全文获取类型
收费全文 | 43112篇 |
免费 | 3870篇 |
国内免费 | 33篇 |
专业分类
47015篇 |
出版年
2023年 | 166篇 |
2022年 | 309篇 |
2021年 | 599篇 |
2020年 | 395篇 |
2019年 | 489篇 |
2018年 | 641篇 |
2017年 | 555篇 |
2016年 | 1044篇 |
2015年 | 1758篇 |
2014年 | 1880篇 |
2013年 | 2456篇 |
2012年 | 2979篇 |
2011年 | 3057篇 |
2010年 | 1946篇 |
2009年 | 1657篇 |
2008年 | 2517篇 |
2007年 | 2508篇 |
2006年 | 2271篇 |
2005年 | 2339篇 |
2004年 | 2291篇 |
2003年 | 2181篇 |
2002年 | 2130篇 |
2001年 | 507篇 |
2000年 | 406篇 |
1999年 | 510篇 |
1998年 | 606篇 |
1997年 | 462篇 |
1996年 | 411篇 |
1995年 | 393篇 |
1994年 | 355篇 |
1993年 | 345篇 |
1992年 | 386篇 |
1991年 | 328篇 |
1990年 | 271篇 |
1989年 | 272篇 |
1988年 | 274篇 |
1987年 | 251篇 |
1986年 | 234篇 |
1985年 | 302篇 |
1984年 | 322篇 |
1983年 | 257篇 |
1982年 | 351篇 |
1981年 | 275篇 |
1980年 | 255篇 |
1979年 | 191篇 |
1978年 | 222篇 |
1977年 | 209篇 |
1976年 | 178篇 |
1974年 | 188篇 |
1973年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Hart LM Traxler GS Garver KA Richard J Gregg JL Grady CA Kurath G Hershberger PK 《Diseases of aquatic organisms》2011,93(2):105-110
Infectious hematopoietic necrosis (IHN) leads to periodic epidemics among certain wild and farmed fish species of the Northeast (NE) Pacific. The source of the IHN virus (IHNV) that initiates these outbreaks remains unknown; however, a leading hypothesis involves viral persistence in marine host species such as Pacific herring Clupea pallasii. Under laboratory conditions we exposed specific pathogen-free (SPF) larval and juvenile Pacific herring to 10(3) to 10(4) plaque-forming units (pfu) of IHNV ml(-1) by waterborne immersion. Cumulative mortalities among exposed groups were not significantly different from those of negative control groups. After waterborne exposure, IHNV was transiently recovered from the tissues of larvae but absent in tissues of juveniles. Additionally, no evidence of viral shedding was detected in the tank water containing exposed juveniles. After intraperitoneal (IP) injection of IHNV in juvenile herring with 10(3) pfu, IHNV was recovered from the tissues of sub-sampled individuals for only the first 5 d post-exposure. The lack of susceptibility to overt disease and transient levels of IHNV in the tissues of exposed fish indicate that Pacific herring do not likely serve a major epizootiological role in perpetuation of IHNV among free-ranging sockeye salmon Oncorhynchus nerka and farmed Atlantic salmon Salmo salar in the NE Pacific. 相似文献
982.
The mitochondrial DNA of Trypanosoma brucei is organized in a complex structure called the kinetoplast. In this study, we define the complete kinetoplast duplication cycle in T. brucei based on three-dimensional reconstructions from serial-section electron micrographs. This structural model was enhanced by analyses of the replication process of DNA maxi- and minicircles. Novel insights were obtained about the earliest and latest stages of kinetoplast duplication. We show that kinetoplast S phase occurs concurrently with the repositioning of the new basal body from the anterior to the posterior side of the old flagellum. This emphasizes the role of basal body segregation in kinetoplast division and suggests a possible mechanism for driving the rotational movement of the kinetoplast during minicircle replication. Fluorescence in situ hybridization with minicircle- and maxicircle-specific probes showed that maxicircle DNA is stretched out between segregated minicircle networks, indicating that maxicircle segregation is a late event in the kinetoplast duplication cycle. This new view of the complexities of kinetoplast duplication emphasizes the dependencies between the dynamic remodelling of the cytoskeleton and the inheritance of the mitochondrial genome. 相似文献
983.
Geoffrey S. Gottlieb Robert A. Smith Ndeye Mery Dia Badiane Selly Ba Stephen E. Hawes Macoumba Toure Alison K. Starling Fatou Traore Fatima Sall Stephen L. Cherne Joshua Stern Kim G. Wong Paul Lu Moon Kim Dana N. Raugi Airin Lam James I. Mullins Nancy B. Kiviat Papa Salif Sow for the UW-Dakar HIV- Study Group 《PloS one》2011,6(7)
Background
Antiretroviral therapy for HIV-2 infection is hampered by intrinsic resistance to many of the drugs used to treat HIV-1. Limited studies suggest that the integrase inhibitors (INIs) raltegravir and elvitegravir have potent activity against HIV-2 in culture and in infected patients. There is a paucity of data on genotypic variation in HIV-2 integrase that might confer intrinsic or transmitted INI resistance.Methods
We PCR amplified and analyzed 122 HIV-2 integrase consensus sequences from 39 HIV-2–infected, INI-naive adults in Senegal, West Africa. We assessed genetic variation and canonical mutations known to confer INI-resistance in HIV-1.Results
No amino acid-altering mutations were detected at sites known to be pivotal for INI resistance in HIV-1 (integrase positions 143, 148 and 155). Polymorphisms at several other HIV-1 INI resistance-associated sites were detected at positions 72, 95, 125, 154, 165, 201, 203, and 263 of the HIV-2 integrase protein.Conclusion
Emerging genotypic and phenotypic data suggest that HIV-2 is susceptible to the new class of HIV integrase inhibitors. We hypothesize that intrinsic HIV-2 integrase variation at “secondary” HIV-1 INI-resistance sites may affect the genetic barrier to HIV-2 INI resistance. Further studies will be needed to assess INI efficacy as part of combination antiretroviral therapy in HIV-2–infected patients. 相似文献984.
Black swallowtail females laid more eggs on plant models treated with contact stimulants and volatiles from carrot leaves than on models treated only with contact stimulants. The volatiles enhanced landing rates and females alighted more frequently on artificial leaves treated with host volatiles than on adjacent control leaves. Volatiles from cabbage, a nonhost, inhibited landing rates on artificial leaves treated with carrot contact stimulants. Examination of antennae revealed two major types of sensilla, believed to be olfactory in function. Electroantennogram preparations responded more strongly to carrot volatiles than to cabbage volatiles and several shared responses at particular retention times to carrot volatile components eluting from a gas chromatograph. Our results are consistent with a long-standing hypothesis that behavioral responses to essential oil components characteristic of the larval food plants have facilitated host shifts in the genus Papilio. 相似文献
985.
Paul J. Chuba Myron A. Leon Amit Banerjee Sunil Palchaudhuri 《Molecular & general genetics : MGG》1989,216(2-3):287-292
Summary
Escherichia coli K12 cells carrying a cloned 1.4 kb HindIII fragment from plasmid ColV2-K94, showed increased survival in guinea pig serum. The recombinant plasmid also conferred group II surface exclusion, i.e. the cells were reduced in recipient ability towards the incoming plasmid R538drd in conjugation experiments. Southern blotting suggested homology with bacteriophage lambda DNA and to the insertion element IS2. Determination of the DNA sequence of the fragment demonstrated the presence of a truncated IS2 (165 bp), separated by 250 bp from a 900 bp stretch of homology with lambda DNA, beginning within the Rz gene and continuing in the rightward direction on the lambda map. A 97 amino acid open reading frame (ORF) adjacent to Rz and on the opposite strand, remained intact in iss, with several amino acid changes. The ORF in iss is preceded by sequences resembling prokaryotic ribosome binding sites and promoters. 相似文献
986.
Ca2+-calmodulin-dependent protein kinase II (CaMKII) is a key regulator of glutamatergic synapses and plays an essential role
in many forms of synaptic plasticity. It has recently been observed that stimulating dendrites locally with a single glutamate/glycine
puff induces a local translocation of CaMKII into spines that subsequently spreads in a wave-like manner towards the distal
dendritic arbor. Here we present a mathematical model of the diffusion, activation and translocation of dendritic CaMKII.
We show how the nonlinear dynamics of CaMKII diffusion-activation generates a propagating translocation wave, provided that
the rate of activation is sufficiently fast. We also derive an explicit formula for the wave speed as a function of physiological
parameters such as the diffusivity of CaMKII and the density of spines. Our model provides a quantitative framework for understanding
the spread of CaMKII translocation and its possible role in heterosynaptic plasticity. 相似文献
987.
Water resource management encompasses a variety of regulations and mandates relevant to water protection and restoration. Awareness of the value-added biological monitoring and assessment to water resource management
is increasing worldwide, but especially in countries that have implemented proactive water law and regulatory frameworks for
protection of surface waters. Biological communities provide an integrated response to pollutants and human disturbance within
watersheds through their continuous exposure to the magnitude, duration, and frequency of stressors, and, thus, are important
for assessing ecosystem health. The selection of proper bioindicators can provide additional benefits through their use in
causal analysis of impaired waters and measurement of ecosystem recovery after restoration. A process for implementing biological
indicators in a monitoring and assessment framework is outlined for managers and practitioners of water resource protection
and restoration. 相似文献
988.
989.
Gravitt PE Paul P Katki HA Vendantham H Ramakrishna G Sudula M Kalpana B Ronnett BM Vijayaraghavan K Shah KV;CATCH Study Team 《PloS one》2010,5(10):e13711
Background
While many studies have compared the efficacy of Pap cytology, visual inspection with acetic acid (VIA) and human papillomavirus (HPV) DNA assays for the detection cervical intraepithelial neoplasia and cancer, few have evaluated the program effectiveness.Methods and Findings
A population-based sample of 5603 women from Medchal Mandal in Andhra Pradesh, India were invited to participate in a study comparing Pap cytology, VIA, and HPV DNA screening for the detection of CIN3+. Participation in primary screening and all subsequent follow-up visits was rigorously tracked. A 20% random sample of all women screened, in addition to all women with a positive screening test result underwent colposcopy with directed biopsy for final diagnosis. Sensitivity, specificity, positive and negative predictive values were adjusted for verification bias. HPV testing had a higher sensitivity (100%) and specificity (90.6%) compared to Pap cytology (sensitivity = 78.2%; specificity = 86.0%) and VIA (sensitivity = 31.6%; specificity = 87.5%). Since 58% of the sample refused involvement and another 28% refused colposcopy or biopsy, we estimated that potentially 87.6% of the total underlying cases of CIN3 and cancer may have been missed due to program failures.Conclusions
We conclude that despite our use of available resources, infrastructure, and guidelines for cervical cancer screening implementation in resource limited areas, community participation and non-compliance remain the major obstacles to successful reduction in cervical cancer mortality in this Indian population. HPV DNA testing was both more sensitive and specific than Pap cytology and VIA. The use of a less invasive and more user-friendly primary screening strategy (such as self-collected swabs for HPV DNA testing) may be required to achieve the coverage necessary for effective reduction in cervical cancer mortality. 相似文献990.