Number 612, November 6, 2002
by Phil Schewe, James Riordon, and Ben Stein
Individual Base Pairs
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; paper
NS/BI/MoA5)
Noise Can Improve Human Balance Control
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, 2 December 2002).
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/cm3,
compared to 0.92 g/cm3 for ordinary ice and 1.0 g/cm3
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.
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)
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