Number 570, December 21, 2001
by Phil Schewe, James Riordon, and Ben Stein
The Bacterial Divide
Roughly once an hour, the rod-shaped bacterium E. coli multiplies
by producing a copy of its DNA and then splitting into two daughter
bacteria, each carrying a complete set of genetic information. It is
vital that the division occur very close to the bacterium's midpoint
to ensure the viability of the daughter cells, but it has long been
a mystery as to how a bacterium locates its middle in preparation for
division. Researchers from Simon Fraser University (British Columbia)
and Dalhousie University (Nova Scotia) believe they have solved the
riddle by studying the interactions of three proteins that flow from
end to end inside the bacterium (Martin Howard, 31-71-527-5515, mjhoward@lorentz.leidenuniv.nl;
Andrew Rutenberg, 902-494-2952, adr@fizz.phys.dal.ca).
Biologists have known for several years that the proteins MinC, MinD,
and MinE are important in cell division; the absence or incorrect distribution
of any one of the three can corrupt cell division, or inhibit the process
altogether. Experiments have shown that these Min proteins oscillate
from end to end of the bacterium every minute or so. The effect of the
oscillation is that MinC and MinD have the their highest concentration
at the bacterial ends. Because MinC inhibits division, the bacterium
will divide at the center, where MinC is minimized.
The nagging question concerns how these protein oscillations are driven.
Jostling molecules in gases and liquids tend to spread concentrated
substances around in a diffusion process; it's the reason a fragrance
can drift across a room even in still air. Diffusion is also the principle
transport mechanism inside bacteria, but acting alone it should evenly
distribute compounds throughout the cell. As the researchers' new model
shows, however, it's when protein diffusion is combined with the binding
and release of proteins from the cell membrane that oscillating patterns
in E. coli occur. The effect is closely related to the Turing
model reaction-diffusion equations often championed as the mechanism
behind complex patterns in nature, such as tiger stripes and ladybug
spots (Update
558). In the case of E. coli, oscillation of the Min protein
self-organizing behavior causes the division site to be at the cell
midpoint. (M.
Howard, A.D. Rutenberg, and S. de Vet, Physical Review Letters,
31 Dec 2001.)
Dendrimer Lasers
Dendrimer lasers have as their active medium fluorescing dye molecules
lodged at the heart of hyper-structured, tree-shaped polymers (see figure).
In most dye lasers the dye concentration cannot go above a millimole/liter
without quenching the fluorescence process. But in a new experiment
by scientists at the Communications Research Laboratory and PRESTO Japan
Science and Technology Corporation, both in Japan, a dye concentration
of 9 millimoles/liter showed no diminution of laser output, but rather
an increase. Furthermore, the spectral linewidth (the spread in wavelengths)
is narrow, only 0.1 nm. The laser output was so potent that end mirrors
were not used. This, combined with other organic-laser properties such
as flexibility and tunability, will soon result in 100-nm-sized lasers.
The researchers (Shiyoshi Yokoyama, 81-789-692-254, syoko@crl.go.jp)
are now at work on extending their dendrimer structures in producing
solid state waveguides, fibers, and photonic crystals. (Yokoyama et
al., Applied Physics Letters,
7 Jan 2002.)
A Tiny Microphone Diaphragm Based on Fly Ears
A tiny microphone diaphragm based on fly ears has been built by researchers
(Ronald Miles, Binghamton University, 607-777-4038, miles@binghamton.edu),
offering such possibilities as compact hearing aids that respond only
to sound in front of the wearer.
The diaphragm is the part of a microphone that vibrates in response
to incoming sound waves; other components then convert the diaphragm's
vibrations into electrical signals which can then be amplified or recorded.
The researchers based their novel diaphragm on Ormia ochracea,
a small parasitic fly that uses sound to track down its cricket host
even in complete darkness. The fly can detect changes as small as two
degrees in the direction of an incoming sound, as good as humans. This
is remarkable since the fly's ears are just a couple hundred microns
apart. Mammals, on the other hand, rely on the fact that their ears
are well separated from one another, so that sound can arrive at each
ear at sufficiently different times and with sufficiently different
intensities.
What's even more remarkable about the fly is that its hearing organs,
a pair of rectangle-shaped membranes, are connected to each other. Specifically,
they are "torsionally coupled" so that a sound wave that lands
on one membrane can deflect the other membrane. The connection between
the membranes enables them to vibrate in several different ways so that
the fly can obtain both the average pressure of an incoming sound and
its pressure gradient, the change in sound pressure as you move from
one ear to the other. This provides lots of information with which to
determine the direction of the sound.
The researchers built a silicon nitride prototype microphone diaphragm
that closely reproduces the characteristics of the fly ears. While the
researchers face challenges in mass-producing such a design, they hope
that its unconventional approach to localizing sound will inspire lots
of applications. (Paper
2aEA1 at Acoustical Society of America
meeting in Ft. Lauderdale, 3-7 Dec 2001.)
A Quantum Computer Has Factored the Number
15
This may sound like a trivial achievement, but it is actually a considerable
physics milestone. It represents the most complex calculation yet performed
in quantum computing, which offers a radically different means of information
processing through the use of quantum mechanics. Even more noteworthy,
it is the first experimental demonstration of Shor's algorithm, a quantum-computer
program which can potentially factor large numbers in a fraction of
the time needed for the world's currently fastest supercomputers. Such
large numbers are used as the basis of encryption codes; the codes are
broken by finding the prime-number factors of the large numbers.
IBM-Almaden and Stanford University researchers (Isaac Chuang, now
at MIT, ichuang@cba.mit.edu) built a quantum computer whose working
substance was a liquid consisting of a billion billion molecules. The
molecules were specially designed to contain 7 nuclear "spins"--5
from fluorine nuclei and 2 from carbon-13 nuclei. Analogous to a bar
magnet which could point north or south, each spin could represent the
binary digits "0" or "1" (or both 0 and 1 at the
same time through the subtleties of quantum mechanics) and could be
controlled by magnetic fields and radio waves (i.e., nuclear
magnetic resonance techniques). By manipulating the 7 qubits, the computer
could take advantage of quantum computing's unique parallel processing
capabilities to determine that the factors of 15 were 3 and 5. Enormous
challenges must be surmounted to build larger-scale quantum computers
which could factor very large numbers, and this is an early step forward.
(Vandersypen et al., Nature,
20/27 December 2001; also see IBM-Almaden
news release.)
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