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151.
Considerable work in evolutionary biology has focused on the question of why sex persists. Both advantages to sex and constraints limiting a return to asexual reproduction are hypothesized to maintain sex once it evolves. Developmental constraints would limit asexual reproduction from a sexual species if it were difficult for females to switch from making eggs that do not develop without fertilization to making zygotes that are capable of developing in the absence of fertilization. Nauphoeta cinerea is an ovoviviparous cockroach in which some females are capable of switching from a sexual mode of reproduction to an asexual mode when isolated from males. Yet, while facultative parthenogenesis can occur in individuals, few females make the switch. Thus, this cockroach provides an ideal system for examining the potential role of developmental constraints in maintaining sex. Here we compare the cytogenetics and embryonic development of sexual and parthenogenetic offspring in N. cinerea. We find that deviations from normal ploidy levels are associated with abnormal development. All viable N. cinerea embryos exhibit typically hemimetabolous insect embryogenesis. Although there is no variation among embryos in development within a sexually produced clutch, we see extreme variation in asexually derived clutches. These results suggest that developmental constraints limit the success of asexual reproduction in this facultatively parthenogenetic cockroach. Our data further suggest that the specific constraint occurs in the switch from a meiotic mode of reproduction requiring fertilization to diploid zygotes that develop in the absence of fertilization. 相似文献
152.
In adipocytes, insulin triggers the redistribution of Glut4 from intracellular compartments to the plasma membrane. Two models have been proposed to explain the effect of insulin on Glut4 localization. In the first, termed dynamic exchange, Glut4 continually cycles between the plasma membrane and intracellular compartments in basal cells, and the major effect of insulin is through changes in the exocytic and endocytic rate constants, k(ex) and k(en). In the second model, termed static retention, Glut4 is packaged in specialized storage vesicles (GSVs) in basal cells and does not traffic through the plasma membrane or endosomes. Insulin triggers GSV exocytosis, increasing the amount of Glut4 in the actively cycling pool. Using a flow cytometry-based assay, we found that Glut4 is regulated by both static and dynamic retention mechanisms. In basal cells, 75-80% of the Glut4 is packaged in noncycling GSVs. Insulin increased the amount of Glut4 in the actively cycling pool 4-5-fold. Insulin also increased k(ex) in the cycling pool 3-fold. After insulin withdrawal, Glut4 is rapidly cleared from the plasma membrane (t((1/2)) of 20 min) by rapid adjustments in k(ex) and k(en) and recycled into static compartments. Complete recovery of the static pool required more than 3 h, however. We conclude that in fully differentiated confluent adipocytes, both the dynamic and static retention mechanisms are important for the regulation of plasma membrane Glut4 content. However, cell culture conditions affect Glut4 trafficking. For example, replating after differentiation inhibited the static retention of Glut4, which may explain differences in previous reports. 相似文献
153.
K. A. Rance S. Mayes Z. Price P. L. Jack R. H. V. Corley 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2001,103(8):1302-1310
The development of an oil palm RFLP marker map has enabled marker-based QTL mapping studies to be undertaken. Information
from 153 RFLP markers was used in combination with phenotypic data from an F2 population to estimate the position and effects of quantitative trait loci (QTLs) for traits including yield of fruit and
its components and measures of vegetative growth. The mapping population consisted of 84 palms segregating for the major gene
influencing shell thickness. Marker data were analysed to produce a linkage map consisting of 22 linkage groups. The QTL mapping
analysis was carried out by interval mapping and single-marker analysis for the unlinked markers; significance thresholds
were generated by permutation. Using both single-marker and interval-mapping analysis significant marker associated QTL effects
were found for 11 of the 13 traits analysed. The results of interval-mapping analysis of fruit weight, petiole cross section
and rachis length, and ratios of shell:fruit, mesocarp:fruit and kernel:fruit indicated significant (P<0.05) QTLs at the genome-wide threshold. The putative QTLs were associated with between 8.2% and 44.0% of the phenotypic
variation, with an average of 27% for the single-marker analysis and 19% for the interval-mapping analysis. The higher percentage
of phenotypic variation explained in the single-marker analysis, when compared to the interval-mapping analysis, is likely
to be due to the lower stringency associated with the single-marker analysis. Large dominance deviations were associated with
a sizeable proportion of the putative QTLs. The ultimate objective of mapping QTLs in commercial populations is to utilise
novel breeding strategies such as marker-assisted selection (MAS). The potential impact of MAS in oil palm breeding programmes
is discussed.
Received: 26 June 2000 / Accepted: 24 October 2000 相似文献
154.
155.
We propose models for in vitro grown mammalian prion protein fibrils based upon left handed beta helices formed both from the N-terminal and C-terminal regions of the proteinase resistant infectious prion core. The C-terminal threading onto a β-helical structure is almost uniquely determined by fixing the cysteine disulfide bond on a helix corner. In comparison to known left handed helical peptides, the resulting model structures have similar stability attributes including relatively low root mean square deviations in all atom molecular dynamics, substantial side-chain-to-side-chain hydrogen bonding, good volume packing fraction, and low hydrophilic/hydrophobic frustration. For the N-terminus, we propose a new threading of slightly more than two turns, which improves upon the above characteristics relative to existing three turn β-helical models. The N-terminal and C-terminal beta helices can be assembled into eight candidate models for the fibril repeat units, held together by large hinge (order 30 residues) domain swapping, with three amenable to fibril promoting domain swapping via a small (five residue) hinge on the N-terminal side. Small concentrations of the metastable C-terminal β helix in vivo might play a significant role in templating the infectious conformation and in enhancing conversion kinetics for inherited forms of the disease and explain resistance (for canines) involving hypothesized coupling to the methionine 129 sulfur known to play a role in human disease.Key words: prion, amyloid fibril, domain swap, beta helix, computational biology 相似文献
156.
Background
Alignments of homologous DNA sequences are crucial for comparative genomics and phylogenetic analysis. However, multiple alignment represents a computationally difficult problem. For protein-coding DNA sequences, it is more advantageous in terms of both speed and accuracy to align the amino-acid sequences specified by the DNA sequences rather than the DNA sequences themselves. Many implementations making use of this concept of "translated alignments" are incomplete in the sense that they require the user to manually translate the DNA sequences and to perform the amino-acid alignment. As such, they are not well suited to large-scale automated alignments of large and/or numerous DNA data sets. 相似文献157.
Agents that extract or sequester membrane cholesterol stimulate IkappaB degradation and lead to NF-kappaB activation in a subset of B cells. Although the extraction of cholesterol by methyl-beta-cyclodextrin is the most potent stimulus of NF-kappaB, other agents that sequester cholesterol have similar effects. B cells and B cell lines with an immature phenotype are significantly more sensitive to the effects of cholesterol perturbation than their mature B cell counterparts. NF-kappaB activation does not involve signaling from the B cell receptor complex. Instead, the disruption of membrane cholesterol activates NF-kappaB through a MyD88-dependent pathway involving the pattern recognition receptor, Toll-like receptor 4. We suggest that lipid raft microdomains may serve not only to orchestrate receptor signaling, but to sequester signaling components one from one another, which serves to prevent receptor-mediated signaling from occurring. A role for this process during B cell development is suggested. 相似文献
158.
We investigated the effect on host-parasitoid dynamics of prolonged diapause, a feature of the life history of many animals living in unpredictable environments, by modifying the classical May (J. Anim. Ecol. 47 (1978) 833) host-parasitoid model. We considered three patterns of development of host and parasitoid: (a) prolonged parasitoid diapause controlled by host physiology, (b) parasitoid interference in host development, preventing parasitized hosts from prolonging diapause, and (c) host diapause independent of parasitoid attack. We found that single-year prolonged diapause shifted the boundaries of the May model towards a slight increase in stability. Longer periods of diapause prolongation had a stronger influence, but this influence remained modest if we considered realistic parameter values. In contrast to other recent studies, our results suggest that prolonged diapause does not necessarily compensate for the destabilizing effects of time lags on the influence of parasitoids on population dynamics. 相似文献
159.
Corley Mastick C Sanguinetti AR Knesek JH Mastick GS Newcomb LF 《Experimental cell research》2001,266(1):142-154
Caveolin-1 was originally identified as a tyrosine-phosphorylated protein in v-Src-transformed cells and it was suggested that phosphorylation of this protein could mediate transformation by the tyrosine kinase class of oncogenes (J. R. Glenney, 1989, J. Biol. Chem. 264, 20163--20166). We found that caveolin-1 is also phosphorylated on tyrosine in v-Abl-transformed cells. In fact, caveolin-1 and a caveolin-associated protein of 29 kDa are among the strongest phosphotyrosine signals detected in the Abl-expressing cells. In addition, v-Abl shows a preferential phosphorylation of caveolin-1 and the 29-kDa caveolin-associated protein over other proteins in the caveolin-enriched Triton-resistant cell fraction. These data indicate that caveolin-1 and the 29-kDa caveolin-associated protein may be preferred substrates of the Abl kinase. Caveolin-1 is phosphorylated at tyrosine 14 in v-Abl-expressing cells as has been observed previously in v-Src-expressing cells. However, using a temperature-sensitive allele of v-Abl (ts120 v-Abl) we provide evidence that caveolin-1 phosphorylation is not sufficient to mediate the loss of caveolin expression or loss of cell adhesion induced by v-Abl. 相似文献
160.
Elaine M. Sandeman Maria del Carmen Valdes Hernandez Zoe Morris Mark E. Bastin Catherine Murray Alan J. Gow Janie Corley Ross Henderson Ian J. Deary John M. Starr Joanna M. Wardlaw 《PloS one》2013,8(8)