全文获取类型
收费全文 | 1226篇 |
免费 | 140篇 |
国内免费 | 1篇 |
专业分类
1367篇 |
出版年
2023年 | 20篇 |
2022年 | 31篇 |
2021年 | 51篇 |
2020年 | 22篇 |
2019年 | 38篇 |
2018年 | 36篇 |
2017年 | 39篇 |
2016年 | 46篇 |
2015年 | 95篇 |
2014年 | 96篇 |
2013年 | 107篇 |
2012年 | 106篇 |
2011年 | 90篇 |
2010年 | 50篇 |
2009年 | 47篇 |
2008年 | 56篇 |
2007年 | 35篇 |
2006年 | 29篇 |
2005年 | 34篇 |
2004年 | 21篇 |
2003年 | 23篇 |
2002年 | 23篇 |
2001年 | 20篇 |
2000年 | 19篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1992年 | 13篇 |
1991年 | 14篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1966年 | 7篇 |
1944年 | 3篇 |
1936年 | 4篇 |
1935年 | 4篇 |
排序方式: 共有1367条查询结果,搜索用时 15 毫秒
11.
A viral genetic system was used to map the determinants of the ability of Tomato spotted wilt virus (TSWV) to overcome the R gene (Sw-5) in tomato and the resistance conferred by the nucleocapsid gene of TSWV (N gene) in tobacco. A complete set of reassortant genotypes was generated from TSWV isolates A and D. TSWV-A was able to overcome the Sw-5 gene in tomato and the TSWV N gene in tobacco, whereas TSWV-D was repressed by both forms of resistance. The ability to overcome both forms of resistance was associated with the M RNA segment of TSWV-A (M(A)). Overcoming the Sw-5 gene was linked solely to the presence of M(A), and the ability of M(A) to overcome the TSWV N gene was modified by the L RNA and the S RNA of TSWV-A, which is consistent with previous reports that suggest that the nucleocapsid gene is not the primary determinant for overcoming the nucleocapsid-mediated resistance. Sequence analysis of the M RNA segment of TSWV-A, -D, and the type isolate BR-01 revealed multiple differences in the coding and noncoding regions, which prevented identification of the resistance-breaking nucleotide sequences. 相似文献
12.
Caitlin Sedwick 《The Journal of cell biology》2012,199(5):716-717
13.
Common goals in the development of human-machine interface (HMI) technology are to reduce cognitive workload and increase function. However, objective and quantitative outcome measures assessing cognitive workload have not been standardized for HMI research. The present study examines the efficacy of a simple event-related potential (ERP) measure of cortical effort during myoelectric control of a virtual limb for use as an outcome tool. Participants trained and tested on two methods of control, direct control (DC) and pattern recognition control (PRC), while electroencephalographic (EEG) activity was recorded. Eighteen healthy participants with intact limbs were tested using DC and PRC under three conditions: passive viewing, easy, and hard. Novel auditory probes were presented at random intervals during testing, and significant task-difficulty effects were observed in the P200, P300, and a late positive potential (LPP), supporting the efficacy of ERPs as a cognitive workload measure in HMI tasks. LPP amplitude distinguished DC from PRC in the hard condition with higher amplitude in PRC, consistent with lower cognitive workload in PRC relative to DC for complex movements. Participants completed trials faster in the easy condition using DC relative to PRC, but completed trials more slowly using DC relative to PRC in the hard condition. The results provide promising support for ERPs as an outcome measure for cognitive workload in HMI research such as prosthetics, exoskeletons, and other assistive devices, and can be used to evaluate and guide new technologies for more intuitive HMI control. 相似文献
14.
15.
Branch lengths, support, and congruence: testing the phylogenomic approach with 20 nuclear loci in snakes 总被引:2,自引:0,他引:2
Wiens JJ Kuczynski CA Smith SA Mulcahy DG Sites JW Townsend TM Reeder TW 《Systematic biology》2008,57(3):420-431
Many authors have claimed that short branches in the Tree of Life will be very difficult to resolve with strong support, even with the large multilocus data sets now made possible by genomic resources. Short branches may be especially problematic because the underlying gene trees are expected to have discordant phylogenetic histories when the time between branching events is very short. Although there are many examples of short branches that are difficult to resolve, surprisingly, no empirical studies have systematically examined the relationships between branch lengths, branch support, and congruence among genes. Here, we examine these fundamental relationships quantitatively using a data set of 20 nuclear loci for 50 species of snakes (representing most traditionally recognized families). A combined maximum likelihood analysis of the 20 loci gives strong support for 69% of the nodes, but many remain weakly supported, with bootstrap values for 20% ranging from 21% to 66%. For the combined-data tree, we find significant correlations between the length of a branch, levels of bootstrap support, and the proportion of genes that are congruent with that branch in the separate analyses of each gene. We also find that strongly supported conflicts between gene trees over the resolution of individual branches are common (roughly 35% of clades), especially for shorter branches. Overall, our results support the hypothesis that short branches may be very difficult to confidently resolve, even with large, multilocus data sets. Nevertheless, our study provides strong support for many clades, including several that were controversial or poorly resolved in previous studies of snake phylogeny. 相似文献
16.
Jie Cheng Bryan D Moyer Michal Milewski Johannes Loffing Masahiro Ikeda John E Mickle Garry R Cutting Min Li Bruce A Stanton William B Guggino 《The Journal of biological chemistry》2002,277(5):3520-3529
We identified a novel cystic fibrosis transmembrane conductance regulator (CFTR)-associating, PDZ domain-containing protein, CAL (CFTR associated ligand) containing two predicted coiled-coiled domains and one PDZ domain. The PDZ domain of CAL binds to the C terminus of CFTR. Although CAL does not have any predicted transmembrane domains, CAL is associated with membranes mediated by a region containing the coiled-coil domains. CAL is located primarily at the Golgi apparatus, co-localizing with trans-Golgi markers and is sensitive to Brefeldin A treatment. Immunoprecipitation experiments suggest that CAL exists as a multimer. Overexpression of CAL reduces CFTR chloride currents in mammalian cells and decreases expression, rate of insertion and half-life of CFTR in the plasma membrane. The Na(+)/H(+) exchanger regulatory factor, NHE-RF, a subplasma membrane PDZ domain protein, restores cell surface expression of CFTR and chloride currents. In addition, NHE-RF inhibits the binding of CAL to CFTR. CAL modulates the surface expression of CFTR. CAL favors retention of CFTR within the cell, whereas NHE-RF favors surface expression by competing with CAL for the binding of CFTR. Thus, the regulation of CFTR in the plasma membrane involves the dynamic interaction between at least two PDZ domain proteins. 相似文献
17.
Most orthopoxviruses encode a functional hemagglutinin (HA), which is nonessential for virus growth in cell culture. However, inactivation of the HA gene leads to the formation of polykaryocytes (syncytia) by fusion of infected cells at neutral pH. Fusion is not observed when a functional HA gene is present. Deletion of open reading frames (ORFs) K2, K3, and K4 within the HindIII K fragment of the HA-positive (HA+) vaccinia virus strain WR also led to fusion of cells upon infection at neutral pH. A novel ORF inactivation procedure utilizing the polymerase chain reaction was used to specifically implicate the K2 ORF in this phenomenon. The K2 ORF (the viral SPI-3 gene) encodes a protein resembling serine protease inhibitors (serpins). Inactivation of the SPI-3 gene in any of the HA+ orthopoxviruses tested caused infected cells to fuse in a manner which appeared identical to that seen for HA- mutants, although fusion was most pronounced with cowpox virus. SPI-3-negative strains fused despite the fact that the HA was expressed and processed normally, i.e., cells infected with SPI-3 mutants remained functionally hemadsorption positive, and analysis of the HA protein by Western immunoblot suggested that posttranslational modifications of the HA protein appeared normal. Fusion triggered by SPI-3 mutants, like that for HA- mutants, was inhibited by the monoclonal antibody C3 directed against the vaccinia virus 14-kDa envelope protein. Therefore SPI-3- and HA-mediated fusion share a requirement for the 14-kDa protein, suggesting linkage of the seemingly disparate SPI-3 and HA genes through a common pathway which normally acts to prevent fusion of cells infected with wild-type virus. 相似文献
18.
David L. Kimbro Edwin D. Grosholz Adam J. Baukus Nicholas J. Nesbitt Nicole M. Travis Sarikka Attoe Caitlin Coleman-Hulbert 《Oecologia》2009,160(3):563-575
Although invasive species often resemble their native counterparts, differences in their foraging and anti-predator strategies
may disrupt native food webs. In a California estuary, we showed that regions dominated by native crabs and native whelks
have low mortality of native oysters (the basal prey), while regions dominated by invasive crabs and invasive whelks have
high oyster mortality and are consequently losing a biologically diverse habitat. Using field experiments, we demonstrated
that the invasive whelk’s distribution is causally related to a large-scale pattern of oyster mortality. To determine whether
predator–prey interactions between crabs (top predators) and whelks (intermediate consumers) indirectly control the pattern
of oyster mortality, we manipulated the presence and invasion status of the intermediate and top trophic levels in laboratory
mesocosms. Our results show that native crabs indirectly maintain a portion of the estuary’s oyster habitat by both consuming
native whelks (density-mediated trophic cascade) and altering their foraging behavior (trait-mediated trophic cascade). In
contrast, invasive whelks are naive to crab predators and fail to avoid them, thereby inhibiting trait-mediated cascades and
their invasion into areas with native crabs. Similarly, when native crabs are replaced with invasive crabs, the naive foraging
strategy and smaller size of invasive crabs prevents them from efficiently consuming adult whelks, thereby inhibiting strong
density-mediated cascades. Thus, while trophic cascades allow native crabs, whelks, and oysters to locally co-exist, the replacement
of native crabs and whelks by functionally similar invasive species results in severe depletion of native oysters. As coastal
systems become increasingly invaded, the mismatch of evolutionarily based strategies among predators and prey may lead to
further losses of critical habitat that support marine biodiversity and ecosystem function.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
19.
20.
Madsen DH Ingvarsen S Jürgensen HJ Melander MC Kjøller L Moyer A Honoré C Madsen CA Garred P Burgdorf S Bugge TH Behrendt N Engelholm LH 《The Journal of biological chemistry》2011,286(30):26996-27010
The degradation of collagens, the most abundant proteins of the extracellular matrix, is involved in numerous physiological and pathological conditions including cancer invasion. An important turnover pathway involves cellular internalization and degradation of large, soluble collagen fragments, generated by initial cleavage of the insoluble collagen fibers. We have previously observed that in primary mouse fibroblasts, this endocytosis of collagen fragments is dependent on the receptor urokinase plasminogen activator receptor-associated protein (uPARAP)/Endo180. Others have identified additional mechanisms of collagen uptake, with different associated receptors, in other cell types. These receptors include β1-integrins, being responsible for collagen phagocytosis, and the mannose receptor. We have now utilized a newly developed monoclonal antibody against uPARAP/Endo180, which down-regulates the receptor protein level on treated cells, to examine the role of uPARAP/Endo180 as a mediator of collagen internalization by a wide range of cultured cell types. With the exception of macrophages, all cells that proved capable of efficient collagen internalization were of mesenchymal origin and all of these utilized uPARAP/Endo180 for their collagen uptake process. Macrophages internalized collagen in a process mediated by the mannose receptor, a protein belonging to the same protein family as uPARAP/Endo180. β1-Integrins were found not to be involved in the endocytosis of soluble collagen, irrespectively of whether this was mediated by uPARAP/Endo180 or the mannose receptor. This further distinguishes these pathways from the phagocytic uptake of particulate collagen. 相似文献