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81.
Heritable epigenetic polymorphisms, such as differential cytosine methylation, can underlie phenotypic variation. Moreover, wild strains of the plant Arabidopsis thaliana differ in many epialleles, and these can influence the expression of nearby genes. However, to understand their role in evolution, it is imperative to ascertain the emergence rate and stability of epialleles, including those that are not due to structural variation. We have compared genome-wide DNA methylation among 10 A. thaliana lines, derived 30 generations ago from a common ancestor. Epimutations at individual positions were easily detected, and close to 30,000 cytosines in each strain were differentially methylated. In contrast, larger regions of contiguous methylation were much more stable, and the frequency of changes was in the same low range as that of DNA mutations. Like individual positions, the same regions were often affected by differential methylation in independent lines, with evidence for recurrent cycles of forward and reverse mutations. Transposable elements and short interfering RNAs have been causally linked to DNA methylation. In agreement, differentially methylated sites were farther from transposable elements and showed less association with short interfering RNA expression than invariant positions. The biased distribution and frequent reversion of epimutations have important implications for the potential contribution of sequence-independent epialleles to plant evolution. 相似文献
82.
The authors review and compare the papers in this issue on saturation and logistic growth with special emphasis on the structure of the underlying probability models. Influence diagrams are used to illustrate the dependence and independence assumptions and the way in which uncertainty is reflected in key model assumptions and parameters characterizing the models. 相似文献
83.
Mammalian hairs are formed by differentiation and keratinization of cells produced in the epidermal matrix (Figs 3, 4). Using the rodent vibrissa follicle as a model, transplantation studies have shown that the dermal papilla, a discrete population of specialized fibroblasts, is of prime importance in the growth of hair. Papillae induce hair growth when implanted into follicles and can interact with skin epidermis to form new hair follicles. When grown in culture, papilla cells display singular morphological and behavioural characteristics compared with connective tissue cells from other skin sources. We report here that serially cultured adult papilla cells can induce the growth of hair when implanted into follicles which otherwise would not grow hairs. This finding presents an opportunity to characterize properties distinguishing the papilla cell population from other skin fibroblasts, and, more specifically, those which control hair growth. The eventual application of this work to human hair replacement techniques can also be envisaged. 相似文献
84.
P Giraud P Gillioz B Conte-Devolx C Oliver 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1979,288(1):127-129
High concentrations of Thyrotrophin-Releasing Hormone (TRH) have been found in the dorsal skin of the Frog Rana esculenta, lower levels being measured in the ventral skin. alpha-MSH and somatostatin were undetectable in these tissues. Nor was TRH detected in the blood of these animals. The concentration of somatostatin in the pancreas was similar to that of the hypothalamus and twice or one hundred times higher than in the intestine or stomach respectively. 相似文献
85.
86.
Berns DM Rudner MS Valenzuela SO Berggren KK Oliver WD Levitov LS Orlando TP 《Nature》2008,455(7209):51-57
The energy-level structure of a quantum system, which has a fundamental role in its behaviour, can be observed as discrete lines and features in absorption and emission spectra. Conventionally, spectra are measured using frequency spectroscopy, whereby the frequency of a harmonic electromagnetic driving field is tuned into resonance with a particular separation between energy levels. Although this technique has been successfully employed in a variety of physical systems, including natural and artificial atoms and molecules, its application is not universally straightforward and becomes extremely challenging for frequencies in the range of tens to hundreds of gigahertz. Here we introduce a complementary approach, amplitude spectroscopy, whereby a harmonic driving field sweeps an artificial atom through the avoided crossings between energy levels at a fixed frequency. Spectroscopic information is obtained from the amplitude dependence of the system's response, thereby overcoming many of the limitations of a broadband-frequency-based approach. The resulting 'spectroscopy diamonds', the regions in parameter space where transitions between specific pairs of levels can occur, exhibit interference patterns and population inversion that serve to distinguish the atom's spectrum. Amplitude spectroscopy provides a means of manipulating and characterizing systems over an extremely broad bandwidth, using only a single driving frequency that may be orders of magnitude smaller than the energy scales being probed. 相似文献
87.
Scheerer P Park JH Hildebrand PW Kim YJ Krauss N Choe HW Hofmann KP Ernst OP 《Nature》2008,455(7212):497-502
Opsin, the ligand-free form of the G-protein-coupled receptor rhodopsin, at low pH adopts a conformationally distinct, active G-protein-binding state known as Ops*. A synthetic peptide derived from the main binding site of the heterotrimeric G protein-the carboxy terminus of the alpha-subunit (GalphaCT)-stabilizes Ops*. Here we present the 3.2 A crystal structure of the bovine Ops*-GalphaCT peptide complex. GalphaCT binds to a site in opsin that is opened by an outward tilt of transmembrane helix (TM) 6, a pairing of TM5 and TM6, and a restructured TM7-helix 8 kink. Contacts along the inner surface of TM5 and TM6 induce an alpha-helical conformation in GalphaCT with a C-terminal reverse turn. Main-chain carbonyl groups in the reverse turn constitute the centre of a hydrogen-bonded network, which links the two receptor regions containing the conserved E(D)RY and NPxxY(x)(5,6)F motifs. On the basis of the Ops*-GalphaCT structure and known conformational changes in Galpha, we discuss signal transfer from the receptor to the G protein nucleotide-binding site. 相似文献
88.
The ability to actively remodel membranes in response to nucleotide hydrolysis has largely been attributed to GTPases of the dynamin superfamily, and these have been extensively studied. Eps15 homology (EH)-domain-containing proteins (EHDs/RME-1/pincher) comprise a less-well-characterized class of highly conserved eukaryotic ATPases implicated in clathrin-independent endocytosis, and recycling from endosomes. Here we show that EHDs share many common features with the dynamin superfamily, such as a low affinity for nucleotides, the ability to tubulate liposomes in vitro, oligomerization around lipid tubules in ring-like structures and stimulated nucleotide hydrolysis in response to lipid binding. We present the structure of EHD2, bound to a non-hydrolysable ATP analogue, and provide evidence consistent with a role for EHDs in nucleotide-dependent membrane remodelling in vivo. The nucleotide-binding domain is involved in dimerization, which creates a highly curved membrane-binding region in the dimer. Oligomerization of dimers occurs on another interface of the nucleotide-binding domain, and this allows us to model the EHD oligomer. We discuss the functional implications of the EHD2 structure for understanding membrane deformation. 相似文献
89.
V Smetacek C Klaas VH Strass P Assmy M Montresor B Cisewski N Savoye A Webb F d'Ovidio JM Arrieta U Bathmann R Bellerby GM Berg P Croot S Gonzalez J Henjes GJ Herndl LJ Hoffmann H Leach M Losch MM Mills C Neill I Peeken R Röttgers O Sachs E Sauter MM Schmidt J Schwarz A Terbrüggen D Wolf-Gladrow 《Nature》2012,487(7407):313-319
Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence-although each with important uncertainties-lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments. 相似文献
90.
中国股票市场的股票收益与波动关系研究 总被引:21,自引:0,他引:21
检验中国证券交易四类股票收益与波动的时间序列特征,以及收益与波动之间的关系。首先应用广义自回归条件异方差模型(GARCH)和指数GARCH模型以获得合适的条件方差序列。应用结果发现,波动性随时间变化的证据,并且波动高/低的时期趋向于聚集,显示出高度持续性和可预测性。然后,应用均值GARCH(GARCH-M)模型检验预期收益与预期风险的关系。研究结果认为,每日交易量作为每日信息到达时刻的代理变量对于中国股票市场每日收益的条件波动的解释力度不显著。 相似文献