INDIVIDUAL BASE PAIRS, being added to a DNA undergoing replication, have been monitored in real time using DNA polymerase and fluorescently tagged nucleotides for the first time. Polymerase is the enzyme which replicates DNA in living organisms. In vitro it does this at a rate of about 700 base-pairs per second. This is far too fast a process for current equipment to watch at the level of a single base, much less determine which of the four canonical bases (A, C, T, and G) is being incorporated at that moment. But at Harold Craighead's lab at Cornell the addition of single bases can be detected (but not yet identified) as it happens. First a polymerase is stuck to the bottom of a channel of a microfluidic device. Then single-stranded DNA floats by and the polymerase begins a replication session by sequentially adding complementary base units to the DNA strand. A burst of fluorescence is emitted each time the polymerase incorporates a base, so like a series of flash bulbs going off the synthesis can be watched in real time (contact Mathieu Foquet, 607-255-6286, mf37@cornell.edu). Ideally the fluorescence would employ four different colors, one for each type of base. The Cornell group so far can muster two colors. With four-color fluorescence this microfluidic process might help to speed up greatly the task of genome sequencing. The new results were presented at the meeting of the AVS Science and Technology Society in Denver, November 3-8 (http://www.avssymposium.org/Overview.asp; paper NS/BI/MoA5)
NOISE CAN IMPROVE HUMAN BALANCE CONTROL, to the point that it may enable elderly subjects to steady themselves as well as their young counterparts, researchers in New England have demonstrated (Jim Collins, Boston University, 617-353-0390, jcollins@bu.edu). Noise, in this case, refers to random mechanical vibrations applied to the feet. In physics, noise denotes any random or seemingly useless fluctuation. Static on a radio station, peripheral conversations in a crowded room, and flashing neon lights along a busy thoroughfare all tend to obscure or distract one from receiving the desired information. But more and more studies in a wide variety of systems - global climate models, electronic circuits and sensory neurons, to name a few - have shown that certain levels of noise can actually enhance the detection and transmission of weak signals, through a mechanism known as stochastic resonance (SR).
Here the authors show that postural sway, the slight movements exhibited by the body when it is erect, can be significantly reduced for both young and elderly individuals. The authors achieved this by randomly applying subtle mechanical vibrations, just below the threshold of sensory perception, under the subjects' feet. The random vibrations likely act to enhance the sensation of pressure on the soles of the feet.
The authors further demonstrate a trend in elderly subjects towards reducing their postural sway to the level of young subjects, suggesting that noise may be a "fountain of youth" for human balance. These results indicate that the random vibrations may ameliorate age-related impairments in balance control. Noise may provide similar beneficial effects in individuals with marked sensory deficits, such as patients who have suffered a stroke or a disorder in the peripheral nervous system. In the future, the authors speculate, noise-based devices, such as randomly vibrating shoe inserts, may enable people to overcome functional difficulties due to age- or disease-related sensory loss (Priplata et al., Physical Review Letters, upcoming).
This paper comes on the heels of another recent finding, that the random hand motions generated by noise in the human nervous system make it possible for people to balance a stick on a finger (Cabrera and Milton, Physical Review Letters, 7 October) .
EXTRA DENSE GLASSY ICE. Scientists have worked out the structure for so-called very high density amorphous ice (VHDA). The density of this ice is 1.25 g/cm^3, compared to 0.92 g/cm^3 for ordinary ice and 1.0 g/cm^3 for liquid water (at sea level and at a temperature of 4 C). This means that VHDA ice would sink in water, not float like regular ice. Most solids are denser than their corresponding liquids. In this respect water is unusual, and this has made all the difference in the world when it comes to the meteorological, chemical, and biological look of things on Earth. Trying to understand why water is so unusual is why physicists have spent so much time squeezing and freezing water in so many ways. To date, 13 different forms of crystalline water ice (each varying, to some degree, in its internal structure) have been identified (http://www.cmmp.ucl.ac.uk/people/finney/jlf.html). As for amorphous ices, in which the molecules don't adopt a regular array, a fifth type was recently discovered. This last species, VHDA, is notable since it retains its structure even at ambient pressure (although it is made at a pressure of 14 kilo-bar), at liquid nitrogen temperatures, 77 K.
The team (University College London, Rutherford Appleton Lab, University of Innsbruck) that has now worked out the structure for VHDA by diffracting a beam of neutrons from the material suggests that VHDA may be a candidate structure for the hypothetical second kind of liquid water whose existence some think is necessary to explain the important anomalies of water. However, their work also raises problems for the two-liquid scenario by implying that rather than there being a single high density structure, a potentially large number of them might exist. (Finney et al., Physical Review Letters, 11 November 2002; contact John Finney at 44 20 7679 7850, j.finney@ucl.ac.uk; background article, Mishima and Stanley, Nature, 26 Nov 1998, p. 329)
The American Institute of Physics Bulletin of Physics News Number 612 November 6, 2002 by Phillip F. Schewe, Ben Stein, and James Riordon.