75,661 research outputs found

    A peridynamic theory for linear elastic shells

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    A state-based peridynamic formulation for linear elastic shells is presented. The emphasis is on introducing, possibly for the first time, a general surface based peridynamic model to represent the deformation characteristics of structures that have one physical dimension much smaller than the other two. A new notion of curved bonds is exploited to cater for force transfer between the peridynamic particles describing the shell. Starting with the three dimensional force and deformation states, appropriate surface based force, moment and several deformation states are arrived at. Upon application on the curved bonds, such states beget the necessary force and deformation vectors governing the motion of the shell. Correctness of our proposal on the peridynamic shell theory is numerically assessed against static deformation of spherical and cylindrical shells and flat plates

    Graviton scattering and matter distribution

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    In this model gravitation results from the emission and absorption of quanta (gravitons) that are scattered a few times in crossing a typical galaxy. Many features of the universe can be explained in terms of this model, although theoretical justification for the scattering of gravitons is lacking. Gravitons follow a random walk and diffuse through the outer regions of a galaxy. As a result the force of attraction follows a 1/R law, matching observed galactic rotation curves and explaining galactic dynamics without the need of dark matter. The model makes predictions regarding early stages in the expansion of the universe and the establishment of the mass distribution. It may be assumed that a nearly uniform expanding cloud of gas was present that was subject to collapse under gravitational forces. The 1/R law of attraction due to graviton diffusion is orders of magnitude more effective for initiation of collapse than the inverse square law, and it applies to blocks of gas larger than the graviton mean free path. Delay in the spread of gravitational attraction by diffusion sets a time-dependent range beyond which the attractive force is zero. In the model this causes arrays of matter to collapse locally into zones with a spacing set by the length of the range of the attractive force. An initial examination indicates that under these conditions the background radiation could have been released from a nearly uniform distribution at the time of decoupling of radiation and matter, followed by gravitational collapse into blocks of galactic mass. In the model the diffusion of gravitons continued and collapse became possible on a larger scale, initiating the formation of galactic clusters and still larger structures. The slow rate of diffusion then prevented the largest structures from attracting each other and permitted the formation of the voids on a very large scale. The model predicts that on the largest scale there is a three-dimensional repeated array of structures separated by voids. Ultimately structures larger than galactic clusters outran the diffusion of the gravitons and have since been freely expanding

    Transposable element insertions have strongly affected human evolution

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    Comparison of a full collection of the transposable element (TE) sequences of vertebrates with genome sequences shows that the human genome makes 655 perfect full-length matches. The cause is that the human genome contains many active TEs that have caused TE inserts in relatively recent times. These TE inserts in the human genome are several types of young Alus (AluYa5, AluYb8, AluYc1, etc.). Work in many laboratories has shown that such inserts have many effects including changes in gene expression, increases in recombination, and unequal crossover. The time of these very effective changes in the human lineage genome extends back about 4 million years according to these data and very likely much earlier. Rapid human lineage-specific evolution, including brain size is known to have also occurred in the last few million years. Alu insertions likely underlie rapid human lineage evolution. They are known to have many effects. Examples are listed in which TE sequences have influenced human-specific genes. The proposed model is that the many TE insertions created many potentially effective changes and those selected were responsible for a part of the striking human lineage evolution. The combination of the results of these events that were selected during human lineage evolution was apparently effective in producing a successful and rapidly evolving species

    Message from the president

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    As an institution committed to civil interchange about the important topics of our time, we believe that discourse, no matter how passionate, can and must be conducted with reason and respect because we also celebrate those values as inherently important to our community

    Idiosyncratic evolution of conserved eukaryote proteins that are similar in sequence to archaeal or bacterial proteins

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    	Sequence comparisons have been made between the proteins of 571 prokaryote species including 46 archaea and 525 bacteria and the set of human proteins. Highly conserved eukaryotic proteins are often strikingly similar in sequence to archaeal and bacterial proteins. Yet in many cases similarity to archaeal proteins is not correlated to the similarity to bacterial proteins. In these comparisons there are hundreds of eukaryote proteins that match well archeal proteins, but do not match recognizably to bacterial proteins, while thousands of proteins match well to bacterial proteins but not recognizably to archeal proteins. Forty percent of the 21,440 human proteins that significantly match prokaryote proteins are in this extreme idiosyncratic category. These relationships have been preserved over billions of years since the last common ancestor or sharing of protein genes between prokaryotes and eukaryotes. For each of the 21,440 members of this set of human proteins (that make significant matches to any of the 1.8 million proteins in this set of prokaryote species protein libraries) it is certain that each protein has important functions both in prokaryotes and eukaryotes and the precursor proteins have been important in the precursor species of both. That is the only explanation for the preservation of amino acid sequence similarity for the billions of years since the last common ancestor or period of sharing of proteins. Comparisons were made between the proteins of Arabidopsis thaliana and Saccharomyces cerevisiae to the proteins of the 571 prokaryote species. The results agreed with the human comparisons indicating that the conclusions apply to eukaryotes generally

    The Physician, the Hospital and Our Obligation to Teach

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    Message from the president

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    As an institution committed to civil interchange about the important topics of our time, we believe that discourse, no matter how passionate, can and must be conducted with reason and respect because we also celebrate those values as inherently important to our community

    Precise sequence complementarity between yeast chromosome ends and two classes of just-subtelomeric sequences

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    The terminal regions (last 20 kb) of Saccharomyces cerevisiae chromosomes universally contain blocks of precise sequence similarity to other chromosome terminal regions. The left and right terminal regions are distinct in the sense that the sequence similarities between them are reverse complements. Direct sequence similarity occurs between the left terminal regions and also between the right terminal regions, but not between any left ends and right ends. With minor exceptions the relationships range from 80% to 100% match within blocks. The regions of similarity are composites of familiar and unfamiliar repeated sequences as well as what could be considered "single-copy" (or better "two-copy") sequences. All terminal regions were compared with all other chromosomes, forward and reverse complement, and 768 comparisons are diagrammed. It appears there has been an extensive history of sequence exchange or copying between terminal regions. The subtelomeric sequences fall into two classes. Seventeen of the chromosome ends terminate with the Y' repeat, while 15 end with the 800-nt "X2" repeats just adjacent to the telomerase simple repeats. The just-subterminal repeats are very similar to each other except that chromosome 1 right end is more divergent
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