全文获取类型
收费全文 | 386篇 |
免费 | 63篇 |
专业分类
449篇 |
出版年
2023年 | 2篇 |
2021年 | 9篇 |
2020年 | 5篇 |
2019年 | 11篇 |
2018年 | 9篇 |
2017年 | 6篇 |
2016年 | 14篇 |
2015年 | 17篇 |
2014年 | 19篇 |
2013年 | 23篇 |
2012年 | 29篇 |
2011年 | 17篇 |
2010年 | 15篇 |
2009年 | 18篇 |
2008年 | 19篇 |
2007年 | 14篇 |
2006年 | 11篇 |
2005年 | 17篇 |
2004年 | 9篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2001年 | 14篇 |
2000年 | 16篇 |
1999年 | 7篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 8篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1972年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有449条查询结果,搜索用时 15 毫秒
71.
Phylogenetic analyses of chloroplast DNA sequences, morphology, and combined data have provided consistent support for many of the major branches within the angiosperm clade Dipsacales. Here we use sequences from three mitochondrial loci to test the existing broad scale phylogeny and in an attempt to resolve several relationships that have remained uncertain. Parsimony, maximum likelihood, and Bayesian analyses of a combined mitochondrial data set recover trees broadly consistent with previous studies, although resolution and support are lower than in the largest chloroplast analyses. Combining chloroplast and mitochondrial data results in a generally well-resolved and very strongly supported topology but the previously recognized problem areas remain. To investigate why these relationships have been difficult to resolve we conducted a series of experiments using different data partitions and heterogeneous substitution models. Usually more complex modeling schemes are favored regardless of the partitions recognized but model choice had little effect on topology or support values. In contrast there are consistent but weakly supported differences in the topologies recovered from coding and non-coding matrices. These conflicts directly correspond to relationships that were poorly resolved in analyses of the full combined chloroplast-mitochondrial data set. We suggest incongruent signal has contributed to our inability to confidently resolve these problem areas. 相似文献
72.
E. Carvalho C. Sindt A. Verdier C. Galan L. O’Donoghue S. Parks M. Thibaudon 《Aerobiologia》2008,24(4):191-201
The Coriolis δ air sampler manufactured by Bertin Technologies (France) is a continuous air sampler, dedicated to outdoor monitoring of
airborne spores and pollen grains. This high-volume sampler is based on patented Coriolis technology delivering a liquid sample.
The air is drawn into a conical vial in a whirling type motion using suction; particles are pulled against the wall by centrifugal
force. Airborne particles are separated from the air and collected in a liquid medium. This innovative solution allows rapid
analysis by several techniques including PCR assay and serological assay in order to measure the antigenicity/allergenicity
of pollen grains and fungal spores. Also, traditional counting of pollen grains or taxa identification by optical microscopy
can be done. A study has been carried out by the Health Protection Agency (HPA), Porton Down, UK, to measure the physical
efficiency of the Coriolis air sampler. The physical efficiency of the sampler for collection of micro-organism-laden particles
of various sizes has been compared with that of membrane filter samplers using the techniques described by ISO 14698-1. The
Coriolis was operated simultaneously with membrane filter samplers in a controlled room where they were challenged with uniform-sized
particles of different diameters containing bacterial spores. For the larger particle sizes, it was found that the physical
efficiency of the Coriolis was 92% for 10-μm particles. The biological performance of the Coriolis in the collection of airborne
fungal spores and pollen grains was evaluated in comparison with a Hirst spore trap (one-week tape-on-drum type sampler) which
is one of the most frequently used traps in the measurement of outdoor pollen grain concentrations. The advantages and limitations
of both technologies are discussed. The Coriolis was operated simultaneously with a Hirst spore trap in the sampling station
of Réseau National de Surveillance Aérobiologique, France (RNSA); the pollen grain and fungal spore counts were analysed by
optical microscopy. The pollen grain count m−3 collected was compared for both devices. The dispersion values were obtained and statistical analysis was carried out. This
study shows that the Coriolis air sampler provided equivalent recovery of pollen grain and fungal spores compared with the
volumetric trap standard method (not significantly different, W test, α = 0.05). Nowadays, the French-led project, acronym MONALISA, with financial support from the European Commission––Life-Environment
(LIFE05 ENV/F/000068), is testing this innovative air sampler in order to measure the antigenicity/allergenicity of the main
aeroallergen particles, i.e. Betula (birch), Poaceae (grasses), Parietaria (pellitory), Olea spp (olive tree), and Artemisia (mugwort) pollen grains, and Alternaria (fungal spores) to validate a new approach of monitoring instead of quantifying pollen grains by their morphology. The robustness
and efficiency of the MONALISA system is being demonstrated at a national level throughout Europe in eight different countries
with different bio-climatic and topography characteristics: France, UK, Finland, Poland, Spain, Portugal, Switzerland, and
Italy. 相似文献
73.
Erin M. O’Donoghue Sheryl D. Somerfield Lyn M. Watson David A. Brummell Donald A. Hunter 《Planta》2009,229(3):709-721
Galactose was the major non-cellulosic neutral sugar present in the cell walls of ‘Mitchell’ petunia (Petunia axillaris × P. axillaris × P. hybrida) flower petals. Over the 24 h period associated with flower opening, there was a doubling of the galactose content of polymers
strongly associated with cellulose and insoluble in strong alkali (‘residual’ fraction). By two days after flower opening,
the galactose content of both the residual fraction and a Na2CO3-soluble pectin-rich cell wall fraction had sharply decreased, and continued to decline as flowers began to wilt. In contrast,
amounts of other neutral sugars showed little change over this time, and depolymerisation of pectins and hemicelluloses was
barely detectable throughout petal development. Size exclusion chromatography of Na2CO3-soluble pectins showed that there was a loss of neutral sugar relative to uronic acid content, consistent with a substantial
loss of galactose from rhamnogalacturonan-I-type pectin. β-Galactosidase activity (EC 3.2.1.23) increased at bud opening,
and remained high through to petal senescence. Two cDNAs encoding β-galactosidase were isolated from a mixed stage petal library.
Both deduced proteins are β-galactosidases of Glycosyl Hydrolase Family 35, possessing lectin-like sugar-binding domains at
their carboxyl terminus. PhBGAL1 was expressed at relatively high levels only during flower opening, while PhBGAL2 mRNA accumulation occurred at lower levels in mature and senescent petals. The data suggest that metabolism of cell wall-associated
polymeric galactose is the major feature of both the opening and senescence of ‘Mitchell’ petunia flower petals. 相似文献
74.
Background
Most studies examining the commensal human oral microbiome are focused on disease or are limited in methodology. In order to diagnose and treat diseases at an early and reversible stage an in-depth definition of health is indispensible. The aim of this study therefore was to define the healthy oral microbiome using recent advances in sequencing technology (454 pyrosequencing). 相似文献75.
Background
The past several years have seen a flurry of papers seeking to clarify the utility and limits of DNA barcoding, particularly in areas such as species discovery and paralogy due to nuclear pseudogenes. Heteroplasmy, the coexistence of multiple mitochondrial haplotypes in a single organism, has been cited as a potentially serious problem for DNA barcoding but its effect on identification accuracy has not been tested. In addition, few studies of barcoding have tested a large group of closely-related species with a well-established morphological taxonomy. In this study we examine both of these issues, by densely sampling the Hawaiian Hylaeus bee radiation. 相似文献76.
Donoghue HD Spigelman M 《Proceedings. Biological sciences / The Royal Society》2006,273(1587):641-2; discussion 643
77.
Debate over the origin and evolution of vertebrates has occupied biologists and palaeontologists alike for centuries. This debate has been refined by molecular phylogenetics, which has resolved the place of vertebrates among their invertebrate chordate relatives, and that of chordates among their deuterostome relatives. The origin of vertebrates is characterized by wide‐ranging genomic, embryologic and phenotypic evolutionary change. Analyses based on living lineages suggest dramatic shifts in the tempo of evolutionary change at the origin of vertebrates and gnathostomes, coincident with whole‐genome duplication events. However, the enriched perspective provided by the fossil record demonstrates that these apparent bursts of anatomical evolution and taxic richness are an artefact of the extinction of phylogenetic intermediates whose fossil remains evidence the gradual assembly of crown gnathostome characters in particular. A more refined understanding of the timing, tempo and mode of early vertebrate evolution rests with: (1) better genome assemblies for living cyclostomes; (2) a better understanding of the anatomical characteristics of key fossil groups, especially the anaspids, thelodonts, galeaspids and pituriaspids; (3) tests of the monophyly of traditional groups; and (4) the application of divergence time methods that integrate not just molecular data from living species, but also morphological data and extinct species. The resulting framework will provide for rigorous tests of rates of character evolution and diversification, and of hypotheses of long‐term trends in ecological evolution that themselves suffer for lack of quantitative functional tests. The fossil record has been silent on the nature of the transition from jawless vertebrates to the jawed vertebrates that have dominated communities since the middle Palaeozoic. Elucidation of this most formative of episodes likely rests with the overhaul of early vertebrate systematics that we propose, but perhaps more fundamentally with fossil grades that await discovery. 相似文献
78.
Soltis DE Smith SA Cellinese N Wurdack KJ Tank DC Brockington SF Refulio-Rodriguez NF Walker JB Moore MJ Carlsward BS Bell CD Latvis M Crawley S Black C Diouf D Xi Z Rushworth CA Gitzendanner MA Sytsma KJ Qiu YL Hilu KW Davis CC Sanderson MJ Beaman RS Olmstead RG Judd WS Donoghue MJ Soltis PS 《American journal of botany》2011,98(4):704-730
79.
Benjamin M Akiyama Monica E Graham Zoe ODonoghue J
David Beckham Jeffrey
S Kieft 《Nucleic acids research》2021,49(12):7122
Mosquito-borne flaviviruses (MBFVs) including dengue, West Nile, yellow fever, and Zika viruses have an RNA genome encoding one open reading frame flanked by 5′ and 3′ untranslated regions (UTRs). The 3′ UTRs of MBFVs contain regions of high sequence conservation in structured RNA elements known as dumbbells (DBs). DBs regulate translation and replication of the viral RNA genome, functions proposed to depend on the formation of an RNA pseudoknot. To understand how DB structure provides this function, we solved the x-ray crystal structure of the Donggang virus DB to 2.1Å resolution and used structural modeling to reveal the details of its three-dimensional fold. The structure confirmed the predicted pseudoknot and molecular modeling revealed how conserved sequences form a four-way junction that appears to stabilize the pseudoknot. Single-molecule FRET suggests that the DB pseudoknot is a stable element that can regulate the switch between translation and replication during the viral lifecycle by modulating long-range RNA conformational changes. 相似文献