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101.
102.
Abstract Left–right asymmetry in Dexiothetica (= echinoderms + chordates) results mainly from dexiothetism—an episode in their ancestry when an animal resembling the Recent pterobranch Cephalodiscus lay right-side-downwards on the sea floor. Castericystis sprinklei belongs to the dexiothete stem group. The history of the echinoderm stem group is reconstructed. Chordate bilateral symmetry evolved by six successive steps. Tail–head overlap occurred independently in craniates and acraniates. The neural crest would have existed in the latest common ancestor of extant chordates, or even earlier. Gross asymmetries occur in extant chordates in organs derived from the calcichordate head, but not in those derived from the calcichordate tail. The anterior boundary of hox gene expression in vertebrates corresponds to the anterior end of the calcichordate tail. Left–right organ pairing (an important step in the origin of chordate bilateral symmetry) may have involved the interaction of a symmetrizing morphogen, produced from the anterior end of the tail, with a lateral morphogen (Wilhelmi's morphogen), produced in ontogeny at first from the left and later from the right. This mechanism may still act in the metamorphosis of amphioxus and in mirror-imaging in vertebrate twins. Wilhelmi's morphogen may be related to one or more members of the dorsal cascade of Drosophila. 相似文献
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104.
Rapamycin suppresses 5''TOP mRNA translation through inhibition of p70s6k. 总被引:15,自引:1,他引:14
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H B Jefferies S Fumagalli P B Dennis C Reinhard R B Pearson G Thomas 《The EMBO journal》1997,16(12):3693-3704
105.
Caccone A; Moriyama EN; Gleason JM; Nigro L; Powell JR 《Molecular biology and evolution》1996,13(9):1224-1232
Drosophila melanogaster belongs to a closely related group of eight species
collectively known as the melanogaster subgroup; all are native to
sub-Saharan Africa and islands off the east coast of Africa. The
phylogenetic relationships of most species in this subgroup have been well
documented; however, the three most closely related species, D. simulans,
D. sechellia, and D. mauritiana, have remained problematic from a
phylogenetic standpoint as no data set has unambiguously resolved them. We
present new DNA sequence data on the nullo and Serendipity-alpha genes and
combine them with all available nuclear DNA sequence data; the total data
encompass 12 genes and the ITS of rDNA. A methodological problem arose
because nine of the genes had information on intraspecific polymorphisms in
at least one species. We explored the effect of inclusion/exclusion of
polymorphic sites and found that it had very little effect on phylogenetic
inferences, due largely to the fact that 82% of polymorphisms are
autapomorphies (unique to one species). We have also reanalyzed our
previous DNA-DNA hybridization data with a bootstrap procedure. The
combined sequence data set and the DNA-DNA hybridization data strongly
support the sister status of the two island species, D. sechellia and D.
mauritiana. This at least partially resolves what had been a paradox of
parallel evolution in these two species.
相似文献
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107.
INTRoDUCTIoNlYho1iumrePensL,whiteclover,isaneconomicallyimportantplantspeciesintemperatepastures.Asbrieflyreportedby[1],ithas16pairsofchromosomes(2n=32).Asyet,nodetailedcytologicalexaminationofthisspecies,suchasC-banding,hasbeenrep0rted.Inthelastdecade,thetechnique0fC-bandinghasbeenusedt0examinehighlyrepeatedsequencesinplantchrom0s0mesandhasprovidedausefultoolf0rtheanalysis0fcyt0geneticstructureincr0pplants[2-71.Inplants,thechr0m0s0mall0calizationofhighlyrepeatedDNAsequencesbyinsituhybr… 相似文献
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The period (per) locus has received much attention in molecular evolution
studies because it is one of the best studied "behavioral genes" and
because it offers insight into the evolution of repetitive sequences. We
studied most of the coding region of per in Drosophila willistoni and
confirmed previously observed patterns of conservation and divergence among
distantly related species. Five regions are so highly diverged that they
cannot be aligned, whereas a region encompassing the PAS domain is very
conserved. Structural and nucleotide polymorphism patterns in the
willistoni group are not the same as those observed in previously studied
species. We sequenced the region homologous to the highly polymorphic
threonine-glycine repeat of D. melanogaster in multiple strains of D.
willistoni, as well as in other members of willistoni group, and found an
unusual amount of conservation in this region. However, the next
nonconserved region downstream in the sequence is quite variable and
polymorphic for the number of repeated glycines. The glycine codon usage is
significantly different in this glycine repeat as compared to other parts
of the gene. We were able to plot the directionality of change in the
glycine repeat region onto a phylogeny and find that the addition of
glycines is the general trend with the diversification of the willistoni
group.
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