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31.
Group I introns are proposed to have become mobile following the
acquisition of open reading frames (ORFs) that encode highly specific DNA
endonucleases. This proposal implies that intron ORFs could behave as
autonomously mobile entities. This was supported by abundant circumstantial
evidence but no experiment of ORF transfer from an ORF- containing intron
to its ORF-less counterpart has been described. In this paper we present
such experiments, which demonstrate the efficient mobility of the
mitochondrial nad1-i4-orf1 between two Podospora strains. The homing of
this mobile ORF was accompanied by a bidirectional co-conversion that did
not systematically involve the whole intron sequence. Orf1 acquisition
would be the most recent step in the evolution of the nad1-i4 intron, which
has resulted in many strains of Podospora having an intron with two ORFs
(biorfic) and four splicing pathways. We show that two of the splicing
events that operate in this biorfic intron, as evidenced by PCR
experiments, are generated by a 5'-alternative splice site, which is most
probably a remnant of the monoorfic ancestral form of the intron. We
propose a sequential evolution model that is consistent with the four
organizations of the corresponding nad1 locus that we found among various
species of the Pyrenomycete family; these organizations consist of no
intron, an intron alone, a monoorfic intron, and a biorfic intron.
相似文献
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33.
Observations by Electron Microscopy on Pig Muscle Inoculated and Incubated with Pseudomonas fragi 总被引:3,自引:3,他引:0 下载免费PDF全文
T. R. Dutson A. M. Pearson J. F. Price G. C. Spink P. J. V. Tarrant 《Applied microbiology》1971,22(6):1152-1158
Myofibrils from pig muscle inoculated and incubated with Pseudomonas fragi showed an extremely disrupted appearance as compared to uninoculated controls. There was an almost complete absence of material in the H zone, marked disruption of the A band (probably myosin), and some loss of dense material from the Z line. These changes indicated that marked proteolysis had occurred. Bacteria observed in spoiled muscle tissue exhibited protrusions or blebs on the outer surface of the cell walls. The blebs appeared to form detached globules that migrated into the muscle mass. Bacteria grown in non-muscle-containing media did not produce blebs, which indicates the blebs were induced by growth on muscle tissue. The possibility that the blebs and globules may contain a proteolytic enzyme responsible for myofibrillar disruption is discussed. 相似文献
34.
Marmoset phylogenetics, conservation perspectives, and evolution of the mtDNA control region 总被引:2,自引:1,他引:2
Tagliaro CH; Schneider MP; Schneider H; Sampaio IC; Stanhope MJ 《Molecular biology and evolution》1997,14(6):674-684
Marmosets (genus Callithrix) are a diverse group of platyrrhine primates
with 13-15 purported taxa, many of them considered endangered.
Morphological analyses constitute most of the basis for recognition of
these forms as distinct taxa. The purpose of this study was to provide a
molecular view, based on mitochondrial control region sequences, of the
evolutionary history of the marmosets, concomitant with a molecular
phylogenetic perspective on species diversity within the group. An
additional purpose was to provide the first comparative examination of a
complete New World monkey control region sequence with those of other
mammals. The phylogenetic analyses provide convincing support for a split
between the Atlantic forest and Amazonian marmosets, with the inclusion of
the pygmy marmoset (Cebuella pygmaea) at the base of the Amazonian clade.
The earliest branch of the Atlantic forest group was C. aurita. In the
Amazonian group, the analyses do not support the recognition of C.
humeralifer and the recently described C mauesi as distinct taxa. They do,
however, support a clear distinction between C. argentata and a strongly
supported mixed clade of C. humeralifer and C. mauesi. In the Atlantic
forest group, the phylogenetic tree suggests mixing between C. penicillata,
C. kuhli, and possibly C. jacchus. Most of the sequence features
characteristic of other mammal control regions were also evident in
marmosets, with the exception that conserved sequence blocks (CSBs) 2 and 3
were not clearly identifiable. Tandem repeat units often associated with
heteroplasmy in a variety of other mammals were not evident in the marmoset
sequences.
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35.
Hormonal Regulation of Pedicel Abscission in Begonia Flower Buds 总被引:1,自引:0,他引:1
CH. H. HÄNISCH TEN CATE J. BERGHOEF A. M. H. VAN DER HOORN J. BRUINSMA 《Physiologia plantarum》1975,33(4):280-284
In order to analyse the hormonal regulation of flower bud shedding in Begonia, levels of indoleacetic acid (IAA), abscisic acid (ABA) and ethylene were determined in buds and pedicels. The translocation and metabolism of 14C-labeled IAA in pedicel segments were also studied. In a monoecious Begonia fuchsioides hybrid, abscising male flower buds contain about 1% of the IAA present in non-abscising female flowers. In a male Begonia davisii hybrid, the seasonal variation in bud drop coincides with changes in the IAA content of the buds, while also the release of IAA from the bud to the pedicel is hampered. Abscission zones of these pedicels always contain abscission promoting ethylene concentrations. The tissue is prevented from responding with abscission by IAA from the flower buds. The buds also contain ABA but without influencing abscission considerably. Pretreatment with ethylene or ABA does not affect IAA transport in pedicel segments. The rate of this transport is 4–6 mm × h–1:; the capacity increases with the transverse area. In young segments, IAA is decarboxylated and also otherwise metabolized. 相似文献
36.
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The vitelline coat (VC) surrounding coelomic eggs of the frog, Rana japonica , comprises bundles of filaments running both parallel and perpendicular to the egg surface. The coat gives little or no staining reaction with PA-CrA-Silver methenamine. In contrast, in the VC of uterine eggs the filament bundles are less conspicuous. and the interstices between the filament bundles stain strongly for carbohydrate. This alteration occurs during passage of the eggs down the first 1/20 th of the oviduct, the pars recta. The epithelium of the p. recta contains secretory cells, which contain electron-dense granules distinct from those in the jelly-secreting cells in more caudal portions of the oviduct. Treatment of coelomic eggs with an extract of p. recta followed by exposure to a sperm suspension resulted in marked swelling and softening of the VC. These results indicate that the contents of the granules secreted from the epithelial cells in the p. recta are deposited in the VC to increase its susceptibility to a fertilizing sperm. 相似文献
39.
40.
The Function of Urease in Citrullus Seeds 总被引:1,自引:0,他引:1
Urease is present in considerable quantity in the cotyledonsof Citrullus, though elsewhere in the plant it is present onlyin traces or is absent; urease activity in the cotyledons changesduring growth, showing an initial rise followed by an abruptdrop almost to zero. These changes, under a wide variety ofconditions, are not correlated with those in the major nitrogenfractions; they are, however, closely correlated with cell extensionand the associated changes in water content and respiration.A connexion with chlorophyll formation is possible but unlikely.It is suggested that the changes in cotyledonary urease constitutemerely one aspect of the protoplasmic differentiationthat takes place as a cell matures. 相似文献