2,502 research outputs found
Studies on Selection in Natural and Experimental Populations of Drosophila Pseudoobscura
Two genetic characters in Drosophila pseudoobscura were utilized in an investigation of some of the genetic mechanisms in evolution. The characters are (1) body size, which is a continuously-varying, polygenic trait, and (2) the arrangement of genes along the third chromosome, which is a Mendelizing, discrete trait. Collections of Drosophila pseudoobscura were taken in many localities in the American West. The two characters vary\u27 regularly with the physiographic division of the West. This variation is evidence that the frequencies of the genes controlling each character are strongly regulated by selection; such variation is the first stage in the genetic divergence which leads to the formation of new species. The frequencies of the gene arrangements on the third chromosomes are contrasted with those obtained in previous samples dating back as far as thirty years. A consistent pattern of change is apparent. The agent of selection responsible for these changes cannot be decided at present, although several possibilities are discussed. The system of inversions on the third chromosome is shown to be independent of that on the X-chromosome. One of the commonest geographic variations of insects is that of body size with temperature, the genetically larger strains coming from the cooler regions. Body size was studied in six experimental populations of Drosophila pseudoobscura which had been exposed to different temperatures. These populations were genetically identical at their inception but were maintained thereafter at different temperatures. After six years a striking genetic divergence in body size was found. The populations kept at the lower temperature had genetically larger flies than those kept at the higher temperatures. Crosses between the populations showed that the genes for larger size are partially dominant. The temperature-directed selection tor body size in these experimental populations may well be similar to that which has produced the temperature-oriented gradients for body size in natural populations of several species of Drosophila
Kondo effect in real quantum dots
Exchange interaction within a quantum dot strongly affects the transport
through it in the Kondo regime. In a striking difference with the results of
the conventional model, where this interaction is neglected, here the
temperature and magnetic field dependence of the conductance may become
non-monotonic: its initial increase follows by a drop when temperature and
magnetic field are lowered
Rotons and Quantum Evaporation from Superfluid 4He
The probability of evaporation induced by and rotons at the
surface of superfluid helium is calculated using time dependent density
functional theory. We consider excitation energies and incident angles such
that phonons do not take part in the scattering process. We predict sizable
evaporation rates, which originate entirely from quantum effects. Results for
the atomic reflectivity and for the probability of the roton change-mode
reflection are also presented.Comment: 11 pages, REVTEX, 3 figures available upon request or at
http://anubis.science.unitn.it/~dalfovo/papers/papers.htm
Theory of the Fano Resonance in the STM Tunneling Density of States due to a Single Kondo Impurity
The conduction electron density of states nearby single magnetic impurities,
as measured recently by scanning tunneling microscopy (STM), is calculated,
taking into account tunneling into conduction electron states only. The Kondo
effect induces a narrow Fano resonance in the conduction electron density of
states, while scattering off the d-level generates a weakly energy dependent
Friedel oscillation. The line shape varies with the distance between STM tip
and impurity, in qualitative agreement with experiments, but is very sensitive
to details of the band structure. For a Co impurity the experimentally observed
width and shift of the Kondo resonance are in accordance with those obtained
from a combination of band structure and strongly correlated calculations.Comment: 4 pages, ReVTeX + 4 figures (Encapsulated Postscript), submitted to
PR
Quantum dots with even number of electrons: Kondo effect in a finite magnetic field
We study a small spin-degenerate quantum dot with even number of electrons,
weakly connected by point contacts to the metallic electrodes, and subject to
an external magnetic field. If the Zeeman energy B is equal to the
single-particle level spacing in the dot, the ground state of the dot
becomes doubly degenerate, and the system exhibits Kondo effect, despite the
fact that B exceeds by far the Kondo temperature . A possible
realization of this in tunneling experiments is discussed
Kondo effect in quantum dots
We review mechanisms of low-temperature electronic transport through a
quantum dot weakly coupled to two conducting leads. Transport in this case is
dominated by electron-electron interaction. At temperatures moderately lower
than the charging energy of the dot, the linear conductance is suppressed by
the Coulomb blockade. Upon further lowering of the temperature, however, the
conductance may start to increase again due to the Kondo effect. We concentrate
on lateral quantum dot systems and discuss the conductance in a broad
temperature range, which includes the Kondo regime
Holocene pollen records from the central Arctic Foothills, northern Alaska: testing the role of substrate in the response of tundra to climate change
1   To explore the role of edaphic controls in the response of arctic tundra to climate change, we analysed Holocene pollen records from lakes in northern Alaska located on glaciated surfaces with contrasting soil texture, topography and tundra communities. Using indicator taxa, pollen accumulation rates (PARs) and multivariate comparison of fossil and modern pollen assemblages, we reconstructed the vegetational changes at Upper Capsule Lake (Sagavanirktok surface) and Red Green Lake (Itkillik II surface) in response to increased effective moisture between the early and middle Holocene. 2   In the Red Green record, low PARs and the continuous presence of taxa indicative of prostrate-shrub tundra (PST; Equisetum , Polypodiaceae, Thalictrum and Rosaceae) indicate that the vegetation resembled PST throughout the Holocene. During the warm, dry early Holocene (11 300–10 000 cal years BP), PST also occurred on Sagavanirktok surfaces, as evidenced by PST indicators (Bryidae, Polypodiaceae, Equisetum and Rosaceae) in this interval of the Upper Capsule record. However, PARs increased, suggesting increased vegetation cover, PST taxa declined and taxa indicative of dwarf-shrub tundra (DST; Rubus chamaemorus and Lycopodium annotinum ) increased between 10 000 and 7500 cal years BP. 3   We hypothesize that between the early and middle Holocene the fine-textured soils and smooth topography of Sagavanirktok surfaces led to increased soil moisture, greater vegetation cover, permafrost aggradation, anoxic and acidic soil conditions, slower decomposition and the development of a thick organic layer. In contrast, soil moisture remained low on the better-drained Itkillik II surface, and vegetational changes were minor. 4   Landscape-scale substrate variations have an effect on how tundra responds to climate change, suggesting that the response of arctic ecosystems to future variability may be spatially heterogeneous. Journal of Ecology (2003) 91 , 1034–1048Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73204/1/j.1365-2745.2003.00833.x.pd
The transcriptional repressor bs69 is a conserved target of the e1a proteins from several human adenovirus species
Early region 1A (E1A) is the first viral protein produced upon human adenovirus (HAdV) infection. This multifunctional protein transcriptionally activates other HAdV early genes and reprograms gene expression in host cells to support productive infection. E1A functions by interacting with key cellular regulatory proteins through short linear motifs (SLiMs). In this study, the molecular determinants of interaction between E1A and BS69, a cellular repressor that negatively regulates E1A transactivation, were systematically defined by mutagenesis experiments. We found that a minimal sequence comprised of MPNLVPEV, which contains a conserved PXLXP motif and spans residues 112–119 in HAdV-C5 E1A, was necessary and sufficient in binding to the myeloid, Nervy, and DEAF-1 (MYND) domain of BS69. Our study also identified residues P113 and L115 as critical for this interaction. Furthermore, the HAdV-C5 and-A12 E1A proteins from species C and A bound BS69, but those of HAdV-B3,-E4,-D9,-F40, and-G52 from species B, E, D, F, and G, respectively, did not. In addition, BS69 functioned as a repressor of E1A-mediated transactivation, but only for HAdV-C5 and HAdV-A12 E1A. Thus, the PXLXP motif present in a subset of HAdV E1A proteins confers interaction with BS69, which serves as a negative regulator of E1A mediated transcriptional activation
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