Catálogo de publicaciones - revistas

<jats:p>Rare antimatter hypernuclei can form when an ultrahigh-energy plasma of quarks and gluons cools down.</jats:p>

<jats:p>How are parental and newly synthesized histones distributed into nucleosomes during eukaryotic cell division?</jats:p>

<jats:title>Forming Antimatter Nuclei</jats:title> <jats:p> Atomic nuclei are everywhere and form all the matter visible to us in the universe. Their counterparts, however—antinuclei or antimatter—are relatively shy about making an appearance. <jats:bold> Chen <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="58" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1183980">58</jats:related-article> published online 4 March; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5974" page="55" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1187769">Cohen</jats:related-article> </jats:bold> ) used the Relativistic Heavy-Ion Collider to coax them into existence by colliding high-energy beams of gold ions head-on. Within the debris from the collisions, evidence was found in the decay paths and particle tracks that suggested the formation of nuclei from antimatter. The ability to form these exotic particles in abundance should help to probe fundamental aspects of nuclear physics, astrophysics, and cosmology. </jats:p>

<jats:title>Sperm Production-Line Maintenance</jats:title> <jats:p> The average man makes upwards of 1500 sperm per heartbeat. Such a feat requires a robust stem cell system. Using mice, <jats:bold> Nakagawa <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="62" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1182868">62</jats:related-article> , published online 18 March) shed light on some of the properties of the murine germline stem cell system that contribute to its robustness. During steady-state spermatogenesis, the majority of the stem cell population lies within a subset of cells called type A spermatogonia. However, during regeneration (for example, during recovery of the stem cell pool after drug exposure) the system in essence hijacks early-differentiating cells back into the stem cell compartment. Lineage analysis and live-imaging also suggests that during sperm production there is more than one path from a stem cell to differentiation. </jats:p>

<jats:title>Open and Closed Case</jats:title> <jats:p> Voltage-dependent ion channels are gated by voltage sensors that show a switchlike response to voltage differences across the membrane. <jats:bold> Tao <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="67" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185954">67</jats:related-article> ; see the cover) used mutagenesis, electrophysiology, and x-ray crystallography to gain insight into the molecular basis of this response in voltage-dependent potassium channels. An occluded site was identified that catalyzes translation of positive charges across the membrane. The closed channel appears to be associated with a distribution of conformations, depending on the degree of hyperpolarization of the membrane, whereas the open channel appears to be associated with a specific conformation. Thus, the transition of the ion channel from open to closed occurs over a very small voltage difference. </jats:p>

<jats:title>On the Origins of Magnetism</jats:title> <jats:p> The magnetic fields in galaxies and galaxy clusters are thought to result from the amplification of weak primordial magnetic fields, which, according to one class of theories, should exist in the voids between galaxies and galaxy clusters. <jats:bold>Neronov and Vovk</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="73" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1184192">73</jats:related-article> ) present evidence for the existence of intergalactic magnetic fields and derive a lower limit for their strength, based on an analysis of data from the Fermi Large Area Telescope. The results place constraints on magnetogenesis models and suggest that magnetic fields originated in the early universe before galaxy formation took place. </jats:p>

<jats:title>Thin Friction</jats:title> <jats:p> The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? <jats:bold> Lee <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="76" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1184167">76</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="5974" page="52" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1188086"> <jats:bold>Müser and Shakhvorostov</jats:bold> </jats:related-article> </jats:bold> ) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. </jats:p>

<jats:title>Oceans Before the Dawn</jats:title> <jats:p> Although the fossil record shows a clear explosion of animal diversity in the oceans at the beginning of the Cambrian period (∼542 million years ago), the evolutionary pressures driving this increase in diversity remain unclear. The likely scenario involves drastic changes in the distribution of oxygen in ocean basins, but global glaciations and poor preservation of rocks from this time often prevent a clear picture of global ecology before the dawn of animal life. <jats:bold> Li <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="80" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1182369">80</jats:related-article> , published online 11 February; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5974" page="53" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1188688">Narbonne</jats:related-article> </jats:bold> ) characterized the geochemical makeup of sedimentary rocks from south China that indicate alternating layers of sulfide- and iron-rich (i.e., sulfate-limited) anoxic waters in the several million years leading up to the Cambrian. These conditions may have set the stage for an eventual increase in productivity of photosynthetic life that oxygenated the oceans and allowed for rapid animal evolution. </jats:p>

<jats:title>Sinking Sea Floors</jats:title> <jats:p> The depths of ocean bottoms are constantly fluctuating at a very slow rate in response to the generation (at mid-ocean ridges) and consumption (at subduction zones) of sea-floor material. Because older sea floor is susceptible to sinking as it cools, it has been assumed that sea-floor depth varies directly with its age. However, <jats:bold>Adam and Vidal</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="83" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185906">83</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="5974" page="54" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1188090">Tolstoy</jats:related-article> </jats:bold> ) now show that the depth of the Pacific Ocean actually varies in response to the mantle underlying the oceanic crust. This effect is clear when sea-floor depth was measured along lithospheric flow lines—which represent the movement of oceanic crust triggered by mantle convection. Because the ocean bottom does not flatten as predicted by previous models, there is no need to invoke any additional heat supply to sustain old oceanic crust in thermal models of the mantle. </jats:p>

<jats:title>Flower Power</jats:title> <jats:p> The transcription factor APETALA1 (AP1) controls the transition from vegetative growth to flower production in the plant <jats:italic>Arabidopsis</jats:italic> . A handful of factors that control AP1 have been identified, as well as some targets that AP1 controls. <jats:bold> Kaufmann <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="85" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185244">85</jats:related-article> ) now apply genome-wide microarray analysis to identify over a thousand genes whose transcription is regulated by AP1. By proximity to AP1 binding sites, over two thousand genes are implicated as putative AP1 targets. Analysis of this network of interactions indicates that AP1 functions first to repress vegetative identity, then to help establish floral primordia, and finally to shape the differentiation of floral parts. </jats:p>