全文获取类型
收费全文 | 1067篇 |
免费 | 191篇 |
出版年
2021年 | 11篇 |
2019年 | 9篇 |
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 20篇 |
2015年 | 34篇 |
2014年 | 33篇 |
2013年 | 36篇 |
2012年 | 46篇 |
2011年 | 53篇 |
2010年 | 32篇 |
2009年 | 39篇 |
2008年 | 33篇 |
2007年 | 43篇 |
2006年 | 58篇 |
2005年 | 39篇 |
2004年 | 41篇 |
2003年 | 48篇 |
2002年 | 35篇 |
2001年 | 28篇 |
2000年 | 37篇 |
1999年 | 22篇 |
1998年 | 23篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 10篇 |
1994年 | 14篇 |
1992年 | 24篇 |
1991年 | 28篇 |
1990年 | 20篇 |
1989年 | 14篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 19篇 |
1985年 | 22篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 22篇 |
1977年 | 14篇 |
1976年 | 11篇 |
1974年 | 12篇 |
1973年 | 25篇 |
1972年 | 18篇 |
1971年 | 8篇 |
1970年 | 8篇 |
1969年 | 10篇 |
1968年 | 11篇 |
排序方式: 共有1258条查询结果,搜索用时 15 毫秒
41.
William D. Pearse Mark W. Chase Michael J. Crawley Konrad Dolphin Michael F. Fay Jeffrey A. Joseph Gary Powney Chris D. Preston Giovanni Rapacciuolo David B. Roy Andy Purvis 《PloS one》2015,10(5)
Conservation biologists have only finite resources, and so must prioritise some species over others. The EDGE-listing approach ranks species according to their combined evolutionary distinctiveness and degree of threat, but ignores the uncertainty surrounding both threat and evolutionary distinctiveness. We develop a new family of measures for species, which we name EDAM, that incorporates evolutionary distinctiveness, the magnitude of decline, and the accuracy with which decline can be predicted. Further, we show how the method can be extended to explore phyogenetic uncertainty. Using the vascular plants of Britain as a case study, we find that the various EDAM measures emphasise different species and parts of Britain, and that phylogenetic uncertainty can strongly affect the prioritisation scores of some species. 相似文献
42.
Habitat fragmentation in coastal southern California disrupts genetic connectivity in the cactus wren (Campylorhynchus brunneicapillus) 下载免费PDF全文
Kelly R. Barr Barbara E. Kus Kristine L. Preston Scarlett Howell Emily Perkins Amy G. Vandergast 《Molecular ecology》2015,24(10):2349-2363
Achieving long‐term persistence of species in urbanized landscapes requires characterizing population genetic structure to understand and manage the effects of anthropogenic disturbance on connectivity. Urbanization over the past century in coastal southern California has caused both precipitous loss of coastal sage scrub habitat and declines in populations of the cactus wren (Campylorhynchus brunneicapillus). Using 22 microsatellite loci, we found that remnant cactus wren aggregations in coastal southern California comprised 20 populations based on strict exact tests for population differentiation, and 12 genetic clusters with hierarchical Bayesian clustering analyses. Genetic structure patterns largely mirrored underlying habitat availability, with cluster and population boundaries coinciding with fragmentation caused primarily by urbanization. Using a habitat model we developed, we detected stronger associations between habitat‐based distances and genetic distances than Euclidean geographic distance. Within populations, we detected a positive association between available local habitat and allelic richness and a negative association with relatedness. Isolation‐by‐distance patterns varied over the study area, which we attribute to temporal differences in anthropogenic landscape development. We also found that genetic bottleneck signals were associated with wildfire frequency. These results indicate that habitat fragmentation and alterations have reduced genetic connectivity and diversity of cactus wren populations in coastal southern California. Management efforts focused on improving connectivity among remaining populations may help to ensure population persistence. 相似文献
43.
Michelle W. Clark Anna M. Yie Elizabeth K. Eder Richard G. Dennis Preston J. Basting Keith A. Martinez II Brian D. Jones Joan L. Slonczewski 《PloS one》2015,10(12)
Under certain kinds of cytoplasmic stress, Escherichia coli selectively reproduce by distributing the newer cytoplasmic components to new-pole cells while sequestering older, damaged components in cells inheriting the old pole. This phenomenon is termed polar aging or cell division asymmetry. It is unknown whether cell division asymmetry can arise from a periplasmic stress, such as the stress of extracellular acid, which is mediated by the periplasm. We tested the effect of periplasmic acid stress on growth and division of adherent single cells. We tracked individual cell lineages over five or more generations, using fluorescence microscopy with ratiometric pHluorin to measure cytoplasmic pH. Adherent colonies were perfused continually with LBK medium buffered at pH 6.00 or at pH 7.50; the external pH determines periplasmic pH. In each experiment, cell lineages were mapped to correlate division time, pole age and cell generation number. In colonies perfused at pH 6.0, the cells inheriting the oldest pole divided significantly more slowly than the cells inheriting the newest pole. In colonies perfused at pH 7.50 (near or above cytoplasmic pH), no significant cell division asymmetry was observed. Under both conditions (periplasmic pH 6.0 or pH 7.5) the cells maintained cytoplasmic pH values at 7.2–7.3. No evidence of cytoplasmic protein aggregation was seen. Thus, periplasmic acid stress leads to cell division asymmetry with minimal cytoplasmic stress. 相似文献
44.
Detection of Borrelia burgdorferi and Anaplasma phagocytophilum in the black‐legged tick,Ixodes scapularis,within southwestern Pennsylvania 下载免费PDF全文
Prevalence studies of Borrelia burgdorferi and Anaplasma phagocytophilum have been rare for ticks from southwestern Pennsylvania. We collected 325 Ixodes scapularis ticks between 2011 and 2012 from four counties in southwestern Pennsylvania. We tested for the presence of Borrelia burgdorferi and Anaplasma phagocytophilum using PCR. Of the ticks collected from Pennsylvania, B. burgdorferi (causative agent of Lyme disease) was present in 114/325 (35%) and Anaplasma phagocytophilum (causative agent of Human Granulocytic Anaplasmosis) was present in 48/325 (15%) as determined by PCR analysis. 相似文献
45.
46.
Single cell tuning of Myc expression by antigen receptor signal strength and interleukin‐2 in T lymphocytes 下载免费PDF全文
47.
Kaya Forest Peter Wan Caroline M Preston 《Photochemical & photobiological sciences》2004,3(5):463-472
The photochemistry of several model plant-derived compounds has been studied in aqueous solution. In particular, the reactions of catechin as a model tannin and methoxy-substituted hydroxybenzhydrols as model lignin functionalities were investigated. Tannins and lignins constitute a significant portion of the humic substances in aquatic systems, which are themselves the main component of dissolved organic matter thought to be responsible for the absorption and attenuation of light in these environments. Catechin (1) was found to undergo a reversible photoisomerization reaction to give epicatechin (2). Such a reaction is an explicit example of a photon absorbing process that enables catechin (1) and its derivatives to act as natural sunscreens by attenuating light energy through non-destructive reactions. The methoxy-substituted hydroxybenzhydrols were found to undergo photosolvolysis reactions via efficient generation of quinone methide intermediates. The intermediate quinone methides were observed to be longer lived, and thus more stable, than previously studied hydroxybenzhydrol derivatives. The meta-hydroxybenzhydrol isomer (5) was found to undergo additional chemistry leading to the production of a ring-closed fluorene from the quinone methide intermediate. 相似文献
48.
Dan E. Robertson Jennifer A. Chaplin Grace DeSantis Mircea Podar Mark Madden Ellen Chi Toby Richardson Aileen Milan Mark Miller David P. Weiner Kelvin Wong Jeff McQuaid Bob Farwell Lori A. Preston Xuqiu Tan Marjory A. Snead Martin Keller Eric Mathur Patricia L. Kretz Mark J. Burk Jay M. Short 《Applied microbiology》2004,70(4):2429-2436
Nitrilases are important in the biosphere as participants in synthesis and degradation pathways for naturally occurring, as well as xenobiotically derived, nitriles. Because of their inherent enantioselectivity, nitrilases are also attractive as mild, selective catalysts for setting chiral centers in fine chemical synthesis. Unfortunately, <20 nitrilases have been reported in the scientific and patent literature, and because of stability or specificity shortcomings, their utility has been largely unrealized. In this study, 137 unique nitrilases, discovered from screening of >600 biotope-specific environmental DNA (eDNA) libraries, were characterized. Using culture-independent means, phylogenetically diverse genomes were captured from entire biotopes, and their genes were expressed heterologously in a common cloning host. Nitrilase genes were targeted in a selection-based expression assay of clonal populations numbering 106 to 1010 members per eDNA library. A phylogenetic analysis of the novel sequences discovered revealed the presence of at least five major sequence clades within the nitrilase subfamily. Using three nitrile substrates targeted for their potential in chiral pharmaceutical synthesis, the enzymes were characterized for substrate specificity and stereospecificity. A number of important correlations were found between sequence clades and the selective properties of these nitrilases. These enzymes, discovered using a high-throughput, culture-independent method, provide a catalytic toolbox for enantiospecific synthesis of a variety of carboxylic acid derivatives, as well as an intriguing library for evolutionary and structural analyses. 相似文献
49.
Single‐cell genomics based on Raman sorting reveals novel carotenoid‐containing bacteria in the Red Sea 下载免费PDF全文
Yizhi Song Anne‐Kristin Kaster John Vollmers Yanqing Song Paul A. Davison Martinique Frentrup Gail M. Preston Ian P. Thompson J. Colin Murrell Huabing Yin C. Neil Hunter Wei E. Huang 《Microbial biotechnology》2017,10(1):125-137
Cell sorting coupled with single‐cell genomics is a powerful tool to circumvent cultivation of microorganisms and reveal microbial ‘dark matter’. Single‐cell Raman spectra (SCRSs) are label‐free biochemical ‘fingerprints’ of individual cells, which can link the sorted cells to their phenotypic information and ecological functions. We employed a novel Raman‐activated cell ejection (RACE) approach to sort single bacterial cells from a water sample in the Red Sea based on SCRS. Carotenoids are highly diverse pigments and play an important role in phototrophic bacteria, giving strong and distinctive Raman spectra. Here, we showed that individual carotenoid‐containing cells from a Red Sea sample were isolated based on the characteristic SCRS. RACE‐based single‐cell genomics revealed putative novel functional genes related to carotenoid and isoprenoid biosynthesis, as well as previously unknown phototrophic microorganisms including an unculturable Cyanobacteria spp. The potential of Raman sorting coupled to single‐cell genomics has been demonstrated. 相似文献
50.
Jeremiah D. Degenhardt Paola de Candia Adrien Chabot Stuart Schwartz Les Henderson Binhua Ling Meredith Hunter Zhaoshi Jiang Robert E. Palermo Michael Katze Evan E. Eichler Mario Ventura Jeffrey Rogers Preston Marx Yoav Gilad Carlos D. Bustamante 《PLoS genetics》2009,5(1)
Variation in genes underlying host immunity can lead to marked differences in susceptibility to HIV infection among humans. Despite heavy reliance on non-human primates as models for HIV/AIDS, little is known about which host factors are shared and which are unique to a given primate lineage. Here, we investigate whether copy number variation (CNV) at CCL3-like genes (CCL3L), a key genetic host factor for HIV/AIDS susceptibility and cell-mediated immune response in humans, is also a determinant of time until onset of simian-AIDS in rhesus macaques. Using a retrospective study of 57 rhesus macaques experimentally infected with SIVmac, we find that CCL3L CNV explains approximately 18% of the variance in time to simian-AIDS (p<0.001) with lower CCL3L copy number associating with more rapid disease course. We also find that CCL3L copy number varies significantly (p<10−6) among rhesus subpopulations, with Indian-origin macaques having, on average, half as many CCL3L gene copies as Chinese-origin macaques. Lastly, we confirm that CCL3L shows variable copy number in humans and chimpanzees and report on CCL3L CNV within and among three additional primate species. On the basis of our findings we suggest that (1) the difference in population level copy number may explain previously reported observations of longer post-infection survivorship of Chinese-origin rhesus macaques, (2) stratification by CCL3L copy number in rhesus SIV vaccine trials will increase power and reduce noise due to non-vaccine-related differences in survival, and (3) CCL3L CNV is an ancestral component of the primate immune response and, therefore, copy number variation has not been driven by HIV or SIV per se. 相似文献