全文获取类型
收费全文 | 129篇 |
免费 | 1篇 |
出版年
2017年 | 1篇 |
2016年 | 1篇 |
2015年 | 2篇 |
2014年 | 4篇 |
2013年 | 8篇 |
2012年 | 10篇 |
2011年 | 3篇 |
2010年 | 4篇 |
2009年 | 7篇 |
2008年 | 7篇 |
2007年 | 2篇 |
2006年 | 2篇 |
2005年 | 5篇 |
2004年 | 4篇 |
2003年 | 2篇 |
2001年 | 1篇 |
1999年 | 10篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1971年 | 2篇 |
1959年 | 1篇 |
1951年 | 1篇 |
1950年 | 1篇 |
1930年 | 1篇 |
排序方式: 共有130条查询结果,搜索用时 15 毫秒
101.
Fellow travellers: a concordance of colonization patterns between mice and men in the North Atlantic region 总被引:1,自引:0,他引:1
EP Jones K Skirnisson TH McGovern MTP Gilbert E Willerslev JB Searle 《BMC evolutionary biology》2012,12(1):1-8
Background
In the Calvin cycle of eubacteria, the dephosphorylations of both fructose-1, 6-bisphosphate (FBP) and sedoheptulose-1, 7-bisphosphate (SBP) are catalyzed by the same bifunctional enzyme: fructose-1, 6-bisphosphatase/sedoheptulose-1, 7-bisphosphatase (F/SBPase), while in that of eukaryotic chloroplasts by two distinct enzymes: chloroplastic fructose-1, 6-bisphosphatase (FBPase) and sedoheptulose-1, 7-bisphosphatase (SBPase), respectively. It was proposed that these two eukaryotic enzymes arose from the divergence of a common ancestral eubacterial bifunctional F/SBPase of mitochondrial origin. However, no specific affinity between SBPase and eubacterial FBPase or F/SBPase can be observed in the previous phylogenetic analyses, and it is hard to explain why SBPase and/or F/SBPase are/is absent from most extant nonphotosynthetic eukaryotes according to this scenario.Results
Domain analysis indicated that eubacterial F/SBPase of two different resources contain distinct domains: proteobacterial F/SBPases contain typical FBPase domain, while cyanobacterial F/SBPases possess FBPase_glpX domain. Therefore, like prokaryotic FBPase, eubacterial F/SBPase can also be divided into two evolutionarily distant classes (Class I and II). Phylogenetic analysis based on a much larger taxonomic sampling than previous work revealed that all eukaryotic SBPase cluster together and form a close sister group to the clade of epsilon-proteobacterial Class I FBPase which are gluconeogenesis-specific enzymes, while all eukaryotic chloroplast FBPase group together with eukaryotic cytosolic FBPase and form another distinct clade which then groups with the Class I FBPase of diverse eubacteria. Motif analysis of these enzymes also supports these phylogenetic correlations.Conclusions
There are two evolutionarily distant classes of eubacterial bifunctional F/SBPase. Eukaryotic FBPase and SBPase do not diverge from either of them but have two independent origins: SBPase share a common ancestor with the gluconeogenesis-specific Class I FBPase of epsilon-proteobacteria (or probably originated from that of the ancestor of epsilon-proteobacteria), while FBPase arise from Class I FBPase of an unknown kind of eubacteria. During the evolution of SBPase from eubacterial Class I FBPase, the SBP-dephosphorylation activity was acquired through the transition ??from specialist to generalist??. The evolutionary substitution of the endosymbiotic-origin cyanobacterial bifunctional F/SBPase by the two light-regulated substrate-specific enzymes made the regulation of the Calvin cycle more delicate, which contributed to the evolution of eukaryotic photosynthesis and even the entire photosynthetic eukaryotes. 相似文献102.
Prevost, I. and Le Page-Degivry, M. Th. 1985. Inverse correlationbetween ABA content and germinability throughout the maturationand the in vitro culture of the embryo of Phaseolus vulgaris.J.exp. Bot. 36: 14571464. Changes in embryo abscisic acid (ABA) content during the maturationof the seed of Phaseolus vulgaris cv. Contender were followed,using a radio-immunoassay. The pattern of change is similarto that already described in several species: a rapid increase(from the 18th to 29th day after anthesis), was followed bya decrease, the ABA level being ten times lower on the 48thday than on the 29th. Embryos isolated from the 18th to the48th day after anthesis were able to germinatewhen cultivated on a mineral medium supplemented with sucroseand agar. The development pattern varied throughout the embryogenesisand could be correlated with the differentiation of the embryoat the time of isolation. Before germination could take place,we observed a lag phase, the duration of which could be correlatedwith embryo ABA content. As ABA content increased in the youngestembryos the duration of the lag phase increased. In the sameway, the number of days to germination was shown to diminishas ABA content decreased. Inverse correlation between ABA contentand germinability was thus demonstrated throughout the developmentof the embryo. During in vitro culture, free ABA content decreased in the embryoand reached low values a few days before germination occurred.So the beginning of root elongation in culture was again wellcorrelated with the disappearance of free endogenous ABA. Atransfer experiment inducing an earlier germination associatedwith a more limited development suggests that the lag phaseis associated with an active continuation of embryonic development Key words: Embryo maturation, abscisic acid, germinability 相似文献
103.
Wolterbeek, H. Th. 1987. Relationships between adsorption, chemicalstate and fluxes of cadmium applied as Cd(NO3)2 in isolatedxylem cell walls of tomato.J. exp. Bot. 38: 419432. Isolated xylem cell wall pieces were applied as membranes inion diffusion experiments. The cell walls were isolated fromtomato internodes (Lycopersicon esculentum Mill, cv. Tiny Tim)and sealed in a two-compartment diffusion system. In flux andadsorption calculations, the cell wall was regarded as a leakymembrane with parallel fluxes through Donnan Free Space (DFS)and Water Free Space (WFS). During the experiments absorptioninto and diffusion across the walls was determined of Cd2 +, applied as 115Cd(NO3)2. Flux experiments with 82Brindicated that excluded volume effects and path tortuosity resultedin apparent WFS diffusion coefficients in the walls which were0·012 times as high as in water. The free proton concentration in the DFS was shown to be relatedto a complex formation between fixed charges and Cd2 +. Thecell wall permeability for Cd2 + and NO3 varied withapplied and absorbed concentrations, and the Cd2 + flux curveshowed an inflexion point coinciding with a buffered degreeof dissociation of fixed charges in the DFS. The necessary couplingof fluxes of opposite charges resulted in relatively high NO3and small Cd2 + permeability of the DFS for strongly dilutedsolutions (P = 104 m s1 and 1011 m s1for NO3 and Cd2 + respectively). The results demonstratethe possible regulatory effects of the cell wall in processesof ion transfer from xylem vessels, or ion uptake in plant tissues. Key words: Cadmium, chemical state, DFS, WFS, ion flux, permeability, xylem cell walls, tomato, bromium, nitrate 相似文献
104.
Wolterbeek, H. Th. and De Bruin, M. 1986. The import and redistributionof several cations and anions in tomato leaves.J. exp.Bot. 37: 331340. The upward movements in the xylem and redistribution from theleaf of Na+ , K+ , Rb+, Cs+ and four anions were examined insub-systems of tomato plants (Lycopersicon esculentum, Mill.cv. Tiny Tim). There was a delay with respect to the redistributionof newly imported elements from the source leaf of about 1620h for all four alkali ions. This is considerably less than theapparent delay for the anions Sb(SO4) WO42 Mo7O246and AsO43 The prolonged delay for the anions is suggestedto be a consequence of metabolic transformation in the leaf.Reduction of the source-sink activity ratio did not decreasethe delay period from the source leaf, but apparently causedincreased Na+ transfer from the xylem. It is concluded thatthe application of a detailed mathematical descnption of upwardelement movement has considerable potential possibilities forunderstanding circulation of nutrients in the plant. Key words: Alkali ions, anions, xylem, phloem, redistribution, tomato 相似文献
105.
106.
N. TOUZET M. SENSENBRENNER TH. LENDER P. MANDEL 《Differentiation; research in biological diversity》1975,4(1-3):183-187
Dissociated cells from cerebral hemispheres of chick embryo at stages 17–18, 12–13 and 9–10, were cultivated for seven days. The cells were cultivated either completely covered with the nutrient medium in an atmosphere containing 5 percent CO2 or they were covered by only a thin film of nutrient medium in contact with air.
For the embryos at stages 17–18 or 12–13, under both culture conditions neurons differentiated after 3 or 4 days in culture, while for the embryos at stage 9–10, no neuronal differentiation occurred under either condition. The cells remained morphologically undifferentiated and formed aggregates of about 50 cells. Some fibroblasts were found to grow on the collagen matrix.
It is concluded that dissociated cells from embryos at stage 9–10 are incapable of auto differentiation under the present culture conditions. The possible causes of this inability to differentiate are discussed. 相似文献
For the embryos at stages 17–18 or 12–13, under both culture conditions neurons differentiated after 3 or 4 days in culture, while for the embryos at stage 9–10, no neuronal differentiation occurred under either condition. The cells remained morphologically undifferentiated and formed aggregates of about 50 cells. Some fibroblasts were found to grow on the collagen matrix.
It is concluded that dissociated cells from embryos at stage 9–10 are incapable of auto differentiation under the present culture conditions. The possible causes of this inability to differentiate are discussed. 相似文献
107.
A single lineage of r2 retrotransposable elements is an active, evolutionarily stable component of the Drosophila rDNA locus 总被引:1,自引:0,他引:1
R2 elements are non-long-terminal-repeat (non-LTR) retrotransposons that
insert specifically in the 28S rRNA genes of many insects. Previous reports
concerning this element in the genus Drosophila have suggested that R2
elements are absent from many species of this genus, particularly those
species from the subgenus Drosophila. In this report, we present an
extensive study of the distribution and evolution of R2 elements in
Drosophila. A PCR survey of 59 species from 23 species groups of the two
major Drosophila subgenera found that R2 elements are present in all but
two species of the melanogaster species subgroup. Phylogenetic analysis
based on partial nucleotide sequences of R2 elements from 23 species
demonstrates that the relationships of R2 elements are congruent with those
of the Drosophila species phylogeny, suggesting that these elements have
been vertically inherited since the divergence of this genus some 60 MYA.
Sequence variation between different copies of R2 elements within each
species was less than 0.16%, indicating that these elements are undergoing
concerted evolution similar to that of the 28S genes. Several properties of
the R2 sequences suggest that these elements depend on retrotransposition
in addition to simple recombination to remain within the rDNA locus: the
rates of synonymous substitutions averaged 4.8 times the rate of
replacement substitutions, 82 of 83 R2 copies partially sequenced contained
intact open reading frames, and, finally, length variation associated with
the poly(A) 3' tails indicated that many R2 copies are the direct result of
retrotransposition.
相似文献
108.
Analysis of a genetic hitchhiking model, and its application to DNA polymorphism data from Drosophila melanogaster 总被引:6,自引:0,他引:6
Begun and Aquadro have demonstrated that levels of nucleotide variation
correlate with recombination rate among 20 gene regions from across the
genome of Drosophila melanogaster. It has been suggested that this
correlation results from genetic hitchhiking associated with the fixation
of strongly selected mutants. The hitchhiking process can be described as a
series of two-step events. The first step consists of a strongly selected
substitution wiping out linked variation in a population; this is followed
by a recovery period in which polymorphism can build up via neutral
mutations and random genetic drift. Genetic hitchhiking has previously been
modeled as a steady-state process driven by recurring selected
substitutions. We show here that the characteristic parameter of this
steady-state model is alpha v, the product of selection intensity (alpha =
2Ns) and the frequency of beneficial mutations v (where N is population
size and s is the selective advantage of the favored allele). We also
demonstrate that the steady-state model describes the hitchhiking process
adequately, unless the recombination rate is very low. To estimate alpha v,
we use the data of DNA sequence variation from 17 D. melanogaster loci from
regions of intermediate to high recombination rates. We find that alpha v
is likely to be > 1.3 x 10(-8). Additional data are needed to estimate
this parameter more precisely. The estimation of alpha v is important, as
this parameter determines the shape of the frequency distribution of
strongly selected substitutions.
相似文献
109.
In Papanicolaou-stained smears certain structures such as nucleoli, Pneumocystis carinii , Charcot-Leyden crystals, bacteria and fungi show a brilliant fluorescence. the morphological characteristics of microorganisms which can be detected by this system, especially mycobacteria, are described. This screening method offers the possibility of providing the clinician with a provisional diagnosis within hours. Proof of the nature of the organisms should be obtained by culture. 相似文献
110.
Lathe WC rd; Burke WD; Eickbush DG; Eickbush TH 《Molecular biology and evolution》1995,12(6):1094-1105
R1 is a non-long terminal repeat (non-LTR) retrotransposable element that
inserts into a specific sequence of insect 28S ribosomal RNA genes. We have
previously shown that this element has been maintained through vertical
transmission in the melanogaster species subgroup of Drosophila. To address
whether R1 elements have been vertically transmitted for longer periods of
evolutionary time, the analysis has been extended to 11 other species from
four species groups of the genus Drosophila (melanogaster, obscura,
testecea, and repleta). All sequenced elements appeared functional on the
basis of the preservation of their open-reading frames and consistently
higher rate of substitution at synonymous sites relative to replacement
sites. The phylogenetic relationships of the R1 elements from all species
analyzed were congruent with the species phylogenies, suggesting that the
R1 elements have been vertically transmitted since the inception of the
Drosophila genus, an estimated 50-70 Mya. The stable maintenance of R1
through the germ line appears to be the major mechanism for the widespread
distribution of these elements in Drosophila. In two species, D.
neotestecea of the testecea group and D. takahashii of the melanogaster
group, a second family of R1 elements was also present that differed in
sequence by 46% and 31%, respectively, from the family that was congruent
with the species phylogeny. These second families may represent occasional
horizontal transfers or, alternatively, they could reflect the ability of
R1 elements to diverge into new families within a species and evolve
independently.
相似文献