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1.
2.
High rate of DNA loss in the Drosophila melanogaster and Drosophila virilis species groups 总被引:3,自引:3,他引:3
We recently proposed that patterns of evolution of non-LTR
retrotransposable elements can be used to study patterns of spontaneous
mutation. Transposition of non-LTR retrotransposable elements commonly
results in creation of 5' truncated, "dead-on-arrival" copies. These
inactive copies are effectively pseudogenes and, according to the neutral
theory, their molecular evolution ought to reflect rates and patterns of
spontaneous mutation. Maximum parsimony can be used to separate the
evolution of active lineages of a non-LTR element from the fate of the
"dead-on-arrival" insertions and to directly assess the relative
frequencies of different types of spontaneous mutations. We applied this
approach using a non-LTR element, Helena, in the Drosophila virilis group
and have demonstrated a surprisingly high incidence of large deletions and
the virtual absence of insertions. Based on these results, we suggested
that Drosophila in general may exhibit a high rate of spontaneous large
deletions and have hypothesized that such a high rate of DNA loss may help
to explain the puzzling dearth of bona fide pseudogenes in Drosophila. We
also speculated that variation in the rate of spontaneous deletion may
contribute to the divergence of genome size in different taxa by affecting
the amount of superfluous "junk" DNA such as, for example, pseudogenes or
long introns. In this paper, we extend our analysis to the D. melanogaster
subgroup, which last shared a common ancestor with the D. virilis group
approximately 40 MYA. In a different region of the same transposable
element, Helena, we demonstrate that inactive copies accumulate deletions
in species of the D. melanogaster subgroup at a rate very similar to that
of the D. virilis group. These results strongly suggest that the high rate
of DNA loss is a general feature of Drosophila and not a peculiar property
of a particular stretch of DNA in a particular species group.
相似文献
3.
DNA sequences were determined for three to five alleles of the bride-of-
sevenless (boss) gene in each of four species of Drosophila. The product of
boss is a transmembrane receptor for a ligand coded by the sevenless gene
that triggers differentiation of the R7 photoreceptor cell in the compound
eye. Population parameters affecting the rate and pattern of molecular
evolution of boss were estimated from the multinomial configurations of
nucleotide polymorphisms of synonymous codons. The time of divergence
between D. melanogaster and D. simulans was estimated as approximately 1
Myr, that between D. teissieri and D. yakuba as approximately 0.75 Myr, and
that between the two pairs of sibling species as approximately 2 Myr. (The
boss genes themselves have estimated divergence times approximately 50%
greater than the species divergence times.) The effective size of the
species was estimated as approximately 5 x 10(6), and the average mutation
rate was estimated as 1-2 x 10(-9)/nucleotide/generation. The ratio of
amino acid polymorphisms within species to fixed differences between
species suggests that approximately 25% of all possible single-step amino
acid replacements in the boss gene product may be selectively neutral or
nearly neutral. The data also imply that random genetic drift has been
responsible for virtually all of the observed differences in the portion of
the boss gene analyzed among the four species.
相似文献
4.
The contractile basis of amoeboid movement: V. The control of gelation, solation, and contraction in extracts from dictyostelium discoideum 总被引:7,自引:22,他引:7 下载免费PDF全文
Motile extracts have been prepared from Dictyostelium discoideum by homogenization and differential centrifugation at 4 degrees C in a stabilization solution (60). These extracts gelled on warming to 25 degrees Celsius and contracted in response to micromolar Ca++ or a pH in excess of 7.0. Optimal gelation occurred in a solution containing 2.5 mM ethylene glycol-bis (β-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA), 2.5 mM piperazine-N-N'-bis [2-ethane sulfonic acid] (PIPES), 1 mM MgC1(2), 1 mM ATP, and 20 mM KCI at ph 7.0 (relaxation solution), while micromolar levels of Ca++ inhibited gelation. Conditions that solated the gel elicited contraction of extracts containing myosin. This was true regardless of whether chemical (micromolar Ca++, pH >7.0, cytochalasin B, elevated concentrations of KCI, MgC1(2), and sucrose) or physical (pressure, mechanical stress, and cold) means were used to induce solation. Myosin was definitely required for contraction. During Ca++-or pH-elicited contraction: (a) actin, myosin, and a 95,000-dalton polypeptide were concentrated in the contracted extract; (b) the gelation activity was recovered in the material sqeezed out the contracting extract;(c) electron microscopy demonstrated that the number of free, recognizable F-actin filaments increased; (d) the actomyosin MgATPase activity was stimulated by 4- to 10-fold. In the absense of myosin the Dictyostelium extract did not contract, while gelation proceeded normally. During solation of the gel in the absense of myosin: (a) electron microscopy demonstrated that the number of free, recognizable F- actin filaments increased; (b) solation-dependent contraction of the extract and the Ca++-stimulated MgATPase activity were reconstituted by adding puried Dictyostelium myosin. Actin purified from the Dictyostelium extract did not gel (at 2 mg/ml), while low concentrations of actin (0.7-2 mg/ml) that contained several contaminating components underwent rapid Ca++ regulated gelation. These results indicated : (a) gelation in Dictyostelium extracts involves a specific Ca++-sensitive interaction between actin and several other components; (b) myosin is an absolute requirement for contraction of the extract; (c) actin-myosin interactions capable of producing force for movement are prevented in the gel, while solation of the gel by either physical or chemical means results in the release of F-actin capable of interaction with myosin and subsequent contraction. The effectiveness of physical agents in producting contraction suggests that the regulation of contraction by the gel is structural in nature. 相似文献
5.
V KW Wong T Li B YK Law E DL Ma N C Yip F Michelangeli C KM Law M M Zhang K YC Lam P L Chan L Liu 《Cell death & disease》2013,4(7):e720
Autophagy is an important cellular process that controls cells in a normal homeostatic state by recycling nutrients to maintain cellular energy levels for cell survival via the turnover of proteins and damaged organelles. However, persistent activation of autophagy can lead to excessive depletion of cellular organelles and essential proteins, leading to caspase-independent autophagic cell death. As such, inducing cell death through this autophagic mechanism could be an alternative approach to the treatment of cancers. Recently, we have identified a novel autophagic inducer, saikosaponin-d (Ssd), from a medicinal plant that induces autophagy in various types of cancer cells through the formation of autophagosomes as measured by GFP-LC3 puncta formation. By computational virtual docking analysis, biochemical assays and advanced live-cell imaging techniques, Ssd was shown to increase cytosolic calcium level via direct inhibition of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase pump, leading to autophagy induction through the activation of the Ca2+/calmodulin-dependent kinase kinase–AMP-activated protein kinase–mammalian target of rapamycin pathway. In addition, Ssd treatment causes the disruption of calcium homeostasis, which induces endoplasmic reticulum stress as well as the unfolded protein responses pathway. Ssd also proved to be a potent cytotoxic agent in apoptosis-defective or apoptosis-resistant mouse embryonic fibroblast cells, which either lack caspases 3, 7 or 8 or had the Bax-Bak double knockout. These results provide a detailed understanding of the mechanism of action of Ssd, as a novel autophagic inducer, which has the potential of being developed into an anti-cancer agent for targeting apoptosis-resistant cancer cells. 相似文献
6.
7.
Typical preparation of seed samples for infrared (IR) microspectroscopy involves imbibition of the seed for varying time periods followed by cryosectioning. Imbibition, however, may initiate germination even at 4° C with associated changes in the chemistry of the sample. We have found that it is possible to section seeds that are sufficiently hard, such as soybeans, on a standard laboratory microtome without imbibition. The use of dry sectioning of unimbibed seeds is reported here, as well as a comparison of different mounting media and modes of analysis. Glycerol, Tissue-Tek, and ethanol were used as mounting media, and the quality of the resulting spectra was assessed. Ethanol was the preferred mountant, because it dried quickly with no residue and thus did not interfere with the spectrum of interest. Analysis in transmission mode using barium fluoride windows to hold the samples was compared with transmission-reflection analysis with sections mounted on special infrared-reflecting slides. The two modes of analysis performed well in different regions of the spectrum. The mode of analysis (transmission vs. transmission-reflection) should be based on the components of greatest interest in the sample. 相似文献
8.
C. AUSTIN FARLEY 《The Journal of eukaryotic microbiology》1967,14(4):616-625
SYNOPSIS. A developmental sequence is proposed for the haplosporidan Minchinia nelsoni Haskin, Stauber and Mackin, 1966, based on study of oyster infections over the past 5 years in Chesapeake Bay. Uninucleate stages develop by nuclear division into multinucleate plasmodia which proliferate in the tissues by plasmotomy. Relatively small plasmodia containing what are considered to be gametic nuclei originate by unequal plasmotomy of large plasmodia. These have been interpreted to aggregate and fuse to form large plasmodia which contain prozygotes. Pairing and fusion of nuclei occur within each plasmodium to produce zygote nuclei (synkaryons) which undergo division, possibly meiotic, to form sporonts. Sporoblasts differentiate into spores with the development of spore walls and opercula. Cystoid plasmodia develop during times of unfavorable conditions. An anomalous but common sequence involving sexuality and mitosis is described, and the occurrence of various life cycle stages within the host thruout the year is discussed. 相似文献
9.
The genomes of homeothermic (warm-blooded) vertebrates are mosaic
interspersions of homogeneously GC-rich and GC-poor regions (isochores).
Evolution of genome compartmentalization and GC-rich isochores is
hypothesized to reflect either selective advantages of an elevated GC
content or chromosome location and mutational pressure associated with the
timing of DNA replication in germ cells. To address the present controversy
regarding the origins and maintenance of isochores in homeothermic
vertebrates, newly obtained as well as published nucleotide sequences of
the insulin and insulin-like growth factor (IGF) genes, members of a
well-characterized gene family believed to have evolved by repeated
duplication and divergence, were utilized to examine the evolution of base
composition in nonconstrained (flanking) and weakly constrained (introns
and fourfold degenerate sites) regions. A phylogeny derived from amino acid
sequences supports a common evolutionary history for the insulin/IGF family
genes. In cold- blooded vertebrates, insulin and the IGFs were similar in
base composition. In contrast, insulin and IGF-II demonstrate dramatic
increases in GC richness in mammals, but no such trend occurred in IGF- I.
Base composition of the coding portions of the insulin and IGF genes across
vertebrates correlated (r = 0.90) with that of the introns and flanking
regions. The GC content of homologous introns differed dramatically between
insulin/IGF-II and IGF-I genes in mammals but was similar to the GC level
of noncoding regions in neighboring genes. Our findings suggest that the
base composition of introns and flanking regions is determined by
chromosomal location and the mutational pressure of the isochore in which
the sequences are embedded. An elevated GC content at codon third positions
in the insulin and the IGF genes may reflect selective constraints on the
usage of synonymous codons.
相似文献
10.
ROGER D. FARLEY 《Invertebrate reproduction & development.》2013,57(2-3):193-208
Summary The scanning electron microscope was used to study changes in the ventral mesosoma of the vaejovid scorpion, Paruroctonus mesaensis. Observations are compared with those from scorpion fossils. The oldest fossils are from the Silurian period; migration from water to land occurred in the Carboniferous and Permian periods. All recent scorpions are terrestrial with four pairs of booklungs and spiracles in mesosomal sternites. Ancient eurypterids and scorpions had flap-like abdominal plates attached to the ventral surface of five mesosomal segments. The abdominal plates were apparently an aquatic adaptation, and authors have described possible gill tissue in the chamber above. In scorpion embryos, rectangular (holostem) plate-like structures precede the formation of sternites in the ventral mesosoma. Transverse folds were seen in the space above the abdominal plates. The lack of elaborate gill-like structures here supports an earlier hypothesis that aquatic scorpions had other mesosomal respiratory sites (e.g., pectines), resulting in less reliance on respiratory tissues above the abdominal plates. Spiracles initially appear as round or ovoid patterns in the epidermis at the latero-posterior margins of the ventral plates. The booklung spiracles are positioned farther anterior in sternites, but the developmental sequence for this transition is still unclear and may occur later than the stages of this study. The abdominal plates lengthen and enlarge laterally and/or epidermis is added at the lateral edges so that broad, overlapping sternites eventually cover the ventral surface of the mesosoma. 相似文献