Solving the riddle of codon usage preferences: a test for translational selection

Translational selection is responsible for the unequal usage of synonymous codons in protein coding genes in a wide variety of organisms. It is one of the most subtle and pervasive forces of molecular evolution, yet, establishing the underlying causes for its idiosyncratic behaviour across living kingdoms has proven elusive to researchers over the past 20 years. In this study, a statistical model for measuring translational selection in any given genome is developed, and the test is applied to 126 fully sequenced genomes, ranging from archaea to eukaryotes. It is shown that tRNA gene redundancy and genome size are interacting forces that ultimately determine the action of translational selection, and that an optimal genome size exists for which this kind of selection is maximal. Accordingly, genome size also presents upper and lower boundaries beyond which selection on codon usage is not possible. We propose a model where the coevolution of genome size and tRNA genes explains the observed patterns in translational selection in all living organisms. This model finally unifies our understanding of codon usage across prokaryotes and eukaryotes. Helicobacter pylori, Saccharomyces cerevisiae and Homo sapiens are codon usage paradigms that can be better understood under the proposed model

Positive Magneto-Resistance in Quasi-1D Conductors

We present here a simple qualitative model that interpolates between the high and low temperature properties of quasi-1D conductors. At high temperatures we argue that transport is governed by inelastic scattering whereas at low temperatures the conductance decays exponentially with the electron dephasing length. The crossover between these regimes occurs at the temperature at which the elastic and inelastic scattering times become equal. This model is shown to be in quantitative agreement with the organic conductor $TTT_2I_{3-\delta}$. Within this model, we also show that on the insulating side, the positive magnetoresistance of the form $(H/T)^2$ observed in $TTT_2I_{3-\delta}$ and other quasi-1D conductors can be explained by the role spin-flip scattering plays in the electron dephasing rate.Comment: 4 pages, Latex, no figure

Eye-Safe Solid-State Quasi-CW Raman Laser with Millisecond Pulse Duration

We demonstrate the first quasi-CW (ms-long pulses, pump duty cycle of 10%) end-diode pumped solid state laser generating eye-safe radiation via intracavity Raman conversion. The output power at the first Stokes wavelength (1524 nm) was 250 mW. A theoretical model was applied to analyze the laser system and provide routes for optimization. The possibility of true CW operation was discussed.Comment: Preprint accepted for publication in Optics Communications on Feb 6, 201

Negative magneto-resistance of electron gas in a quantum well with parabolic potential

We have studied the electrical conductivity of the electron gas in parallel electric and magnetic fields directed along the plane of a parabolic quantum well (across the profile of the potential). We found a general expression for the electrical conductivity applicable for any magnitudes of the magnetic field and the degree of degeneration of the electron gas. A new mechanism of generation of the negative magnetoresistance has been revealed. It has been shown that in a parabolic quantum well with a non-degenerated electron gas the negative magnetoresistance results from spin splitting of the levels of the size quantization.Comment: 15 pages, 3 figure

Ytterbium and chromium doped composite Y3Al5O12 ceramics self-Q-switched laser

Composite Yb:Y3Al5O12/Cr:Y3Al5O12 ceramics were fabricated by using vacuum sintering technique and nanocrystalline technology. Self-Q-switched composite Yb:Y3Al5O12/Cr:Y3Al5O12 ceramic lasers with pulse energy of 125 mu J and peak power of over 105 kW at repetition rate of 3.8 kHz have been demonstrated. Nearly diffraction-limited beam quality with M-2 less than 1.35 was achieved in this composite Yb:Y3Al5O12/Cr:Y3Al5O12 ceramic self-Q-switched lasers. (C) 2007 American Institute of Physics