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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
November 9, 2001 -- Vol. 5 Number 45
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There must be no barriers to freedom of inquiry.
There is no place for dogma in science. The scientist
is free, and must be free to ask any question,
to doubt any assertion, to seek for any evidence,
to correct any error.
-- J. Robert Oppenheimer (1904-1967)
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. Self-Organization in Surface Reactions
2. Inhibition of Fusion in Nucleus-Nucleus Collisions
3. On the Interactions of Gold Atoms
4. Compositions of US Physics Faculties
5. On Forces at the Level of Single Molecules
6. Interactions Between Aromatic Moieties
7. On the Molecular Basis of Segmentation in Animals
8. Chronic Myelogenous Leukemia
9. Tracing the Evolutionary Origins of Diseases
10. Molecular Motors, Neurons, and Human Disease
11. Climate Change and Habitat Expansion
12. On the Utilization of Minerals by Bacteria
13. In Focus: On the Childhood of Isaac Newton
14. From PRAXIS: Rotavirus Vaccine: Risks and Controversy
15. Sources
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Section 2
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1. SELF-ORGANIZATION IN SURFACE REACTIONS
C. Sachs et al (Fritz Haber Institute MPG, DE) discuss self-
organization in surface reactions. Spatiotemporal pattern
formation in *open systems far from equilibrium is the basis of
self-organization of matter. Catalytic reactions between small
molecules on well-defined solid surfaces are probably the
simplest model systems that show such phenomena, with
concentrations patterns formed on mesoscopic length scales
(typically 1 micron to 1 millimeter) that can be imaged with
techniques such as *photoemission electron microscopy.
Nonequilibrium patterns can be observed if the reaction kinetics
fulfill certain criteria. The minimum requirements are the
presence of nonlinearities and of spatial coupling. These
patterns can be modeled with *reaction-diffusion equations in
which the properties of the individual particles are replaced by
continuum variables such as adsorbate concentrations, and this
approach often provides a good qualitative description. The
authors report that *scanning tunneling microscopy data revealed
the atomic processes in propagating reaction fronts that occur in
the catalytic oxidation of hydrogen on platinum. The fronts were
also characterized on mesoscopic length scales with respect to
their velocity and width. Simulations on the basis of a reaction-
diffusion model reproduce the experimental findings
qualitatively, but quantitative comparison reveals the
limitations of this traditional approach to modeling
spatiotemporal pattern formation in nonlinear dynamics.
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SCI 2001 293:1635
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Notes:
... ... *open systems: In general, an "open system" is any system
that can exchange both matter and energy with its surroundings.
... ... *photoemission electron microscopy: A technique in which
electron photoemission from a metal surface is projected on a
fluorescent screen.
... ... *reaction-diffusion equations: In general, in theoretical
chemistry, a "reaction-diffusion system" involves the coupling
between a chemical reaction and molecular diffusion, with the
coupling modifying the rate of the reaction. In a reaction-
diffusion system, according to theory, nonlinear dynamic
processes may result in the appearance of density fluctuations.
... ... *scanning tunneling microscopy: In scanning tunneling
microscopy, an atomically sharp metal tip is brought in atomic
proximity (e.g., 0.5 to 1 nanometer) to a flat surface so that
electrons can *tunnel between the two systems. The probe is
slowly moved across the surface and raised and lowered so as to
keep the tunneling current constant. A computer-generated contour
map of the surface is thus produced. The technique can resolve
individual atoms, but requires electrically conducting materials.
... ... *tunnel: "Tunneling" is a quantum mechanical phenomenon
involving an effective penetration of an energy barrier by a
particle resulting from the width of the barrier being less than
the wavelength of the particle.
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2. INHIBITION OF FUSION IN NUCLEUS-NUCLEUS COLLISIONS
A.C. Berriman et al (Australian National University, AU) discuss
nucleus-nucleus collisions. Unstable heavy atomic nuclei not
found in nature can be created by fusing two stable nuclei in a
process analogous to colliding charged droplets of liquid.
Recently, the formation of a handful of super-heavy nuclei with
atomic numbers 114 and 116 has been achieved by fusion of heavy
nuclei. The electrostatic energy of such systems is very large
(which is the reason super-heavy nuclei are unstable), so
although the two nuclei may initially be captured by the nuclear
potential, rather than fusing, they almost always separate after
transfer of mass to the lighter nucleus. This process, called
"quasi-fission", can inhibit fusion by many orders of magnitude,
and understanding this inhibition may hold the key to forming
more super-heavy elements. Theoretically, inhibition is predicted
when the product Z(sub1)Z(sub2) of the charges of the projectile
and target nuclei is larger than approximately 1600. The authors
report measurements of three fusion reactions with Z(sub1)Z(sub2)
approximately half this value, each reaction forming
(sup216)(sub88)Ra. The authors report they find convincing model-
independent evidence both of inhibition of fusion and of the
presence of quasi-fission. The authors suggest these results defy
interpretation within the standard picture of nuclear fusion and
fission.
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NAT 2001 413:144
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3. ON THE INTERACTIONS OF GOLD ATOMS
Hubert Schmidbaur (Munich Technical University, DE) discusses the
chemistry of gold atoms. Molecules containing gold atoms are not
expected to form metallic bonds, but there is growing evidence
that there are interactions between the gold atoms that are
similar in strength to hydrogen bonds. When gold forms a bond
with another atom, it simply has to donate one electron to attain
an extremely stable electronic state. Conventional wisdom says
that atoms in such a state should not engage in any further
external bonding. But accumulating experimental evidence and
quantum-chemical calculations have shown that there are
surprisingly strong metal-metal interactions between gold atoms
in molecular complexes. These interactions appear to be similar
in strength to hydrogen bonds, and chemists have started to use
these interactions to design new structures with unusual physical
properties. For example, R.E. Bachman et al (2001) report the
construction of striking assemblages of gold complexes based
solely on their metal-metal interactions. The behavior of gold
complexes in these assemblages could potentially be modified to
produce interesting and useful structures, such as metal-based
liquid crystals or photoluminescent materials. Most gold
compounds have a linear rod-like structure. As more and more
crystal structures of complexes containing such rod-like
structural elements became known in the 1980s and 1990s, chemists
were surprised by the tight aggregation of the components, which
always appeared to minimize the distances between the gold atoms.
It seemed gold was literally always drawn to gold, and the term
"aurophilicity" was coined to describe this new phenomenon in
structural chemistry.
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NAT 2001 413:31
JACS 2001 123:5376
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4. COMPOSITIONS OF US PHYSICS FACULTIES
Toni Feder (PT) discusses the compositions of US physics
faculties. A new report presents data from a survey of 50
universities with a total of 1987 physics faculty members. Some
of the results are tabulated as follows:
11.2 percent of the total US physics faculty are Asian-
Americans (10 percent of these are women).
6.6 percent of the total US physics faculty are women.
0.6 percent are African-American (all men).
0.5 percent are Hispanic (half women).
There is 1 Native-American physics professor.
Most black academic physicists are at historically black
colleges and universities.
Full data can be viewed at URL:
http://www.awis.org/statistics/physicsTable.html
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PT 2001 October
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Related Background:
PHYSICS FACULTIES: Despite the rise in the total number
of new physics faculty in the US, the fraction of jobs going to
women has fallen. Only 14 percent of academic physicists hired in
2000 were women, down from 17 percent in 1998. In contrast, there
was an increase in the number of new physics faculty that earned
their PhDs outside the US -- such faculty now make up more than a
third of new full-time physics faculty in US PhD-granting
departments.
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PT 2001 May
SW 2001 6 Jul
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5. ON FORCES AT THE LEVEL OF SINGLE MOLECULES
T. Strick et al (Cold Spring Harbor Laboratories, US) discuss the
manipulation of single biomolecules and the range of forces at
the level of single molecules. Biophysics is currently undergoing
a transformation due to the development of new tools for
manipulating, visualizing, and studying single molecules and
their interactions. The smallest measurable forces at the
molecular level are the *Langevin forces responsible for the
Brownian motion of bacteria, pollen grains, and other small
objects in water at room temperature. The average force buffeting
a bacterium every second is comparable to its weight and is
approximately 10^(-14) newtons. Almost a thousand times stronger
are the forces typical of molecular motors, which convert
chemical energy from adenosine triphosphate (ATP) into mechanical
work. ATP is the common coin of stored chemical energy in all
life on Earth. The hydrolysis of an ATP molecule yields an energy
of approximately 14 kT, where the thermal energy (kT) at body
temperature is 4 x 10^(-21) joules, and the molecular dimensions
are of the order of 10 nanometers. Thus, the characteristic
forces of such motors are of the order of 10^(-11) newtons. Next
on the way up the force scale are the cohesion forces associated
with hydrophobic interactions and cooperative hydrogen bonding.
Such interactions contribute to the stability of biomolecules and
their native folded configurations. These forces are of the order
of 10^(-10) newtons, the typical force required to break a
noncovalent bond and denature a protein. The strongest forces at
the molecular level are the forces of the order of 10^(-9)
newtons required to break covalent bonds with dimensions of the
order of an angstrom and typical binding energies of 1
electronvolt.
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PT 2001 October
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Notes:
... ... *Langevin forces: Named after Paul Langevin (1872-1946).
In this context, the term "Langevin forces" refers to two forces
appearing in the Langevin equation of random motion. The two
forces are a frictional force resulting from the viscosity of the
surrounding fluid and a random force describing the average
effect of Brownian motion.
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Related Background:
APPLICATIONS OF SINGLE-MOLECULE SPECTROSCOPY
Only a few decades ago, most scientists believed that
individual molecules would not come within the domain of
experimental observations within their lifetime, if ever, and
that the statistical ensemble properties of molecules were
therefore the only properties of relevance. That view has now
undergone a dramatic alteration as a consequence of technological
advances, and there is much excitement evident in many
laboratories over the prospects of single-molecule explorations
in physics, chemistry, and biology.
As an experimental technique, single-molecule spectroscopy
is only a few years old, but already research reports are
appearing in a variety of applications as diverse as low-
temperature dynamics of single dye molecules embedded in
crystals, optical tracking of the entry of individual viruses
into living cells, single photon light sources from single
molecules, polymer conformations and dynamics, the mechanisms of
single enzymatic motors.
In a commentary on new research involving single-molecule
spectroscopy, A.M. Kelley et al point out that one reason for the
burgeoning interest in single-molecule optical techniques is that
photons may be the least perturbing probe of the state of a
molecule. "In combination with single-molecule manipulation,
microfluidics, and microelectromechanical systems, [single-
molecule optical studies] will open up ever more possibilities
for new discoveries."
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SCI 2001 292:1671
PRAX 2001 6 Aug
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Related Background:
ON THE NANOSCALE SCIENCE OF SINGLE MOLECULES
In recent years, experiments on individual molecules using
scanning probe microscopies [*Note #1] have demonstrated a
diversity of physical, chemical, mechanical, and electronic
phenomena. These techniques have permitted deeper insight into
the quantum electronics of molecular systems and have provided
unique information about the conformational and mechanical
properties of these systems. Concomitant developments in
experimentation and theory have allowed a diverse range of
molecules to be studied, molecules varying in complexity from
simple diatomic systems to biological macromolecular systems.
... ... J.K. Gimzewski and C. Joachim (2 installations, CH FR)
present an extensive review of current single-molecule research,
the authors making the following points: 1) The very nature of
proximal probe methods encourages exploration of the nanoworld
beyond conventional microscopic imaging. Scanning probes now
allow us to perform "engineering" operations on single molecules,
atoms, and bonds, thereby providing a tool that operates at the
ultimate limits of fabrication. These techniques have also
enabled explorations of molecular properties on an individual
basis as opposed to explorations restricted to the statistical
properties of large populations of molecules. 2) The
nanomechanical properties of individual molecules take the form
of vibrations, rotations, conformational changes, and
translations. *Inelastic tunneling processes, probe-tip-induced
forces, and Brownian motion have been found to drive mechanical
responses in individual molecules, and these aspects are the
focus of current research. The important role of thermal noise at
room temperature in nanoscale systems suggests that future
technologies for building small energy-efficient devices will
need to use ambient temperature fluctuations rather than fight
against them. 3) Future developments in single-molecule nanoscale
science call for a close integration of chemistry, biology,
physics, and technology in terms of synthesis, theoretical
modeling, and advanced scanning probe microscope techniques.
Although scanning probe microscopy has been shown to be an
ultimate probe for investigating the properties of individual
molecules, it is still an open question whether these techniques
have the intrinsic capabilities to be useful fabrication tools in
technology. The recent development of massive micromechanical
arrays of thousands of scanning probe microscopy probes suggests
that such a possibility is becoming more real each day.
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SCI 1999 283:1683
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Notes:
... ... *Note #1: The general approach in scanning probe
microscopy research is illustrated by consideration of two major
techniques, scanning tunneling microscopy (STM) and atomic force
microscopy (AFM). In scanning tunneling microscopy, an atomically
sharp metal tip is brought in atomic proximity (e.g., 0.5 to 1
nanometer) to a flat surface so that electrons can *tunnel
between the two systems. The probe is slowly moved across the
surface and raised and lowered so as to keep the tunneling
current constant. A computer-generated contour map of the surface
is thus produced. The technique can resolve individual atoms, but
requires electrically conducting materials. In atomic force
microscopy, a tip is fixed to a cantilever whose position is
monitored while the tip scans the surface. The force between the
tip and the surface determines the position of the cantilever.
When recorded in atomic resolution, the image represents a map of
atomic forces at the surface. The advantage of atomic force
microscopy is that the probed surface does not need to be
electrically conducting.
... ... *tunnel: "Tunneling" is a quantum mechanical
phenomenon involving an effective penetration of an energy
barrier resulting from the width of the barrier being less than
the wavelength of the particle.
... ... *Inelastic tunneling processes: In general, an
"inelastic" process is a process which results in a permanent
change in the properties of a system. In this context, the term
"inelastic tunneling process" refers to a technique involving the
input of energy into a single-molecule system to selectively
excite chemical bonds or to perform spectroscopic studies of the
system.
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SW 1999 21 May
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6. ON INTERACTIONS BETWEEN AROMATIC MOIETIES
C-Y. Kim et al (University of Pennsylvania, US) discuss aromatic
interactions. The fundamental basis of attractive interaction
between aromatic moieties is not thoroughly understood, partly
because of apparent differences in the preferred geometries of
such interactions. In DNA, for example, nucleotide bases are
stacked in a face-to-face manner, which facilitates interstrand
hydrogen bonding and stabilizes the double helix. In contrast,
aromatic side chains in refined protein structures tend to prefer
an edge-to-face geometry, which contributes to tertiary structure
stabilization. Because nucleic acid bases are heteroaromatic with
varied structures and electronic properties, it is likely that
the electronic properties of the aromatic ring as well as its
structural context play a significant role in directing the
preferred geometries of intermolecular interactions. Theoretical
calculations demonstrate that the benzene homodimer
preferentially forms an edge-to-face complex because of the large
quadrupole moment associated with benzene: a face-to-face
arrangement would result in electrostatic repulsion. Consistent
with preferred edge-to-face aromatic-aromatic interactions,
benzene molecules pack in the crystal lattice with an edge-to-
face herringbone-like pattern. Recently, researchers have
reported elaborate small molecules in which two substituted
benzene rings are brought into close contact by a semirigid
linker. Surprisingly, these experiments suggest that the
intramolecular aromatic-aromatic interactions are dominated not
by the interactions between aromatic ring quadrupoles but instead
by dispersion forces.
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JACS 123:9620
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7. ON THE MOLECULAR BASIS OF SEGMENTATION IN ANIMALS
C.J. Tabin and R.L. Johnson (Harvard University, US) discuss
segmentation in animals. In vertebrates, the spine, ribcage, and
breastbone are derived from repeated blocks of tissue that begin
as identical units in early development and are then modified
into unique shapes with different purposes. Some segments, for
example, allow the head to move; some are sites of attachment for
the muscles involved in breathing; and some segments protect the
organs in the chest. To produce such a body plan, there must be
mechanisms both for generating the segments and for giving each
segment its distinct identity. For vertebrates, the task of
producing repeated units is apparently controlled partly by a
molecular timing mechanism in the unsegmented paraxial *mesoderm
-- the tissue from which the units arise. The identity of the
units is controlled by the differential expression of genes known
as "Hox genes", the expression occurring in a nested pattern from
the head to the tail. Essentially, segmentation in vertebrate
embryos occurs as follows: On each side of the neural tube (which
forms the spinal cord) is a strip of unsegmented ("presomitic")
mesoderm. Cells from this tissue progressively bud off,
contributing to "somites", the units of cells that will later
develop into vertebrae and associated muscles. This
differentiation process occurs in a wave that moves gradually
from the head to the tail (i.e., down the anterior-posterior
axis), with presomitic mesoderm in front of the wave and somites
in its wake. The specific molecular and genetic mechanisms that
control vertebrate segmentation are not yet understood.
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NAT 2001 412:780
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Notes:
... ... *mesoderm: 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. These layers, via further cell
differentiation and proliferation, determine the development of
all the major body systems and organs.
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Related Background:
COMPUTER MODELS OF INVERTEBRATE SEGMENTATION
Eors Szathmary (Eotvos University, HU) discusses body
segmentation in invertebrates. It has long been known that the
beginning and the end of embryological development are variable
traits in evolution. Diversity at an early developmental stage
(in the mechanism of gastrulation, for example) can be attributed
to evolutionary adaptations to the ecological setting in which
the embryo begins to unfold. The later phases must also differ,
otherwise species would all look the same. For example,
development in parasitic wasps has diverged widely. Early cell
divisions of the fertilized egg, establishment of head-to-tail
polarity, and the genetic circuit for body segmentation have all
been modified, apparently as adaptations to the parasitic life-
style. But developmental malleability can go beyond this.
Although the external appearance of an organism may be fixed, the
genetic network, in which genes switch one another on and off so
that programmed development runs successfully, seems to be
changeable. This is analogous to rewiring a computer without
changing the housing. Segmentation, including stripes on animal
coats, fascinated the mathematician Alan Turing (1912-1954), who
in 1952 proposed a mechanism for pattern formation. Turing
demonstrated that in a chemical system that begins as spatially
homogeneous, a diffusing activator and an inhibitor could give
rise to stationary wave-like concentration profiles of chemicals.
Similar reaction-diffusion mechanisms may be at work in some
biological systems, but other systems, previously thought to be
Turing systems, such as stripe formation in the fruit fly
Drosophila, apparently use a mechanism in which stripe identity
is determined by differing combinations of regulatory elements,
following an initial spatial heterogeneity, with the underlying
genetic circuit having a hierarchical structure.
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NAT 2001 411:143
SW 2001 12 Oct
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Related Background:
IN FOCUS: ON HOMEOBOX GENES
"In the 1980s, the door to incorporating developmental insights
into evolutionary theory was finally opened with the discovery of
a class of highly conserved regulatory genes, called homeobox
genes. Homeobox genes control an organism's development by means
of sending signals from one to another in the form of the
proteins they produce. As demonstrated in the fruit fly, the cell
that eventually gives rise to the egg cell receives the messages
that determine what will be the head and tail and up, down,
right, and left sides of a potential offspring by a back-and-
forth signaling, carried by proteins, between homeobox genes in
this cell and the cells of the ovary around it. The animal that
then emerges from the egg that derives from this predetermined,
pre-egg cell obtains its specific features through the process of
turning on and off certain homeobox genes at different times in
different regions of its developing body. All animals, from
unsegmented worms to fruit flies, starfish, tunicates, zebra
fish, chickens, mice, and humans, share essentially the same
basic homeobox genes. Since all of an organism's genes are
contained in each and every one of its cells, the striking
morphological difference between animals lies basically in which
cells and when during development one or more homeobox genes are
active... It is mind-boggling to realize that, for all intents
and purposes, many differences between a fruit fly and a human
may lie pretty much in where and when certain homeobox genes are
activated. To be sure, there are some other differences between a
fruit fly and a human at the molecular level. But, fundamentally,
the main difference between organisms lies in alterations in
development that result from differences in the timing of
homeobox gene activity."
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Jeffrey H. Schwartz: _Sudden Origins: Fossils, Genes, and the
Emergence of Species_
(John Wiley & Sons, New York 1999, p.12)
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8. CHRONIC MYELOGENOUS LEUKEMIA
M. Kalidas et al (Anderson Cancer Center Houston, US) discuss
chronic myelogenous leukemia (CML). This disease is a disorder of
*hematopoietic stem cells, accounting for 15 percent of adult
leukemias. The median age at presentation is between 50 and 60
years; 12 to 30 percent of patients at diagnosis are older than
60 years. The disease classically progresses through 3 phases,
becoming more resistant to treatment in each successive phase.
The majority of patients present in the chronic phase, which may
last 4 to 6 years and is often asymptomatic at diagnosis. In the
accelerated phase, symptoms become worse and immature *blasts
increase in the peripheral blood, the duration of this phase
lasting as long as a year. The final and fatal blastic phase,
with greater than 30 percent blasts in the bone marrow or
peripheral blood, has features of an acute leukemia, including
fever, weight loss, bleeding, and anemia. This phase may last 3
to 6 months. In 1960, patients with CML were described as having
a shortened chromosome ("Philadelphia chromosome) that was later
discovered to be chromosome 22. The molecular defect is a gene
fusion that produces an active cytoplasmic specific tyrosine
*kinase that results in the initiation of multiple signal
cascades involved in cell growth, differentiation, adhesion, and
death, the cells bypassing regulated growth and undergoing a
malignant transformation to leukemia. CML is one of the few
diseases in which a specific cytogenetic and molecular
abnormality has been implicated in the pathogenesis of the
disease. The presence of a specific active tyrosine kinase in the
tumor cells has led to the first targeted molecular therapy of
its kind, and clinical trials have shown very promising results
in all phases of this leukemia.
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JAMA 2001 286:895
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Notes:
... ... *hematopoietic: The process by which the cells of the
blood are formed is called "hemopoiesis" (hematopoiesis).
... ... *blasts: In general, a "blast" is any immature or
precursor cell.
... ... *kinase: In general, a "kinase" is any enzyme involved in
the transfer of a phosphate group.
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9. TRACING THE EVOLUTIONARY ORIGINS OF DISEASES
Carl Zimmer (SCI) discusses the use of molecular techniques in
tracing the origins of diseases. Researchers can now trace the
evolution of human diseases from millions of years ago, well
before the origin of our species. For example, consider the
flagellated protozoans called "trypanosomes". Trypanosoma brucei
causes sleeping sickness, which kills an estimated 300,000 people
in Africa each year. In South America, the closely related
pathogen T. cruzi causes Chagas' disease, which affects 20
million people each year. Recent analyses of DNA sequences from
62 different species of the genus Trypanosoma have revealed
evidence of a common ancestor for T. cruzi and T. brucei -- 100
million years ago. At that time, Africa, South America, and
Australia were joined in the supercontinent Gondwana. Africa
split off first, and its drift is reflected in the evolutionary
tree of trypanosomes: T. brucei and almost all other African
trypanosomes belong to a single branch. By contrast, certain
trypanosomes that infect kangaroos in Australia show a kinship
with South American forms such as T. cruzi. African trypanosomes
probably co-evolved with human ancestors for millions of years.
Like baboons and several other African primates, humans carry an
anti-trypanosome factor in their blood that prevents many African
species of trypanosomes from infecting them. Researchers
speculate that the ancestors of both baboons and humans were apes
that lived on the African plains, where they were plagued by the
tsetse fly that carries trypanosomes.
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SCI 2001 292:1090
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10. MOLECULAR MOTORS, NEURONS, AND HUMAN DISEASE
L.S. Goldstein (University of California San Diego, US) discusses
*molecular motors in neurons. The large size and extreme polarity
of neurons presents these cells with an unusual and substantial
transport challenge. Materials synthesized in the cell body must
be transported down long *axons to *presynaptic sites of
utilization. These distances can reach 1 meter or more in the
case of humans and larger animals, and axonal volumes can exceed
the volume of the cell body by 1000 times or more. In addition,
axons and *dendrites can be highly branched, and in some cases
have very small diameters, which can limit transport rate and
volume. The polarity of neurons presents analogous problems.
Structural and signaling components destined for the axon must
somehow be sorted from components needed in dendrites, and the
transport system appears to play a critical role in these
processes. The combination of the substantial pressure of
distance and volume, coupled to the enormous branching and narrow
caliber of many neuronal processes, suggests that the
intracellular transport system may be a major vulnerability of
these large and complex cells -- a system easily disturbed by
environmental insult, mutation, or other trauma to cause
neurodegenerative disease. *Kinesin molecular motor proteins are
responsible for many of the major *microtubule-dependent
transport pathways in neuronal and non-neuronal cells, and there
is some evidence that these pathways and proteins may be involved
in several human diseases, including ciliary dyskinesias, situs
inversus, and retinitis pigmentosa. There is also some evidence
of a functional involvement of kinesins in Alzheimer's disease.
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PNAS 2001 98:6999
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Notes:
... ... *molecular motors: In this context, the term "molecular
motor" refers to any biomolecular complex involved in directed
motion.
... ... *axons: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. In some types of nerve cells,
axons are extensively branched into a multitude of fine fibers
that make contact (synapses) with other nerve cells.
... ... *presynaptic: The presynaptic part of a synapse is the
region associated with the axon terminal providing input to the
junction.
... ... *dendrites: The general input extensions of nerve cells
are called "dendrites", and they may be extensively branched. In
general, dendrites are considered to receive input and axons to
propagate output, but the electrical architecture of most neurons
is complicated, and in many types of nerve cells activation of
the axon produces electrical activity that not only propagates
down the axon but also propagates backward through the cell body
and dendrites.
... ... *Kinesin: "Motor proteins" are mechanico-chemical enzymes
involved in locomotion of cells or transport of materials in
cells, and there are three families of such proteins: kinesins,
dyneins, and myosins.
... ... *microtubule: Composed of the protein tubulin,
microtubules are part of the cytoskeleton of biological cells,
the quasi-rigid matrix that among other things determines cell
shape. The microtubules are 25 nanometers in diameter and occur
in regular arrays. Cortical microtubules are microtubules
structuring the surfaces of cells.
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Related Background:
CRYSTAL STRUCTURES OF A BIOMOLECULAR MOTOR
... Kinesins and dyneins are microtubule-based motor proteins,
while myosin is a microfilament-based motor protein. Kinesin is
apparently present in all eukaryotic cells. The form of kinesin
originally discovered is a soluble rod-shaped molecule composed
of two polypeptide chains, the molecule travelling toward one
specific end of microtubules (depending on the type of kinesin,
either the so-called "plus" end or "minus" end. Kinesin motors
apparently power many cellular motile processes by converting ATP
energy into unidirectional motion along microtubules. Although
numerous biochemical and biophysical studies have accumulated
much data that link microtubule-assisted ATP hydrolysis to
kinesin motion, the structural view of kinesin movement has
remained unclarified. A new study of a monomeric kinesin motor
combines x-ray crystallography and cryo-electron microscopy to
allow analysis of force-generating conformational changes at
atomic resolution. The kinesin motor is revealed in its two
functionally critical states -- complexed with adenosine
diphosphate and with a non-hydrolyzable analog of adenosine
triphsophate. The conformational change is apparently modular,
extending to all kinesins, and is similar to the conformational
change used by myosin motors and G proteins.
-----------
NAT 2001 411:439
SW 2001 3 Aug
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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11. CLIMATE CHANGE AND HABITAT EXPANSION
C.D. Thomas et al (University of Leeds, UK) discuss ecological
and evolutionary processes at expanding range margins. Many
animals are regarded as relatively sedentary and specialized in
marginal parts of their geographical distributions, and they are
expected to be slow at colonizing new habitats. Despite this, the
cool margins of the distributions of many species have expanded
rapidly in association with recent climate warming. The authors
examined 4 insect species that have expanded their geographical
ranges in Britain over the past 20 years, and they report that 2
butterfly species have increased the variety of habitat types
that they can colonize, and that 2 bush cricket species show
increased fractions of longer-winged (dispersive) individuals in
recently founded populations. The authors suggest that both
ecological and evolutionary processes are probably responsible
for these changes. Increased habitat breadth and dispersal
tendencies have resulted in approximately 3- to 15-fold increases
in expansion rates, allowing these insects to cross habitat
disjunctions that would have represented major or complete
barriers to dispersal before the expansion started. The authors
suggest that the emergence of dispersive phenotypes will increase
the speed at which species invade new environments, and probably
underlies the responses of many species to both past and future
climate change.
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NAT 2001 411:577
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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Related Background:
GLOBAL WARMING: ON ERRORS IN PREDICTING SPECIES RANGE SHIFTS
In the context of this report, "microcosm experiments" are
experiments involving a small-scale model of a larger scale
system. The "climate envelope approach" in the study of biotic
responses to climate change involves mapping the current
distribution of a species in the relevant climate-space, and upon
a change in the position of that climate-space, using the
previous mapping to predict shifts in the distribution of that
species. ... ... J. Davis et al (5 authors at 2 installations,
UK) report microcosm experiments on simple assemblages involving
3 fruit fly species to demonstrate how misleading the climate
envelope approach can be. The authors suggest that population
dispersal and population interactions, which are important
elements of population dynamics, must be included in predictions
of biotic responses to climate change, and in particular in
predictions of the effects of global warming on conservation,
medical considerations, and agricultural pest control.
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NAT 1998 19 Feb
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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Related Background:
PUBLIC HEALTH: EXPECTED CONSEQUENCES OF GLOBAL WARMING
Climate change produced by global warming is expected to result
in melting ice caps, rising sea levels, torrential floods,
devastating droughts, and severe harvest failures. What are often
not considered in discussions of global warming are the effects
of global warming on public health.
... ... Pim Martens (University of Maastricht, NL) presents a
review of the expected effects of global warming on public
health, the author making the following points:
1) Concerning heat stress: The author suggests that perhaps
the most immediate consequence of increasing global temperatures
will be a rise in the number of heat waves and heat-related
illnesses. Such temperature extremes can, for example, increase
the sensitivity of asthmatics to their condition. There will also
be an increasing number of deaths from heat stress brought about
by high ambient temperatures lasting days on end. On the other
hand, the milder winters associated with global warming will
offer a better chance of survival for at-risk groups such as the
elderly during the coldest months. Research into the effect of a
gradual temperature increase has revealed that we can expect a
decline in mortality from cardiovascular and pulmonary disease in
the winter. Whether the milder winters could offset the mortality
during the summer heat waves is not clear.
2) Concerning malaria: The spread of this disease is limited
by conditions that favor the disease vector (the malarial
mosquito Anopheles) and the protozoan parasite (Plasmodium). The
malarial mosquito is most comfortable at temperatures of
approximately 20 to 30 degrees centigrade and at a relative
humidity of at least 60 percent. Also, the malaria parasite
develops more rapidly inside the mosquito as the temperature
rises, and the development ceases entirely below approximately 15
degrees centigrade. Increased rainfall and increased surface
water, expected to result from global warming, will produce more
breeding grounds for the mosquito. Malaria currently kills 1 to 2
million people each year.
3) Concerning schistosomiasis (bilharzia): The enormous
expanse of irrigation systems in many tropical countries has
doubled the incidence of this disease in the past 50 years. There
are some estimates that nearly 200 million people are infected
worldwide. The disease is caused by a parasitic worm (a
trematode; also called a "fluke"; a type of flatworm) whose eggs
enter the water supply by way of human urine or feces. Infected
water snails serve as hosts for the parasites while they develop
into free-swimming "mini-worms" (larvae; cercaria). The circle
closes when a larva penetrates the skin of a human who comes in
contact with the contaminated water. The development of the
parasite and the population of the host snails are both governed
by the ambient temperature, with warm waters favoring their
growth. Also, the warmer the ambient temperature, the more often
people come into contact with water. In places where the disease
is endemic, it is known that the number of infected snails
declines sharply during the winter months. A temperature rise of
only a few degrees will ensure that this disease is transmitted
throughout the year. It is estimated that currently worldwide
approximately 500 million people are at risk of infection by this
pathogen.
4) Concerning dengue: Like malaria, this disease is
transmitted by mosquitoes (Aedes aegypti, which also transmits
yellow fever), but the pathogen is a virus (dengue virus, a
flavivirus). The dengue virus is currently restricted to the
tropics, approximately between latitudes 30 degrees south and 20
degrees north. Temperature affects the development of both the
mosquito and the virus as well as the frequency of mosquito
bites. A warmer climate may increase not only the elevations
above sea level at which the disease occurs, but also its
northern and southern ranges. Dengue hemorrhagic fever, a severe
form of the disease, has a mortality of 6 to 30 percent, with
most deaths occurring in infants less than 1 year old.
5) Concerning various water-borne diseases: Changes in the
amount of precipitation will accompany the temperature changes to
a warmer Earth. Many disease-causing organisms require water for
survival, and increases in rainfall and flooding will encourage
the wider distribution of such pathogens, with higher
temperatures increasing the chances of pathogen survival. Various
bacteria (e.g., Salmonella and Shigella), viruses (e.g.,
rotavirus), protozoa (e.g., Giardia and Cryptosporidium) can
cause diarrhea, which kills more than 3 million children every
year.
6) In general, many factors will interact with a changing
climate in a nonlinear way, so their effects on human health are
extremely difficult to quantify. Despite the uncertainties, there
are increasing indications that a changed global climate may be a
major factor in the global distribution of many diseases.
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AS 1999 87:534
SW 2000 21 Jan 00
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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12. ON THE UTILIZATION OF MINERALS BY BACTERIA
Dianne K. Newman (California Institute of Technology, US)
discusses bacterial respiration of minerals. Microbiologists have
long known that solid surfaces are a welcome home for bacteria.
If a sterile glass slide is left in any water body, within a few
days it will be become entirely covered by microorganisms. By
attaching to and transforming minerals, microbes play an
important role in the weathering of rocks near the surface of the
Earth and perhaps even at depth. Yet the mechanisms underlying
these transformations are not well understood. One of the most
intriguing examples of microbial interactions with rocks is the
use of minerals for "cellular respiration", i.e., the use of
minerals for the oxidation of organic molecules to fuel chemical
reactions. How bacteria do this has remained a mystery, in part
because we have not been able to observe what goes on at the
molecular level. High-resolution studies of the microbial-mineral
interface have been done with techniques such as transmission
electron microscopy, but the activity of the organism is
destroyed by sample preparation in such techniques. But a new
technique has now appeared: S.K. Lower et al (2001) report the
use of a modified atomic force microscope that allows the
observation of bacteria while they respire minerals. Lower et al
accomplish this by linking fully functional cells to a small bead
at the end of a cantilevered atomic force microscope tip and then
measuring the forces exerted on the tip in response to its
deflection or attraction to an oriented mineral crystal. The
fundamental mysteries are not yet answered, but this new
technique appears to be extremely promising.
-----------
SCI 2001 292:1312,1360
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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Related Background:
SILVER-BASED CRYSTALLINE NANOPARTICLES FABRICATED BY BACTERIA
Heavy metal ions are usually toxic to biological systems,
although certain heavy metal ions, in trace quantities, serve as
enzymatic catalyst cofactors. Silver is highly toxic to most
microbial cells and can be used as a biocide or antimicrobial
agent. It has been reported, however, that several bacterial
strains are silver-resistant and may even accumulate silver at
the cell wall to as much as 25 percent of the dry weight biomass.
... ... T. Klaus et al (4 authors at Uppsala University, SE)
report the biological synthesis of silver-based crystals in a
bacterial species, the authors making the following points:
1) The authors report the biosynthesis of silver-based
single crystals with well-defined compositions and shapes, such
as equilateral triangles and hexagons, in the bacterium
Pseudomonas stutzeri AG259, a bacterial strain originally
isolated from a silver mine. The crystals were up to 200
nanometers in size and were often located at the cell poles.
Electron microscopy, x-ray analysis, and electron diffraction
studies established that the crystals comprise at least 3
different types in both whole cells and thin sections.
2) In contrast to many other metal-resistant bacteria, for
which efflux of toxic ions is the main detoxification mechanism,
the majority of the accumulated silver in P. stutzeri AG259 is
deposited as particles in vacuole-like granules between the outer
membrane and the plasma membrane (periplasmic space). X-ray
analysis shows silver and sulfur in the proportion 2:1, thus
suggesting Ag(sub2)S (silver sulfide). The electron diffraction
pattern suggests acanthite, a stable crystalline form of silver
sulfide. The authors suggest the formation of acanthite crystals
may be caused by a reaction of silver particles with H(sub2)S gas
produced by P. stutzeri.
3) The authors conclude: "Metal-insulator composite
materials have interesting optical and electrical properties that
favor their application in microelectronics, for example, or as
functional optical thin-film coatings. The possibility of
synthesizing metal particles directly in an organic matrix points
toward new uses of metal-containing bacteria as precursors in
thin film and surface coating technology, for which a composite
or *cermet structure can yield controlled optical, electrical,
and other properties."
-----------
PNAS 1999 96: 13611
-----------
Notes:
... ... *cermet structure: The term "cermet" refers to any of a
group of composite materials made by mixing, pressing, and
sintering CERamic with METal material.
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SW 2000 18 Feb
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Related Background:
IN FOCUS: ON THE WORKINGS OF THE BACTERIAL CELL
"A few years ago a well-known physical chemist interested in
bacterial physiology wrote this remarkable sentence: 'The
structure of the bacterial cell is simple.' This is true of
course from the bacterial point of view. The bacterial machine
works, synthesizes, grows, and divides. And, as the bacterium is
devoid of brain, it has no problems. But for us, the suffering
human beings who try to penetrate the intimate nature of life,
the bacterial cell, despite being small, is far from simple. In
this machine of around 1 micron diameter, corresponding to a
volume of 10^(-12) milliliters, a few thousand specific molecular
species are at work, manufacturing more of their specific kind.
And we would rather be inclined to say with Anton van Leewenhoek,
who in 1676 discovered the bacterial world, 'Dear God, what
marvels there are in so small a creature'... At first sight the
problem seems formidable and the situation hopeless. A cell
contains some 2000 to 5000 species of macromolecules. Moreover,
nature has produced an immense variety of categories of different
organisms. Yet, when the living world is considered at the
cellular level, one discovers unity. Unity of plan: each cell
possesses a nucleus embedded in protoplasm. Unity of function:
the metabolism is essentially the same in each cell. Unity of
composition: the main macromolecules of all living beings are
composed of the same small molecules. For, in order to build the
immense diversity of the living systems, nature has made use of a
strictly limited number of building blocks. The problem of
diversity of structures and functions, the problem of heredity,
and the problem of diversification of species have been solved by
the elegant use of a small number of building blocks organized
into specific macromolecules... Each macromolecule is endowed
with a specific function. The machine is built for doing
precisely what it does. We may admire it, but we should not lose
our heads. If the living system did not perform its task, it
would not exist. We have simply to learn how it performs its
task."
-----------
Andre Lwoff: _Biological Order_
(MIT Press, Cambridge MA 1962)
[Microbiologist Andre Lwoff (1902-1994) demonstrated that enzymes
produced by some genes regulate the function of other genes.
Lwoff shared the 1965 Nobel Prize in Physiology and Medicine with
Jacques Monod and Francois Jacob.]
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13. IN FOCUS: ON THE CHILDHOOD OF ISAAC NEWTON (1642-1727)
"The Newtons were moderately successful farmers. They lived in
the gentle river valley of Lincolnshire, England. Isaac, Newton's
father, owned a farm in the small village of Woolsthorpe, which
was a half dozen miles south of the town of Grantham. None of the
Newtons before Isaac Newton was literate. However, on his
mother's side, Isaac's uncle, William Ayscough, had graduated
with an M.A. from Cambridge University. In October 1642, before
the birth of his son, Newton's father died, leaving Newton's
mother, Hannah Ayscough Newton, a pregnant widow. That Christmas
Day when Isaac Newton was born he was so small and sickly it was
said he would fit into a quart mug. He was not expected to live
out the day. Yet he did, developing a sufficiently strong
constitution to survive into his 80s. How young Isaac would have
grown into manhood with a loving father and mother we will never
know. When Isaac was only three years old his mother married a
sixty-three-year-old reverend by the name of Barnabas Smith,
whose church was a mere mile and a half south of Woolsthorpe in
the village of North Witham. Hannah left her son in Woolsthorpe
with his maternal grandmother while she moved into Barnabas
Smith's church in North Witham. Fatherless at birth, and
abandoned by his mother at three, Isaac did not grow up with the
love and nurturing many children take for granted. That he was
not close to his Grandmother Ayscough is attested by the fact
that he never mentions her with affection in any of his
voluminous writings. It may seem strange that a young mother
would leave a three-year-old son to live fewer than two miles
away. Perhaps this same thought occurred to young Newton. Under
the care of his grandmother, he lived with the knowledge that his
mother was only a short walk south on the road. Abandoned and
very bright, Isaac grew up in isolation."
-----------
Calvin C. Clawson: _Mathematical Sorcery: Revealing the Secrets
of Numbers_
(Perseus Publishing, Cambridge MA 1999, p.233)
http://www.amazon.com/exec/obidos/ASIN/073820496X/scienceweek
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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14. FROM PRAXIS:
ROTAVIRUS VACCINE: RISKS AND CONTROVERSY
Reoviruses are a genus within the Reoviridae family of medium-
sized viruses (60 to 80 nanometers in diameter), the viruses in
this family having a double-stranded RNA genome. The family
includes human rotaviruses, the most important cause of infant
gastroenteritis around the world.
... ... Jon Cohen (SCI) discusses rotavirus vaccine. Two years
ago, the manufacturer of a vaccine (Rotashield) to prevent
rotavirus infection -- a diarrheal disease that kills up to
800,000 children worldwide each year -- pulled the product off
the market after researchers had linked the vaccine to a rare but
dangerous bowel obstruction called "intussusception". Analysts
calculated that the vaccine was too risky, and their argument
resulted in the removal of the vaccine from the market. But the
scientific and ethical controversy continued, and the controversy
has apparently now been renewed. Two new studies suggest that the
risk-benefit calculations in 1999 may have been in error,
intensifying questions about the decision to withdraw the
vaccine. The stakes are high, not just for North America, where
the vaccine was briefly available, but for developing countries,
where most rotavirus deaths occur. Although rotavirus sends
55,000 US children to hospitals each year, 20 to 40 of whom die,
for most children the virus causes only a mild diarrhea, and many
physicians view the rotavirus vaccine as a "convenience vaccine".
But in developing countries, as many as 1 in 200 infected
children die, mainly from dehydration due to the diarrhea caused
by the virus. The problem is that developing countries cannot
afford the lifesaving rehydration therapy used to treat the
disease, and for such countries the vaccine could provide a
tremendous benefit.
-----------
SCI 2001 293:1577
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PRAXIS 5 Nov 2001 http://scienceweek.com/praxis
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SCIENCE-WEEK 9 Nov 2001 http://scienceweek.com
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15. SOURCES:
------------
AS: American Scientist; CEN: Chemical & Engineering News;
CR: Chemical Reviews; GD: Genes & Development;
GR: Genome Research; JACS: J. Amer. Chemical Society;
JAMA: J. Amer. Medical Association; JCE: J. Chem. Education;
MMWR: CDC Morbidity and Mortality Weekly Report; NAT: Nature;
NATM: Nature Medicine; NEJM: New England J. Medicine;
NS: New Scientist; NYT: New York Times; NYR: New York Review;
PNAS: Proceedings of the National Academy of Sciences;
PRL: Physical Review Letters; PT: Physics Today; PRAX: PRAXIS;
SA: Scientific American; SCI: Science; SW: ScienceWeek;
TS: The Scientist.
In the text, the affiliation following the names of authors is
the affiliation of the lead author.
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