from 1.5 +/- 0.2 microF/cm2 in 2-d fibers to 2.9 +/- 0.4 microF/cm2 in 44-d fibers. The increases in peak Islow, Qmax, and normalized linear capacitance all had similar time courses. Although the function of Islow is unknown, the substantial postnatal increase in its magnitude suggests that it plays an important role in the physiology of skeletal muscle. 相似文献
49.
To explore the role of the glutathione oxidation-reduction cycle in altering the sensitivity of rats to the effects of hyperbaric hyperoxia, we administered N,N-bis(2-chloroethyl)-N-nitrosourea (BCNU) to decrease tissue glutathione reductase activity. We then exposed these animals and their matched vehicle-treated controls to 100% O2 at 4 ATA. Animals that received BCNU and were immediately exposed to hyperbaric O2 showed enhanced toxicity by seizing earlier in the exposure than controls. Animals that received BCNU 18 h before the hyperbaric O2 exposure were paradoxically protected from the effects of the exposure with a prolongation of their time to initial seizure and a marked increase in their survival time during the exposure. Tissue glutathione concentrations were also measured in the various groups and the hyperbaric O2 exposure produced marked decreases in hepatic glutathione levels in all control animals. In animals treated with BCNU 18 h before exposure, hepatic glutathione concentrations also decreased, but the concentrations had significantly increased during the 18-h waiting period, allowing these animals to maintain hepatic levels in the normal range even during their hyperbaric exposures. We conclude that treatment of rats with BCNU 18 h before exposure to hyperbaric hyperoxia results in enhanced protection of the animals during the exposure. 相似文献
50.
Lawrence E. Shapiro Neil Wagner 《In vitro cellular & developmental biology. Plant》1988,24(4):299-303
Summary Serum-free tissue culture medium consisting of a 1∶1 mixture of Dulbecco's modified Eagle's medium (DMEM) and Ham's F12 medium
is herein shown to support growth of Reuber H-35 cells over several days in culture. Cells were initially plated in serum
containing DMEM medium for 3 h. After cell attachment, serum is removed and replaced with a serum-free 1∶1 mixture of these
two commercially available tissue culture media. The doubling time of cell growth in this unsupplemented serum-free medium
was 46 h in lightly plated cultures over the first 5 d. The presence of transferrin (5 μg/ml) and insulin (3.3 nM) results in a cell doubling time of 17 h, which equaled the growth rate in medium containing 10% fetal bovine serum. In the
absence of transferrin, growth rates in serum-free medium were correlated with the cell density of cultures. Conditioned medium
from dense, serum-free cultures has growth-stimulating activity in recipient lightly plated cultures. This simple, serum-free
culture medium will facilitate studies on the growth regulation of H-35 rat hepatoma cells.
This work was funded by a feasibility grant from the American Diabetes Association, as well as by the National Institutes
of Health grants CA 24604-09 and CA 16463-14. 相似文献