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ScienceWeek
SCIENCE-WEEK - March 29, 2002 - Vol. 6 Number 13
An Email Research Digest Published Weekly Since 1997
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Science is a community with an attitude: people who
rejoice when a new truth defeats their past confusions,
people who would rather know reality than superstitions,
people who believe that with their minds and hearts and
hands they can shape their own destiny. Since the beginning
of human time, this attitude has threatened those whose
life and fortune are based on illusion.
-- Anonymous
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Section 1
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Contents of this Issue (Full reports in Section 2):
[(*) = includes background reports]
Basic Sciences
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1. Evidence for Nuclear Emissions During Acoustic Cavitation (*)
2. Large-Scale Patterns in Empirically Derived Cellular Automata
3. Topology of Polymers
4. On Global Earth Deformation
5. History of Dendrimer Polymers
6. Quantum Physics: On Generic Entanglement Generation (*)
7. Energetics of Ion Conduction Through the Potassium Channel
8. Cell Cytoskeleton: Regulation by Protein Kinase
9. Mutations and the Odds of Developing Cancer (*)
10. On the Nucleolus
11. On Signaling Complexes in Biological Cells
12. Embryogenesis and Gene Expression
Praxis
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13. Materials Science: On Clay-Polymer Nanocomposites
14. Crystal-Control of X-Ray Switching
15. On Water and Oxygen in Amorphous Silicon Dioxide
16. On Triple-Point Wetting on Surfaces
17. On Nuclear Power
18. Early History of High-Temperature Superconductors
19. Hormone Replacement Therapy and Breast Cancer
20. Cognitive Activity and the Risk of Alzheimer Disease (*)
21. On Legionnaire's Disease
22. Hypothermia and Treatment after Cardiac Arrest
23. Genetic Factors and Crohn's Disease
24. On the Total Artificial Heart
Miscellany
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25. In Focus: Geology: On the Oldest Parts of the Lithosphere
26. SW Archive: On the Standard Model and a Unified Physics (*)
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Section 2
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1. EVIDENCE FOR NUCLEAR EMISSIONS DURING ACOUSTIC CAVITATION
In general, the term "cavitation" refers to the formation of gas-
or vapor-filled cavities in liquids in motion when the pressure
is reduced to a critical value while the ambient temperature
remains constant.
... ... R.P. Taleyarkhan et al (Oak Ridge National Laboratory, US
discuss acoustic cavitation, the authors making the following
points:
1) The intense implosive collapse of gas or vapor bubbles,
including acoustically forced cavitation bubbles, can lead to
ultrahigh compressions and temperatures and to the generation of
light flashes attributed to sonoluminescence. The authors report
a study of ultrahigh compression and temperatures in bubbles
nucleated by means of fast neutrons, whereby bubble nucleation
centers with an initial radius of 10 to 100 nanometers are
created, and the bubbles grow in an acoustic field to a maximum
radius of approximately 1 millimeter before an implosive
collapse. This report builds on observations that increasing the
maximum radius modestly (e.g., by 50 percent), or increasing the
rate of collapse, can result in very large increases in peak gas
temperatures and produce light emission during implosions. In
contrast to single-bubble experiments, in which the initial
bubble radius typically increases to the maximum bubble radius by
a factor of only approximately 10 (e.g., from 10 microns to 100
microns), the neutron-induced nucleation technique used by the
authors results in bubble-radius increases of approximately
10^(5). For a spherical bubble, a radius-ratio increase by a
factor of 10^(4) implies a related volumetric-ratio increase of
10^(12) over that produced by conventional cavitation techniques.
The authors state their expectation was that such an approach,
with its vastly increased energy concentration potential during
implosions, should give rise to significant increases in the peak
temperatures within the imploding bubbles, leading to fusion and
detectable levels of nuclear particle emissions in suitable
fluids.
2) The authors report that in such cavitation experiments
with deuterated acetone, tritium decay activity above background
levels was detected. In addition, evidence for neutron emission
near 2.5 million eV was also observed, as would be expected by
deuterium-deuterium fusion. Control experiments with normal
acetone did not result in tritium activity or neutron emissions.
Hydrodynamic shock code simulations supported the observed data
and indicated highly compressed and hot (10^(6) to 10^(7)
kelvins) bubble implosion conditions, as required for nuclear
fusion reactions.
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Science 2002 295:1868
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Editor's note: This paper has caused considerable controversy in
the physics community, including reported pressure against the
journal _Science_ not to publish the paper. In an editorial on
the paper, the editor of the journal, Donald Kennedy, states: "We
see no good reason for abandoning our plans to publish the paper,
and we can see no merit whatsoever in the efforts to discredit it
in advance. Both the premature critics and those who believe in
the result would do well to wait for the scientific process to do
its work." ScienceWeek fully concurs, particularly since the
experimental results reported above were already theoretically
predicted nearly 5 years ago, as indicated below.
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Related Background:
SONOLUMINESCENCE MODEL SUGGESTS THERMONUCLEAR FUSION POSSIBILITY
Sonoluminescence is an energy transduction phenomenon associated
with the nucleation, growth, and collapse of small gas-filled
bubbles in a liquid, the transduction involving the conversion of
sound (mechanical) energy to light energy, which means the
conversion of energy of motion to light energy. The luminescence
is extremely short in duration, =< 50 picoseconds [10^(-12)
seconds], and synchronous with the periodic acoustic driving
field. Experimental measurements of the spectra of the emitted
light pulses reveal interior gas temperatures as great as the
surface of the sun and maybe as much as a million degrees
centigrade. There is no question about the experimental
observations: the luminescence occurs, it can be measured, it can
be altered by doping the bubbles with various noble gases.
Experiments can also be done with a single levitated bubble,
rather than with a population of bubbles, and in this case the
experimental observations are quantitatively but not
qualitatively different. The problem for physicists has been to
explain the phenomenon, and there are half a dozen different
theories ranging from the sonoluminescing bubble as a macroscopic
demonstration of quantum vacuum radiation to the idea that the
sonoluminescing bubble is a microscopic high temperature reaction
chamber. This week William C. Moss et al (Lawrence Livermore
National Laboratory, Livermore CA US) presented a mathematical
model of the single sonoluminescing bubble as a thermally
conducting, partially ionized, two-component plasma, and the
calculations from this model appear to be consistent with the
idea that the sonoluminescent bubble may indeed be a high
temperature reaction chamber in which temperatures as high as a
million degrees centigrade may be achieved, at which temperatures
thermonuclear fusion would be possible. The result has caused
excitement in the physics community.
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Science 1997 30 May
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2. LARGE-SCALE PATTERNS IN EMPIRICALLY DERIVED CELLULAR AUTOMATA
J. Timothy Wootton (University of Chicago, US) discusses cellular
automata, the author making the following points:
1) An important unanswered question in ecology is whether
processes such as species interactions that occur at a local
scale can generate large-scale patterns seen in nature. Because
of the complexity of natural ecosystems, developing an adequate
theoretical framework to scale up local processes has been
challenging. Models of complex systems can produce a wide array
of outcomes, so that model parameter values must be constrained
by empirical information to usefully narrow the range of
predicted behavior. Under some conditions, spatially explicit
models of locally interacting objects (e.g., cells, sand grains,
car drivers, or organisms), variously termed "cellular automata"
or interacting particle models, can self-organize to develop
complex spatial and temporal patterning at larger scales in the
absence of any externally imposed pattern. When these models are
based on transition probabilities of moving between ecological
states at a local level, relatively complex versions of these
models can be readily linked to empirical information on
ecosystem dynamics.
2) Mussel beds characterize many temperate rocky intertidal
shores throughout the world, and can exhibit complex patterning
thought to result from the interplay between interactions among
sessile species and external disturbance agents, particularly
large waves. The author demonstrates that an empirically derived
cellular automaton model of a rocky intertidal mussel bed based
on local interactions correctly predicts large-scale spatial
patterns observed in nature.
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Nature 2001 413:841
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3. ON THE TOPOLOGY OF POLYMERS
Y. Tezuka and H. Oike (Tokyo Institute of Technology, JP) discuss
the topology of polymers, the authors making the following
points;
1) The topology of polymer molecules is often a basis for
control of their properties and functions in static and dynamic
states both in bulk and in solution. Thus, a novel strategy to
design unprecedented polymer topologies is an ongoing challenge
in polymer science and polymer technology. Remarkable
achievements within recent decades include such branched polymer
topologies as "star" polymers, "H-shaped" polymers, "super H-
shaped" polymers, "pom-pom-shaped" polymers, as well as "comb"
polymers (polymacromonomers) and "dendrimers", in addition to
such cyclic, multicyclic, and cyclic-branched combined polymers
as "ring" polymers, "8-shaped" polymers, and "tadpole" polymers.
These entities have been described in terms of their shapes.
Catenanes, rotaxanes, and knots are another class of
topologically unique macromolecules of growing interest designed
through noncovalent interactions of precursor components.
2) A systematic classification of topologically unique
macromolecules may provide useful insight into structural
relationships between different compounds, and studies on the
classification of dendrimers, as well as knots, catenanes, and
rotaxanes, have been reported. In particular, the fundamental
mathematical theory of knots has been a subject of active
investigation, providing an impetus to elucidate topological
features in macromolecular chemistry. The authors propose a
systematic classification of a series of well-defined cyclic and
branched polymer architectures by reference to constitutional
isomerism in alkanes and in a series of mono- and
polycycloalkanes. On the basis of their classification of polymer
topologies, the authors suggest a novel synthetic strategy
involving "electrostatic self-assembly and covalent fixation" for
the construction of a variety of topologically unique polymer
architectures.
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J. Am. Chem. Soc. 2001 123:11570
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4. ON GLOBAL EARTH DEFORMATION
What is called the "Global Positioning System" consists of a
total of 24 satellites in different orbits, with 4 of the
satellites visible at any time from any point on the surface of
the Earth. This system provides a locational accuracy of less
than 1 centimeter for receiving points on the surface, and the
system is used (in addition to certain military applications) to
measure the motions of small blocks of the Earth's crust. For
example, global positioning system surveys in 1989 and 1993
determined that southwestern Greece moved systematically to the
southwest relative to Italy at a mean annual rate of 2 to 4
centimeters per year. Thus, movements of the Earth's crust are
now being measured directly by satellite systems rather than
inferred from models and paleogeological data.
... ... G. Blewitt et al (University of Nevada, US) discuss
global Earth deformation, the authors making the following
points:
1) Redistribution of mass over Earth's surface generates
changes in gravitational and surface forces that produce a stress
response in the solid Earth, accompanied by characteristic
patterns of surface deformation. Previous investigations in
space-based geodesy have detected displacements of surface height
at the 10-millimeter level in response to variation in
atmospheric pressure and large-scale terrestrial water storage.
The change in Earth's shape due to the gravitational and pressure
stresses of surface loading is theoretically characterized by
spherical harmonic potential perturbations first formulated by
Augustus Love (1863-1940) (Love numbers and Love waves).
2) The authors report they have detected a global mode of
Earth deformation that is predicted by theory. Precise
positioning of Global Positioning System sites distributed
worldwide reveals that during February to March, the Northern
Hemisphere compresses (and the Southern Hemisphere expands) such
that sites near the North Pole move downward by 3.0 millimeters
and sites near the equator are pulled northward by 1.5
millimeters. The opposite pattern of deformation occurs during
August to September. The authors identify this pattern as the
degree-one spherical harmonic response of an elastic Earth to
increased winter loading of soil moisture, snow cover, and
atmosphere. Analysis reveals the load moment's trajectory as a
great circle traversing the continents, peaking at 6.9 x 10^(22)
kilogram meters near the North Pole in winter, indicating
interhemispheric mass exchange of 1.0 x 10^(16) +- 0.2 x 10^(16)
kilograms.
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Science 2001 294:2342
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5. ON THE HISTORY OF DENDRIMER POLYMERS
S.M. Grayson and J.M. Frechet (University of California Berkeley,
US) discuss dendrimer polymers, the authors making the following
points:
1) Dendrimers represent a key stage in the ongoing evolution
of macromolecular chemistry. From the origins of polymer
chemistry until 20 years ago, a major focus was the synthesis and
characterization of linear polymers. Although the molecular
interactions and the many conformations of linear polymers
involve three dimensions, their covalent assembly is strictly a
1-dimensional process. Half a century ago, in theoretical
studies, P.J. Flory (1910-1985) was among the first to examine
the potential role of branched units in macromolecular
architectures, but it was not until the mid-1980s that methods
for the orderly preparation of these polymers became sufficiently
developed to enable the practical study of these entities.
2) In 1978, Vogtle developed an iterative cascade method for
the synthesis of low molecular weight branched amines. Using
chemistry and conditions less prone to cyclization side-reactions
and therefore more suitable for repetitive growth, Tomalia et
disclosed the synthesis and characterization of the first family
of dendrimers in 1984-1985. The synthesis was initiated by an
addition reaction (Michael addition) of a "core" molecule of
ammonia to three molecules of methyl acrylate, followed by
exhaustive amidation of the triester adduct, using a large excess
of ethylenediamine, a process that generates a molecule with 6
terminal amine groups. Iterative growth is then continued, using
alternate Michael addition and amidation steps with appropriate
excess of reagents, and optimization of this procedure enables
the synthesis of globular poly(amidoamine) dendrimers on a
commercial scale with molecular weights well above 25,000.
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Chem Revs. 2001 101:3819
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6. QUANTUM PHYSICS: ON GENERIC ENTANGLEMENT GENERATION
S. Bose and D. Home (University of Oxford, UK) discuss
entanglement, the authors making the following points:
1) Recent years have witnessed a great surge of interest in
the applications of entanglement, and this context it is
important to explore efficient and general ways of preparing
entanglement. Most known mechanisms for obtaining entangled
states are dependent on the specific nature of the systems
involved. The authors propose a very general scheme for
entangling the spins (or any spin-like degree of freedom) of two
particles of any type (bosons or fermions) by a combination of
two-particle interferometry and which-way detection. The
fractional yield of entangled pairs for a given number of input
pairs can be arbitrarily increased by a recursive procedure using
just one beam splitter and two detectors. The authors suggest the
main application of their setup will be in entangling material
objects such as neutrons, electrons, atoms, or macromolecules.
This will enable testing quantum nonlocality through separate
measurements on far separated massive particles.
2) A salient feature of the setup of the authors is the fact
that two independent identical do not need to interact directly
in order to become entangled. They need only interact
individually with beam splitters and detectors, and their
indistinguishability can be exploited to yield entanglement. This
is thus useful for entangling those particles which interact
weakly (or which do not interact) with other particles of the
same species.
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Phys. Rev. Lett. 2002 88:050401
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Related Background:
QUANTUM ENTANGLEMENT AND QUANTUM OPTICS
Paul Kwiat (University of Illinois Urbana-Champaign, US)
discusses quantum entanglement. Quantum entanglement between two
particles means that measuring the behavior of one particle
instantly determines the behavior of the other particle, even
when they are physically far apart. Erwin Schroedinger (1887-
1961) once described this peculiar connection as "_the_
characteristic trait of quantum mechanics, the one that enforces
its entire departure from classical lines of thought."
Entanglement describes a system with several components in which
the individual parts carry no information but nevertheless share
quantum correlations with each other that are stronger than those
allowed by classical physics. For example, photons can be
polarized -- the polarization describes the oscillation direction
of the electric field associated with a light wave. Polarization
filters, such as Polaroid sunglasses, will let through photons
polarized in one plane but block those polarized at right angles,
and so can be used to measure photon polarization. If two photons
have entangled polarizations, each photon individually would
appear completely unpolarized (with no particular oscillation
direction) and yet measuring the polarization of one completely
determines to polarization of the other. It is as if you flipped
two coins, each of which was equally likely to come up heads or
tails, and yet they always gave the same results -- that is,
both heads or both tails. Although normal coins do not behave
like this, it has been known for some time how to produce pairs
of photons that do display such bizarre quantum-mechanical
correlations.
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Nature 2001 412:866
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Related Background:
ENTANGLEMENT, DECOHERENCE, AND THE QUANTUM-CLASSICAL BOUNDARY
Quantum mechanical entanglement is a phenomenon that has caught
the imagination of the public as one of the more bizarre
consequences of fundamental physical theory. Entanglement is
unique to quantum mechanics, and involves a relationship (a
"superposition of states") between the possible quantum states of
two entities such that when the possible states of one entity
collapse to a single state as a result of suddenly imposed
boundary conditions, a similar and related collapse occurs in the
possible states of the entangled entity no matter where or how
far away the entangled entity is located. Entanglement arises
from the wave function equation of quantum mechanics, which has
an array of possible function solutions rather than a single
function solution, with each possible solution describing a set
of possible probabilistic quantum states of the physical system
under consideration. Upon fixation of the appropriate boundary
conditions, the array of possible solutions collapses into a
single solution. For many quantum mechanical physical systems,
the fixation of boundary conditions is a theoretical and
fundamental consequence of some interaction of the physical
system with something outside that system, e.g., an interaction
with the measuring device of an observer. In this context, two
entities that are described by the same array of possible
solutions to the wave function equation are said to be
"coherent", and when events decouple these entities, the
consequence is said to be "decoherence". As a physical
phenomenon, entanglement was discussed many years ago, most
particularly following the publication in 1935 of the often
quoted Einstein-Podolsky-Rosen paper (*Physical Review* 1935
47:777). These discussions have been in the form of "gedanken"
(thought) experiments involving two quantum-mechanical entangled
entities. More recently, however, there have been laboratory
constructions of actual quantum mechanical systems exhibiting
such entanglement phenomena, and the reportage of these
laboratory arrangements by the media have engaged the public
fancy. Essential here is that any purely verbal account of
quantum mechanical phenomena is severely limited by the
constraint that the properties of quantum mechanical systems can
be precisely described only by the equations relevant for those
systems, and all other descriptions usually introduce serious
ambiguities. ... ... Serge Haroche (Ecole Normale Superieure
Paris, FR) reviews quantum mechanical entanglement, decoherence,
and the question of the boundary between the physics of quantum
phenomena and the physics of classical phenomena. Haroche makes
the following points: 1) In quantum mechanics, a particle can be
delocalized (simultaneously occupy various probable positions in
space), can be simultaneously in several energy states, and can
even have several different identities at once. This apparent
"weirdness" behavior is encoded in the wave function of the
particle. 2) Recent decades have witnessed a rash of experiments
designed to test whether nature exhibits implausible nonlocality.
In such experiments, the wave function of a pair of particles
flying apart from each other is entangled into a non-separable
superposition of states. The quantum formalism asserts that
detecting one of the particles has an immediate effect on the
other, even if they are very far apart, even far enough apart to
be out of interaction range. The experiments clearly demonstrate
that the state of one particle is always correlated to the result
of the measurement performed on the other particle, and in just
the strange way predicted by quantum mechanics. 3) An important
question is: Why and how does quantum weirdness disappear
(decoherence) in large systems? In the last 15 years, entirely
solvable models of decoherence have been presented by various
authors (e.g., Leggett, Joos, Omnes, Zeh, Zurek), these models
based on the distinction in large objects between a few relevant
macroscopic observables (e.g., position or momentum) and an
"environment" described by a huge number of variables, such as
positions and velocities of air molecules, number of black-body
radiation photons, etc. The idea of these models, essentially, is
that the environment is "watching" the path followed by the
system (i.e., interacting with the system), and thus effectively
suppressing interference effects and quantum weirdness, and the
result of this process is that for macroscopic systems only
classical physics obtains. 4) In mesoscopic systems, which are
systems between macroscopic and microscopic dimensions,
decoherence may occur slowly enough to be observed. Until
recently, this could only be imagined in a gedanken experiment,
but technological advances have now made such experiments real,
and these experiments have opened this field to practical
investigation.
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Physics Today 1999 July
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Related Background:
EXPERIMENTAL QUANTUM TELEPORTATION
Quantum teleportation is the transmission and reconstruction over
arbitrary distances of the state of a quantum system, an effect
first suggested by Bennett et al in 1993 (Phys. Rev. Lett.
70:1895). The achievement of the effect depends on the phenomenon
of entanglement, an essential feature of quantum mechanics.
Entanglement is unique to quantum mechanics, and involves a
relationship (a "superposition of states") between the possible
quantum states of two entities such that when the possible states
of one entity collapse to a single state as a result of suddenly
imposed boundary conditions, a similar and related collapse
occurs in the possible states of the entangled entity no matter
where or how far away the entangled entity is located.
Polarization is essentially a condition in which the properties
of photons are direction dependent, a condition that can be
achieved by passing light through appropriate media. Bouwmeester
et al (6 authors, Univ. of Innsbruck, AT) now report an
experimental demonstration of quantum teleportation involving an
initial photon carrying a polarization that is transferred to one
of a pair of entangled photons, with the polarization-acquiring
photon an arbitrary distance from the initial one. The authors
suggest quantum teleportation will be a critical ingredient for
quantum computation networks.
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Nature 1997 11 Dec
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Related Background:
REPORT OF FIRST QUANTUM MECHANICAL ENTANGLEMENT OF ATOMS
... In the past, evidence of quantum mechanical entanglement has
been restricted to elementary particles such as protons,
electrons, and photons. Now E. Hagley et al, using rubidium
atoms prepared in circular Rydberg states (which means the outer
electrons of the atom have been excited to very high energy
states and are far from the nucleus in circular orbits), have
shown quantum mechanical entanglement at the level of atoms.
What is involved is that the experimental apparatus produces two
entangled atoms, one atom in a ground state and the other atom
in an excited state, physically separated so that the
entanglement is non-local, and when a measurement is made on one
atom, let us say the atom in a ground state, the other atom
instantaneously presents itself in the excited state -- the
result of the second atom wave function collapse thus determined
by the result of the first atom wave function collapse. There is
talk that before long quantum mechanical entanglement may be
demonstrated for molecules and perhaps even larger entities.
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Phys. Rev. Lett. 1997 79:1
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Related Background:
QUANTUM PHOTON ENTANGLEMENT AT A DISTANCE OF SEVEN MILES
Whether or not the quantum mechanical behavior of elementary
particles is called mysterious depends, more or less, on the
attitude one has. If there is a demand that the behavior of these
particles be explainable with the logistic structure of human
language, then some aspects of their behavior seem mysterious
indeed. On the other hand, if there is a willingness to admit
that the logical structure of human language may not at present
be isomorphic with the logical structure of the laws that govern
the behavior of these particles, then it is probably best to put
off notions of mysteries and take the behavior for what it is.
This week there was announced to the popular press, before
publication, the results of a twin-photon experiment in
Switzerland. Nicolas Gisin et al (University of Geneva, CH)
reported that a pair of twin photons split and sent along two
diverging paths, when arriving at terminals seven miles apart,
exhibit the phenomenon of quantum "entanglement". The gist of it
is that the detection of one of the photons effectively causes
the collapse of the spectrum of its wave-function solutions to a
single solution, and this collapse instantaneously causes the
collapse of the possible quantum states of the other photon, in
this case seven miles away. The melodramatic notion (purveyed by
the press) is that information has somehow travelled from one
photon to the other at a speed greater than the speed of light,
with the result that great canons of thought are thereby
destroyed. But perhaps the more prosaic reality is that any
attempt to describe non-classical events with language based on
classical laws and perceptions cannot succeed.
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New York Times 1997 22 Jul
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7. ENERGETICS OF ION CONDUCTION THROUGH THE POTASSIUM CHANNEL
S. Berneche and B. Roux (Cornell University, US) discuss the
potassium ion channel, the authors making the following points:
1) Potassium ion channels are transmembrane proteins
essential for the transmission of nerve impulses. The ability of
these proteins to conduct potassium ions at levels near the limit
of diffusion is traditionally described in terms of concerted
mechanisms in which ion-channel attraction and ion-ion repulsion
have compensating effects, as several ions are moved
simultaneously in single file through the narrow pore. The
efficiency of such a mechanism, however, relies on a delicate
energy balance -- the strong ion-channel attraction must be
perfectly counterbalanced by the electrostatic ion-ion repulsion.
2) Although the available experimental data provide a wealth
of information concerning the structure and function of potassium
ion channels, theoretical considerations are necessary for
understanding the energetics of ion conduction at the atomic
level. To elucidate the mechanism of ion conduction at the atomic
level, the authors performed molecular dynamics free energy
simulations on the basis of the x-ray structure of a specific
type of potassium ion channel (KcsA). The authors report that ion
conduction in this system involves transitions between two main
states, with two and three potassium ions occupying the
selectivity filter, respectively. The authors suggest this
process is reminiscent of the "knock-on" mechanism proposed by
Hodgkin and Keynes in 1955. The largest free energy barrier is
evidently on the order of 2 to 3 kilocalories per mole, implying
that the process of ion conduction in this system is limited by
diffusion. The authors report that ion-ion repulsion, although
essential for rapid conduction, acts only at very short
distances. The calculations also demonstrate that the rapidly
conducting pore is selective.
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Nature 2001 414:73
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8. CELL CYTOSKELETON: REGULATION BY PROTEIN KINASE
In general, a "kinase" is any enzyme involved in the transfer of
a phosphate group. A tyrosine kinase is an enzyme that transfers
the terminal phosphate of adenosine triphosphate (ATP) to a
specific tyrosine residue on its target protein: the enzyme
catalyzes the phosphorylation by ATP of protein-tyrosine to form
protein-tyrosine phosphate with release of adenosine diphosphate
(ADP).
... ... Paul A. Janmey (University of Pennsylvania, US) discusses
the cell cytoskeleton, the author making the following points:
1) One of the most apparent differences between normal cells
and their malignantly transformed counterparts, and between
immature and fully differentiated cells, is the shape of the
cells. Cell morphology is controlled largely by the structure of
the cytoskeleton, a system of 3 distinct types of filamentous
polymers that assemble into networks and bundles of various kinds
to link the cell interior physically with the plasma membrane and
endow the cell with viscoelastic properties.
2) The transforming of cytoplasm from a liquid to a solid
was observed by early microscopists to be tightly associated with
the ability of cells to move, and this "sol-gel transition" is
now known to be caused by changes in the state of polymerization
and organization of the protein actin, one of the 3 types of
filaments comprising the cell cytoskeleton.
3) When something goes wrong in the complex system of
control proteins and messengers that signals for changes in the
actin system, as can happen with genetic mutations or by the
insertion of viral genes expressing malignantly altered control
proteins, cells may migrate and divide inappropriately if the
signals for division or motility cannot be stopped. One such
control factor is the proto-oncogene protein Ab1, a tyrosine
kinase. In addition to its implication in human leukemias, this
protein is also an important regulator of the actin system
involved in neuronal cell function and early neuronal
development. Ab1 is part of the signaling pathways that control
the actin cytoskeleton and associates with large actin-containing
cellular structures.
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Proc. Nat. Acad. Sci. 2001 98:14745
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9. ON MUTATIONS AND THE ODDS OF DEVELOPING CANCER
K.C. Quon and A. Berns (The Netherlands Cancer Institute, NL)
discuss mutations and cancer, the authors making the following
points:
1) During the course of cancer development, a normal cell
progresses toward malignancy by acquiring a specific series of
mutations. These include mutations that activate otherwise
innocuous proto-oncogenes, and other mutations that inactivate
recessive tumor suppressor genes. By acquiring these mutations, a
cell progressively alters its phenotype, and thereby eludes the
various controls that normally prevent malignant growth in an
organism.
2) Based on epidemiological data, and consistent with
in-vitro experimental data, it is estimated that between 4 and 8
rate-limiting mutations occur during the development of most
human cancers. But this raises a conundrum. The incidence of
cancer should be proportional to the number of rate-limiting
events necessary for tumorigenesis, the frequency of these
events, and the size of the target-cell population for these
events. Therefore, given that somatic mutations arise at a
frequency of less than 6 x 10^(-6) per locus, an overly
simplistic calculation would suggest that even a tumor requiring
only 4 mutations would only arise at a frequency of approximately
1 in 10^(-21) cells, a vanishingly low frequency even in an
organism composed of approximately 10^(14) cells, as humans are.
Why, then, are the odds of developing cancer during one's
lifetime approximately 1 in 3, and what does this tell us about
the mechanisms that operate during tumorigenesis?
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Genes & Development 2001 15:2917
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Related Background:
MEDICAL BIOLOGY: ON MUTATIONS AND CANCER
The term "cancer", which means "crab" in Latin, was introduced by
Hippocrates (460-370 B.C.) to describe diseases in which tissues
grow and spread unrestrained throughout the body, eventually
causing death. Cancers can originate in almost any tissue of the
body, including nerve, muscle, blood, connective tissue, etc.
Depending on the cell type involved, cancers are grouped into 3
main categories: a) carcinomas, the most common types of cancer,
arise from the *epithelial cells that cover external and internal
body surfaces, with lung, breast, and colon cancers the most
frequent cancers of this type; b) sarcomas originate in
supporting tissues of *mesodermal origin, such as bone,
cartilage, fat, connective tissue, and muscle; c) lymphomas and
leukemias arise from cells of blood and *lymphatic origin, the
term "leukemia" used when such cancer cells circulate in large
numbers in the bloodstream rather than growing mainly as solid
masses of tissue. Cancer is a disease of the genomic apparatus of
the cell, in particular of the growth-regulation apparatus, and
considering the vast number of activities that must be
coordinated and regulated by the genomic apparatus during the
lifetime of each cell, it is not surprising that malfunctions
arise. In general, cancer is the most prominent of the many
diseases arising from aberrations in cell function, with more
than 25 percent of people in the US now expected to develop
cancer in their lifetime.
... ... C.R. Boland and L. Ricciardiello (2 installations, US)
present a review of current research on the genomic basis of
cancer, the authors making the following points:
1) It has been known during most of this century that cancer
is often associated with visible derangements in the nucleus of
the cell. The cells of solid tumors commonly exhibit chromosome
duplications, deletions, and rearrangements, but before the
organization of the human cell nucleus was understood, these
chromosome aberrations were difficult to categorize and were of
little help in understanding the biological basis of cancer.
2) Within a few decades after the discovery of the structure
of DNA, cancer-related genes (oncogenes) were isolated, and these
were frequently found to be mutant versions of normal cellular
genes in which an activating *point mutation or an aberrant
*genetic amplification process resulted in a gain of function for
that gene product, and a growth advantage for that aberrant cell.
But as more and more oncogenes were identified, researchers
realized that tumor growth was also associated with loss of
function of certain "tumor suppressor genes". These tumor
suppressor genes were often inactivated by their deletion from
the nucleus, and the phrase "loss of heterozygosity" (LOH) was
applied to genetic loci in which both *alleles were present in
normal tissues, but one copy was lost in tumor tissue. In many
instances, tumor suppressor genes were first identified by virtue
of germ-line mutations that were present at a high frequency in a
rare tumor, e.g., retinoblastoma, but it soon became apparent to
researchers that many tumor suppressor genes were associated with
a variety of different tumors, many of which were not rare at
all.
3) There are no oncogenes or tumor suppressor genes that are
activated or deleted in and from all cancers. Even tumors of a
single organ rarely have uniform genetic alterations, although
tumor types from one specific organ do have a tendency to share
mutations in certain genes or in different genes within a single
growth-regulatory pathway.
4) At the present time, it is not known how many critical
mutations are required to convert a single normal cell into a
malignant cell. Human cells have been difficult to transform in
vitro, and the basis for this difficulty is not yet understood.
The simplest model of tumorigenesis is as follows:
... ... a) Human cells experience a certain number of mutations
each day as a result of exposure to carcinogens or as a result of
ordinary biological degradation, both of which can alter
nucleotide sequences. Errors will also occur during new DNA
synthesis and in the process of disentangling the chromosomes
during *mitosis. Most of these errors would be either irrelevant
to the life of the cell or deleterious because of the loss of a
gene critical for cellular viability.
... ... b) By chance, an occasional genomic mutation might create
a growth advantage for a cell, permitting increased net cellular
growth, because of increased proliferation or a reduction in
programmed cell death (reduction in apoptosis), with a resulting
*clonal expansion of that lineage. A second genomic alteration
might then occur within this expanding clone, again by chance,
providing an additional growth advantage for that cell and its
progeny. By virtue of these two advantages, the cells of this
clone would eventually overgrow neighboring cells, creating yet
another expanding clone. This scenario would repeat as a
consequence of each new mutation that provided an additional
growth advantage. The accumulation of these growth promoting
mutations is the basis of the current view of "multistep
carcinogenesis".
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Proc. Nat. Acad. Sci. 1999 96:14675
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Notes:
... ... *epithelial cells: In animals, "epithelial cells"
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
... ... *mesodermal: In the embryos of higher animals, there
occurs the transformation of a single-layer "blastula" into a
3-layered "gastrula" consisting of ectoderm (outermost layer),
mesoderm (middle layer), and endoderm (innermost layer)
surrounding a cavity with one opening. The 3 layers are called
the "germ layer", and these layers, via further cell
differentiation and proliferation, determine the development of
all the major body systems and organs.
... ... *lymphatic: The lymphatic system is a complex network
for the distribution of lymph fluid (which is similar to blood
plasma -- blood without red cells). Lymph is collected by
drainage from the tissues throughout the body, flows in the
lymphatic vessels through the lymph nodes, and is eventually
added to the venous blood circulation.
... ... *point mutation: A minor changes in the genome; a single
base-pair substitution.
... ... *genetic amplification process: The production, by
various means, of additional copies of a stretch of genomic DNA.
... ... *alleles: One of two or more forms of a given gene that
control a particular characteristic, with the alternative forms
occupying corresponding loci on homologous chromosomes.
... ... *mitosis: Programmed division of the nucleus during cell
replication.
... ... *clonal expansion: This refers to the expansion of a
population of cells all derived from repeated replications of
progeny of a single cell.
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Related Background:
ON GENETICS AND HUMAN CANCERS
The current consensus is that cancer results from the
accumulation of mutations in the genes that directly control the
birth and death of biological cells. But the mechanisms through
which these mutations are generated are the subject of continuing
debate and much research. It has been argued that an underlying
genetic instability is absolutely essential for the generation of
the multiple mutations that underlie cancer. On the other hand,
it has also been suggested that normal rates of mutation, coupled
with waves of *clonal expansion, are sufficient for the cancer
process to occur in humans. ... ... C. Lengauer et al (Johns
Hopkins University, US) present a review of observations
concerning the stability of the genome of human cancer cells, the
authors making the following points:
... 1) Numerous genetic alterations that affect growth-
controlling genes have been identified in neoplastic cells over
the past 15 years, and these observations provide persuasive
evidence for the genetic basis of human cancer. The alterations
can be divided into 4 major categories:
... ... a) Subtle sequence changes: These changes involve
nucleotide base substitutions or deletions or insertions of a few
nucleotides in the genome, and unlike the alterations described
below, they cannot be detected via cytogenetic analysis. Such
mutations, for example, occur in over 80 percent of pancreatic
cancers.
... ... b) Alterations in chromosome number: Such alterations
involve losses or gains of whole chromosomes. Such changes are
found in nearly all major human tumor types.
... ... c) Chromosome translocations: These alterations can be
detected cytogenetically as fusions of different chromosomes or
of normally non-contiguous segments of a single chromosome. At
the molecular level, such translocations produce fusions between
two different genes, endowing the fused genetic entity with
tumorigenic properties. Such translocations are known to occur in
the *chronic myelogenous leukemias.
... ... d) Gene amplifications: These are seen at the cytogenetic
level as homogeneously stained regions, and at the molecular
level they involve multiple copies of a gene. An example of gene
amplification occurs in advanced *neuroblastomas.
... 2) All 4 of the alterations described above occur commonly in
specific tumor types but are rarely or never observed in normal
cells. However, the existence of genetic alterations in a tumor,
even when frequent, does not mean that the tumor is genetically
unstable. By definition, instability is a matter of rate, and the
existence of a mutation provides no information about the rate of
its occurrence. The higher prevalence of mutations in tumor cells
compared with normal cells still requires explanation.
... The authors conclude: "One can argue persuasively that all
chemotherapeutic compounds used at present are more toxic to
cancer cells than to normal cells only and specifically because
of the defective *checkpoints that occur in cancer cells. This
line of reasoning suggests that, although instability may be
essential for neoplasia to develop, it may also prove to be its
Achilles' heel when the tumor is attacked by the right agents.
Further research to define the molecular and physiological bases
of instability may, therefore, yield entirely new approaches to
treating common forms of cancer."
-----------
Nature 1998 396:643
-----------
Notes:
... ... *clonal expansion: In this context, this refers to the
expansion of a population of cells all deriving from a single
mutated cell.
... ... *chronic myelogenous leukemias: (granulocytic leukemias)
These leukemias are characterized by an uncontrolled
proliferation of myelopoietic cells (blood cells derived from
bone marrow).
... ... *neuroblastomas: Neuroblastomas are malignant neoplasms
characterized by only slightly differentiated immature nerve
cells of embryonic type.
... ... *checkpoints: In this context, the term "checkpoint"
refers to a point in the eukaryotic cell division cycle where the
cycle can be halted until conditions are suitable for the cell to
proceed to the next stage. (eukaryotic = containing membrane-
bound organelles such as a nucleus.)
-----------
Related Background:
ANEUPLOIDY AND GENETIC INSTABILITY OF CANCER CELLS
In general, germ cells (egg cells and sperm cells) and somatic
cells (non-germ cells) carry different numbers of chromosomes,
with germ cells carrying exactly half the number (haploid number)
of somatic cell chromosomes (diploid number). The term
"aneuploidy" (heteroploidy) refers to a condition in which the
number of chromosomes in a cell is not an integer multiple of the
haploid number typical for that cell or organism. For example,
the haploid human chromosome number is 23; the normal somatic
cell contains 46 chromosomes; a somatic cell with 47 or 44
chromosomes is aneuploid. Some authors, however, use the term
"aneuploidy" to indicate merely an abnormal number of
chromosomes. In cell biology, the term "karyotype" refers to the
characteristics profile (number, size, and shape) of a set of
chromosomes of a cell or organism. In this context, the term
"phenotype" refers to the total appearance of a cell as
determined by the interaction during development between its
genetic constitution (genotype) and the cell's environment.
Genetic and phenotypic instability are hallmarks of cancer cells,
but the cause of the instability is not clear. The leading
hypothesis suggests that a poorly defined gene mutation generates
genetic instability and that one or more of the many subsequent
mutations then cause cancer [*Note #1]. ... ... P. Duesberg et al
(2 installations, DE US) report an investigation of the
hypothesis that genetic instability of cancer cells is caused by
aneuploidy, which they define as "an abnormal balance of
chromosomes". The authors point out that because symmetrical
segregation of chromosomes during mitosis depends on exactly two
copies of the genes involved in mitosis ("mitosis genes"),
aneuploidy involving chromosomes bearing mitosis genes will
destabilize the karyotype. The authors propose that the
aneuploidy hypothesis predicts that the degree of genetic
instability should be proportional to the degree of aneuploidy,
and it should thus be difficult to maintain the particular
karyotype of a highly aneuploid cancer cell on *clonal
propagation. The authors report this prediction is confirmed with
clonal cultures of chemically transformed aneuploid Chinese
hamster embryo cells. Defining the "ploidy factor" as the
quotient of the modal chromosome number divided by the normal
number of the species, it was found that the higher the ploidy
factor of a clone, the more unstable was its karyotype. The
authors point out that work by others has established an exact
correspondence between the karyotype instability of human colon
cancer cell lines and the degree of aneuploidy. The present
authors suggest that, independent of gene mutation, aneuploidy is
sufficient to explain genetic instability and the resulting
karyotypic and phenotypic heterogeneity of cancer cells. The
authors further suggest that because aneuploidy has also been
proposed to cause cancer, their hypothesis "offers a common,
unique mechanism of altering and simultaneously destabilizing
normal cellular phenotypes."
-----------
Proc. Nat. Acad. Sci. 1998 95:13692
-----------
Notes:
... ... *Note #1: In 1976, Peter Nowell postulated that a
precancerous mutation generates exceptional "genetic instability"
or "mutability", and that the highly mutable "premalignant" cell
then suffers many further gene mutations, including those that
cause cancer (P.C. Nowell, Science 194:21 1976).
... ... *clonal propagation: In general, in this context, a
"clone" is a line of identical cells produced from one or a few
originating cells.
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10. ON THE NUCLEOLUS
An argument can be made that after the genome of the living
cell, the most important cellular entity is the ribosome, or more
exactly the ribosomes of the cell, since although each cell
contains only one genome, each cell contains many thousands of
ribosomes. The reason for the importance of ribosomes is simple:
the genome contains programs for the ultimate synthesis of
proteins, the molecules that do the work of the cell, but these
programs are translated into chemical action in the ribosomes --
it is the ribosomes that do the actual synthesis of proteins.
There is no DNA in ribosomes. The term "ribosomal DNA"
refers to any DNA sequence that codes for ribosomal RNA.
It is now believed that all important functions of the
ribosome actively involve the ribosomal RNAs as major players,
whereas the ribosomal proteins act either as structural "glue" or
as "helpers" that promote specific binding reactions. This
represents a complete reversal of the long-held view that the
proteins of the ribosome perform all the important tasks.
The nucleolus, a large structure within the nucleus of
eukaryotic cells, consists of numerous loops of chromatin-bound
DNA that contain clusters of often tandemly repeated ribosomal
RNA genes: the nucleolus is thus the structure responsible for
the production of ribosomal RNA in eukaryotic cells.
... ... J.K. Ospina and A.G. Matera (Case Western Reserve
University, US) discuss the nucleolus, the authors making the
following points:
1) The nucleolus, first described over 150 years ago,
functions in ribosomal RNA synthesis, processing, and ribosome
assembly. Nucleoli are also believed to play a role in various
other activities, such as the maturation of certain messenger
RNAs, the sequestration of regulatory molecules, and the
maturation of telomerase and signal-recognition-particle
ribonucleoproteins. Thus, the identification of proteins that
localize to the nucleolus will provide a crucial first step to
many studies focused on understanding nucleolar function,
dynamics, and interaction with other subnuclear structures.
2) The mammalian nucleolus is a large (5 to 10 microns)
structure that disassembles in late prophase and reforms in
telophase. Nucleoli form in response to transcription of
ribosomal DNA repeats that are often tandemly arrayed in so-
called "nucleolar organizer regions". There are 3 distinct
subdomains within nucleoli, called the "dense fibrillar
component", "granular component", and "fibrillar center".
Transcription likely occurs near the border of the fibrillar
center and dense fibrillar component. Processing of new pre-
ribosomal RNAs occurs in the dense fibrillar component, followed
by assembly of ribosomal subunits in the granular component.
3) Aside from these well-established functions, new roles
for the nucleolus are becoming evident, and the occasional
identification of previously unknown nucleolar proteins has
provided the raw materials for studies of novel nucleolar
functions. As nucleoli are not bounded by membranes, and the
intact structure is not differentially extractable by salts, the
usual biochemical fractionation methods are not relevant.
Instead, nucleoli must be detached from the nucleoplasmic milieu
by extensive sonication, followed by sucrose centrifugation. Once
a suitably pure fraction has been obtained, identification of the
proteins can proceed.
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Current Biology 2002 12:R29
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11. ON SIGNALING COMPLEXES IN BIOLOGICAL CELLS
F.D. Smith and J.D. Scott (Oregon Health and Sciences University,
US) discuss signaling complexes, the authors making the following
points:
1) An enigmatic yet fundamental principle of signal
transduction in biological cells is that parallel signaling
pathways assembled from a common repertoire of enzymes are able
to propagate diverse physiological responses. A key feature of
such a mechanism is that separate signaling pathways are
organized into localized transduction units, each tailored to
respond optimally to a particular signal. Protein-protein
interactions maintained by anchoring, adaptor, and scaffolding
proteins provide the molecular glue that holds these signal
transduction units together. A major objective of signaling
researchers is to ascertain how signals flow through
compartmentalized transduction units that contain transmembrane
receptors, protein kinases, phosphatases, and their substrates.
2) One surprising outcome of the human genome project is
that the total number of genes expressed in an individual cell is
less than was anticipated. Yet the complexity of biological
processes requires these genes to be used in different ways to
meet the demand for cellular diversity. Scaffolding proteins
contribute to this diversity via the assembly of signaling
networks in which enzymes are compartmentalized with a subset of
their substrates. There are several implications of this
combinatorial model. First, diversity may be obtained by the
selective assembly of signaling enzymes that associate with a
multivalent anchoring protein. Diversity may also be achieved by
the preferential use of targeting signals that are contained
within the anchoring protein. Diversity in targeting can also be
obtained through alternative splicing of targeting signals. And
additional diversity can be achieved via the dynamic assembly of
signaling networks.
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Current Biology 2002 12:R32
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12. EMBRYOGENESIS AND GENE EXPRESSION
T. Kudoh et al (National Institutes of Health, US) discuss
embryogenesis and gene expression, the authors making the
following points:
1) Embryonic development is accompanied by regulated changes
in the expression of large sets of genes, and determining how the
interplay of these changes influences the progress of development
at the cellular and organismic level is a major aim of
developmental biology. In the past several decades it has become
clear that the expression and function of a variety of regulatory
genes guide developmental processes such as cell differentiation
and pattern formation, and it has also emerged that a highly
effective way of approaching questions of developmental mechanism
is to study the properties of differentially regulated gene
expression during embryogenesis.
2) This approach has been applied to vertebrate systems in a
variety of ways. Earlier studies emphasized specific aspects of
developmental control by selecting genes for study according to
various criteria, including temporal patterns of expression,
regional restriction, and functional characteristics. Such
approaches have led to a wealth of information about gene
expression patterns, providing useful regional markers, and
yielding insights into regulatory factors that control
differentiation and pattern formation.
3) As developmental biology entered the genomic era, a
notion that gained currency was that it may not only be desirable
but also feasible to characterize the regulated expression of the
entire population of genes that affect embryogenesis, rather than
focus on selected subsets of genes. But even in cases where a
complete genome sequence is available, this aim is quite large.
Instead, by placing the focus on those genes whose expression is
spatially and temporally regulated during development, the total
numbers of genes that need to be studied is reduced and the yield
of useful information is increased. Screens of this type have now
been carried out with Xenopus (African clawed toad), mouse
embryos, and zebrafish (Danio rerio).
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Genome Research 2001 11:1979
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13. MATERIALS SCIENCE: ON CLAY-POLYMER NANOCOMPOSITES
S. Stackhouse et al (University of London, UK) discuss clay-
polymer nanocomposites, the authors making the following points:
1) In recent years there has been considerable interest in
clay-polymer nanocomposites due to the novel material qualities
that they exhibit. They offer enhanced mechanical and thermal
properties, which have led to a wide range of applications in the
automotive, electronics, and furnishings industries.
2) The intrinsic structure of clay-polymer nanocomposites
varies depending on their constituents and method of preparation.
Two extremes may be used to define the range of possible
structures. a) At one extreme, the intercalated polymer chains
sit within the clay mineral layers, which are stacked together in
a well-ordered manner. Such materials are called "intercalated"
or "non-exfoliated" nanocomposites. b) At the other extreme, the
clay mineral layers have lost their order and are well-dispersed
in a continuous polymer matrix. Such materials are called
"delaminated" or "exfoliated" nanocomposites. Some nanocomposites
may contain both ordered and disordered phases. Recent research
has mainly focused on the preparation of exfoliated rather than
non-exfoliated materials because of their lower density.
3) Currently, three established methods of clay-polymer
nanocomposite preparation exist: a) exfoliation-adsorption; b) in
situ intercalative polymerization; and c) direct polymer melt
intercalation. Exfoliation-adsorption involves exfoliation of the
clay mineral using a solvent in which the polymer is soluble. In
situ intercalative polymerization involves mechanical mixing of
the clay mineral with a monomer, which intercalates within the
interlayer and promotes delamination. Direct polymer melt
intercalation involves mechanical mixing of the clay mineral into
a polymer melt.
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J. Am. Chem. Soc. 2001 123:11764
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14. CRYSTAL-CONTROL OF X-RAY SWITCHING
F. Krausz and C. Spielmann (Vienna University of Technology, AT)
discuss x-ray switching, the authors making the following points:
1) Since their discovery approximately 100 years ago by
Wilhelm Roentgen (1845-1923), x-rays have found important uses in
hospitals, laboratories, and the exploration of space. They have
proved particularly useful in explorations of the microscopic
structure of matter. By scattering x-rays from small molecules,
large biopolymers, or macroscopic crystals, researchers can
determine how the constituent atoms are arranged relative to each
other. But obtaining fundamental information about the dynamics
of molecular reactions is much more difficult, since that
requires the motions of constituent atoms to be followed over
interatomic distances. To achieve this, x-rays must be switched
on and off within a time period short enough to "freeze" atomic
motion. This is an enormous challenge.
2) Traditional x-ray structural analysis, although it cannot
follow the progression of reactions, provides insight into how
molecules work, and is thus a key experimental technique for the
chemical and life sciences. The equilibrium positions of atoms in
complex molecules can be measured with an accuracy of 10^(-13)
meters (approximately 0.001 of a molecular bond length). Knowing
the equilibrium structure of molecules allows researchers to
predict how they might behave in various situations, but definite
answers to many important questions require the direct
observation of molecular dynamics.
3) DeCamp et al (2001) have now demonstrated how to control
the proportion of x-rays transmitted through a crystal on a
picosecond timescale, and with this method it may be possible to
develop a sub-picosecond x-ray switch that would be fast enough
to track dynamic changes in molecular structure during chemical
and biochemical reactions.
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Nature 2001 413:784,825
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15. ON WATER AND OXYGEN IN AMORPHOUS SILICON DIOXIDE
T. Bakos et al (Vanderbilt University, US) discuss amorphous
silicon dioxide, the authors making the following points:
1) Amorphous silicon dioxide is a key component of metal-
oxide semiconductor field-effect transistors, solar cells, and
optical fibers, all of which are basic elements of modern
technology. Practical applications of silicon dioxide are often
limited by point defects that can change the mechanical,
electrical, and optical properties of the oxide. In addition,
many impurities, such as hydrogen and alkali metals, are known to
affect the properties of the oxide.
2) Water and oxygen molecules are also known to play
important roles in determining the properties of amorphous
silicon dioxide, but the relevant atomic-scale configurations and
processes remain unresolved. There exists no detailed account of
whether these molecules remain intact in the interstitial
regions, if they diffuse as molecules without breaking up, and if
they attach whole to the network.
3) In the case of water, vibrational data reveal signatures
of both interstitial water molecules and silanol groups (SiOH),
with the ratios depending on sample preparation, total water
uptake, and temperature. Experiments using water molecules
containing O-18 found that O-18 exchanges with network oxygen
atoms even at 400 degrees C., when oxidation processes are
negligible. Other experiments using tritiated water found that
tritium diffuses with an activation energy in the range of 0.6 to
0.8 electron volts, which is in agreement with the value
extracted from thermal oxidation data, although the
identification of the diffusing species is uncertain. The above
experiments are consistent, but suggest a rather complex
diffusing mechanism that involves reactions with the network.
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Phys. Rev. Lett. 2002 88:055508
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16. ON TRIPLE-POINT WETTING ON SURFACES
In this context, the term "triple point" refers to the
temperature and pressure at which the gas, liquid, and solid
phases of a substance are in equilibrium.
... ... A. Esztermann et al (Heinrich-Heine University, DE)
discuss surface wetting, the authors making the following points:
1) Wetting of a solid substrate exposed to a gas in
thermodynamic equilibrium is a ubiquitous phenomenon with both
fundamental aspects and important applications. Microscopically,
substrate wetting by a liquid film is caused by a strong
substrate-particle attraction mediated by van der Waals forces.
At present, an almost complete microscopic understanding of
wetting on flat solid substrates is available, predicting the
thickness of the liquid film as a function of the substrate-
particle and interparticle interactions for given thermodynamic
parameters such as temperature and pressure.
2) The following basic theoretical predictions have been
confirmed by experiment using, for example, noble gases on
different substrates:
a) For fixed thermodynamic conditions, the thickness of
the wetting layer grows for increasing substrate-particle
attraction.
b) Complete wetting (i.e., a diverging thickness of the
liquid layer) occurs if the substrate-particle attraction is
stronger than the interparticle attraction and the thermodynamic
conditions approach liquid-gas coexistence.
3) The latter condition (complete wetting) can be achieved
only if the system temperature is above the triple point
temperature. For a system temperature less than the triple point
temperature, on the other hand, a solid film shows up near the
sublimation line. Various experiments have demonstrated that the
width of the solid layer always remains finite when approaching
gas-solid coexistence. It is only near the triple point that a
liquid layer on top of the solid sheet is formed, with a
diverging width as the triple point is approached. This universal
behavior is called "triple point wetting".
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Phys. Rev. Lett. 2002 88:055702
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17. ON NUCLEAR POWER
J.A. Lake et al (Department of Energy, US) discuss nuclear power,
the authors making the following points:
1) Most of the world's nuclear power plants are pressurized
water reactors. In these systems, water placed under high
pressure (155 atmospheres) to suppress boiling serves as both the
coolant and the working fluid. Initially developed in the US
based on experience gained from the US naval reactor program, the
first commercial pressurized light-water reactor began operation
in 1957.
2) The reactor core of a pressurized water reactor consists
of arrays of zirconium alloy-clad fuel rods composed of small
cylinders (pellets) of mildly enriched uranium oxide with the
diameter of a dime. A typical 17 x 17 square array of fuel rods
constitutes a fuel assembly, and approximately 200 fuel
assemblies are arranged to form a reactor core. These cores,
which are typically approximately 3.5 meters in diameter and 3.5
meters high, are contained within steel pressure vessels 15 to 20
centimeters thick.
3) The nuclear fission reactions produce heat that is
removed by circulating water. The coolant is pumped into the core
at approximately 290 degrees C. and exits the core at
approximately 325 degrees C. To control the power level, control
rods are inserted into the fuel arrays. Control rods are made of
materials that moderate the fission reaction by absorbing the
slow (thermal) neutrons emitted during fission. They are raised
out of or lowered into the core to control the rate of the
nuclear reaction. To change the fuel or in the case of an
accident, the control rods are lowered all the way into the core
to shut down the reaction.
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Scientific American 2002 January
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18. EARLY HISTORY OF HIGH-TEMPERATURE SUPERCONDUCTORS
D. Larbalestier et al (University of Wisconsin, US) discuss high-
temperature superconductors, the authors making the following
points:
1) The potential applications of superconductors -- the name
Kamerlingh Onnes (1853-1926) gave to materials that lose electric
resistance on cooling below a specific transition temperature --
were apparent to Onnes almost immediately. In 1913, just two
years after his discovery, Onnes talked in Chicago about the
design of very powerful magnets far exceeding the fields
achievable by iron; these would cost as much as a battleship if
made from copper and cooled with liquid air, but be affordable if
made from superconducting wires. By that time he had already
tested a nickel alloy coated with lead-rich superconducting
solder, but this lost superconductivity at fields of less than 50
millitesla. He ascribed this unexpected setback to bad places in
the wire, a problem he anticipated would soon be fixed without
difficultly.
2) In fact, applications had to wait 50 more years,
particularly because the physics of superconductivity in magnetic
fields was seriously misunderstood. This need not have been so,
because London and Shubnikov made important breakthroughs in
understanding the magnetic properties of superconductors in the
1930s. By careful alloying experiments, Shubnikov pointed out the
vital distinction between type I superconductors, in which
currents flow only at the surface and superconductivity is
destroyed by weak fields, as in Onnes's 1913 experiment, and a
new type of superconductor, now called a type II superconductor,
capable of carrying bulk supercurrent at high fields. The key
understanding that the behavior of type II superconductors is due
to quantized magnetic vortices was achieved by Abrikosov in the
1950s.
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Nature 2001 414:368
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19. HORMONE REPLACEMENT THERAPY AND BREAST CANCER
C-L. Chen et al (Fred Hutchinson Cancer Research Center Seattle,
US) discuss hormone replacement therapy and breast cancer, the
authors making the following points:
1) The possible association between the use of estrogen
replacement therapy or combined estrogen-progestin replacement
therapy and the incidence of breast cancer has been assessed in
numerous studies. In 1997, the Collaborative Group on Hormonal
Factors in Breast Cancer reported a pooling and reanalysis of
data from 52,705 women with breast cancer and 108,411 women
without breast cancer, that study reporting a modest increase in
the risk of breast cancer associated with ever use of estrogen
replacement therapy, with evidence of an increasing relative risk
with increasing duration of use. The risk of breast cancer was
increased among current users, but not among past users. It was
also reported in that study that among women whose duration of
current combination therapy was more than 5 years, the risk
appeared to be increased relative to never users, but the
evidence was imprecise. In addition, there is recent evidence
that use of hormone replacement therapy may differentially affect
the incidence of lobular cancer relative to other types of breast
cancer.
2) The authors conducted a nested case-control study to
examine the relationship between postmenopausal hormone
replacement therapy and risk of breast cancer by histologic type
among female enrollees of a group health cooperative in the
Seattle area. The study involved 705 postmenopausal women, age 50
to 74 years, who had primary invasive breast cancer diagnosed
between July 1, 1990 and December 31, 1995 (called "cases"), and
692 randomly selected aged-matched female members of the same
health cooperative (called "controls").
3) The authors report their data add to the growing body of
evidence that recent long-term use of hormone replacement therapy
is associated with an increased risk of breast cancer, and that
such use may by related particularly to lobular tumors.
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J. Am. Med. Assoc. 2002 287:734
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20. COGNITIVE ACTIVITY AND THE RISK OF ALZHEIMER DISEASE
Alzheimer's disease is now tabulated as the 12th leading cause of
death in the US, with a 1997 death-rate per 100,000 population of
8.4, which is higher than homicide and legal intervention (7.0),
AIDS (6.2), and atherosclerosis (6.2). It is estimated that by
2025 more than 20 million people worldwide will be afflicted with
the disease. In general, Alzheimer's disease is a degenerative
brain disorder that develops in mid- to late-adult life, the
disease resulting in a progressive and irreversible decline in
memory coupled with a decline in various other cognitive
functions. In terms of general pathology, the disease is
characterized by the destruction of nerve cells and neural
connections in the cerebral cortex of the brain and by a visible
and significant loss of brain mass.
... ... R.S. Wilson et al (Rush-Presbyterian-St. Luke's Medical
Center Chicago, US) discuss Alzheimer disease, the authors making
the following points:
1) Alzheimer disease is the leading cause of dementia in
older persons, but few risk factors for the disease have been
identified. Frequent participation in cognitively stimulating
activities has been hypothesized to reduce the risk of
Alzheimer's disease, but this hypothesis has not been tested
prospectively in longitudinal studies of incident disease.
Support for the hypothesis currently comes mainly from
retrospective case-control studies suggesting that mid-life
cognitive activity is associated with disease risk, and from
cross-sectional research showing an association between frequency
of cognitive activity and level of cognitive function in old age.
2) The authors report they used a previously established
measure of frequency of participation in common cognitive
activities and tested its association with incident Alzheimer
disease and decline in cognitive function in a large cohort of
older Catholic clergy members examined annually for up to 7
years.
3) The authors report their results suggest that frequent
participation in cognitively stimulating activities is associated
with reduced risk of Alzheimer disease.
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J. Am. Med. Assoc. 2002 287:742
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Related Background:
MEDICAL BIOLOGY: MIDLIFE REDUCED ACTIVITY IN ALZHEIMER'S DISEASE
... ... R.P. Friedland et al (8 authors at 2 installations, US)
present a study of activity of Alzheimer's disease patients in
midlife compared with a healthy control-group, the authors making
the following points:
1) The authors point out that research in North America,
Europe, Asia, and the Middle East has demonstrated that the
*incidence and prevalence of Alzheimer's disease is lower in
subjects with relatively higher levels of education. According to
one study (the East Boston study), each year of education reduces
the risk of Alzheimer's disease by 17 percent. Although the
protection against the development of Alzheimer's disease
provided by education could be an artifact produced by the
ability of more highly educated persons to perform better on
cognitive tests, many studies have used functional rather than
psychometric measures for diagnosis and have documented the
protective effect of education. Although the mechanisms of
education protection remain unknown, it has been proposed that
the protective effects of education are related to neuronal
reserve, with individuals with higher levels of education more
resistant to the effects of the disease on cognition because of
enhanced synaptic complexity. Occupational attainment also has
been demonstrated to be protective against the disease.
2) The authors point out that education protection also may
be induced by lifelong patterns of neuronal activation associated
with exposure to education. But education and occupation are not
the only reflection of these lifelong patterns: recreational
activities are also indications of the ways in which cognitive
and other skills are used in daily life. The authors have
hypothesized that recreational tasks, in addition to education
and occupation, are protective against the development of
Alzheimer's disease. Leisure endeavors are reflective of the
intrinsic value of an activity for an individual -- they may be
more reflective of neurological factors than education or
occupation, which are strongly influenced by socioeconomic
determinants, especially in the earlier years of this century,
when economic, social, and military factors often determined who
went to school and for how long. Recreational activities thus may
provide a reflection of neuronal reserve and activation that may
be relatively independent of these economic, social, and military
factors.
3) The authors report their results indicate that patients
with Alzheimer's disease are less active in midlife (early and
middle adulthood) in terms of intellectual, passive, and physical
activities than members of the control group used in this study.
The lower activity levels prior to onset of disease (premorbid
levels) in patients with Alzheimer's disease persisted in
measures of intellectual, passive, and physical activities,
calculated by using an independently developed scale following
statistical correction for year of birth, sex, education, and
income adequacy. These differences were not explained by
differing educational levels in the two groups. The study
minimized the influence of early disease on participation in
activities by the collection of data only concerning the period
of midlife ending at age 60 or ending 5 years before disease
onset (whichever was earlier). The authors suggest their results
indicate that low participation in activities in midlife (in
addition to low levels of educational and occupational
achievement) is a risk factor for the disease.
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Proc. Nat. Acad. Sci. 2001 98:3440
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Notes:
... ... *incidence and prevalence: In this context, the term
"incidence" refers to the number of new cases during a specified
time period; the term "prevalence" refers to the total number of
existing cases at a specified time or during a specified time
period.
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21. ON LEGIONNAIRE'S DISEASE
The pathogen that causes Legionnaire's disease, Legionella
pneumophila, was first discovered during an investigation of an
outbreak of acute febrile respiratory illness among members of
the American Legion in Philadelphia in 1976. Subsequent
retrospective studies identified cases as early as 1943, and a
variety of related organisms have been tentatively classified in
the genus Legionella. There are approximately 30 proposed species
of Legionella, and at least 19 of these species have been
implicated as agents of pneumonia in humans.
Macrophages (phagocytes) are amoeba-like white blood cells
(leukocytes) of the immune system that are able to surround and
digest foreign entities such as bacteria and protozoa.
In this context, a "transporter" is a membrane protein
catalyzing the passage of molecules from one face of a membrane
to the other.
... ... H. Nagai et al (Yale University, US) discuss
Legionnaire's disease, the authors making the following points:
1) Legionella pneumophila are aquatic bacteria that infect
and grow within protozoan hosts in most freshwater ecosystems.
When these bacteria are inhaled by humans, L. pneumophila will
replicate in specific macrophages in the lung (alveolar
macrophages), resulting in a severe pneumonia known as
"Legionnaire's disease".
2) L. pneumophila replicate within phagocytes by first
creating a specialized vacuole that is similar morphologically to
the endoplasmic reticulum of its host. Biogenesis of this
replicative vacuole requires a specific transporter (Dot/Icm), a
so-called type IV protein secretion apparatus. Pathogens such as
Agrobacterium tumefaciens (the soil bacterium causing crown gall
disease in trees and plants) and Helicobacter pylori (the
bacterium implicated in human stomach ulcers and stomach cancer)
use type IV transporters to inject bacterial proteins directly
into the cytoplasm of eukaryotic host cells, and it is thought
that the Dot/Icm transporter is used by L. pneumophila to inject
proteins into host cells in order to control the biogenesis of a
replicating organelle by modulating the activity of host factors
involved in vesicle traffic. However, genetic screens that have
been successful in isolating virulence determinants required for
growth of L. pneumophila in host cells, including the genes
encoding the Dot/Icm apparatus, have not revealed any injected
proteins.
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Science 2002 295:679
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22. HYPOTHERMIA AND TREATMENT AFTER CARDIAC ARREST
In general, "ischemia" is a sudden loss of blood supply to a
tissue caused by blockage of a blood vessel or by cardiac arrest.
The term "ischemia/reperfusion injury" refers to the damage that
can occur to a tissue when it is reperfused after a prolonged
period of ischemia. The phenomenon is of considerable clinical
importance, especially in connection with heart attacks and
strokes.
... ... M. Holzer et al (Universitaetsklinik fuer Notfallmedizin
Vienna, AT) discuss cardiac arrest, the authors making the
following points:
1) An estimated 375,000 people in Europe undergo sudden
cardiac arrest each year, and recovery without residual
neurologic damage after cardiac arrest with global cerebral
ischemia is rare. After cardiac arrest with no blood flow for
more than 5 minutes, the generation of free radicals, together
with other mediators, during reperfusion creates chemical
cascades that result in cerebral injury. Until recently there was
no therapy with documented efficacy in preventing brain damage
after cardia arrest.
2) Several studies have demonstrated that in dogs moderate
systemic hypothermia (30 degrees C.) or mild hypothermia (34
degrees C.) markedly mitigate brain damage after cardiac arrest.
The exact mechanism for this cerebral resuscitative effect is not
clear. A reduction in cerebral oxygen consumption and other
multifactorial chemical and physical mechanisms during and after
ischemia have been postulated. These mechanisms include
retardation of destructive enzymatic reactions, suppression of
free-radical reactions, protection of the fluidity of lipoprotein
membranes, reduction of oxygen demand in low-flow regions,
reduction of intracellular acidosis, and inhibition of the
biosynthesis, release, and uptake of excitatory
neurotransmitters.
3) The authors report a study comparing mild hypothermia
with standard normothermia in human patients who had had cardiac
arrest due to ventricular fibrillation. The authors report that
in such patients who have been successfully resuscitated,
therapeutic mild hypothermia increased the rate of favorable
neurologic outcome and reduced mortality. The authors suggest
that of the 30,000 cardiac arrest cases in Europe each year who
would meet the inclusion criteria for this study, "we can be 95
percent confident that treatment with hypothermia would prevent
an unfavorable neurologic outcome in 1200 to 7500 of these
patients."
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New Engl. J. Med. 2002 346:549
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23. GENETIC FACTORS AND CROHN'S DISEASE
Charles O. Elson (University of Alabama Birmingham, US) discusses
Crohn's disease, the author making the following points:
1) Crohn's disease and ulcerative colitis, collectively
known as "inflammatory bowel disease", affect up to 1 million
people in the US. These related but distinct diseases are complex
disorders with immunologic, environmental, and genetic
components, each of which is the subject of intensive
investigation. In 1996, a gene conferring susceptibility to
Crohn's disease was identified on chromosome 16 in families with
multiple affected members. Recently, two groups working
independently and using different strategies, have identified the
relevant gene at that locus.
2) The gene is named NOD2, and is similar to the R factor
genes of plants that confer resistance to infection. NOD2 was
originally identified in a search for genes encoding proteins
that mediate apoptosis. Indeed, the NOD1 protein has two copies
of a domain found in proteins that mediate apoptosis, termed the
"caspase recruitment domain". Other regions of NOD2 include a
nucleotide-binding domain and a region of 10 leucine-rich repeats
at the carboxy terminal. A key disease-related mutation truncates
the 10th leucine-rich repeat, and this mutation was found in 8
percent of patients with familial Crohn's disease, as compared
with 4 percent of controls. However, only patients with Crohn's
disease were homozygous for this mutation, and homozygosity
increased the risk of Crohn's disease by a factor of 20 to 40.
Mutations were not seen more frequently in patients with
ulcerative colitis than in controls, supporting the concept that
ulcerative colitis and Crohn's disease are related by distinct
disorders.
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New Engl. J. Med. 2002 346:614
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24. ON THE TOTAL ARTIFICIAL HEART
P.M. McCarthy and W.A. Smith (Kaufman Center for Heart Failure,
US) discuss the total artificial heart, the authors making the
following points:
1) Each year in the US, approximately 2000 patients with
end-stage heart failure receive heart transplants. Yet 400,000 or
more individuals develop heart failure annually, of whom 30,000
to 100,000 potentially could benefit from a transplant. This has
led to a system of rationing, with many patients rejected for
transplantation for one reason or another. One goal is to develop
a machine that is as good or even better than a donor heart, thus
avoiding the need for a transplant altogether.
2) As a result of the highly public reintroduction of the
total artificial heart in July 2001, the interest of the medical
community and the public has been drawn again to mechanical
circulatory support systems for the treatment of end-stage heart
disease. A total artificial heart implant is a dramatic event
that removes the patient's heart and that results in complete
dependence on the most sophisticated technology ever implanted in
humans. A total artificial heart must "beat" approximately 35
million to 40 million times a year, providing a cardiac output of
5 to 6 liters per minute of blood. Current design goals are 90
percent reliability after 5 years of operation, which is
comparable to the 70 percent 5-year survival for current heart
transplant patients. The device that has made news headlines is a
titanium and polymer construct that uses an electrohydraulic
actuator system to shuttle blood alternately between the right
and left pumps. Currently under development are electromechanical
total artificial heart devices that directly convert electrical
energy into mechanical action. A recent improvement has been the
use of a transcutaneous energy transmission system that transmits
power through intact skin, thus eliminating the risk of infection
entering the body along the percutaneous power line used with
earlier implanted blood pumps. A rechargeable implanted battery
provides brief periods of freedom from the external battery and
other external electronics, allowing the patient to shower, or
theoretically even to swim.
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Science 2002 295:998
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25. IN FOCUS: GEOLOGY: ON THE OLDEST PARTS OF THE LITHOSPHERE
"Today, plate tectonics is the dominant process behind the
creation and destruction of Earth's lithosphere, the stiff outer
layer that includes the crust and the uppermost part of the
mantle. For example, as the South American Plate and the African
Plate are moving away from one another, lithosphere is created in
the central South Atlantic and begins to be destroyed in the
subduction zone beneath the Andes. Was plate tectonics also a
dominant process in Earth's early history? To answer this
question, we must study the oldest parts of today's lithosphere.
The oldest rocks on Earth are found in the Slave Province, a
geological region in the Northwestern Territories of Canada.
These rocks formed 4030 million years ago -- only about 500
million after the formation of the Solar System and 3900 million
years before Gondwanaland broke up into Africa and South America.
Practically all rocks and geological structures of the Slave
Province are older than 2500 million years, and the area has been
tectonically undisturbed ever since. Geophysical observations
indicate that North American Provinces that are older than 1000
million years, including the Slave Province, generally have a
cold, stiff lithosphere that is 250 kilometers thick on average.
Some rocks, called mantle xenoliths, have been brought to the
surface from deep within this thick lithosphere by rapidly
ascending gas-rich magmas. In the Slave Province, these magmas
also carried diamonds to the surface, suggesting that the deep
lithosphere could be as old as the crust. Indeed, recent analyses
of mantle xenoliths from the Slave Province indicate that the
deep lithosphere is over 2500 million years old."
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Suzan van der Lee: Deep Below North America
Science 2001 294:1297
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26. FROM THE SW ARCHIVE:
ON THE STANDARD MODEL AND A UNIFIED PHYSICS
In particle physics, the "Standard Model" is a theoretical
framework whose basic idea is that all the visible matter in the
universe can be described in terms of the elementary particles
leptons and quarks and the forces acting between them. Leptons
are a class of point-like fundamental particles showing no
internal structure and no involvement with the strong forces.
Electrons and neutrinos are among the particles classified as
leptons. The strong force (nuclear strong force) is one of the
four fundamental forces: the gravitational force, the
electromagnetic force, the nuclear strong force, and the nuclear
weak force (see below), with the strong force approximately 100
times stronger than the electromagnetic force. A quark is a
hypothetical fundamental particle, having charges whose
magnitudes are one-third or two-thirds of the electron charge,
and from which the elementary particles may in theory be
constructed. At the present time, ongoing experimental projects
in particle physics are expected to permit a completion of the
Standard Model, but a unified theory of all forces known to
physics is not yet in sight.
... ... Steven Weinberg (University of Texas Austin, US) presents
the following considerations concerning the Standard Model and
current attempts to achieve a unified physics:
1) In physics, the greatest advances of the past have been
steps involving unification: a) the unification of terrestrial
and celestial mechanics by Isaac Newton (1642-1727) in the 17th
century; b) the unification of optics with the theories of
electricity and magnetism by James Clerk Maxwell (1831-1879) in
the 19th century; c) the unification of space-time geometry and
the theory of gravitation by Albert Einstein (1879-1955) in the
years 1905 to 1916; d) the unification of chemistry and atomic
physics by quantum mechanics in the 1920s.
2) Our current theory of elementary particles and forces,
known as the Standard Model of particle physics, has achieved a
unification of electromagnetism with the weak interactions, the
forces responsible for the change of neutrons and protons into
each other in radioactive processes and in the stars. The
Standard Model also gives a separate by similar description of
the strong interactions, the forces that hold quarks together
inside protons and neutrons, and that hold protons and neutrons
together inside atomic nuclei. We have ideas about how the theory
of strong interactions can be unified with the theory of weak and
electromagnetic interactions, but the approach may only work if
gravity is included, and the inclusion of gravity presents
serious theoretical difficulties.
3) The Standard Model is a quantum field theory. Its basic
ingredients are fields, including the electric and magnetic
fields of 19th century electrodynamics. Perturbations in these
fields carry energy and momentum from place to place, and quantum
mechanics indicates these perturbations come in bundles, or
quanta, which are recognized in the laboratory as elementary
particles. For example the quantum of the electromagnetic field
is the photon. The Standard Model includes a field for each type
of elementary particle that has been observed in high-energy
physics laboratories.
4) The Standard Model is a quantum field theory of a special
kind, one that is "renormalizable". This term goes back to the
1940s, when physicists were learning how to use the first quantum
field theories to calculate small shifts of atomic energy levels.
They found that calculations using quantum field theory kept
producing infinite quantities, a situation which usually
indicates a theory is badly flawed or is being pushed beyond its
limits of validity. Eventually, physicists discovered a way to
deal with the infinite quantities by absorbing them into a
redefinition, or "renormalization", of just a few physical
constants, such as the charge and mass of the electron.
5) Although the profoundest advances in fundamental physics
tend to occur when the principles of different types of theories
are reconciled within a single new framework, we do not yet know
what guiding principle underlies the unification of quantum field
theory, as embodied in the Standard Model, with general
relativity. The quantum nature of space and time must be dealt
with in a unified theory. At the shortest distance scales, space
may be replaced by a continually reconnecting structure of
*strings and membranes -- or by something stranger still.
6) The author suggests it is impossible to state when these
problems will be overcome. "They may be solved in a preprint put
out tomorrow by some young theorist. They may not be solved by
2050, or even 2150. But when they are solved... we will not have
any trouble in recognizing the truth of the fundamental unified
theory. The test will be whether the theory successfully accounts
for the measured values of the physical constants of the Standard
Model, along with whatever effects beyond the Standard Model may
have been discovered by then."
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Scientific American December 1999
-----------
Notes:
... ... *strings and membranes: See background material below.
-----------
Related Background:
ON FIELD THEORY IN PHYSICS
In physics, a field is an entity that acts as intermediary in
interactions between particles, and which is distributed over
part or all of space, and whose properties are functions of space
coordinates, and except for static fields, also functions of
time. There is also a quantum-mechanical analog of this entity,
in which the function of space and time is replaced by an
operator at each point in space-time. ... ... Roman Jackiw
reviews present theoretical work in the theory of elementary
particles and forces, and the author makes the following points:
1) Present-day theory for fundamental processes (i.e.,
descriptions of elementary particles and forces) is phenomenally
successful. Experimental data confirms theoretical prediction,
and where accurate calculation and experiments are attainable,
agreement is achieved to 6 or 7 figures. Two examples: a) The
helium atom ground state energy (*Rydbergs) is experimentally
measured as -5.8071394 and theoretically calculated as
-5.8071380. b) The muon magnetic dipole moment is experimentally
measured as 2.00233184600 and theoretically calculated as
2.00233183478. 2) The theoretical structure within which this
success has been achieved is *local field theory, which offers a
wide variety of applications, and which provides a model for
fundamental physical reality as described by our theories of
*strong, electroweak, and gravitational processes. No other
framework exists in which one can calculate so many phenomena
with such ease and accuracy. 3) But is spite of these successes,
today there is little confidence that field theory will advance
our understanding of nature at its fundamental workings beyond
what has already been achieved. Although in principle all
observed phenomena can be explained by present-day field theory,
these accounts are still imperfect, requiring ad hoc inputs.
Moreover, because of conceptual and technical obstacles,
classical gravity theory has not been integrated into the
*quantum field description of nongravitational forces:
*quantizing the *metric tensor of Einstein's theory produces a
quantum field theory beset by infinities that apparently cannot
be controlled. 4) These shortcomings are actually symptoms of a
deeper lack of understanding concerning *symmetry and symmetry
breaking... Physicists are happy in the belief that Nature in its
fundamental workings is essentially simple, but observed physical
phenomena rarely exhibit overwhelming regularity. Therefore, at
the very same time that we construct a physical theory with
intrinsic symmetry, we must find a way to break the symmetry in
physical consequences of the model. 5) These problems have
produced a theoretical impasse for over two decades, and in the
absence of new experiments to channel theoretical speculation,
some physicists have concluded that it will not be possible to
make progress on these questions within field theory, and they
have turned to a new structure, "*string theory". In field
theory, the quantized excitations are point particles with point
interactions, and this gives rise to the infinities. In string
theory, the excitations are extended objects -- strings -- with
nonlocal interactions; there are no infinities in string theory,
and that enormous defect of field theory is absent. 6) Yet in
spite of its positive features, until now string theory has
provided a framework rather than a definite structure, and a
precise derivation of the *Standard Model has yet to be given.
The author concludes: "On previous occasions when it appeared
that quantum field theory was incapable of advancing our
understanding of fundamental physics, new ideas and new
approaches to the subject dispelled the pessimism. Today we do
not know whether the impasse within field theory is due to a
failure of imagination or whether indeed we have to present
fundamental physical laws in a new framework, thereby replacing
the field theoretic one, which has served us well for over 100
years."
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Proc. Nat. Acad. Sci. 1998 95:12776
-----------
Notes:
... ... *Rydbergs: A unit of energy used in atomic physics,
value = 13.605698 electronvolts.
... ... *local field theory: In this context, "locality" is the
condition that two events at spatially separated locations are
entirely independent of each other, provided that the time
interval between the events is less than that required for a
light signal to travel from one location to the other. For
example, the quantum mechanical wave function is a "local" field.
... ... *strong, electroweak, and gravitational processes: The
fundamental forces comprise the gravitational force, the
electromagnetic force, the nuclear strong force, and the nuclear
weak force. The "electroweak" interactions are a unification of
the electromagnetic and nuclear weak interactions, and are
described by the Weinberg-Salam theory (sometimes called "quantum
flavordynamics"; also called the Glashow-Weinberg-Salam theory).
... ... *quantum field description: In general, a quantum field
theory is a quantum mechanical theory applied to systems having
an infinite number of *degrees of freedom.
... ... *degrees of freedom: In general, the number of
independent parameters required to specify the configuration of a
system.
... ... *quantizing: In experimental physics, a quantized
variable is a variable taking only discrete multiple values of a
quantum mechanical constant. In theoretical physics, "quantizing"
means the consistent application of certain rules that lead from
classical to quantum mechanics. In general, "quantization" is a
transition from a classical theory or a classical quantity to a
quantum theory or the corresponding quantity in quantum
mechanics.
... ... *metric tensor: The mathematical statement (involving a
set of quantities) that describes the deviation of the Pythagoras
theorem in a curved space.
... ... *symmetry and symmetry breaking: If a theory or process
does not change when certain operations are performed on it, the
theory or process is said to possess a symmetry with respect to
those operations. For example, a circle remains unchanged under
rotation or reflection, and a circle therefore has rotational and
reflection symmetry. The term "symmetry breaking" refers to the
deviation from exact symmetry exhibited by many physical systems,
and in general, symmetry breaking encompasses both "explicit"
symmetry breaking and "spontaneous" symmetry breaking. Explicit
symmetry breaking is a phenomenon in which a system is not quite,
but almost, the same for two configurations related by exact
symmetry. Spontaneous symmetry breaking refers to a situation in
which the solution of a set of physical equations fails to
exhibit a symmetry possessed by the equations themselves.
... ... *string theory: In particle physics, string theory is a
theory of elementary particles based on the idea that the
fundamental entities are not point-like particles but finite
lines (strings), or closed loops formed by strings, the strings
one-dimensional curves with zero thickness and lengths (or loop
diameters) of the order of the Planck length of 10^(-35) meters.
... ... *Standard Model: See main report.
-----------
Related Background:
THEORETICAL PHYSICS: ON STRING THEORY
... ... B.R. Greene et al present a short review of recent
developments in string theory and make the following points: 1)
Particle physics has spent much of this century grappling with
one basic question in various forms: What are the fundamental
*degrees of freedom needed to describe nature, and what are the
laws that govern their dynamics. 2) The current "Standard Model"
of particle physics -- which is nearly 25 years old and which has
much experimental evidence in its favor -- involves 6 *quarks, 6
*leptons, 4 *forces, and the as yet unobserved *Higgs boson. But
this model contains internal indications that it too may be just
another step along the path of uncovering the truly fundamental
degrees of freedom. The Standard Model is valid to distances as
small as 10^(-16) cm, and there is some evidence that the next
level of structure will be detected only at a distance scale of
the order of 10^(-32) cm, which is far beyond our abilities to
measure in the laboratory. 3) A related important issue concerns
the unification of general relativity and quantum mechanics. A
serious problem arises when general relativity is extrapolated to
small distance scales of the order of 10^(-32) cm where quantum
effects must be taken into account: the relevant theoretical
equations produce uncontrollable divergences, and the history of
particle physics suggests this is an indication of a new physics
occurring at these distance scales. 4) String theory offers hope
of addressing both of these issues. There is only one known way
to cure the divergence problem in the quantum-mechanical
expansion of general relativity, and that is to model the
particles in the theory not as points but as one-dimensional
loops of "string". Every consistent such string model necessarily
contains a special kind of particle -- the "*graviton" -- whose
long-distance interactions are described by general relativity.
So in a sense, string theory predicts gravity. 5) An exciting new
frontier was opened during the past few years with the discovery
of "string duality", which predicts equivalences among various
different physical systems. This discovery has its roots in the
properties of "supersymmetry", a novel type of symmetry that all
consistent string theories possess. Briefly, supersymmetry
relates properties of two basic types of particles -- bosons and
*fermions -- which cannot be related by ordinary symmetry. There
is a current belief that supersymmetry will play a role in the
structure of particle physics beyond the Standard Model. One of
the important achievements of string duality has been the
determination of the behavior of the 5 consistent string theories
when interactions become strong. All the consistent string
theories are apparently related to each other, and to an
elaboration known as "membrane theory" (M-theory). String duality
has produced hope that there may be only one possible string-
theoretic model of the universe, and that it may be possible to
eventually predict such features as particle masses and
interaction strengths directly from such a theory. The authors
conclude: "Development has been rapid on many fronts since string
duality was introduced. We may be seeing glimpses of the
underlying principle manifested in these new results. The
challenging task that lies ahead is to discover that principle
and thereby find what may well be the truly fundamental degrees
of freedom in our universe."
-----------
Proc. Nat. Acad. Sci. US 1998 95:11039
-----------
Notes:
... ... *degrees of freedom: See previous report.
... ... *quarks: See previous report.
... ... *leptons: A class of elementary particles. Although they
are affected by electromagnetic and gravitational forces, apart
from that they are involved only with weak interactions, acted
upon by weak forces but not by strong forces, as opposed to
quarks, which are acted upon by strong forces but not by weak
forces. One further difference between leptons and quarks is that
leptons can be isolated as single particles, whereas quarks
apparently cannot. The leptons include the electron, the muon,
the tau, and their associated neutrinos. The mass of the tau is
approximately 3484 times the mass of the electron; the mass of
the muon is intermediate.
... ... *forces: See previous report (fundamental forces).
... ... *Higgs boson: Higgs fields (named after Peter W. Higgs,
University of Edinburgh, UK) constitute a set of fundamental
theoretical fields that induce spontaneous symmetry breaking. In
general, spontaneous symmetry breaking occurs in systems whose
underlying symmetry state is unstable. A Higgs particle is
associated with a Higgs field in the same way that a photon is
associated with the electromagnetic field. Higgs bosons are
massive mesons whose existence is predicted by certain theories.
Mesons are apparently composed of quark and anti-quark pairs;
they are produced by various high-energy interactions and decay
into stable particles.
... ... *graviton: Several quantum field theories consistent with
both quantum mechanics and special relativity postulate that the
gravitational force between two quantum domain particles is
generated by the exchange of an intermediate particle called a
graviton.
... ... *fermions: Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
-----------
Related Background:
ON RECENT DEVELOPMENTS IN SUPERSTRING THEORY
Bose-Einstein statistics is the statistical mechanics of a system
of indistinguishable particles for which there is no restriction
on the number of particles that may simultaneously exist in the
same quantum energy state. Bosons are particles that obey Bose-
Einstein statistics, and they include photons, pi mesons, all
nuclei having an even number of particles, and all particles with
integer spin. Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
In particle physics, string theory is a theory of elementary
particles based on the idea that the fundamental entities are not
point-like particles but finite lines (strings), or closed loops
formed by strings, the strings one-dimensional curves with zero
thickness and lengths (or loop diameters) of the order of the
Planck length of 10^(-35) meters. The fundamental forces comprise
the gravitational force, the electromagnetic force, the nuclear
strong force, and the nuclear weak force, and the "grand unified
theories" are theories that aim to provide a mathematical frame-
work in which the electromagnetic forces, strong forces, and weak
forces emerge as parts of a single unified force, with the three
forces related by symmetry. Supersymmetry is an aspect of an
extension of the grand unified theories, an attempt to unify all
the four fundamental forces, i.e., linking gravitation to the
electromagnetic force, the strong force, and the weak force
through a supersymmetry scheme, and superstrings are strings in
this scheme that obey supersymmetry. ... ... John H. Schwarz
(California Institute of Technology, US) presents a brief
overview of some of the advances in understanding super-
string theory that have been achieved in the last few years.
String theories that have a symmetry relating bosons and
fermions, called "supersymmetry", are called "superstring"
theories. Major advances in understanding of the physical world
have been achieved during the past century by focusing on
apparent contradictions between well-established theoretical
structures. In each case the reconciliation required a better
theory, often involving radical new concepts and striking
experimental predictions. Four major advances of this type were
the discoveries of special relativity, quantum mechanics, general
relativity, and quantum field theory. This was quite an
achievement for one century, but there is one fundamental
contradiction that still needs to be resolved, namely the clash
between general relativity and quantum field theory. Many
theoretical physicists are convinced that superstring theory will
provide the answer.
-----------
Proc. Nat. Acad. Sci. US 17 Mar 98
-----------
Related Background:
ON THE EVOLUTION OF STRING THEORY TO MEMBRANE THEORY
... Membrane theory (M-theory) is a recent extension of string
theory in which the fundamental physical entities are considered
as surfaces in a many-dimensional space (membranes) rather than
as lines or loop elements (open or closed strings). Given all of
the above, some caution is necessary: the translation of a highly
abstract mathematical model of physical reality into
non-mathematical language is often an exercise of limited
usefulness, and in this case in particular, we are presenting
only the ghost of the theoretical scheme. String theory was
originally invented in the 1960s as a theory of the strong force,
became overshadowed by the strong force theory of gluons and
quarks, then had a revival in the 1980s -- but with the history
more dependent on new work than on fashion. ... ... M. Duff
(Texas A & M Univ., US), who is active in string theory and
membrane theory, in a review of various aspects of the history
and essentials of string theory and membrane theory, suggests
that future historians may judge the 20th century as "a time when
theorists were like children playing on the seashore, diverting
themselves with the smoother pebbles or prettier shells of
superstrings, while the great ocean of M-theory lay undiscovered
before them."
-----------
Scientific American February 1998
-----------
ScienceWeek 29 Mar 2002 www.scienceweek.com
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