全文获取类型
收费全文 | 3509篇 |
免费 | 408篇 |
出版年
2021年 | 50篇 |
2020年 | 29篇 |
2019年 | 40篇 |
2018年 | 30篇 |
2017年 | 37篇 |
2016年 | 69篇 |
2015年 | 108篇 |
2014年 | 136篇 |
2013年 | 140篇 |
2012年 | 163篇 |
2011年 | 192篇 |
2010年 | 86篇 |
2009年 | 83篇 |
2008年 | 126篇 |
2007年 | 141篇 |
2006年 | 135篇 |
2005年 | 116篇 |
2004年 | 123篇 |
2003年 | 120篇 |
2002年 | 108篇 |
2001年 | 129篇 |
2000年 | 114篇 |
1999年 | 100篇 |
1998年 | 43篇 |
1997年 | 50篇 |
1996年 | 44篇 |
1995年 | 42篇 |
1994年 | 27篇 |
1993年 | 29篇 |
1992年 | 91篇 |
1991年 | 84篇 |
1990年 | 49篇 |
1989年 | 77篇 |
1988年 | 57篇 |
1987年 | 68篇 |
1986年 | 38篇 |
1985年 | 44篇 |
1984年 | 48篇 |
1983年 | 41篇 |
1982年 | 27篇 |
1981年 | 36篇 |
1979年 | 35篇 |
1978年 | 40篇 |
1977年 | 38篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 40篇 |
1973年 | 27篇 |
1972年 | 49篇 |
1971年 | 35篇 |
排序方式: 共有3917条查询结果,搜索用时 359 毫秒
21.
Inhibition of in vitro nuclear transport by a lectin that binds to nuclear pores 总被引:88,自引:42,他引:46 下载免费PDF全文
Selective transport of proteins is a major mechanism by which biochemical differences are maintained between the cytoplasm and nucleus. To begin to investigate the molecular mechanism of nuclear transport, we used an in vitro transport system composed of a Xenopus egg extract, rat liver nuclei, and a fluorescently labeled nuclear protein, nucleoplasmin. With this system, we screened for inhibitors of transport. We found that the lectin, wheat germ agglutinin (WGA), completely inhibits the nuclear transport of fluorescently labeled nucleoplasmin. No other lectin tested affected nuclear transport. The inhibition by WGA was not seen when N-acetylglucosamine was present and was reversible by subsequent addition of sugar. When rat liver nuclei that had been incubated with ferritin-labeled WGA were examined by electron microscopy, multiple molecules of WGA were found bound to the cytoplasmic face of each nuclear pore. Gel electrophoresis and nitrocellulose transfer identified one major and several minor nuclear protein bands as binding 125I-labeled WGA. The most abundant protein of these, a 63-65-kD glycoprotein, is a candidate for the inhibitory site of action of WGA on nuclear protein transport. WGA is the first identified inhibitor of nuclear protein transport and interacts directly with the nuclear pore. 相似文献
22.
The influence of foot position on standing balance 总被引:3,自引:0,他引:3
To test the hypothesis that variations in foot position would significantly affect standing balance, we studied ten normal subjects on a Kistler force platform which measured the travel and center of pressure displacement. With the feet together there was substantially more mediolateral (ML) travel than with the axes of the feet 15, 30 or 45 cm apart and the mean ML position of the center of pressure was displaced toward the right; there was no consistent effect on anteroposterior (AP) travel or position. As the right foot was placed 10 and 30 cm forward or back, the least amount of ML and AP travel occurred with the feet even or at 10 cm either direction; the mean AP and ML position moved toward the foot which was placed more posteriorly. Of the five foot angles ranging from toes-out 45 degrees to toes-in 45 degrees, the extent of ML and AP travel was lowest in the toes-out 25 degrees position and greatest in the toes-in 45 degrees position; the mean AP and ML position was farthest forward and to the right with toes-in 45 degrees. These findings have implications for the prosthetic replacement of the lower limbs, sports, ergonomics and postural sway studies. 相似文献
23.
K H Grabstein L S Park P J Morrissey H Sassenfeld V Price D L Urdal M B Widmer 《Journal of immunology (Baltimore, Md. : 1950)》1987,139(4):1148-1153
The proliferation of mitogen-activated primary T cells, antigen-activated memory T cells from mixed leukocyte culture, and antigen-dependent alloreactive T cell clones in response to purified murine recombinant B cell stimulatory factor-1 (also known as interleukin 4) was examined. We found that B cell stimulatory factor-1 (BSF-1) stimulated optimal proliferation of these T cells only after their recent activation by antigen or mitogen. Analysis of cell surface BSF-1 receptor expression indicated that although T cell activation is accompanied by a small increase in BSF-1 receptor expression, the cells also express BSF-1 receptors prior to activation at a time when they do not proliferate in response to BSF-1. BSF-1 was as effective a stimulus as interleukin 2 for inducing proliferation of the Lyt-2+ subpopulation of concanavalin A-activated murine spleen cells and an alloreactive cytolytic T cell clone. However, the L3T4+ subpopulation of concanavalin A-activated spleen and an alloreactive helper T cell clone were less responsive to BSF-1 than to interleukin 2. Taken together, the data indicate an important role for BSF-1 in the regulation of normal T cell proliferation. 相似文献
24.
Homozygous paracentric inversion 12 in a mentally retarded boy: a case report and review of the literature 总被引:1,自引:1,他引:0
Summary A mentally retarded male was found to be homozygous for a paracentric inversion of the long arm of chromosome 12(inv(12)(q21.1q23.2)). His parents, who are first cousins, and his phenotypically normal younger brother are inversion heterozygotes. Homozygous structural rearrangements are discussed and cases of paracentric inversions, including a further nine previously unpublished, are reviewed. 相似文献
25.
Summary We tested the Enemy Impact Hypothesis, which predicts that communities of one tropic level are organized by the tropic level above. In the case of gallforming insect communities, the hypothesis predicts that gall morphology will diverge, minimizing the number of parasitoids shared among species. We used the monophyletic group of gallforming cecidomyiids (Asphondylia spp.) on creosote bush (Larrea tridentata) to test this hypothesis, predicting that species with thicker gall walls should exclude species of parasitoids with shorter ovipositors and have lower levels of parasitism. Of 17 parasitoid species reared from Asphondylia galls on creosote bush, 9 accounted for over 98% of parasitism. Seven of these 9 species had ovipositors long enough to penetrate 10 of 13 gall morphs measured. There was no significant relationship between gall wall thickness and number of associated parasitoid species (r
2=0.01, P>0.05, n=13). There was no relationship between gall wall thickness and types of parasitoid species colonizing galls: parasitoids with the shortest ovipositors colonized all types of gall morphs and were dominant members of the parasitoid assemblages in galls with the thickest walls. Ultimately, there were no significant differences in percent parasitism among Asphondylia species, regardless of gall wall thickness. We found no difference in numbers of associated parasitoids or percent parasitism in galls with different textures (e.g. hairy versus smooth), different locations on the plant or different phenologies. Our results suggest that enemy impact has not influenced the diversity of this gall community. Gall wall thickness, phenology, location on the plant and surface structure do not appear to influence the distribution of parasitoid species. Other explanations are offered to account for diversity in gall morphology among these species. 相似文献
26.
Ethoxyzolamide Inhibition of CO(2)-Dependent Photosynthesis in the Cyanobacterium Synechococcus PCC7942 总被引:5,自引:4,他引:1 下载免费PDF全文
Cells of the cyanobacterium, Synechococcus PCC7942, grown under high inorganic carbon (Ci) conditions (1% CO2; pH 8) were found to be photosynthetically dependent on exogenous CO2. This was judged by the fact that they had a similar photosynthetic affinity for CO2 (K0.5[CO2] of 3.4-5.4 micromolar) over the pH range 7 to 9 and that the low photosynthetic affinity for Ci measured in dense cell suspensions was improved by the addition of exogenous carbonic anhydrase (CA). The CA inhibitor, ethoxyzolamide (EZ), was shown to reduce photosynthetic affinity for CO2 in high Ci cells. The addition of 200 micromolar EZ to high Ci cells increased K0.5(CO2) from 4.6 micromolar to more than 155 micromolar at pH 8.0, whereas low Ci cells (grown at 30 microliters CO2 per liter of air) were less sensitive to EZ. EZ inhibition in high and low Ci cells was largely relieved by increasing exogenous Ci up to 100 millimolar. Lipid soluble CA inhibitors such as EZ and chlorazolamide were shown to be the most effective inhibitors of CO2 usage, whereas water soluble CA inhibitors such as methazolamide and acetazolamide had little or no effect. EZ was found to cause a small drop in photosystem II activity, but this level of inhibition was not sufficient to explain the large effect that EZ had on CO2 usage. High Ci cells of Anabaena variabilis M3 and Synechocystis PCC6803 were also found to be sensitive to 200 micromolar EZ. We discuss the possibility that the inhibitory effect of EZ on CO2 usage in high Ci cells of Synechococcus PCC7942 may be due to inhibition of a `CA-like' function associated with the CO2 utilizing Ci pump or due to inhibition of an internal CA activity, thus affecting CO2 supply to ribulose bisphosphate carboxylase-oxygenase. 相似文献
27.
Transgenic HLA-DR alpha faithfully reconstitutes IE-controlled immune functions and induces cross-tolerance to E alpha in E alpha 0 mutant mice 总被引:5,自引:0,他引:5
We have constructed transgenic mice that express the human class II MHC molecule HLA-DR alpha on a genetic background in which the equivalent endogenous gene, H-2 IE alpha, is not expressed. In these mice, DR alpha complemented the E beta chain such that tissue-specific expression of an interspecies hybrid DR alpha-E beta heterodimer was obtained. Despite 25% amino acid differences between DR alpha and E alpha, immune responsiveness to IE-controlled antigens, clonal deletion of IE-reactive T cells, and alloantigenicity were quantitatively and qualitatively indistinguishable in IE-positive mice and in mice that had integrated at least four copies of the transgene. These results demonstrate a remarkable degree of structural, regulatory, and functional conservation. They also suggest that tolerance induction involves only discrete portions of MHC molecules. 相似文献
28.
Cellular and molecular biology of Alzheimer's disease 总被引:4,自引:0,他引:4
D L Price E H Koo A Unterbeck 《BioEssays : news and reviews in molecular, cellular and developmental biology》1989,10(2-3):69-74
Alzheimer's disease results from the degeneration of neurons. Degenerating nerve cells express atypical proteins, and amyloid is deposited. We suggest that some of these events are strongly influenced by genetic factors and age. Animal models should be useful in investigating the pathogenic mechanisms that lead to the brain abnormalities seen in this disease. 相似文献
29.
Prostaglandins play an important role during the maintenance of pregnancy and the initiation of parturition. Prostaglandin endoperoxide synthase activity has been demonstrated in human fetal membranes and decidua. Using immunohistochemical techniques, we identified in these tissues the cell types that contain prostaglandin endoperoxide synthase. A total of 33 specimens, ranging from 8 wk to 42 wk gestation, were studied. Decidualized stromal cells stained the most intensely and consistently of all cell types. Cytotrophoblast of the chorion and early placental villi and syncytotrophoblast of all gestational ages demonstrated a lighter, more variable staining pattern. Regardless of gestational age, amnion stained in a heterogeneous fashion, with some cells demonstrating an intense staining and other cells having no staining. There were no observable differences in laboring compared to nonlaboring term specimens. In summary, the specific cell types that contain immunoreactive prostaglandin endoperoxide synthase have been identified in fetal membranes and decidua. 相似文献
30.
Genetic Correlations and Maternal Effect Coefficients Obtained from Offspring-Parent Regression 总被引:4,自引:1,他引:3 下载免费PDF全文
Additive genetic variances and covariances of quantitative characters are necessary to predict the evolutionary response of the mean phenotype vector in a population to natural or artificial selection. Standard formulas for estimating these parameters, from the resemblance between relatives in one or two characters at a time, are biased by natural selection on the parents and by maternal effects. We show how these biases can be removed using a multivariate analysis of offspring-parent regressions. A dynamic model of maternal effects demonstrates that, in addition to the phenotypic variance-covariance matrix of the characters, sufficient parameters for predicting the response of the mean phenotype vector to weak selection are the additive genetic variance-covariance matrix and a set of causal coefficients for maternal effects. These can be simultaneously estimated from offspring-parent regressions alone, in some cases just from the daughter-mother regressions, if all of the important selected and maternal characters have been measured and included in the analysis. 相似文献