全文获取类型
收费全文 | 51047篇 |
免费 | 4166篇 |
国内免费 | 2280篇 |
专业分类
57493篇 |
出版年
2023年 | 494篇 |
2022年 | 1204篇 |
2021年 | 2017篇 |
2020年 | 1288篇 |
2019年 | 1757篇 |
2018年 | 1675篇 |
2017年 | 1196篇 |
2016年 | 1807篇 |
2015年 | 2644篇 |
2014年 | 3112篇 |
2013年 | 3498篇 |
2012年 | 4019篇 |
2011年 | 3798篇 |
2010年 | 2249篇 |
2009年 | 2037篇 |
2008年 | 2563篇 |
2007年 | 2482篇 |
2006年 | 2150篇 |
2005年 | 1886篇 |
2004年 | 1711篇 |
2003年 | 1650篇 |
2002年 | 1458篇 |
2001年 | 698篇 |
2000年 | 606篇 |
1999年 | 640篇 |
1998年 | 528篇 |
1997年 | 443篇 |
1996年 | 424篇 |
1995年 | 433篇 |
1994年 | 400篇 |
1993年 | 318篇 |
1992年 | 369篇 |
1991年 | 326篇 |
1990年 | 264篇 |
1989年 | 227篇 |
1988年 | 220篇 |
1987年 | 206篇 |
1986年 | 177篇 |
1985年 | 203篇 |
1984年 | 204篇 |
1983年 | 181篇 |
1982年 | 232篇 |
1981年 | 242篇 |
1980年 | 230篇 |
1979年 | 148篇 |
1978年 | 149篇 |
1977年 | 134篇 |
1976年 | 164篇 |
1974年 | 137篇 |
1973年 | 154篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
1.
Protein Kinase PKR Mediates the Apoptosis Induction and Growth Restriction Phenotypes of C Protein-Deficient Measles Virus 总被引:1,自引:0,他引:1 下载免费PDF全文
Ann M. Toth Patricia Devaux Roberto Cattaneo Charles E. Samuel 《Journal of virology》2009,83(2):961-968
The measles virus (MV) accessory proteins V and C play important roles in MV replication and pathogenesis. Infection with recombinant MV lacking either V or C causes more cell death than infection with the parental vaccine-equivalent virus (MVvac), and C-deficient virus grows poorly relative to the parental virus. Here, we show that a major effector of the C phenotype is the RNA-dependent protein kinase PKR. Using human HeLa cells stably deficient in PKR as a result of RNA interference-mediated knockdown (PKRkd cells), we demonstrated that a reduction in PKR partially rescued the growth defect of C knockout (Cko) virus but had no effect on the growth of either wild-type (WT) or V knockout (Vko) virus. Increased growth of the Cko virus in PKRkd cells correlated with increased viral protein expression, while defective growth and decreased protein expression in PKR-sufficient cells correlated with increased phosphorylation of PKR and the α subunit of eukaryotic initiation factor 2. Furthermore, infection with WT, Vko, or especially Cko virus caused significantly less apoptosis in PKRkd cells than in PKR-sufficient cells. Although apoptosis induced by Cko virus infection in PKR-sufficient cells was blocked by a caspase antagonist, the growth of Cko virus was not restored to the WT level by treatment with this pharmacologic inhibitor. Taken together, these results indicate that PKR plays an important antiviral role during MV infection but that the virus growth restriction by PKR is not dependent upon the induction of apoptosis. Furthermore, the results establish that a principal function of the MV C protein is to antagonize the proapoptotic and antiviral activities of PKR. 相似文献
2.
3.
The functioning of the vertebrate eye depends on its absolute size, which is presumably adapted to specific needs. Eye size variation in lidless and spectacled colubrid snakes was investigated, including 839 specimens belonging to 49 genera, 66 species and subspecies. Variations of adult eye diameters (EDs) in both absolute and relative terms between species were correlated with parameters reflecting behavioral ecology. In absolute terms, eye of arboreal species was larger than in terrestrial and semiaquatic species. For diurnal species, EDs of terrestrial species do not differ from semiaquatic species; for nocturnal species the ED of terrestrial species is larger than fossorial species but not different from semiaquatic species. In relative terms, ED did not differ significantly by habitat for diurnal species. Although the ED of terrestrial species is larger than fossorial species there were no differences for nocturnal species between semiaquatic and fossorial snakes. In contrast to other vertebrates studied to date, colubrid EDs in absolute and relative terms are larger in diurnal than in nocturnal species. These observations suggest that among colubrid snakes, eye size variation reflects adaptation to specific habitats, foraging strategies and daily activities, independently of phylogeny. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc. 相似文献
4.
5.
6.
Charles L. Nunn Peter H. Thrall Kelly Stewart Alexander H. Harcourt 《Evolutionary ecology》2008,22(4):519-543
Emerging infectious diseases threaten a wide diversity of animals, and important questions remain concerning disease emergence
in socially structured populations. We developed a spatially explicit simulation model to investigate whether—and under what
conditions—disease-related mortality can impact rates of pathogen spread in populations of polygynous groups. Specifically,
we investigated whether pathogen-mediated dispersal (PMD) can occur when females disperse after the resident male dies from
disease, thus carrying infections to new groups. We also examined the effects of incubation period and virulence, host mortality
and rates of background dispersal, and we used the model to investigate the spread of the virus responsible for Ebola hemorrhagic
fever, which currently is devastating African ape populations. Output was analyzed using regression trees, which enable exploration
of hierarchical and non-linear relationships. Analyses revealed that the incidence of disease in single-male (polygynous)
groups was significantly greater for those groups containing an average of more than six females, while the total number of
infected hosts in the population was most sensitive to the number of females per group. Thus, as expected, PMD occurs in polygynous
groups and its effects increase as harem size (the number of females) increases. Simulation output further indicated that
population-level effects of Ebola are likely to differ among multi-male–multi-female chimpanzees and polygynous gorillas,
with larger overall numbers of chimpanzees infected, but more gorilla groups becoming infected due to increased dispersal
when the resident male dies. Collectively, our results highlight the importance of social system on the spread of disease
in wild mammals. 相似文献
7.
E. Charles Kerr E Bruce Eaton L. Thomas Netzel 《Nucleosides, nucleotides & nucleic acids》2013,32(5-6):851-866
Abstract Syntheses of two analogs of deoxyuridine with N,N-dialkylaniline chromophores are reported. 5-[3-(N-methylphenylamino)propanoyl]-2′-deoxyuridine (1) and 5-[2-(4-N,N-dimethylaminophenyl)ethyl)]-2′-deoxyuridine (2) are prepared by palladium-mediated coupling. Preparation of 2 was facilitated by in situ transient O4-trimethylsilyl protection during alkynylation which suppressed secondary cyclization of the coupling adduct. 相似文献
8.
9.
10.
Pierre Beuchet Laïla El kihel Michel Dherbomez Georges Charles Yves Letourneux 《Bioorganic & medicinal chemistry letters》1998,8(24):931
Δ7-5-Desaturase catalyses one of the last steps in ergosterol biosynthesis in fungi. Moreover Δ5-unsaturation is necessary for the sparking function. Synthesis of three pairs of C-6 epimeric cholestanol derivatives are described as potential growth inhibitors. Preliminary results suggest that 6β-aminocholestanol is a potent antifungal agent. 相似文献