全文获取类型
收费全文 | 33864篇 |
免费 | 3186篇 |
国内免费 | 3788篇 |
专业分类
40838篇 |
出版年
2024年 | 105篇 |
2023年 | 514篇 |
2022年 | 1054篇 |
2021年 | 1681篇 |
2020年 | 1197篇 |
2019年 | 1428篇 |
2018年 | 1373篇 |
2017年 | 1021篇 |
2016年 | 1321篇 |
2015年 | 2183篇 |
2014年 | 2443篇 |
2013年 | 2587篇 |
2012年 | 3083篇 |
2011年 | 2845篇 |
2010年 | 1946篇 |
2009年 | 1716篇 |
2008年 | 1891篇 |
2007年 | 1743篇 |
2006年 | 1536篇 |
2005年 | 1291篇 |
2004年 | 1163篇 |
2003年 | 1059篇 |
2002年 | 936篇 |
2001年 | 604篇 |
2000年 | 571篇 |
1999年 | 526篇 |
1998年 | 312篇 |
1997年 | 248篇 |
1996年 | 235篇 |
1995年 | 177篇 |
1994年 | 222篇 |
1993年 | 127篇 |
1992年 | 197篇 |
1991年 | 172篇 |
1990年 | 141篇 |
1989年 | 114篇 |
1988年 | 90篇 |
1987年 | 120篇 |
1986年 | 92篇 |
1985年 | 103篇 |
1984年 | 56篇 |
1983年 | 58篇 |
1982年 | 54篇 |
1981年 | 43篇 |
1980年 | 38篇 |
1979年 | 57篇 |
1978年 | 42篇 |
1977年 | 46篇 |
1975年 | 36篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
72.
Expression of Bcl-2 protein in the epiphyseal plate cartilage and trabecular bone of growing rats 总被引:2,自引:0,他引:2
Ying Wang Renée Toury Michelle Hauchecorne N. Balmain 《Histochemistry and cell biology》1997,108(1):45-55
The protooncogene protein, Bcl-2, protects cells from apoptosis and ensures their survival in vitro by inhibiting the action
of the apoptosis-inducer, Bax. Its expression in proliferative and long-lived cells in vivo also indicates that it protects
against cell death. The chondrocytes of the epiphyseal plate cartilage undergo a series of maturation steps and deposit mineral
in the cartilage matrix before dying. The possibility that Bcl-2 helps protect chondrocytes until mineral deposition is completed
was investigated by determining the distribution of Bcl-2 immunoreactivity in the epiphyseal plate cartilage of growing rats
and its subcellular localization, using a specific antibody. The involvement of Bax in the triggering of chondrocyte death
was checked by immunocytochemistry. Bcl-2 expression in the osteoblasts and the final result of their evolution, the osteocytes,
was also examined in trabecular bone. Bcl-2 immunoreactivity was non-uniformly distributed throughout the epiphyseal cartilage.
It was maximal in proliferative chondrocytes, decreased in mature chondrocytes, and low in hypertrophic chondrocytes, whereas
there was Bax immunoreactivity in all chondrocytes examined. Immunolabeling was intense in osteoblasts but considerably lower
in fully differentiated osteocytes. Bcl-2 immunoreactivity was mainly in the cytoplasm of chondrocytes, osteoblasts, and early
osteocytes; the nuclei appeared clear. The subcellular distribution of Bcl-2 immunolabeling in chondrocytes, revealed by gold
particles in the electron microscope, showed that gold particles were frequently concentrated in the mitochondria in all the
cartilage zones and lay mainly within the organelles, not at their periphery. The endoplasmic reticulum contained moderate
immunoreactivity and there were few gold particles in the cytoplasm and nuclei. The number of gold particles decreased in
all the subcellular compartments from proliferative to hypertrophic chondrocytes. In contrast, Bax immunoreactivity changed
little during chondrocyte terminal evolution, and its subcellular distribution mirrored that of Bcl-2. These immunocytochemical
data indicate that Bcl-2 helps maintain chondrocytes and osteoblasts until their terminal maturation.
Accepted: 19 February 1997 相似文献
73.
M.A. Osman F.M. Pinchbeck L.K. Cheng G.J. Wright 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1984,336(1)
A sensitive and selective high-performance liquid chromatographic method has been developed for a new anticonvulsant, fluzinamide, and three of its active metabolites. This method requires only 0.5 ml of plasma, and it involves a single extraction with a mixture of hexane—dichloromethane—butanol (55:40:5). The plasma extract is chromatographed on a 10-μm, C18 reversed-phase column and quantitated by ultraviolet absorbance at 220 nm. The concentration—response curve for all four compounds are linear from 0.05 μg/ml to at least 10 μg/ml. The extraction efficiency of this method is greater than 90%. The accuracy and precision of the method were tested by analyzing spiked unknown samples that had been randomly distributed across the concentration range. The mean concentrations found were within ± 9% of the various amounts added with a standard deviation of ± 3.5%. This method has been successfully applied to the analysis of samples obtained from fluzinamide-dosed dogs, healthy unmedicated volunteers, and patients who were at steady state with phenytoin, carbamazepine, and fluzinamide. 相似文献
74.
The cells of bacteria of the genus Butyrivibrio are universally described as being gram negative, and they produce an unequivocal gram-negative reaction in the standard staining procedure. However, their cell walls contain derivatives of teichoic acid, which are characteristic of gram-positive cells. In this study, the cell walls of two representative strains of Butyrivibrio were of the gram-positive morphological type, as seen by electron microscopy, but they were very thin (12 to 18 nm). The thinness of these cell walls may account for the tendency of these cells to stain gram negatively in the standard staining procedure. Ruthenium red staining revealed an extracellular structure surrounding cells of Butyrivibio sp. (strain C3). This structure was composed of individual "knobs" that sometimes mediated cell-to-cell adhesion in the culture. 相似文献
75.
The fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, has been used to investigate the effects of controlled and uncontrolled growth on the dynamic properties of the lipid regions of hepatocyte plasma membranes. DPH was incubated with plasma membranes derived from quiescent and regenerating liver and Morris hepatoma 7777, and the resulting systems were studied by fluorescence polarization spectroscopy. Membranes from the rapidly growing hepatoma exhibited a significantly lower fluorescence polarization than observed in quiescent liver, suggesting the presence of a more fluid membrane lipid domain. Membranes from regenerating liver exhibited a time-dependent increase in membrane fluidity, reaching a maximum 12 h after growth stimulation. A close correspondence between membrane fluidity and the cholesterol-phospholipid ratio was also observed where a decrease in this ratio resulted in a more fluid lipid matrix. These results suggest that cell cycling, as observed in regenerating liver and Morris hepatoma 7777, results in significant increases in membrane fluidity, a property which may play an important regulatory role in various cell functions. 相似文献
76.
Nature of the antigenic determinants of T locus antigens 总被引:2,自引:0,他引:2
The nature of the antigenic specificities of several antigens associated with the T/t complex in the mouse were analyzed by means of glycosidase and haptene inhibition studies. Results indicate that on testicular cells sugar residues are involved in at least six different T/t antigenic determinants. The immunodominant sugar appears to be different for each of the specificities. The specificity for the following T/t antigens resides predominantly in the sugars indicated: T:sialic acid; t12:beta-D-galactose; tw32:beta-D-galactose; t0:L-fucose; tw1:N-acetyl-D-galactosamine; tw18:L-fucose. It seems probable that these sugars are found at the terminal reducing ends of the carbohydrate portion of T/t-bearing moleculse. These studies imply that at least some of the genes in the T locus code for glycosyltransferases or regulators of glycosyltransferases which modigy oligosaccharide structures and impart specificity to the T/t antigens by alteration of their terminal sugar residues. 相似文献
77.
The inactivation of rat adipocyte Mg2+-dependent phosphatidate phosphohydrolase by noradrenaline. 总被引:5,自引:5,他引:0 下载免费PDF全文
The inactivation of rat adipocyte Mg2+-dependent phosphatidate phosphohydrolase by noradrenaline [Cheng & Saggerson (1978) FEBS Lett. 87, 65--68; Cheng & Saggerson (1978) FEBS Lett. 93, 120--124] persists for at least 40 min in crude defatted homogenates kept at 0 degrees C or 20 degrees C, but is diminished at 37 degrees C. The effect of noradrenaline persists through the isolation of post-105000 g supernatants and is then stable in these preparations at 0 degrees C and 37 degrees C. Inclusion of albumin (10--20 mg/ml) in homogenization buffers abolishes the effect of noradrenaline. The effect of noradrenaline is not removed by dialysis of extracts or by raising the concentrations of Mg2+ or phosphatidate in assays. 相似文献
78.
A new fast assay procedure for increasing deoxyuridine triphosphate nucleotidohydrolase activity was developed. With this assay procedure, this enzyme derived from blast cells of patients with acute lymphocytic leukemia was purified at least 1218-fold. The molecular weight was estimated by gel filtration to be 43,000. The enzyme exhibited optimal activity over a pH range of 7 to 8 and the activation energy was estimated to be 6.5 kcal/mol at pH 7.5. While the enzyme had activity in the absence of added divalent cations, the activity could be inhibited by EDTA but not by phenanthroline. The inhibition caused by EDTA could be reversed by Mg2+ or Zn2+. The enzyme had maximal activity in the presence of Mg2+ (40 muM) and Mg2+ (4 mM) stabilized the enzyme at 37 degrees C. Cupric ion (0.5 mM) inhibited (50%) enzyme activity in the presence or absence of Mg2+. The substrate for the enzyme was dUTP and the apparent Km was 1 muM. No other deoxyribonucleoside or ribonucleoside triphosphate served as a substrate for the enzyme. 相似文献
79.
DNase Induced After Infection of KB Cells by Herpes Simplex Virus Type 1 or Type 2 II. Characterization of an Associated Endonuclease Activity 下载免费PDF全文
Purified preparations of the "exonuclease" specified by herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) possess an endonuclease activity. The exonuclease and endonuclease activities copurify and cosediment in a sucrose density gradient. Endonuclease activity is only observed in the presence of a divalent cation, and Mg(2+) or Mn(2+) is equally effective as a cofactor with an optimal concentration of 2 mM. A slight amount of endonuclease activity is observed in the presence of Ca(2+), whereas no activity occurs in the presence of Zn(2+). In the presence of Mg(2+), Ca(2+) and Zn(2+) are inhibitory. Comparison of exonuclease and endonuclease activity in the presence of various divalent cations revealed that, at concentrations of Mn(2+) greater than 1 mM, only endonuclease activity occurs whereas endonuclease and exonuclease activity occur at all concentrations of Mg(2+). The endonuclease was affected by putrescine and spermidine to the same extent as the exonuclease activity, but in marked contrast the endonuclease was inhibited by a 10-fold-lower concentration of spermine compared to the exonuclease. The activity specified by HSV-1 and HSV-2 has very similar properties. HSV-1 and HSV-2 endonuclease cleave covalently closed circular DNA to yield, firstly, nicked circles and then linear DNA which is subsequently hydrolyzed to small oligonucleotides. Cleavage does not appear to be base sequence specific. Conversion of nicked circles to linear DNA and subsequent degradation of linear DNA occurs more rapidly in the presence of Mg(2+) than Mn(2+) presumably by virtue of the presence of the exonuclease activity. Nonsuperhelical covalently closed circular duplex DNA is cleaved by the endonucleases at a rate 60 times slower than the rate observed on the supercoiled form. These data indicate that the HSV-1 and HSV-2 endonuclease preferentially recognize single-stranded DNA regions. 相似文献
80.
Transfer of purified herpes virus thymidine kinase gene to cultured mouse cells. 总被引:342,自引:0,他引:342
Treatment of Ltk?, mouse L cells deficient in thymidine kinase (tk), with Bam I restriction endonuclease cleaved DNA from herpes simplex virus-1 (HSV-1) produced tk+ clones with a frequency of 10?6/2 μg of HSV-1 DNA. Untreated cells or cells treated with Eco RI restriction endonuclease fragments produced no tk+ clones under the same conditions. The thymidine kinase activities of four independently derived clones were characterized by biochemical and serological techniques. By these criteria, the tk activities were found to be identical to HSV-1 tk and different from host wildtype tk. The tk+ phenotype was stable over several hundred cell generations, although the rate of reversion to the tk? phenotype, as judged by cloning efficiency in the presence of bromodeoxyuridine, was high (1–5 × 10?3). HSV-1 DNA Bam restriction fragments were separated by gel electrophoresis, and virtually all activity, as assayed by transfection, was found to reside in a 3.4 kb fragment. Transformation efficiency with the isolated fragment is 20 fold higher per gene equivalent than with the unfractionated total Bam digest. These results prove the usefulness of transfection assays as a means for the bioassay and isolation of restriction fragments carrying specific genetic information. Cells expressing HSV-1 tk may also provide a useful model system for the detailed analysis of eucaryotic and viral gene regulation. 相似文献