This Week in Science

Science  15 May 1998:
Vol. 280, Issue 5366, pp. 977
  1. End of an era

    The end-Permian extinction is the largest in Earth's history. Its age, 251 million years ago, corresponds to the eruption of the Siberian Traps, the largest flood basalt province on Earth. However, the cause of the extinction remains unclear, in part because the duration of the extinction and of the terminal part of the Permian has not been well resolved. Bowring et al. (p. 1039; see the news story by Kerr, (p. 1007) provide uranium-lead dates on several volcanic tuff horizons straddling the Permian-Triassic boundary in China. The data imply that the final extinction pulse there, corresponding to loss of most of the marine species, lasted less than 1 million years and perhaps occurred within only a few hundred thousand years.

  2. Ocean twister

    The steady venting of fluids from mid-ocean ridge hydrothermal systems at black smokers is dwarfed by event plumes or megaplumes that mark the rapid release of hot water associated with intrusion of new magma. These plumes can extend for tens of kilometers above a ridge and persist for months; they were first recognized from their anomalous helium isotopic composition and have been used to trace ocean circulation. Although there have been several theoretical studies of how they persist, in situ observations have been lacking. Lupton et al. (p. 1052; see the commentary by Speer, p. 1034) were able to insert a neutrally buoyant tracking float into an event plume that formed over the Gorda Ridge in spring 1996. During a 60-day period, the float traced an anticyclonic path marking the circulation of water in the plume. Chemical and physical data collected at the beginning and end of this period imply that the plume remained coherent and maintained concentrations of iron particles and dissolved manganese. The data imply that such plumes could persist for periods greater than 1 year.

  3. Performing PCR on a chip

    The three steps of the polymerase chain reaction (PCR), melting the double-stranded DNA, binding specific primers, and enzymatically extending the primers, have been automated on a chip. Kopp et al. (p. 1046) developed a continuous flow system that temperature-cycles the sample and the necessary reactants through 20 rounds of amplification. At the highest flow rates, the process is complete in 90 s, and successive samples appear to be amplified without cross-contamination.

  4. Fossil record of land bridges

    Several fossil remains of vertebrates from the Upper Cretaceous (about 80 million years ago) from Madagascar, including a nearly complete skull of a predatory dinosaur, are described by Sampson et al. (p. 1048; see the cover). Similar fossils have been so far found only in South America and in India, which was still connected to Madagascar in the Late Cretaceous, but not yet in Africa. This distribution suggests that land bridges remained between South America and India through Antarctica into the Latest Cretaceous.

  5. Fault asymmetries

    The ground motion caused by a thrust (reverse) fault is greater than the ground motion caused by a normal fault for earthquakes of the same magnitude. Oglesby et al. (p. 1055) performed dynamic simulations of earthquakes on different fault geometries and found that the difference in the intensity of the ground motions is caused by time-dependent stress produced by the interaction of the earthquake stress field with Earth's free surface. These simulations will be helpful for characterizing previous earthquakes and modeling ground motion for future events.

  6. Ironing out the lower mantle

    The formation of Earth's core required separation of iron melt from Earth's silicate mantle. Whether this separation involved percolation of melt through a solid silicate matrix or the settling of iron melt through a molten matrix may have depended on the nature of the pore network through MgSiO3 perovskite, the dominant silicate phase in the lower mantle. Shannon and Agee (p. 1059) measured experimentally the wetting angle between iron melts and perovskite. The data imply that percolation of iron through the Earth's lower mantle might have been possible, unlike the case for Earth's upper mantle, where the mineralogy is different.

  7. Forcing ferromagnetism

    Interactions between magnetic ions in oxides can be estimated theoretically to predict whether materials will exhibit ferromagnetism. Although theory predicts that Fe3+ and Cr3+ should be able to form ferromagnets in a perovskite structure, phase separation normally occurs in the bulk oxides and antiferromagnetic materials are formed. Ueda et al. (p. 1064) grew superlattices of alternating layers of LaCrO3 and LaFeO3 on the (111) face of SrTiO3. They show that this material, which forces the proximity of Fe3+ and Cr3+ ions, is ferromagnetic.

  8. Reconnected magnetotails

    The magnetosphere of Jupiter is controlled by currents in the jovian interior, but the volcanically active moon, Io, is expected to upset the magnetosphere by spewing out tons of ions in a magnetized plasma. Theories suggest that one way to release this plasma sheet is to reconnect the jovian magnetotails and allow the plasma to flow out of the system. Russell et al. (p. 1061) have been sampling the magnetospheric field from the Galileo spacecraft since its insertion into orbit around Jupiter in December 1995. They have found evidence for transient reconnections of the magnetotail that supports this theory and helps explain how Jupiter can maintain a balanced magnetic flux in the presence of Io.

  9. Alternative function for cell cycle inhibitor

    The protein p21 inhibits cell division by inhibiting the activity of cyclin-dependent kinases and binding to proliferating cell nuclear antigen (PCNA). Such effects of p21 may contribute to terminal differentiation of cells by promoting withdrawal from the cell division cycle. However, Di Cunto et al. (p. 1069; see the commentary by Jacks and Weinberg, p. 1035) report that the opposite is true in the later stages of differentiation of mouse keratinocytes. In this situation, p21 has an inhibitory effect on expression of markers of terminal differentiation. These results are consistent with observations that the amounts of p21 first increase as the cells begin to differentiate and then diminish. Furthermore, the inhibitory effects of p21 on differentiation appear to be produced independently of its effects on cell division.

  10. Lab error and transient HIV infection

    Numerous cases have been reported in which infants of HIV-infected mothers appear to have been transiently infected. If validated, these findings would have significant implications for thinking about the ability of the host immune response to successfully fight HIV. However, when Frenkel et al. (p. 1073) reanalyzed 42 cases in which a transient infection was suspected, the results were more likely due to mislabeling or laboratory contamination. The authors suggest that criteria for the conclusive demonstration of a transient infection should include phylogenetic analyses on previously unmanipulated portions of the specimens in a manner that can minimize cross-contamination.

  11. SNPing away at the human genome

    New tools are becoming available for the identification of disease genes, characterization of human traits, forensic identification, and studies of human evolution and diversity. Wang et al. (p. 1077) present a proof-of-principle study to show that large-scale analysis of single-nucleotide polymorphisms (SNPs) in human DNA is feasible. They identified more than 3200 candidate SNPs across the entire human genome (with about 40 percent confirmed so far), constructed a map more than 2000 of the loci, and used 500 of these to create a genotyping chip.

  12. Linking to Ras

    The small guanine nucleotide- binding protein Ras participates in control of cell signaling pathways that regulate diverse cellular functions from cell division to neuronal activity in the brain. Ebinu et al. (p. 1082) describe a new guanine nucleotide releasing factor, RasGRP. Ras is active when GTP (guanosine triphosphate) is bound to it, and exchange factors like RasGRP activate Ras by facilitating release of bound guanosine diphosphate and binding of GTP. RasGRP is found primarily in the brain and contains protein domains that bind the intracellular signaling molecules calcium and diacylglycerol. RasGRP appears to link signals that alter intracellular concentrations of calcium and diacylglycerol to changes in Ras activity.

  13. SMADs and cancer predisposition

    Familial juvenile polyposis (JP) is an autosomal dominant disease characterized by a predisposition to hamartomatous polyps and gastrointestinal cancer. Howe et al. (p. 1086; see the commentary by Kinzler and Vogelstein, p. 1036) show that a subset of JP families carry germline mutations in the SMAD4/DPC4 gene on chromosome 18q21.1. SMAD4 is a key player in the transforming growth factor-β (TGF-β)-signaling pathway, which mediates cell growth inhibitory signals from the cell surface to the nucleus.

  14. Jasmonic acid and auxin signaling

    Jasmonic acid and its derivatives affect developmental and defensive responses in a great variety of plants. Xie et al. (p. 1091) have cloned and mapped a gene from Arabidopsis that is required for responses to jasmonates. The coi1 gene encodes a protein that has intriguing similarities the tir1 gene, required for auxin signaling, suggesting that jasmonates and auxin may share some aspects of their signaling pathways. Both proteins include F-box motifs, which are found in proteins that form part of the ubiquitin-dependent protein degradation process.

  15. Surveying the damage

    DNA damage and its repair are key determinants in the early stages of carcinogenesis, cancer therapy, and aging. Le et al. (p. 1066) have developed an ultrasensitive assay for DNA damage that couples immunochemistry with capillary electrophoresis and laser-induced fluorescence detection. This assay, which is specific for radiation-induced thymine glycols, requires only nanogram amounts of DNA and can detect a single base modification among 109 normal bases. With appropriate affinity probes for other types of DNA damage and with further automation, the assay could be adapted for use in risk assessment.

  16. Clues to ATM function

    A key component in understanding how an abnormal gene can act to cause disease can come from insights into its normal function. The neurodegenerative disease ataxia telangiectasia is associated with mutations in the gene ATM. When Herzog et al. (p. 1089) have knocked out the Atm gene in mice, cells of the developing central nervous system were no longer as sensitive to ionizing radiation and showed a dramatic lack of programmed cell death. The authors speculate that Atm may be involved in a developmental checkpoint, and that mutations in this gene may allow damaged cells to survive during development, leading to problems later in life.

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