全文获取类型
收费全文 | 152篇 |
免费 | 9篇 |
专业分类
161篇 |
出版年
2021年 | 1篇 |
2019年 | 1篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 9篇 |
2014年 | 6篇 |
2013年 | 2篇 |
2012年 | 3篇 |
2011年 | 5篇 |
2010年 | 4篇 |
2009年 | 4篇 |
2008年 | 2篇 |
2007年 | 2篇 |
2006年 | 2篇 |
2005年 | 1篇 |
2004年 | 3篇 |
2002年 | 2篇 |
2001年 | 4篇 |
1999年 | 3篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1990年 | 5篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有161条查询结果,搜索用时 15 毫秒
101.
Keyur Doolabh Lucius Caviola Julian Savulescu Michael Selgelid Dominic JC Wilkinson 《Developing world bioethics》2017,17(3):173-204
The 2016 outbreak of the Zika arbovirus was associated with large numbers of cases of the newly‐recognised Congenital Zika Syndrome (CZS). This novel teratogenic epidemic raises significant ethical and practical issues. Many of these arise from strategies used to avoid cases of CZS, with contraception in particular being one proposed strategy that is atypical in epidemic control. Using contraception to reduce the burden of CZS has an ethical complication: interventions that impact the timing of conception alter which people will exist in the future. This so‐called ‘non‐identity problem’ potentially has significant social justice implications for evaluating contraception, that may affect our prioritisation of interventions to tackle Zika. This paper combines ethical analysis of the non‐identity problem with empirical data from a novel survey about the general public's moral intuitions. The ethical analysis examines different perspectives on the non‐identity problem, and their implications for using contraception in response to Zika. The empirical section reports the results of an online survey of 93 members of the US general public exploring their intuitions about the non‐identity problem in the context of the Zika epidemic. Respondents indicated a general preference for a person‐affecting intervention (mosquito control) over an impersonal intervention (contraception). However, their responses did not appear to be strongly influenced by the non‐identity problem. Despite its potential philosophical significance, we conclude from both theoretical considerations and analysis of the attitudes of the community that the non‐identity problem should not affect how we prioritise contraception relative to other interventions to avoid CZS. 相似文献
102.
103.
Molecular phylogeny of the major arthropod groups indicates polyphyly of crustaceans and a new hypothesis for the origin of hexapods 总被引:12,自引:6,他引:12
A phylogeny of the arthropods was inferred from analyses of amino acid
sequences derived from the nuclear genes encoding elongation factor-1 alpha
and the largest subunit of RNA polymerase II using maximum- parsimony,
neighbor-joining, and maximum-likelihood methods. Analyses of elongation
factor-1 alpha from 17 arthropods and 4 outgroup taxa recovered many
arthropod clades supported by previous morphological studies, including
Diplopoda, Myriapoda, Insecta, Hexapoda, Branchiopoda (Crustacea), Araneae,
Tetrapulmonata, Arachnida, Chelicerata, and Malacostraca (Crustacea).
However, counter to previous studies, elongation factor-1 alpha placed
Malacostraca as sister group to the other arthropods. Branchiopod
crustaceans were found to be more closely related to hexapods and myriapods
than to malacostracan crustaceans. Sequences for RNA polymerase II were
obtained from 11 arthropod taxa and were analyzed separately and in
combination with elongation factor-1 alpha. Results from these analyses
were concordant with those derived from elongation factor-1 alpha alone and
provided support for a Hexapoda/Branchiopoda clade, thus arguing against
the monophyly of the traditionally defined Atelocerata (Hexapoda +
Myriapoda).
相似文献
104.
Gutierrez A del Rio JC Martinez MJ Martinez AT 《Applied and environmental microbiology》1999,65(4):1367-1371
Solid-state fermentation of eucalypt wood with several fungal strains was investigated as a possible biological pretreatment for decreasing the content of compounds responsible for pitch deposition during Cl2-free manufacture of paper pulp. First, different pitch deposits were characterized by gas chromatography (GC) and GC-mass spectrometry (MS). The chemical species identified arose from lipophilic wood extractives that survived the pulping and bleaching processes. Second, a detailed GC-MS analysis of the lipophilic fraction after fungal treatment of wood was carried out, and different degradation patterns were observed. The results showed that some basidiomycetes that decreased the lipophilic fraction also released significant amounts of polar extractives, which were identified by thermochemolysis as originating from lignin depolymerization. Therefore, the abilities of fungi to control pitch should be evaluated after analysis of compounds involved in deposit formation and not simply by estimating the decrease in the total extractive content. In this way, Phlebia radiata, Funalia trogii, Bjerkandera adusta, and Poria subvermispora strains were identified as the most promising organisms for pitch biocontrol, since they degraded 75 to 100% of both free and esterified sterols, as well as other lipophilic components of the eucalypt wood extractives. Ophiostoma piliferum, a fungus used commercially for pitch control, hydrolyzed the sterol esters and triglycerides, but it did not appear to be suitable for eucalypt wood treatment because it increased the content of free sitosterol, a major compound in pitch deposits. 相似文献
105.
Norbert Meyer Jan W Dallinga Sarah Janine Nuss Edwin JC Moonen Joep JBN van Berkel Cezmi Akdis Frederik Jan van Schooten Günter Menz 《Respiratory research》2014,15(1)
Background
Several classifications of adult asthma patients using cluster analyses based on clinical and demographic information has resulted in clinical phenotypic clusters that do not address molecular mechanisms. Volatile organic compounds (VOC) in exhaled air are released during inflammation in response to oxidative stress as a result of activated leukocytes. VOC profiles in exhaled air could distinguish between asthma patients and healthy subjects. In this study, we aimed to classify new asthma endotypes by combining inflammatory mechanisms investigated by VOC profiles in exhaled air and clinical information of asthma patients.Methods
Breath samples were analyzed for VOC profiles by gas chromatography–mass spectrometry from asthma patients (n = 195) and healthy controls (n = 40). A total of 945 determined compounds were subjected to discriminant analysis to find those that could discriminate healthy from asthmatic subjects. 2-step cluster analysis based on clinical information and VOCs in exhaled air were used to form asthma endotypes.Results
We identified 16 VOCs, which could distinguish between healthy and asthma subjects with a sensitivity of 100% and a specificity of 91.1%. Cluster analysis based on VOCs in exhaled air and the clinical parameters FEV1, FEV1 change after 3 weeks of hospitalization, allergic sensitization, Junipers symptoms score and asthma medications resulted in the formation of 7 different asthma endotype clusters. We identified asthma clusters with different VOC profiles but similar clinical characteristics and endotypes with similar VOC profiles, but distinct clinical characteristics.Conclusion
This study demonstrates that both, clinical presentation of asthma and inflammatory mechanisms in the airways should be considered for classification of asthma subtypes.Electronic supplementary material
The online version of this article (doi:10.1186/s12931-014-0136-8) contains supplementary material, which is available to authorized users. 相似文献106.
The nucleotide sequences of the cow epsilon 2 and epsilon 4 globin genes
were determined. The sequences were 95% identical. These genes arose via a
four-gene block duplication that also gave rise to the bovine fetal (gamma)
and adult (beta) genes. Their deduced amino acid sequences are unlike any
previously reported fetal or adult globins; rather, comparison to other
mammalian globin genes indicates that they are embryonic in nature. The
sequence data indicate that these two genes have converted each other
during evolution. Pairwise comparison to the corresponding goat genes shows
greater similarity between paralogues than between more directly related
orthologues. This is in direct contrast to the situation between the cow
and goat fetal and adult genes. These observations suggest that the
frequency of DNA conversion or the fixation of conversion events may vary
in different locations of the cow beta-globin cluster.
相似文献
107.
Statistical methods of DNA sequence analysis: detection of intragenic recombination or gene conversion 总被引:32,自引:4,他引:28
Simple but exact statistical tests for detecting a cluster of associated
nucleotide changes in DNA are presented. The tests are based on the linear
distribution of a set of s sites among a total of n sites, where the s
sites may be the variable sites, sites of insertion/deletion, or
categorized in some other way. These tests are especially useful for
detecting gene conversion and intragenic recombination in a sample of DNA
sequences. In this case, the sites of interest are those that correspond to
particular ways of splitting the sequences into two groups (e.g., sequences
A and D vs. sequences B, C, and E-J). Each such split is termed a
phylogenetic partition. Application of these methods to a well-documented
case of gene conversion in human gamma-globin genes shows that sites
corresponding to two of the three observed partitions are significantly
clustered, whereas application to hominoid mitochondrial DNA
sequences--among which no recombination is expected to occur--shows no
evidence of such clustering. This indicates that clustering of
partition-specific sites is largely due to intragenic recombination or gene
conversion. Alternative hypotheses explaining the observed clustering of
sites, such as biased selection or mutation, are discussed.
相似文献
108.
Human responses to propionic acid. I. Quantification of within- and between-participant variation in perception by normosmics and anosmics 总被引:4,自引:3,他引:1
The objective of this study was to fully characterize normosmic perception
of stimuli expected to cause widely varying degrees of olfactory and nasal
trigeminal stimulation and to directly evaluate the possible role of
olfactory nerve stimulation in nasal irritation sensitivity. During each of
four identical test sessions, four anosmic and 31 normosmic participants
were presented with a range of concentrations extending from peri-threshold
for normosmics to supra- threshold for anosmics. For each session, odor (O)
and nasal irritation (NI) sensitivities were summarized in terms of the
concentrations required to produce four sensation levels ('iso-response'
concentrations). Within-participant variation in these iso-response
concentrations was < 10-fold for 95% of normosmics, for both O and NI.
For O but not NI, these apparent fluctuations in sensitivity were largely
accounted for by the uncertainty surrounding the iso-response
concentrations calculated for each session. Anosmics exhibited minimal
within- and between-participant variation in NI and required, for all but
the highest perceptual level, a higher concentration than almost all
normosmics. Between-participant variation, expressed in terms of 90%
confidence interval widths, was approximately 0.5 log units for both O and
NI for the highest perceptual level, but increased to approximately 0.8 and
1.8 log units, respectively, for the lowest (peri- threshold) level. Our
findings suggest that: (i) most apparent variation over time in O
sensitivity is actually a reflection of the uncertainty surrounding
estimates of sensitivity obtained for each session; (ii) within- and
between-participant variation in O sensitivity is far less than is commonly
reported; and (iii) low to moderate levels of NI in normosmics are the
result of relatively weak trigeminal stimulation combined with much greater
olfactory activation.
相似文献
109.
Regier JC; Fang QQ; Mitter C; Peigler RS; Friedlander TP; Solis MA 《Molecular biology and evolution》1998,15(9):1172-1182
Evolution and phylogenetic utility of the period gene are explored through
sequence analysis of a relatively conserved 909-bp fragment in 26
lepidopteran species. Taxa range from tribes to superfamilies, primarily
within the putative clade Macrolepidotera plus near outgroups, and include
both strongly established and problematic groupings. Their divergence dates
probably range from the late Cretaceous through much of the Tertiary.
Comparisons within the same set of closely related species show that amino
acid substitutions in period occur 4.9 and 44 times as frequently as they
do in two other nuclear genes--dopa decarboxylase and elongation factor-1
alpha, respectively. In contrast, rates of observed synonymous substitution
are within 60% of each other for these three genes. Synonymous changes in
period approach saturation by the family level, whereas nonsynonymous and
amino acid divergences across the Macrolepidoptera are less than half the
maximal values reported for this gene. Phylogenetic analyses of period
strongly supported groupings at the family level and below. In contrast to
previous analyses at this level with other nuclear genes, much of the
information lies in nonsynonymous change. Relationships up to the
superfamily level were recovered with decreasing effectiveness, and little,
if any, signal was apparent regarding relationships among superfamilies.
This could reflect rapid radiation of the superfamilies, however, rather
than saturation in the period locus; thus, period, in combination with
other genes, remains a plausible candidate for approaching the difficult
problems of lepidopteran family and superfamily relationships.
相似文献
110.
Multiple haplotypes from each of three nuclear loci were isolated and
sequenced from geographic populations of the American oyster, Crassostrea
virginica. In tests of alternative phylogeographic hypotheses for this
species, nuclear gene genealogies constructed for these haplotypes were
compared to one another, to a mitochondrial gene tree, and to patterns of
allele frequency variation in nuclear restriction site polymorphisms
(RFLPs) and allozymes. Oyster populations from the Atlantic versus the Gulf
of Mexico are not reciprocally monophyletic in any of the nuclear gene
trees, despite considerable genetic variation and despite large allele
frequency differences previously reported in several other genetic assays.
If these populations were separated vicariantly in the past, either
insufficient time has elapsed for neutral lineage sorting to have achieved
monophyly at most nuclear loci, or balancing selection may have inhibited
lineage extinction, or secondary gene flow may have moved haplotypes
between regions. These and other possibilities are examined in light of
available genetic evidence, and it is concluded that no simple explanation
can account for the great variety of population genetic patterns across
loci displayed by American oysters. Regardless of the source of this
heterogeneity, this study provides an empirical demonstration that
different sequences of DNA within the same organismal pedigree can have
quite different phylogeographic histories.
相似文献