Catálogo de publicaciones - revistas

<jats:title>Like Beads on a String</jats:title> <jats:p> The optimal packing of spheres, and the somewhat lower densities obtained in the compaction of granular materials are well studied problems. What is less clear is what happens when the spheres are connected, as in the case of polymeric materials—often represented by connected sphere models. <jats:bold> Zou <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="408" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177114">408</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5951" page="374" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1180973"> <jats:bold>Reichhardt and Lopatina</jats:bold> ) </jats:related-article> </jats:bold> examined the packing of chains of metal beads commonly used for securing bathroom drain plugs or for raising or lowering window blinds. Both the length of the chains, and whether they were linear or looped, influenced the overall packing density. Jamming the chains together captured the key physics of the glass transition of polymeric materials. </jats:p>

<jats:title>Magnetic Monopoles</jats:title> <jats:p> Magnets come with a north and a south pole. Despite being predicted to exist, searches in astronomy and in high-energy particle physics experiments for magnetic monopoles (either north or south on their own) have defied observation. Theoretical work in condensed-matter systems has predicted that spin-ice structures may harbor such elusive particles (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5951" page="375" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1181510">Gingras</jats:related-article> </jats:bold> ). <jats:bold> Fennell <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="415" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177582">415</jats:related-article> , published online 3 September) and <jats:bold> Morris <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="411" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178868">411</jats:related-article> , published online 3 September) used polarized neutron scattering to probe the spin structure forming in two spin-ice compounds—Ho <jats:sub>2</jats:sub> Ti <jats:sub>2</jats:sub> O <jats:sub>7</jats:sub> and Dy <jats:sub>2</jats:sub> Ti <jats:sub>2</jats:sub> O <jats:sub>7</jats:sub> —and present results in support of the presence of magnetic monopoles in both materials. </jats:p>

<jats:title>Magnetic Monopoles</jats:title> <jats:p> Magnets come with a north and a south pole. Despite being predicted to exist, searches in astronomy and in high-energy particle physics experiments for magnetic monopoles (either north or south on their own) have defied observation. Theoretical work in condensed-matter systems has predicted that spin-ice structures may harbor such elusive particles (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5951" page="375" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1181510">Gingras</jats:related-article> </jats:bold> ). <jats:bold> Fennell <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="415" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177582">415</jats:related-article> , published online 3 September) and <jats:bold> Morris <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="411" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178868">411</jats:related-article> , published online 3 September) used polarized neutron scattering to probe the spin structure forming in two spin-ice compounds—Ho <jats:sub>2</jats:sub> Ti <jats:sub>2</jats:sub> O <jats:sub>7</jats:sub> and Dy <jats:sub>2</jats:sub> Ti <jats:sub>2</jats:sub> O <jats:sub>7</jats:sub> —and present results in support of the presence of magnetic monopoles in both materials. </jats:p>

<jats:title>Diddy Dinosaurs</jats:title> <jats:p> Tyrannosaurs were the dominant large dinosaur predator during the Late Cretaceous. They have several distinct specialized features, including an oversized skull, huge hindlimbs, and tiny arms, that have been thought to have evolved in concert with their large size and carnivorous diet. <jats:bold> Sereno <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="418" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177428">418</jats:related-article> , published online 17 September; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5951" page="373" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1181276">Clark</jats:related-article> </jats:bold> ) now describe an earlier, diminutive tyrannosaur from China that also has these common specializations. Thus, these features were not a result of size increase but appear to have been required for feeding efficiency at all sizes. </jats:p>

<jats:title>Balancing the Nitrogen Budget</jats:title> <jats:p> Setting the global budget for elements presents difficult challenges, such as accounting for possibly unknown sources or sinks. An unresolved imbalance in the oceanic nitrogen budget suggests that there may be additional sources of biological nitrogen fixation in the deep sea. Using high-resolution imaging techniques, <jats:bold> Dekas <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="422" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178223">422</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5951" page="377" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1181129">Fulweiler</jats:related-article> </jats:bold> ) observed direct assimilation of isotopically labeled N <jats:sub>2</jats:sub> by anaerobic methane-oxidizing archaea from deep marine sediment and the subsequent transfer of nitrogen to their sulfate-reducing bacterial symbionts. This slow and energetically costly conversion by archaea is dependent upon methane and requires physical contact with the associated bacterial partner. Such syntrophic consortia represent a potential source of nitrogen in the oceans and may help to balance the global nitrogen budget. </jats:p>

<jats:title>A Fix to the Heart</jats:title> <jats:p> Regenerative cardiovascular medicine is a promising avenue for therapeutic application in advanced heart failure. Although clinical trials have suggested some limited benefits in cell transplantation therapy, robust cardiac muscle formation is lacking. <jats:bold> Domian <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="426" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177350">426</jats:related-article> ) examined the developmental processes in normal mature cardiac muscle. A two-color murine reporter system was used to isolate committed ventricular progenitors, which were then used to build functional force-generating cardiac tissue. Such combinations of tissue engineering and stem cell biology may eventually lead to cardiac regenerative therapy. </jats:p>

<jats:title>Haploid Medaka Stem Cells</jats:title> <jats:p> Although diploid embryonic stem cells have been generated by various means, there would also be value in deriving haploid stem cells. In these cells, recessive mutations in essential genes would show phenotypes that would not be apparent in heterozygous animals. <jats:bold> Yi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="430" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1175151">430</jats:related-article> ) used the medaka fish model system to generate haploid stem cells that show stable growth and pluripotency. In addition, a fertile female medaka fish was produced by haploid embryonic stem cell nuclear transfer into a normal egg. This system has potential for analyzing recessive genes, for example, in disease phenotypes or in various cell lineages in culture. </jats:p>

<jats:title>The Taming of the Silkworm</jats:title> <jats:p> Silkworms, <jats:italic>Bombyx mori</jats:italic> , represent one of the few domesticated insects, having been domesticated over 10,000 years ago. <jats:bold> Xia <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="433" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1176620">433</jats:related-article> , published online 27 August) sequenced 29 domestic and 11 wild silkworm lines and identified genes that were most likely to be selected during domestication. These genes represent those that enhance silk production, reproduction, and growth. Furthermore, silkworms were probably only domesticated once from a large progenitor population, rather than on multiple occasions, as has been observed for other domesticated animals. </jats:p>

<jats:title>Coupling Clocks and Metabolism</jats:title> <jats:p> Circadian clocks in mammals coordinate behavior and physiology with daily light-dark cycles by driving rhythmic transcription of thousands of genes. The master clock in the brain is set by light, but clocks in peripheral tissues, such as the liver, are set by daily feeding. Such coupling should allow tissues to “anticipate” food consumption and optimize the timing of metabolic processes, but how nutritional status is communicated to peripheral clocks is unclear. Studying cell culture models and mice, <jats:bold> Lamia <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="437" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1172156">437</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5951" page="378" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1181278"> <jats:bold>Suter and Schibler</jats:bold> </jats:related-article> </jats:bold> ) show that the nutrient-responsive signaling molecule AMPK (AMP-activated protein kinase) provides metabolic information to circadian clocks by triggering phosphorylation and subsequent degradation of the clock component cryptochrome-1. Thus—cryptochromes, which originally evolved as blue-light photoreceptors in plants, act as chemical energy sensors in mammals. </jats:p>

<jats:title>Teaching Teacher</jats:title> <jats:p> High-school science teachers rarely have an opportunity to participate in scientific research. A program from Columbia University studies what happens when these teachers do get laboratory experience. After a summer program spanning 2 years, including professional development programs, as well as laboratory research, <jats:bold> Silverstein <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="440" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177344">440</jats:related-article> ) show that the teachers' students benefit. Students of teachers who participated in this summer research experience showed greater pass rates on the tough New York State Regents exams than did students of teachers who did not participate in the program. </jats:p>