Number 610, October 22, 2002
by Phil Schewe, James Riordon, and Ben Stein
Trapping DNA Through Thermophoresis
Trapping DNA through thermophoresis might have a bearing on the origins
of life, as a new experiment shows. The DNA molecules in our bodies
are protected behind a nuclear membrane and a cellular membrane, but
on the early Earth nascent life forms might have consisted of DNA floating
in a free aqueous environment. How would such fragile entities keep
from diluting themselves to death?
One answer might be thermophoresis, a process (known for almost 150
years) by which heat can repel polymers. Generally the longer the molecule
the greater the thermal repulsion will be, just as molecules or particles
will be separated in a centrifuge according to mass.
An experiment conducted by Dieter Braun and Albert Libchaber at Rockefeller
monitors fluorescent-tagged DNA molecules as they are harried by a laser-generated
heat spot. As expected the DNA was repelled, carried along by a convective
flow away from the heat.
But surprisingly the DNA then came back; the convection, scrutinized
more carefully, was seen to be a circular cell pattern. The DNA had
become trapped in a small zone (20 microns across and with a DNA concentration
enhanced by a factor of 1000) centered around the heat spot.
Braun (212-327-8160, braund@rockefeller.edu) says this is the first
quantitative experimental evidence, on a microscopic level, that biological
molecules (DNA was used rather than RNA because RNA can quickly degrade
in the presence of proteins in the solution) can be trapped in this
way.
Demonstrating a mechanism for confining early metabolic and replicative
life forms in a far-from-equilibrium environment such as localized heat
sources (e.g., hydrothermal vents) immersed in a cold ocean, should
be of interest to biologists who ponder the advent of life. (Braun
and Libchaber, Physical Review Letters, 28 October 2002;
see also research website;
independent thermophoresis expert: Werner Kohler in Bayreuth, Germany,
werner.koehler@uni-bayreuth.de)
Noninvasive EEGs
Conventional electroencephalograms (EEGs) monitor electrical activity
in the brain with electrodes placed either on the scalp (involving hair
removal and skin abrasion) or inserted directly into the brain with
needles.
Now a noninvasive form of EEG has been devised by scientists at the
University of Sussex. Instead of measuring charge flow through an electrode
(with attendant distortions, in the case of scalp electrodes) the new
system measures electric fields remotely, an advance made possible by
new developments in sensor technology.
The device's sensitivity is demonstrated by watching electric activity
change as the ambient relaxed brain signal (the so-called alpha wave,
at a frequency of 8-14 Hz) gives way to the beta wave (14-35 Hz) as
the subject opens his eyes (see figure).
The Sussex researchers (contact Terry Clark, t.d.clark@sussex.ac.uk,
44-127-678087) believe their new sensor will instigate major advances
in the collection and display of electrical information from the brain,
especially in the study of drowsiness and the human-machine interface.
The same group of scientists has made remote-sensing ECG units as well.
(Harland et al.,
Applied Physics Letters, 21 October 2002; see research
website.)
Naval Neutrinos
Naval neutrinos, emitted by nuclear subs as a routine byproduct of
the reactions producing propulsion, will have to be taken into account
when studying neutrino oscillations, suggests a team of Stanford physicists.
Oscillation experiments probe the fascinating process by which one
type of neutrino turns into other types. The power generated by nuclear
submarines (100-200 operating at any one time) is only a few percent
of all nuclear-generated thermal power in the world, and the neutrino
flux from a typical naval reactor is only about 200,000 per sq. cm per
second at a distance of 40 km.
This does not represent much of a background for the current generation
of reactor-based neutrino-oscillation experiments. But for future reactor-based
experiments, trying to perform higher precision measurements or those
using a lower flux from a longer baseline (neutrino flux drops with
the square of the distance), naval-reactor neutrinos will have to be
factored in.
Stanford physicist Giorgio Gratta (650-725-6509, gratta@stanford.edu)
says that, on the other hand, neutrinos from naval reactors may be used
for a new breed of oscillation experiments in which the baseline for
oscillations could be changed by simply "sailing the reactor"
to a new position with respect to the (fixed) large detector. It is
suggested that a nuclear ice-breaker could be chartered for this purpose.
And, no, a sub's neutrino flux is not strong enough to give away its
position. (Detwiler
et al., Physical Review Letters, 4 November 2002.)
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