This Week in Science

Science  02 Apr 2004:
Vol. 304, Issue 5667, pp. 13
  1. Breaking Up Is Hard to Do


    During the cell cycle, cohesins maintain the association between sister chromatids after replication until anaphase. During mitosis, cohesins dissociate from mitotic chromosomes, first during prophase—when the bulk of cohesins along chromosome arms are removed, and then in metaphase—when centromeric cohesins are removed. Now Dynek and Smith (p. 97; see the Perspective by Azzalin and Lingner) suggest that resolution of sister chromatids and mitotic progression in human cells also requires tankyrase 1 to act to promote sister telomere dissociation.

  2. Low Oxygen in Old Oceans

    Earth's early atmosphere was reducing, and atmospheric O2 appeared in appreciable amounts only about 2.3 billion years ago. When did the deep ocean become oxygenated? Banded iron formations, which are indicative of low O2 concentrations, were produced until 1.8 billion years ago, but that does not necessarily mean that O2 appeared in the deep ocean at that point. It has been suggested that the deep oceans remained anoxic or euxinic (low in O2 and high in H2S) until 1 billion years ago, only then reaching O2 concentrations near today's values. Arnold et al. (p. 87) test that hypothesis by measuring the isotopic composition of molybdenum, which is sensitive to redox conditions, in modern and ancient marine sediments. Their results from samples 1.7 and 1.4 billion years old indicate that the deep ocean at those times was largely euxinic, an important finding for understanding Proterozoic ecosystems and biogeochemical cycles.

  3. A Quantum Resonator

    The precision to which the position of an object can be determined is limited by the Heisenberg uncertainty principle. LaHaye et al. (p. 74; see the Perspective by Blencowe) have developed an ultrasensitive displacement detector in which the position of a nanomechanical resonator is detected by the charge state of a single electron transistor. The displacement can be determined with a precision approaching the limit set by quantum mechanics, which should prove of practical use in probe microscopy and metrology applications.

  4. Following Water Films on the Fly


    The dynamics of ice-film formation and its subsequent melting have been followed by Ruan et al. (p. 80), who used picosecond time-resolved electron diffraction to study water adsorbed on a hydrophobic chlorine-terminated silicon substrate. This method allowed the water structure to be determined separately from the substrate. During growth near equilibrium, an epitaxial layer of water grew that adjusted to the Cl termination in a bilayer fashion. For thicker layers, crystallites of cubic ice formed without orientation to the substrate. Time- dependent laser-induced melting studies showed changes on several time scales. Crystalline order was lost in the first 10 to 20 picoseconds, and OO distances became uncorrelated before that for the O-HO distances. The reestablishment of the ice structure took more than 1 nanosecond.

  5. Worth the Weights

    One way to model nonlinear systems is to use a “black-box” approach, in which a series of resonators are trained or weighted to reproduce input and output states. One type of neural network used in these cases are recurrent neural networks (RNNs) that maintain activation even without input stimulation and thus achieve dynamic memory. This approach has often been hampered by slow convergence and sub-par solutions in nonlinear systems. Jaeger and Haas (p. 78) now show that using a large net, in which links between an RNN and the output are changed during learning, rather than those in the whole net, can greatly increase accuracy. They use this approach to equalize a wireless communications channel.

  6. Shotgunning the Sea

    The Sargasso Sea is home to a vast array of microorganisms. Venter et al. (p. 66; see the Perspective by Falkowski and de Vargas) carried out a shotgun sequencing effort to sample microbial diversity within Sargasso Sea samples, and identified more than 1.2 million new genes. Reconstructions of whole genomes were performed, and initial estimates produced for species diversity and abundance. The data set provides raw materials for analyses of the metabolic potential of ocean life and of ocean ecology at an unprecedented scale.

  7. SUMO Wrestling in Huntington's Disease

    Huntington's disease (HD) is a fatal neurodegenerative disease characterized by accumulation of an aberrant form of a protein called huntingtin containing an abnormal polyglutamine repeat expansion. Steffan et al. (p. 100) find that huntingtin can be modified by SUMO-1. Sumoylated huntingtin is stabilized, is less likely to form aggregates, and has an increased capacity to repress transcription. In a Drosophila model of HD, sumoylation of huntingtin on three lysine residues exacerbated neurodegeneration, whereas ubiquitination of the same three lysine residues abrogated neurodegeneration.

  8. The Footsteps of Evolution

    It was long thought that vertebrate feet evolved as early fish began to move on land, but some recent fossils have suggested that tetrapods first evolved in an aquatic environment. Shubin et al. (p. 90; see the cover and the Perspective by Clack) now describe a humerus from an early tetrapod that lived about 370 million years ago, in what is now Pennsylvania, that sheds light on this transition. Analysis of the fossil indicates that limbs evolved in fish fins to support the body and could produce a walking gait. These findings may also help explain some enigmatic early tetrapod trackways preserved in river beds.

  9. Reconstituting Human Immunity

    Reconstitution of an intact human immune system within a laboratory animal model would allow for experimental manipulation of human immune responses in an in vivo setting. Traggiai et al. (p. 104) used CD34+ human cord blood cells to reconstitute newborn immune-deficient mice that lacked lymphocyte subsets. Intact immune cell development, which persisted within the mice for at least 6 months, generated mature T and B lymphocytes as well as dendritic cells. Functional cellular interactions were apparent between the reconstituted cells, as well as with host cells, and these could produce appropriate immune responses. These humanized mice will allow new questions to be addressed about the development and function of the human immune system.

  10. Wired for Weight Control


    Leptin is a fat-derived hormone that plays a key role in regulating body weight. Surprising results from two independent research groups may force revision of current models of when and how leptin exerts it effects on body weight (see the Perspective by Elmquist and Flier). During a restricted period in neonates, Bouret et al. (p. 108) show that leptin functions as a neurotrophic factor by directing the formation of neural projection pathways in the arcuate nucleus of the hypothalamus (ARH). Later in life, leptin targets these same pathways to regulate food intake and energy balance. Pinto et al. (p. 110) find that adult mice deficient in leptin differ significantly from wild-type mice in the number of excitatory and inhibitory synapses in the ARH. Single-dose administration of leptin to the mutant mice induces rapid rewiring of the synaptic connections so that they more closely resemble those seen in wild-type mice.

  11. Fuzzy Face Recognition

    In perception, both intrinsic and contextual cues can be effective when identifying objects. The neural correlates of intrinsically defined object perception have been widely studied. What brain mechanisms underlie the contextual influences on object perception? Cox et al. (p. 115) studied the influence of context on the perception of faces. Bodies with degraded faces activated the brain area involved in face perception, the fusiform face area (FFA), as effectively as intact faces. The same degraded face stimuli appearing in the absence of bodies, misaligned with bodies, or faceless bodies failed to activate FFA.

  12. Complex Response to DNA Damage

    Cells avoid the disastrous effects of DNA damage by activating a checkpoint mechanism to prevent proliferation of cells that contain damaged DNA. The protein kinase ATM is an important component of this signaling mechanism, but precisely how its activity is regulated has not been clear. The protein Nbs1, which is mutated in the human disorder, Nijmegen Breakage Syndrome, forms MRN complexes with two other proteins that participate in DNA repair, Mre11 and Rad50. Nbs is also a substrate for ATM. But now, Lee and Paull (p. 93) suggest that the interaction between Nbs1 and ATM is more complicated. MRN complexes directly activate enzymatic activity of ATM, possibly by causing a conformational change in ATM that alters the affinity of the kinase toward its substrates.

  13. Electron-Capturing Cluster

    A metal cluster with more than a dozen atoms would be expected to be highly reactive, but exceptions occur when the number of valence electrons of a cluster form a closed shell similar to that formed by the electrons of a noble gas. In the “jellium” model used to describe such clusters, a species like the Al13 anion with an electron count of 40 is especially stable. Bergeron et al. (p. 84) show that reaction of aluminum clusters with HI produces among its products Al13I, in which an ionic bond forms between the metal cluster anion and the neutral I atom. Calculations show that the electron localizes opposite the I atom. Reactions with HBr and HCl do not form similar products because Br and Cl have higher electron affinities than I and prevent the formation of a stable cluster by retaining their electron. The Al13 anion has characteristics of both a pseudohalogen like CN and a “superelectron.”

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