This Week in Science

Science  12 Jan 2001:
Vol. 291, Issue 5502, pp. 205
  1. Microfluid Actuation

    For complex microfluid systems (such as transporting chemicals in lab-on-chip devices) that may comprise many thousands of micrometer-sized flow channels, a method for the controlled movement of fluid through individual channels will be required. Prins et al. (p. 277) describe how electrocapillary pressure can be used to actuate fluid flow in an array of such microchannels. In their design, an electrode is embedded within the walls of each flow channel. Application of a bias to an individual channel electrode attracts the conducting liquid to the wall, effectively decreasing the diameter of the capillary channel, and results in upward movement of the liquid.

  2. Magic Wires Under Tension

    The recent discovery of helical multishell gold wires provides a particularly clear demonstration of the effect of size and dimension on nanoscale materials. Regular but noncrystalline nanowires suspended between tips were shown to form discrete multishell structures of specific “magic” sizes. Tosatti et al. (p. 288) have performed density functional calculations of the wires and show that nanowire stability is determined by the wire string tension rather than its total free energy. “Magic” structures correspond to string tension minima; no such minima are found for silver wires.

  3. Superconducting DNA

    Long strands of DNA have been proposed to provide a possible solution for molecular wiring in nanoscale electronics, but the conduction state of DNA remains unclear. Bridging the gap between superconducting rhenium/carbon electrodes with strands of λ-DNA, Kasumov et al. (p. 280) show that, below the transition temperature of the superconducting contacts, a proximity-induced superconductivity effect is produced in the DNA. The observation of this proximity effect, occurring when the superconducting wave function penetrates into a normal conductor, suggests not only that the molecules conduct, but that their phase coherence length is of the order of their actual length.

  4. Turning On Defects in Carbon Nanotubes

    As one of the leading contenders for applications in molecular electronics, much effort is being spent on elucidating the electronic and structural properties of carbon nanotubes. While most experimental and theoretical work has concentrated on defect-free nanotubes, defects can play a role in determining how such materials will operate in device structures. Bockrath et al. (p. 283) present experimental results on metallic single-walled carbon nanotubes showing that defects can result in inhomogeneous electronic properties along the length of the tube. Specifically, when the tubes are gated, the defects are switched on, and electronic properties of the tubes may then differ from what was expected.

  5. Primate Genetic Engineering

    The ability to genetically engineer nonhuman primates would facilitate the development of animal models of human diseases. Chan et al. (p. 309; see the news story by Vogel) transferred the gene encoding green fluorescent protein (GFP) to rhesus macaques oocytes by injecting a pseudotyped retroviral vector. After intracytoplasmic sperm injection and transfer of the embryos to surrogate mothers, one live animal and one stillborn set of twins were observed to be transgenic by polymerase chain reaction analysis and by fluorescence of toenails, hair, and placenta.

  6. A One-Component Molecular Metal

    The formation of a metal requires a source of conduction-band carriers (electrons or holes) and good overlap between their wave functions. Although many elements can form metals as one-component systems (such as silver and sodium), molecular metals usually need two components to create carriers through a charge transfer process. Tanaka et al. (p. 285; see the Perspective by Cassoux) now report that single crystals of [Ni(tmdt)2] show metallic behavior from room temperature down to 0.6 K. Moreover, first-principles calculations indicate that this material has a fully three-dimensional band structure despite the planar nature of the molecule.

  7. Forcing Glacial Cycles

    Milankovitch theory says that the timing of glacial cycles is controlled by July insolation (i.e., the season during which fallen snow either melts or persists to help form ice sheets) at 65°N (a latitude near the center of the range of where the vast Northern Hemisphere continental ice sheets accumulate during glaciations). The timing of the last deglaciation (termination 1, about 20,000 years ago) supports this hypothesis, but some evidence indicates that the penultimate deglaciation (termination 2) occurred thousands of years before the date of about 127,000 years ago predicted by Milankovitch. This apparent contradiction makes it important to determine precisely when other deglaciations happened. Stirling et al. (p. 290) have measured the ages of corals that formed during the high sea-level stands after termination 4 and find that their dates agree with Milankovitch.

  8. Mixed Ancestry

    Accumulating evidence indicates that hominids first migrated out of Africa into Eurasia more than one million years ago and reached lands as distant as parts of Indonesia. Yet molecular evidence suggests that modern humans arose from a separate, much later, migration. Models of human migration and evolution depend on the extent to which these later hominids replaced or interbred with earlier populations, which include, for example, the Neandertals in Europe. As a test, Wolpoff et al. (p. 293; see the news story by Pennisi) examined shared physical characteristics in two populations at far ends of the migration, in Australia and Europe. They conclude that the shared features of both populations argue against a complete replacement model.

  9. Tracing Otolith Signatures

    Natural chemical labels—relative concentrations of stable isotopes and of elements—increasingly are used to monitor migratory patterns of animals. Thorrold et al. (p. 297; see the news story by Malakoff) provide estimates of natal homing in estuarine-spawning marine fish by examining the chemical signatures of otoliths (“earbones”) in juvenile and adult weakfish along the east coast of the United States. They show details of natal homing and population differentiation not revealed by genetic studies, indicating that models used in fisheries management need to incorporate a spatial component.

  10. Guided by the Sun

    Tens of millions of shorebirds migrate from the Southern Hemisphere to breed in the high Arctic and then to return in the fall. Progress has been made on understanding how migrating birds use different compasses based on the sun, light polarization patterns, stars, and geomagnetic fields, but most studies have used cage experiments rather than observations in the field. Alerstam et al. (p. 300; see Perspective by Wehner) investigate this problem close to the magnetic north pole in the high Arctic of Canada, using radar to track migrating flocks. The results indicate that the birds follow sun compass trajectories that—at least in high latitudes—conform closely to cost-efficient great circle routes.

  11. Heart of the Matter

    Dilated cardiomyopathy is a severe cardiac disease in which the ventricles of the heart become enlarged, leading to compromised function. Several studies have suggested that a dysregulated immune system may be involved in some forms of this condition. In the course of analyzing the immunoregulatory molecule, PD-1, Nishimura et al. (p. 319) observed that genetic deficiency for PD-1 resulted in a form of dilated cardiomyopathy. This disease only developed when the mice were bred on a particular genetic strain and when the animals were otherwise immunologically intact.

  12. How to Make Auxin

    Auxin is an essential plant hormone that is involved in regulation of plant growth and development, however the mechanism of auxin biosynthesis in plants is poorly understood. Now Zhao et al. (p. 306) show that the Arabidopsis gene YUCCA encodes a flavin monooxygenase-like enzyme that catalyzes hydroxylation of the amino group of tryptamine. The step appears to be rate-limiting in auxin biosynthesis via a tryptophan-dependent pathway.

  13. Neuronal Representation of Visual Categories

    Categories are the fundamental building blocks of nearly all higher level cognitive functions. Using a sophisticated graphics system, Freedman et al. (p. 312; see the Perspective by Thorpe and Fabre-Thorpe) generated stimuli that were morphs between prototype cats and dogs. The activity of many neurons in the primate lateral prefrontal cortex reflected the category of visual stimuli rather than physical appearance. In addition, categorical representations changed during learning, when stimuli were assigned to new categories.

  14. Export Signal Unveiled

    Newly synthesized plasma membrane proteins enter the secretory pathway at the level of the endoplasmic reticulum (ER). Export from the ER has been thought to be by default for proteins that are correctly assembled and folded. Ma et al. (p. 316) examined the regulation of export of a potassium channel from the ER and found that the protein included a pair of sorting motifs that stimulated export from the ER, thereby controlling the level of surface expression.

  15. Ducking Destruction

    Subclinical cytomegalovirus (CMV) infections are lifelong, can be life-threatening in AIDS, and are associated with arterial disease. Laboratory strains of human CMV do not grow on vascular endothelial cells, but laboratory strains of the mouse virus do. Brune et al. (p. 303) have developed a technique to make mouse virus mutants and have used these to identify a gene (M45) that controls this tissue specificity. They also discovered that, when cells are infected with viruses containing mutated M45, the cells die rapidly by apoptosis. So, not only does M45 govern which cells are infected, it also prevents the host from killing infected cells and thus reducing the spread of infection.

  16. Advances in ESR

    Electron spin resonance (ESR) spectroscopy can be used to study structure and dynamics in membranes and proteins. Borbat et al. (p. 266) review current ESR techniques based on nitroxide spin labeling of biomolecules. High frequency continuous wave ESR allows measurement of internal dynamics in proteins. This can be combined with lower frequency measurements to unravel complex protein dynamics. Two-dimensional Fourier transform ESR allows accurate measurement of distances in biomolecules and can be used to study the structure of membrane domains.

  17. Mechanism of Ribosomal Peptide Bond Formation

    Three papers in the 11 Aug. 2000 issue characterized the atomic structure of the large ribosomal subunit, and used that structural underpinning to explore the mechanisms of peptide bond formation in the ribosome. In a comment, Barta et al. question whether the highly conserved nucleotide A2451 is the sole catalytic site, as suggested in the 11 Aug. papers, and argue that metal ion catalysis may remain a viable alternative to the acid-base catalysis model favored in those studies. Berg and Lorsch, in a separate comment, put forth an alternative mechanism for peptide bond formation that they maintain avoids “three significant difficulties” implicit in the mechanism proposed in the 11 Aug. papers. In their response, Nissen et al. critically review the literature cited by Barta et al. and conclude that it does not substantially undermine their proposals; they also present additional information bearing on the three deficiencies alleged by Berg and Lorsch. The full text of these comments can be seen at

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