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ScienceWeek
SCIENCE-WEEK - December 7, 2001 -- Vol. 5 Number 49
An Email Research Digest Published Weekly Since 1997
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Our biology has made us into creatures who are
constantly recreating our psychic and material
environments, and whose individual lives are the
outcomes of an extraordinary multiplicity of
intersecting causal pathways. Thus, it is our
biology that makes us free.
-- Richard Lewontin
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. Global Climate Models
2. On the Control of Photons
3. Measuring Atomic and Molecular Collisions
4. On Black Holes
5. The Diet of European Paleolithic Man
6. On the Bleaching of Coral Reefs
7. Retrograde Signaling at Central Synapses
8. Regulation of Gene Expression
9. On the Turnover of Stem Cells
10. Complex Functionality of Nerve Cell Dendrites
11. High-Symmetry Molecular Clusters
12. The Earliest Known Fully Quadrupedal Sirenian
13. In Focus: On Bt Cotton
14. From PRAXIS: On Childhood Sunburn and Adult Melanoma
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Section 2
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1. ON GLOBAL CLIMATE MODELS
M. Stute et al (Barnard College, US) discuss global climate
models, the authors making the following points:
1) Global climate is a result of the complex interactions
between the atmosphere, cryosphere (ice), hydrosphere (oceans),
lithosphere (land), and biosphere (life), fueled by the
nonuniform spatial distribution of incoming solar radiation. We
know from climate reconstructions using recorders such as ice
cores, ocean and lake sediment cores, tree rings, corals, cave
deposits, and ground water that the Earth's climate has seen
major changes over its history.
2) An analysis of the temperature variations patched
together from all these data reveals that climate change occurs
in cycles with characteristic periods, for example, 200 million,
100,000, or 4 to 7 years. For some of these cycles, particular
mechanisms have been identified, for example, climate forcing by
changes in the Earth's orbital parameters or internal
oscillations of the coupled ocean-atmosphere system. However,
major uncertainties remain in our understanding of the interplay
of the components of the climate system.
3) Paleoclimate reconstructions, in particular from ice
cores, also have demonstrated that climate can change over
extremely short periods of time such as a few years. Over the
last century, humans have altered the Earth's surface and the
composition of its atmosphere to the extent that these factors
measurably affect current climate conditions, and there is
concern that perhaps during one human generation we will
gradually change climate conditions or even trigger a rapid and
much more dramatic shift: we might be "poking an angry beast".
4) Major progress in our understanding of climate processes
in the past, present, and future has been made by the development
of numerical models that simulate climate at an increasing level
of detail. Recent breakthroughs in spatial coverage and temporal
resolutions of systems recording today's climate, and high-
resolution reconstruction of past climate conditions from diverse
sources using new past-climate indicators (proxies), make it
possible to validate climate models and thus improve their
reliability for future predictions.
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Proc. Nat. Acad. Sci. 2001 98:10529
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SCIENCE-WEEK 7 Dec 2001 http://scienceweek.com
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Related Background:
ICE-CORE EVIDENCE OF ABRUPT CLIMATE CHANGES
Records of changes in Earth's climate are particularly clear in
high-resolution ice cores, which can preserve histories of local
climate (as reflected in snowfall and temperature), regional
climate (as reflected in wind-blown dust, sea salt, etc.), and
broader climate (as reflected in trace gases deposited from the
atmosphere) -- all on a common time scale that can demonstrate
synchrony of climate changes over wide regions.
... ... Richard B. Alley (Pennsylvania State University, US)
reviews current ice-core research, the author making the
following points:
1) Dating and accumulation: On some glaciers and ice sheets,
sufficient snow falls each year to form recognizable annual
layers that are marked by seasonal variations in physical,
chemical, electrical, and isotopic properties. These variations
can be counted to determine ages of the layers, and accuracy of
the determination can be assessed by a number of ways, including
comparison to the chemically identified fallout of historically
dated volcanoes.
2) Paleothermometry: Ice cores are essentially local
paleothermometers. The classic paleothermometer is the stable
isotopic composition of water in the ice core. Natural waters
typically contain a fraction of 1 percent of isotopically heavy
water molecules, and the vapor pressure of this heavy water is
less than ordinary or "light" water. The result is that as an air
mass is cooled and precipitates, it preferentially loses heavy
water and must increasingly precipitate light water. Both
empirically and theoretically, isotopic composition of
precipitation and site temperature are strongly correlated in
time and space.
3) Aerosols: Anything in the atmosphere can eventually end
up in an ice core. Some materials are reversibly deposited, but
most materials remain in the ice unchanged. Although details of
the air-snow transfer process are complex and not yet completely
elucidated, large changes in concentrations of most materials in
ice can with confidence be said to reflect changes in the
atmospheric loading of these materials.
4) Gases: Trapped gases in ice-core bubbles are highly
reliable records of atmospheric composition, as indicated by
comparisons among cores from different ice sheets, and comparison
with instrumental records and the air in the *firn above the
bubble-trapping depth. The slight differences between bubble and
air composition caused by gravitational and thermal effects are
well understood and recognizable.
5) Geographic coverage: The ice-core record of abrupt
climate changes is clearest in Greenland. Although no other
record is available that spans the same time interval with
equally high time resolution, it appears that ice cores from the
Canadian arctic islands, high mountains in South America, and
Antarctica also contain indications of the same abrupt changes.
Dating is considered secure for some of the Antarctic ice cores.
6) The author suggests that as the world slid in and out of
the last ice age, the general cooling and warming trends were
punctuated by abrupt changes, and climate shifts up to half as
large as the entire difference between ice age and modern
conditions occurred over hemispheric or broader regions in mere
years to decades. Such abrupt changes have been absent during the
few key millennia when agriculture and industry have arisen.
7) In summary, ice-core records indicate that climate
changes in the past have been large, rapid, and synchronous over
broad areas extending into low latitudes, with less variability
over historical times. These ice-core records come from high
mountain glaciers and the polar regions, including small ice caps
and the large ice sheets of Greenland and Antarctica.
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Proc. Nat. Acad. Sci. 2000 97:1331
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Notes:
... ... *firn: The term "firn" refers to the transitional layer
between snow and glacier ice. The layer consists of snow that has
melted during one summer melt season, the layer in the process of
transforming to glacier ice as the temperature decreases.
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ScienceWeek 2000 14 Apr
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Related Background:
ON GLOBAL CLIMATE CHANGE
Environmental change involves jumps, fluctuations, and trends,
the environment changing through the operation of the internal
machinery of the *ecosphere (biosphere), and through the external
agencies of cosmic and geological forces. Evidence of past
environmental change, almost always incomplete, derives from
geochemical, physical, biological, historical, and instrumental
sources. In recent years, high-speed computers have allowed
researchers to manipulate complicated and reasonably realistic
models of environmental change, with modelling particularly
useful for studying changes in *sedimentary basins,
biogeochemical cycles, and climate. General circulation models,
run with appropriate boundary conditions, predict climates of the
past, and these predicted climates can be compared with
paleoclimatic indicators.
... ... R.B. Alley et al (3 authors 3 installations, US) present
a review of current research on global climate change, the
authors making the following points:
1) Prediction of climate change requires observational
constraints on the current climate state, knowledge of the way
the coupled air-ocean-ice-earth-life system behaves, and
information on changing forcings such as solar variability.
Studies of past climate are also required to focus model-building
efforts on climate components that are likely to change, and to
allow testing of the ability of models to predict time-evolution
of the system.
2) The last few million years have been generally cold and
icy compared with the previous hundred million years but have
alternated between warmer and colder conditions. These
alternations have been linked to changes over tens of thousands
of years in the seasonal and latitudinal distribution of sunlight
on Earth caused by features of Earth's orbit. Globally
synchronous climate change despite some hemispheric asynchrony of
the forcing is explained at least in part by lowering carbon
dioxide during colder times in response to changes in ocean
chemistry. We live in one of the warmer times of these orbital
cycles; the coolest times brought glaciation to nearly one-third
of the modern land area.
3) Studies of past climate changes indicate that the Earth
system has experienced greater and more rapid changes over larger
areas that was generally believed possible, with jumping between
fundamentally different modes of operation in as little as a few
years. Most of the last 100,000 years or longer has been
characterized by large and abrupt regional-to-global climate
changes, and agriculture and industry have developed during
anomalously stable climatic conditions. New high-resolution
analysis of sediment cores indicates these past changes have been
caused by "*band jumps" between modes of operation of the climate
system. Recurrence of such band jumps is possible and might be
affected by human activities.
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Proc. Nat. Acad. Sci. 1999 96:9987
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Notes:
... ... *ecosphere (biosphere): In general, the term "biosphere"
refers to the portion of the planet capable of supporting life.
It ranges from elevations of approximately 10,000 meters above
sea level to the deep ocean, and a few hundred meters below the
surface of the soil. The biosphere consists of the hydrosphere,
the lower atmosphere (troposphere), and the surface of the
*lithosphere, all three regions inhabited by metabolically active
organisms.
... ... *lithosphere: In current geology, the lithosphere is the
approximately 100 kilometer rigid upper layer of the crust and
upper mantle of the Earth.
... ... *sedimentary basins: The term "sedimentary basin" refers
to a subsiding area of the Earth's crust, which permits the net
accumulation of sediment, i.e., material derived from pre-
existing rock, from biogenic sources, or precipitated by chemical
processes.
... ... *band jumps: In this context, the term "band jump" refers
to an abrupt change from one range of variation to another.
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ScienceWeek 1999 1 Oct
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Related Background:
ON THE POSSIBILITY OF RAPID CLIMATE CHANGE
Over the course of geologic history, the environment on Earth has
been far from static. Geologic evidence suggests that 600 million
years ago the atmosphere lacked sufficient oxygen to support
animal life. More recently, as indicated by sediments recording
conditions over the past 500,000 years, the climate of the planet
varied between at least two different states. The record from the
past 150,000 years is particularly well-preserved, offering
details concerning repeated climate changes. Between
approximately 131,000 and 114,000 years ago, a warm period
similar to the climate of today occurred. This was followed by
what is called the "Wisconsin ice age", which ended approximately
12,000 years ago when the current relatively warm *Holocene
period began. ... ... Kendrick Taylor (Desert Research Institute,
US) presents a review of the research of a large project to
develop a climate record for the past 110,000 years, the author
making the following points:
1) The layerings of glacial ice record seasonal variations
of temperature, snowfall, concentrations of atmospheric gases,
and atmospheric circulation patterns. In general, the weight of
accumulating snow compresses the snow below it, trapping
atmospheric gases, dust, and chemicals, and a deep ice core thus
provides a sequential record amenable to analysis.
2) The author reports that by examining ice cores from
Greenland, he and his colleagues have determined that climate
changes large enough to have extensive impacts on our society
have occurred in a time-frame of less than 10 years. The author
suggests that the climate of Earth could change significantly
during a lifetime, that we are still a long way from being able
to predict such a change, but we are getting closer to an
understanding of how it might occur. A pressing concern is
whether anthropogenic changes in the atmosphere of the planet
might perturb climate stability.
3) The author points out that climate is the result of the
exchange of heat and mass between the land, ocean, atmosphere,
ice sheets, and space. As long as changes to the land, ocean,
atmosphere, and ice sheets stay below certain thresholds, climate
changes will occur slowly. But climate will change rapidly if
those thresholds are crossed. *Greenhouse warming, for example,
by altering ocean circulation and the flow of tropical heat to
the North Atlantic, could lead to rapid cooling in eastern North
America, Europe and Scandinavia. Altered ocean circulation could
lead to much larger changes. We have no experience predicting
climate switches between stable modes.
4) The author suggests human ingenuity would most likely
allow us to adapt to a rapid change in climate, but we would pay
a larger price than our civilization has ever known. The author
poses a scenario: "Imagine the economic and social cost of
moving, in a 20-year period, most of our agricultural activities
500 miles south of their current locations. Imagine the social
cost and famine if agriculture could not be relocated quickly
enough."
5) Although we do not know the critical level of greenhouse
gas concentration that would trigger a rapid climate change, we
do know that reducing the rate of greenhouse emissions would help
in two ways. First, the atmospheric concentration of greenhouse
gases would increase more slowly. Second, numerical models
predict that the climate threshold will occur at a higher
concentration of greenhouse gases if the concentration of
greenhouses increases slowly.
6) The author suggests it will be another 20 years before
the climate changes that are predicted to be associated with the
greenhouse effect becomes large enough to be unambiguously
differentiated from naturally occurring variations in climate.
As a society we have the choice of ignoring the warning signs or
taking some action.
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American Scientist 1999 87:320
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Notes:
... ... *Holocene period: The most recent epoch of the geologic
time scale, from approximately 10,000 years ago to the present.
... ... *Greenhouse warming: The physical basis of the so-called
"greenhouse effect" is essentially simple: carbon dioxide gas is
transparent to visible light but relatively opaque to infrared
radiation. The same is true of glass. Relatively high-energy
visible light radiation from the sun passes inward through the
atmosphere, warms the surface of the Earth, which then radiates
lower energy in the form of infrared radiation (heat) back to the
atmosphere. But if the atmosphere has a concentration of infrared
impenetrable gases such as carbon dioxide, the infrared radiation
cannot pass out, and the surface of the Earth underlying the
atmosphere cannot cool, and the surface of the Earth thus will
continue to grow hotter.
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ScienceWeek 1999 13 Aug
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2. ON THE CONTROL OF PHOTONS
M.D. Lukin and A. Imamoglu (Harvard University, US) discuss the
control of photons, the authors making the following points:
1) Photons normally behave as non-interacting particles, and
this property ensures that information encoded in optical signals
will be insensitive to environmental disturbances. As a
consequence, optics has emerged as the preferred method for
communicating information. In contrast, the processing of
information requires interactions between signal carriers, i.e.,
either between different photons or between photons and
electrons. Many other applications of optics, from medicine to
spectroscopy, also rely on strong light-matter interactions. One
of the main challenges of nonlinear optical science is the
"tailoring" of material properties to enhance such interactions,
while minimizing the role of destructive processes such as photon
absorption.
2) The strength of the interaction between light and atoms
is a function of the wavelength or frequency of light. When the
light frequency matches the frequency of a particular atomic
transition, a resonance condition occurs and the optical response
of the medium is greatly enhanced. Light propagation is then
accompanied by strong absorption and dispersion, as the atoms are
actively promoted into fluorescing excited states.
3) It is well known that a dielectric medium can be used to
manipulate properties of light pulses. However, optical
absorption limits the extent of possible control, and this is
especially important for weak light pulses. Absorption in an
opaque medium can be eliminated via quantum mechanical
interference, an effect known as "electromagnetically induced
transparency". Theoretical and experimental work has demonstrated
that this phenomenon can be used to slow down light pulses
dramatically, or even bring them to a complete halt. Interactions
between photons in such an atomic medium can be many orders of
magnitude stronger than in conventional optical materials.
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Nature 2001 413:273
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Related Background:
EXPERIMENTAL PHYSICS: FIRST EFFECTIVE ZERO-VELOCITY LIGHT
Experiments involving the control of light pulses in a quantum
mechanical regime hold great promise for a future technology of
quantum computing involving optical information storage and
transmission. In 1999, L.V. Hau et al succeeded in slowing light
to a velocity of 30 meters per second in an ultracold sodium gas.
Now the same laboratory reports effectively stopping light
completely for an interval of 1 millisecond before releasing the
pulse to resume normal velocity. Essentially, the phenomenon
involves "storing" the light pulse in the quantum states of the
atoms, with the light pulse reconstituted for propagation at a
later time.
... ... C. Liu et al (4 authors at 2 installations, US) report
observations of halted light pulses, the authors making the
following points:
1) The authors point out that "*electromagnetically induced
transparency" is a quantum interference effect that permits the
propagation of light through an otherwise opaque atomic medium. A
"coupling laser" is used to create the interference necessary to
allow the transmission of resonant pulses from a "probe laser".
This technique has previously been used to slow and spatially
compress light pulses by 7 orders of magnitude, resulting in
their complete localization and containment within an atomic
cloud.
2) The authors report the use of electromagnetically induced
transparency to bring laser pulses to a complete stop in a
magnetically trapped cold cloud of sodium atoms (approximately 11
million sodium atoms at 0.9 microkelvins). Within the spatially
localized pulse region, the atoms are in a *superposition state
determined by the amplitude and phases of the coupling and probe
laser fields. Upon sudden turn-off of the coupling laser, the
compressed probe pulse is effectively stopped, and *coherence
information initially contained in the laser fields is "frozen"
in the atomic medium for up to 1 millisecond. When the coupling
laser is turned back on, the probe pulse is regenerated: the
stored coherence is read out and transferred back into the
radiation field. The authors present a theoretical model from
which it is concluded that the system is self-adjusting to
minimize dissipative loss during the "read" and "write"
operations. The authors state: "We anticipate applications of
this phenomenon for quantum information processing."
... ... In a commentary on this work, Eric A. Cornell (University
of Colorado, US) states: "The key fact here is that as the pulse
of light penetrates into the dense region of the ultracold atomic
cloud, it turns into a "quantum coherence pattern" printed on the
sodium atoms -- the information in the light beam becomes stored
in the quantum phase relationships within the internal atom
states. In the final limit, when the pulse comes to a dead stop,
all the photons have been "imprinted" (absorbed in a fully
reversible way) into the coherence pattern. Later, when the
coupling light is turned back on, the information contained in
the pattern is read out and converted back into propagating
photons that accelerate to the conventional speed of light as
they come to the edge of the atom sample."
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Nature 2001 409:461,490
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Notes:
... ... *electromagnetically induced transparency: In his
commentary, Cornell states: "The key to slowing light is the
presence of a second laser beam, the so-called 'coupling' pulse.
Distinguishable from the propagating (or 'probe') pulse by its
polarization, the coupling light delicately adjusts the internal
energy levels of the atoms, suppressing their ability to absorb
the probe light -- in effect, a single absorption level is split
into two levels that cancel each other out. This phenomenon is
known as 'electromagnetically induced transparency'.
... ... *superposition state: In this context, the general idea
is that the cloud of sodium atoms (335 microns by 55 microns in
this experiment), under these laser conditions, behaves almost as
a single quantum mechanical entity: the quantum states of the
atoms are superposed into a single wave function for the entire
system.
... ... *coherence information: In quantum physics, coherence
involves the locking of phase differences between wave functions:
the wave functions of two or more particles are said to be
coherent if the phase difference between their wave functions
remains constant. In general, a perfectly coherent system of
particles can be described by a single macroscopic wave function.
In general, in optics, the term "coherence" refers to the
existence of a correlation between the phases of two or more
waves. In this context, the term "coherence information" refers
to information contained in and dependent upon the coherence of
the laser pulse.
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ScienceWeek 2001 16 Feb
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3. MEASURING ATOMIC AND MOLECULAR COLLISIONS
F. Fleming Crim (University of Wisconsin Madison, US) discusses
atomic and molecular collision measurements, the author making
the following points:
1) For a chemical reaction to occur, molecular collisions
must deposit energy in a reactant or bring reactants together
such that they can rearrange bonds. Powerful experimental and
theoretical techniques are unraveling the details of these
encounters. The current arsenal of experimental techniques for
studying atomic and molecular collisions comprises scattering
probes, which observe the direction and speed of molecules after
a collision, and spectroscopic probes, which detect the quantum
state of the molecules -- i.e., their particular configuration of
energy, spin, momentum, charge, and other quantum properties.
2) In combination, the two techniques yield detailed
information about molecular motion during a collision. In such a
combined experiment, a beam of molecules prepared in one or a few
quantum states crosses another beam of similarly prepared atoms
or molecules. The pressure is low to ensure that each molecule
collides at most once and at the point where the beams cross.
Collision may result in energy transfer (inelastic scattering) or
a reaction (reactive scattering). Some experiments determine the
recoil speed and angle of the scattered molecules and infer their
total internal energy. Other experiments use spectroscopy to
observe the population of individual quantum states, determine
the internal energy of a single product state, and identify the
molecular motion (such as vibration or rotation) in which this
energy resides.
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Science 2001 293:2014
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4. ON BLACK HOLES
B. Bruegmann et al (Albert Einstein Institute Muhlenberg, DE)
discuss black holes, the authors making the following points:
1) Black holes have become an astrophysical reality. Solid
observational evidence exists for black holes in two mass ranges:
a) Supermassive black holes of 10^(6) to 10^(9) solar masses have
been observed at the centers of many galaxies including our own
Galaxy. b) Stellar-size black holes of approximately 3 to 20
solar masses have been studied in x-ray binaries and
microquasars. In addition, numerical simulations have become
possible for the merger of black hole binaries.
2) Recent high-resolution imaging studies of the stars at
the center of our Galaxy have produced strong dynamical evidence
for a central concentration of dark matter, establishing the
Milky Way as the most convincing case of a galaxy containing a
central supermassive black hole. In these observations, images
obtained over 2 to 6 years provided measurements of the
velocities of stars in the plane of sky, from which a statistical
analysis revealed the existence of 2 to 3 x 10^(6) solar masses
of dark matter contained within radius of 0.015 parsec (2.6 light
weeks). Newer results have reduced stellar velocity uncertainties
by a factor of 3.
3) In contrast to the need for measuring the velocities of
dozens of stars to determine the mass of the black hole in the
Galactic Center, the masses of black holes in x-ray binary
systems can be deduced either from optical/infrared measurements
of just one star, namely the companion of the stellar-mass black
hole, or from x-ray observations of the binary. In x-ray
binaries, a black hole of typically 3 to 10 solar masses and a
normal star (1 to 30 solar masses) orbit each other. Matter is
pulled off the companion star, and because of its angular
momentum this matter forms an accretion disk as it moves toward
the black hole. Before finally falling into the black hole, the
matter heats up to several million degrees at the inner part of
the disk and emits luminous x-ray radiation.
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Proc. Nat. Acad. Sci. 2001 98:10525
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Related Background:
ASTROPHYSICS: ON BLACK HOLES AND EVENT HORIZONS
At the end of the life of a star, when the nuclear fuel has
been exhausted and the outward pressure can no longer balance the
inward-pulling gravitational forces, a series of events occurs,
one event of which is a "blowing-off" of considerable stellar
material. If the terminal stages of star death leave a remnant
star mass greater than approximately 3 solar masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable. Another view of a black hole
is that it is a mass that has collapsed to such a small volume
that its gravity prevents the escape of all radiation. Since
black holes are centers of essentially infinite gravity, they
naturally accrete material from any surrounding astronomical
objects.
In this context, the term "singularity" refers to a
mathematical singularity, in general a point at which a function
takes on infinite values.
So-called "neutron stars" also derive from the terminal
stages of star death. If, following its terminal stages, the
remnant mass of a star is between 1.4 and approximately 3 solar
masses, the star will collapse into a neutron star, a body with a
radius of 10 to 15 kilometers, with a core so dense that its
component protons and electrons have merged into neutrons. The
average density of a neutron star is 10^(15) grams per cubic
centimeter, and the weight of an object on the surface of a
neutron star would be 10^(11) its weight on the surface of the
Earth. Neutron stars apparently have an outer shell of iron, but
it is iron like no Earth iron, an iron of 4 orders of magnitude
greater density.
Another possible consequence of the death of a star is the
formation of a "white dwarf star". If the remnant mass after
star-death blow-off is less than 1.44 solar masses (the
Chandrasekhar limit for a star with no hydrogen content), the
star collapses into a white dwarf. Such stars are approximately
the size of Earth, but with a mass approximately that of the Sun.
Space and time essentially have no meaning in a black hole.
The boundary of the black hole is called the "event horizon",
because any event within the boundary is invisible outside, the
invisibility resulting from the fact that no radiation can escape
to be detected. The radius of the black hole depends upon how
much matter has fallen into the region; it is called the
"Schwarzschild radius", and it is usually a few kilometers.
However, massive black holes are possible and are thought to be
the source of quasars and also the centers of certain galaxies.
Our own galaxy is believed to have a massive black hole at its
center.
Once matter has disappeared into a black hole, only three
original properties can be ascertained: the total mass, the net
electric charge, and the total angular momentum. (So-called
"black-hole emissions" are really emissions of matter immediately
outside the black hole.) Since all black holes must have mass,
there are 4 possible types of black hole, each type derived from
equations of the general theory of relativity:
a) A "Schwarzschild black hole" (first derived 1916) has no
charge and no angular momentum.
b) A "Reissner-Nordstrom" black hole (first derived 1918)
has charge but no angular momentum.
c) A "Kerr black hole" (first derived 1963) has angular
momentum but no charge.
d) A "Kerr-Newman black hole" (first derived 1965) has both
charge and angular momentum.
It is currently believed that real black holes are almost
certainly rotating and have very little electric charge, so that
the Kerr solution should be the most applicable.
The so-called "event horizon" constitutes the effective
surface of a black hole. For a non-rotating black hole it is a
spherical boundary at the Schwarzschild radius of the black hole.
For a rotating black hole, the event horizon is elliptical.
A central problem in the study of black holes has been to
discover how to distinguish them from neutron stars. In recent
years, a method has been developed, a method based on x-ray
emissions from the vicinity of each type of body. This method has
allowed the demonstration that black holes are indeed a reality.
The x-ray emissions of x-ray binary systems have been
particularly revealing. An "x-ray binary" is an x-ray emitting
binary system apparently consisting of a neutron star (or in some
cases a black hole or a white dwarf star) and a normal star. The
x-ray emission is an expected result of accretion of material by
gravity from the normal companion to the compact massive
gravitationally collapsed object.
The term "Doppler effect" is a lengthening (shift to red) of
the wavelengths of electromagnetic radiation from a source caused
by the movement of the source away from the observer, or a
shortening (shift to the blue) of wavelengths as the source moves
towards the observer.
The first indications for the actual existence of black
holes came from x-ray observations of the UHURU satellite
(launched in 1970), but black holes were predicted theoretically
60 years ago, and in fact Laplace in 1796 already noted that
light cannot leave a star when its free-fall velocity as
determined by the star's mass and radius is larger than the speed
of light as we have measured it. At the present time, if the
apparent mass of a discovered compact object exceeds the limiting
mass, and if we hold the general theory of relativity to be
valid, we say we have discovered a black hole. There are now
dozens of such objects that have been identified, many of them
super-massive objects each containing millions and billions of
solar masses and surrounded by dense stellar clusters in the
strong gravity of a black hole.
The "general theory of relativity" is the theory announced
by Albert Einstein (1879-1955) in 1915, the theory describing how
space and time are affected by the gravitational fields of
matter. General relativity was essentially developed from the
principal of equivalence between gravitational and inertial
forces, and testing the various predictions of the theory is a
central focus of much research in astrophysics.
... ... Joseph F. Dolan presents a commentary on current research
on black holes, the author making the following points:
1) The author points out that black holes are now widely
accepted as the cause of many phenomena in astrophysics, from
supermassive black holes at the center of galaxies to stellar-
mass black holes in x-ray binaries. In general, black holes are
the simplest objects that can be conceived to explain the
behavior of these systems. The author points out, however, that
there remains the theoretical possibility that these objects may
be more exotic than black holes.
2) The author points out that every black hole will have an
event horizon, so the naked point singularity determining the
existence of the black hole is forever shielded from our view.
But because any object with an event horizon must be a black
hole, detection of such a horizon would prove the existence of a
black hole.
3) One approach to detecting the event horizon of a black
hole was proposed by W.R. Stoeger in 1980, who suggested that
individual flare patches -- clumps of material whose radiative
characteristics stand out above the mean radiative flux of the
system -- would appear to emit pulsed radiation because of the
aberration of light away from the direction of Earth when the
clumps of material orbit on the far side of the black hole. The
separation between pulses should decrease as the material spirals
into the event horizon, and the peak intensity of the pulses
should also decrease as the material approaches the event horizon
because of the Doppler effect. The last visible pulse should thus
be the weakest. In contrast, if the accreting object was not a
black hole and had a solid surface, as in a neutron star, the
last pulse would be the largest pulse as the material impacted
the surface.
4) The author (Dolan) points out that some theories of
gravity that are consistent with general relativity do not
predict the existence of black holes and event horizons, and this
may provide a test of the general theory of relativity. In these
theories, collapsed objects that are not point singularities can
exist, but only radiation directed nearly radially outward can
escape from their surface. If the impact of accreting material
occurs on a part of their surface not visible from Earth, no
radiation would reach us. If the theoretical orbital topography
near compact objects in these competing theories does not predict
astronomical observations, then detection of event horizons may
be another method of confirming the validity of general
relativity.
-----------
Science 2001 291:1079
ScieneWeek 2001 27 Jul
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5. ON THE DIET OF EUROPEAN PALEOLITHIC MAN
M.P. Richards et al (University of Bradford, UK) discuss the diet
of European Paleolithic man. Human subsistence patterns during
the late Pleistocene have been inferred principally from animal
remains preserved in archeological sites; from the uses of stone
tools based on form, microwear traces, and organic residue
analysis; and occasionally from preserved vegetal remains.
Although it is recognized generally that Late Pleistocene human
diets must have included a variety of plants and animals, the
majority of the studies conducted to date have focused on large
mammal remains and taphonomic arguments about the changing nature
of human exploitation of those animals. By the late Upper
Paleolithic period, however, paleontological, paleobotanical, and
technological evidence all indicate substantial broadening of
human diets in several regions of the Old World. The authors
report new carbon and nitrogen stable isotope values for human
remains dating to the mid-Upper Paleolithic in Europe, the data
indicating significant amounts of aquatic (fish, mollusks, and/or
birds) foods in some of their diets. The authors suggest that
most of this evidence points to exploitation of inland freshwater
aquatic resources in particular. In contrast, European
Neanderthal collagen carbon and nitrogen stable isotope values do
not indicate significant use of inland aquatic foods, but instead
demonstrate that Neanderthals obtained the majority of their
protein from terrestrial herbivores. In agreement with recent
zooarcheological analyses, the authors suggest their isotope
results indicate shifts toward a more broad-spectrum subsistence
economy in inland Europe by the mid-Upper Paleolithic period,
probably associated with significant population increases.
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Proc. Nat. Acad. Sci. 2001 98:6528
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6. ON THE BLEACHING OF CORAL REEFS
Andrew C. Baker (New York Aquarium, US) discusses the bleaching
of coral reefs. Coral reefs are built by symbioses between
scleraactinian (stony) corals and photosynthetic dinoflagellate
algae. These diverse algae are important species because their
loss during bleaching can lead to widespread coral mortality and
degradation of reef ecosystems. Different types of algal symbiont
often show strong zonal patterns within their coral hosts that
correspond to light intensity (shallow, "high-light" algae, or
deep, "low-light" algae). In general, reef corals are flexible
associations that can switch or shuffle symbiont communities in
response to environmental change. The bleaching of coral reefs,
in which symbiont algae are lost from reef-building
invertebrates, is usually considered to be a drastic and damaging
response to adverse environmental conditions. The author reports
results from transplant experiments involving different
combinations of coral host and algal symbiont that support an
alternative view in which bleaching offers a high-risk ecological
opportunity for reef corals to rid themselves rapidly of
suboptimal algae and to acquire new partners. This strategy could
be an advantage to coral reefs that face increasingly frequent
and severe episodes of mass bleaching as a result of projected
climate change. Symbiosis recombination may help to resolve the
paradox of reef corals as environmentally fragile yet
geologically long-lived associations. This counters conventional
wisdom that bleaching is detrimental from all perspectives, and
supports the role of symbionts as adaptive agents.
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Nature 2001 411:765
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SCIENCE-WEEK 7 Dec 2001 http://scienceweek.com
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Related Background:
PALEOBIOLOGY: ON THE ORIGINS OF MODERN CORALS
The term "coral" refers to any of a variety of invertebrate
marine organisms of the class Anthozoa (phylum Cnidaria) that are
characterized by skeletons (external or internal) of a stone-
like, horny, or leathery consistency. The term "coral" is also
applied to the skeletons of these animals, particularly to the
skeletons of the stone-like corals. Stony corals (order
Madreporararia or Scleractinia) number approximately 1000
species; black corals and thorny corals (Antipatharia) number
approximately 100 species; horny corals (Gorgonacea) number
approximately 1200 species; and blue corals (Coenothecalia)
number one living species. The body of a coral animal consists of
a polyp, a hollow cylindrical structure attached at one end to a
surface. At the free end is a mouth surrounded by tentacles which
gather food and which are extensible to varying degrees, and
which are armed with specialized stinging structures
(nematocysts) that paralyze prey. The skeleton of a stony coral
is almost pure calcium carbonate and is deposited in a cup-shaped
form with the polyp inside. The growth rate of the skeleton
varies with age, food supply, water temperature, and species.
Atolls and coral reefs are composed of stony coral, with such
formations growing at an average rate of approximately 0.5 to 2.8
centimeters per year.
The geological period known as the Cambrian is the time
frame from approximately 570 million years ago to 510 million
years ago. Its most outstanding aspect is the rather sudden
appearance of numerous invertebrate fossils, so numerous that
some have termed it an explosion of evolutionary processes. Many
of the life forms that existed during the Cambrian are long
extinct, but their fossils are numerous, and through their
fossils the various Cambrian species have been the subject of
much study by paleobiologists. The Cambrian explosion of life
forms has been a long-standing puzzle for paleobiologists, and at
present there is apparently no single generally accepted
explanation. Among the ideas proposed have been, 1) that the
explosion of new forms resulted from a sudden increase in
atmospheric oxygen; 2) that the explosion is only apparent, and
the Precambrian, the period previous to the Cambrian, lacks
fossils because of heat and pressure associated with important
geological changes; 3) that living forms evolved mostly in
freshwater areas, and are therefore absent in Precambrian
sediments, which are primarily marine; 4) that changes in the
shape and extent of shorelines produced by continental drift
dramatically transformed climate and environment; 5) that the
previous evolution of DNA recombination and regulatory genes
culminated in and sparked the diversity and anatomical complexity
manifested in the explosion; 6) that an exponential increase of
species could become significant only after attaining a threshold
value at the start of the Cambrian; and, 7) that once
multicellular organisms appeared, the intrinsic possibilities for
variation increased enormously with a resultant explosion of
evolved forms. Unfortunately, there is no evidence to suggest a
selection of one of these proposals, although some of them are
less convincing than others. And of course the truth may be that
more than one factor was involved. No matter the origin, the
Cambrian explosion is apparently accepted by most paleobiologists
as a real discontinuity, a period that saw the sudden emergence
of dozens of new orders and phyla.
The "Permian period" comprises the approximate time-frame
290 to 245 million years ago, and the "Triassic period" comprises
the approximate time-frame 245 to 208 million years ago. At the
end of the Permian period, many groups of animals and plants
apparently vanished in the greatest known crisis in the history
of life on Earth. This extinction event was the most severe in
the past 540 million years, killing off over 90 percent of all
marine species, approximately 70 percent of terrestrial
vertebrate genera, and most land plants. Proposed catastrophic
hypotheses for the Permian/Triassic boundary extinction include
an exploding meteor (bolide) (asteroidal or cometary) and massive
volcanic lava flows (flood basalt volcanism). Other extinction
mechanisms involving ocean anoxia, as well as changes in sea
level and climate, have also been proposed.
The so-called "Paleozoic era" comprises the time-frame 570
million to 245 million years ago, thus beginning with the
Cambrian period and terminating at the end of the Permian period.
... ... G.D. Stanley Jr. and D.G. Fautin (2 installations, US)
present a commentary on recent research on the origins of modern
corals, the authors making the following points:
1) The authors point out that most calcifying multicellular
animals made their debut approximately 540 million years ago
during the Cambrian period, soon after the explosion of
biological diversity in the sea. Molecular evidence indicates
that most Paleozoic multicellular animals (metazoans) originated
much earlier, in the Precambrian. Scleractinian corals are
relative latecomers, appearing in the fossil record during the
Triassic period approximately 237 million years ago -- 14 million
years after the Permian extinction.
2) The authors point out that despite a rich fossil record,
the origin of Scleractinia corals has remained shrouded in
controversy. The absence of coral fossils in the first 14 million
years of the Triassic coincides with a time when carbonate
deposition was apparently suppressed globally, long after most
marine life had been extinguished at the end of the Permian. This
gap is a problem for the old theory that scleractinians were
derived from Paleozoic corals. It has been postulated that some
survived but that post-extinction abundances were low, and the
resulting fossils so rare that they have eluded detection.
Another theory is that these corals survived the 14-million-year
gap in as yet undiscovered refuges. The authors suggest both
ideas seem tenuous in light of the intense global scrutiny by
researchers of lower Triassic rocks.
3) The authors suggest that many apparent ambiguities and
conflicts in previous analyses can be reconciled if lineages of
corals have lost and redeveloped skeletons repeatedly through
their history in response to environmental conditions. To test
this idea, the authors suggest that future research should
explore biochemical mechanisms of calcification, establish
phylogenetic relationships between scleractinians and
morphologically similar soft-bodied animals, analyze if and how
skeletal structure of scleractinian groups corresponds with
molecular data, and seek evidence of geochemical changes in
geologic history that correlate with changes in the robustness of
coral skeletons and appearances or disappearances of various
coral groups.
-----------
Science 2001 291:1913
ScienceWeek 2001 11 May
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SCIENCE-WEEK 7 Dec 2001 http://scienceweek.com
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Related Background:
IN FOCUS: ON THE GREAT BARRIER REEF
"On a trip to the Great Barrier Reef early in 1999 I went
into a dive shop to hire some dive gear. 'Have a good holiday.
Enjoy the reef while you can,' said the shop assistant. I thought
then that he meant 'Enjoy it while you can,' in the 'life is
fleeting' sense. Later I realized he meant 'Enjoy it while it's
still here.' In 1998 a dramatic wave of coral bleaching spread
across the tropical oceans of the world. Corals from most species
bleached to white, suddenly and within weeks. First noticed in
December 1997 in the Galapagos Islands, the bleaching swept
across the Pacific Ocean to the Great Barrier Reef and onward,
ultimately affecting corals in the Caribbean some nine months
later. The colors of corals bring pleasure; their widespread
bleaching brought dismay. While some corals have recovered,
others have since died. The situation is disturbing. The Great
Barrier Reef has existed in its present form for roughly 6000
years, which is only a moment in geological time, but the reef's
moment may be passing...
"The 1998 International Year of the Oceans was the hottest
year on record for a thousand years; it was also the year of
death of corals on a scale never seen before, through coral
bleaching. In bleaching, coral loses its color and much more. The
brilliant colors of corals come from tiny single-celled algae,
the zooxanthellae or symbiotic dinoflagellates, which live in the
tissues of the corals in great numbers; between 6 million and 12
million organisms to a square inch of coral tissue. When the
zooxanthellae get stressed, they collect together in the hollow
column of the coral polyp and leave their host. They bail out
into the ocean to take up an independent life. The coral skeleton
becomes visible through the now transparent polyp. Normally, the
algae live in symbiosis with the polyps and produce up to 60
percent of their energy; when they bail out, ill health and often
death of corals follow.
"Coral bleaching is relatively new as a cause of massive
death and destruction of corals. It was observed on the Great
Barrier Reef before 1998 but on isolated reefs in isolated
patches. There have been five similar if less serious outbreaks
since 1979, though, curiously, few reports before then. The
bleaching effect was linked to a number of causes, both increased
sea temperature and decreased water salinity caused by heavy rain
can prompt corals to release large numbers of algae. Laboratory
studies in the 1980s showed that other stresses such as increased
ultraviolet light, sedimentation, and toxic chemicals also may
cause bleaching..."
-----------
Rosaleen Love: _Reefscape: Reflections on the Great Barrier Reef_
(Joseph Henry Press, Washington 2001, p.4,175.)
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ScienceWeek 2001 11 May
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7. ON RETROGRADE SIGNALING AT CENTRAL SYNAPSES
H.W. Tao and M-M. Poo (University of California Berkeley, US)
discuss retrograde signaling at synapses and make the following
points:
1) Neural information coded by the action potential is
transmitted through a chemical synapse in the anterograde
direction by release of neurotransmitters, neuropeptides, and
other protein factors from the presynaptic terminal. These
molecules produce immediate changes in the membrane potential as
well as long-term structural and metabolic changes in the
postsynaptic cell.
2) Over the past several decades, it has become increasingly
clear that information exchange at the synapse is bidirectional:
the postsynaptic cells also provides a variety of retrograde
signals to the presynaptic neuron, and this reciprocal
interaction is crucial for the differentiation and maintenance of
the presynaptic cell as well as for the formation and maturation
of the synapse.
3) The general notion of retrograde signaling involves
postsynaptic production of a signal, either constitutively or
triggered by synaptic activity, that acts on the presynaptic
neuron through various mechanisms: a) The retrograde signal can
be carried by a membrane-permeant molecule that diffuses across
the plasma membranes from the postsynaptic cell directly into the
presynaptic nerve terminal. b) Membrane-impermeant but soluble
factors can be packaged and secreted via exocytotic vesicles by
the postsynaptic cell and exert the retrograde action by binding
and activation of receptors on the presynaptic membrane. c)
Direct signaling through the synaptic cleft may be accomplished
through the mediation of physically coupled pre- and postsynaptic
membrane-bound proteins.
-----------
Proc. Nat. Acad. Sci. 2001 98:11009
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SCIENCE-WEEK 7 Dec 2001 http://scienceweek.com
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Related Background:
RETROGRADE AND ANTEROGRADE CONDUCTION IN NEURON DENDRITES
Nerve cells, in biological systems which have nerve cells, come
in all shapes and sizes, with the various shapes and sizes
correlated or not yet correlated with various functions. The
generalized neuron is more or less modeled after the vertebrate
motor neuron, a nerve cell with a particular morphology and a
particular relation of its morphology to its function, but all
neurobiologists are aware of the multiplicities of nerve cell
design actually found in nature, and the multiplicities of the
way various types of nerve cell behave. In the classical
generalized neuron, the idea is that the various membrane
depolarizations and hyperpolarizations that are the inputs to the
neuron are summated as excitatory and inhibitory inputs with an
end result at the initial segment of the axon hillock just beyond
the cell body that determines whether the nerve cell fires an
action potential that will be propagated along its axon to
another neuron or to a muscle cell. This paradigm is the classic
paradigm, supremely useful as a conceptual scheme (and a scheme
that produced at least 4 Nobel Prizes), but the fact is many
neuron types, particularly in the vertebrate brain, are
exceptions to the generalization. This may be particularly true
of the dendritic arborizations of central nervous system neurons,
and one of the fundamental questions of neurobiology is to
unravel the electrophysiology of nerve cell dendrites, and in
particular to determine if and when dendrites of particular types
of neurons actually conduct action potentials. Now Wei R. Chen et
al (3 authors at 2 installations, US, DK) report that
electrophysiological observations of the rat olfactory mitral
nerve cell indicate that the action potential can be initiated
either in the soma-axon hillock or in the distal primary
dendrite, and that the initiation site is controlled by
excitatory synaptic inputs to the distal dendrite and inhibitory
synaptic inputs near the cell body. The authors suggest that
mitral cells provide a model that widens the view of how
dendritic excitability contributes to information processing in
different types of neurons in the vertebrate brain.
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Science 1997 17 Oct
ScienceWeek 1997 7 Nov
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8. ON THE REGULATION OF GENE EXPRESSION
C. Peter Verrijzer (Leiden University, NL) discusses the
regulation of gene expression. The process that switches on the
appropriate genes in the correct cells at the right time is
central to cellular differentiation and the development of
multicellular organisms. Gene expression is controlled
predominantly by regulating transcription, the process that
copies the gene's DNA instructions into messenger RNA (mRNA),
which is then translated into protein. The molecular machinery
that drives the transcription of genes comprises RNA polymerase
II (the actual enzyme that makes mRNA) and a group of basal or
general transcription factors. This basal transcription factor
machinery assembles on a DNA sequence, termed the "core
promotor", located at the beginning of a gene. In addition,
regulated gene expression requires DNA sequences, termed
"enhancers", that bind to sequence-specific DNA-binding proteins
("activators"), which in turn activate transcription. Each gene
is controlled by a unique array of binding sites for distinct
activators that ensure its expression at the right time and
place. The communication between the enhancer-bound activators
and the basal transcription machinery factors depends on a third
class of transcription factors, the so-called "co-activators". It
is generally believed that a universal invariant basal
transcription apparatus integrates the signals from enhancer-
bound gene-specific activators expressed only in particular
cells. However, new studies reveal that tissue-specific
components of the basal transcription machinery can also be gene-
specific regulators of development.
-----------
Science 2001 293:2010
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9. ON THE TURNOVER OF STEM CELLS
S. Ro and B. Rannala (University of Alberta, CA) discuss stem-
cell turnover. To understand the normal aging process, as well as
the role of cellular aging in diseases such as cancer, it is
essential to understand the process of somatic cell development
and renewal. Stem cells are undifferentiated cells normally
residing in a specific location (a "niche") within a tissue, and
such cells are capable of producing a variety of somatic cell
types needed for periodic tissue renewal and tissue regeneration
after injury. To accomplish this, stem cells produce intermediate
progenitors, called "transit amplifying cells", that can divide
rapidly and differentiate into various types of tissue cells.
Because stem cells are the only cells capable of continuous
tissue renewal, the population of stem cells must be maintained.
It is still largely a mystery how stem cells maintain their
numbers, but two competing models have been proposed: a) The
deterministic model proposes that a small number of stem cells
reside in a niche, each generating exactly one stem cell and one
transit amplifying cell at each (symmetrical) cell division. The
daughter transit amplifying cell leaves the niche to proliferate
for tissue renewal, while the daughter stem cell remains in the
niche. Each stem cell is "immortal" under this model. b) The
stochastic model proposes that many stem cells exist in a niche
with each stem cell division producing either two, one, or zero
stem cells (and either zero, one, or two proliferating transit
amplifying cells, respectively). This leads to "drift" in the
numbers of descendants of each stem cell lineage over time.
Eventually, a single common ancestral stem cell exists from which
all stem cells in a niche are descended. Under the stochastic
model, the stem cell population is most likely to persist if the
probabilities that a stem cell division produces either two stem
cells or zero stem cells are equal.
-----------
Proc. Nat. Acad. Sci. 2001 98:10519
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10. ON THE COMPLEX FUNCTIONALITY OF NERVE CELL DENDRITES
D-S. Wei et al (University of Maryland, US) discuss the dendrites
of neurons. Dendrites are not passive antennae that simply
receive synaptic inputs: instead, dendrites actively process and
transform inputs as they are received. The apical dendritic arbor
can be divided into 3 morphologically distinct regions: the thick
main apical trunk, a set of short intermediate branches, and a
set of long and thin terminal branches. The apical trunk of
pyramidal cell dendrites is relatively thick and contains
sufficient densities of sodium channels to mediate forward and
backward propagation of action potentials. Terminal branches, in
contrast, have diameters one-fourth that of the apical trunk and
do not decrease over distance. Terminal dendritic segments
constitute 70 to 90 percent of the combined length of the apical
dendritic arbor. Despite being the main recipient of excitatory
synaptic inputs, little is known about the passive and active
transformations an individual terminal segment performs on its
inputs. The authors report that the excitability of terminal
apical dendrites in pyramidal cells of the rat hippocampus
differs from that of the apical trunk. In response to
fluorescence-guided focal photolysis of caged glutamate,
individual terminal apical dendrites generated cadmium-sensitive
all-or-none responses that were subthreshold for somatic action
potentials. Calcium transients produced by all-or-none responses
were not restricted to the sites of photolysis, but occurred
throughout individual distal dendritic compartments, indicating
that electrogenesis is mediated primarily by voltage-gated
calcium channels. The authors suggest that compartmentalized and
binary behavior of parallel-connected terminal dendrites can
greatly expand the computational power of a single neuron.
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Science 2001 293:2272
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11. ON HIGH-SYMMETRY BIOMOLECULAR CLUSTERS
D.L. Caulder et al (University of California Berkeley, US)
discuss high-symmetry biomolecular clusters. Nature provides
stunning examples of non-covalently linked molecular clusters of
high symmetry. Studied by electron microscopy and x-ray
diffraction, the high symmetry of viruses has been recognized
since the 1950s. The protein coat of the human rhinovirus, for
example, is composed of 60 copies of each of 4 protein subunits
arranged in icosahedral symmetry. Viral protein coats are not the
only high-symmetry natural clusters, however. The iron-storage
protein ferritin is composed of 24 identical protein subunits
arranged in octahedral symmetry. Of interest is the fact that in
most cases these clusters spontaneously self-assemble from their
identical subunits. Particularly intriguing is the fact that
these high-symmetry structures are utilized as protective shells:
the virus coats protect genetic material, while the ferritin
shell stores several thousand iron atoms as polymeric ferric oxy
hydroxide. Under the assumption that evolutionary pressure drove
optimization of the shell design, rationales have been put
forward to explain the frequent occurrence of such self-assembled
structures. Subunit economy concomitant with maximization of the
enclosed space and mechanical rigidity of the resulting structure
have been cited concerning viral protein coats. As little as one
gene is required to encode the protein that in turn contains all
the information necessary for multiple copies of the protein to
self-assemble into the complete shell.
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J. Am. Chem. Soc. 2001 123:8923
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12. THE EARLIEST KNOWN FULLY QUADRUPEDAL SIRENIAN
D.P. Domning (Howard University, US) discusses the sirenian
fossil record. Modern seacows (manatees and dugongs; Mammalia,
Sirenia) are completely aquatic, with flipper-like forelimbs and
no hindlimbs. Since 1990, abundant remains of sirenians, together
with other marine taxa of early middle Eocene age (approximately
50 million years ago), have been collected from Seven Rivers,
Jamaica. The sediments (silt-stones and sandstones) represent a
lagoonal, estuarine, or deltaic depositional environment. The
author reports Eocene fossils from Jamaica that represent nearly
the entire skeleton of a new genus and species of sirenian -- the
most primitive for which extensive postcranial remains are known.
This animal was fully capable of locomotion on land, with 4 well-
developed legs, a multivertebral sacrum, and a strong sacroiliac
articulation that could support the weight of the body out of
water as in land mammals. Aquatic adaptations demonstrate,
however, that this animal probably spent most of its time in the
water. Its intermediate form thus illustrates the evolutionary
transition between terrestrial and aquatic life. Similar to
contemporary primitive cetaceans, the animal probably swam by
spinal extension with simultaneous pelvic paddling, unlike later
sirenians and cetaceans, which lost the hindlimbs and enlarged
the tail to serve as the main propulsive organ. Together with
fossils of later sirenians elsewhere in the world, these new
specimens document one of the most marked examples of
morphological evolution in the vertebrate fossil record.
-----------
Nature 2001 413:625
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13. IN FOCUS: ON Bt COTTON
[Editor's note: The term "Bt cotton" refers to cotton genetically
engineered to contain and express a gene from the bacterium
Bacillus thurengiensis, the expressed protein toxic to the
tobacco budworm, a major insect larval pest of cotton plants.]
------------------------------------------------------
"The very first year that Bt cotton was sold commercially, cotton
farmers in Alabama planted it on 75 percent of their acres, even
though the gene was only available in cotton varieties that were
suited for the longer growing seasons of Mississippi or
Louisiana. Bt was their lifeline; the previous year, tobacco
budworms had almost wiped out their cotton crop. In the cotton
heartland of Georgia, Mississippi, and eastern Arkansas, Bt
cotton accounted for more than a third of the crop in 1996. Two
years later it covered 60 percent of those areas. Bt cotton
didn't just clobber the tobacco budworm; it also delivered a
staggering blow to the companies that sell insecticides to cotton
farmers, and the to the crop dusters who spray those chemicals on
fields. Before 1996 [cotton farmer] Frank Mitchener was spending
up to $140 per acre fighting insects. After 1996 he spent on
average $90 an acre, and a third of that was the "technology fee"
he paid for the right to use Monsanto's Bt gene. Statistics on
the frequency of insecticide spraying tell an even more dramatic
story. In 1995, cotton farmers in the Mississippi Delta sprayed
their fields 4.5 times on average to control the tobacco budworm
and the cotton bollworm. A year later, the average had dropped to
2.5, and farmers who planted Bt cotton sprayed even less. In the
hill country of Mississippi and in Alabama the drop was
astonishing: Farmers sprayed an average of 8 times for the
budworm and bollworm in 1995, and only 1.5 times in 1996."
-----------
Daniel Charles: _Lords of the Harvest: Biotech, Big Money, and
the Future of Food_
(Perseus Publishing, Cambridge MA 2001, p.174)
For more information about this book:
http://www.amazon.com/exec/obidos/ASIN/0738202916/scienceweek
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SCIENCE-WEEK 7 Dec 2001 http://scienceweek.com
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14. FROM PRAXIS: ON CHILDHOOD SUNBURN AND ADULT MELANOMA
F.P. Noonan et al (George Washington University, US) discuss
early sunburn and adult melanoma. Retrospective epidemiological
data have indicated that cutaneous malignant melanoma may arise
as a consequence of intense and intermittent exposure of the skin
to ultraviolet radiation, particularly in children, rather than
from cumulative lifetime exposure associated with other forms of
skin cancer. The authors report the use of a genetically
engineered mouse model to demonstrate that a single dose of
burning ultraviolet radiation in neonates, but not adults, is
necessary and sufficient to induce tumors with high penetrance
that are reminiscent of human melanoma. The authors suggest their
results provide experimental support for epidemiological evidence
that childhood sunburn poses a significant risk for development
of this potentially fatal disease. Concerning the question of why
neonatal mouse skin is so sensitive to ultraviolet-induced
melanogenesis, the authors point out that since melanocyte-
precursor cells are more abundant in neonatal than in adult skin
and more proliferative under stress, ultraviolet light exposure
may stimulate proliferation of DNA-damaged neonatal progenitors
and thus facilitate melanogenesis. Also, early exposure to
intense ultraviolet radiation might affect the developing immune
system, promoting future tolerance to arising melanoma. However,
the authors also suggest that caution is called for in
extrapolating these results to sunburn in children. The
difference in thickness between mouse and human skin could affect
the penetration of ultraviolet radiation. Moreover, the human age
equivalent to a 3.5-day-old neonatal mouse cannot be precisely
calculated.
-----------
Nature 2001 413:271
-----------
PRAXIS 3 Dec 2001 http://scienceweek.com/praxis
-----------
SCIENCE-WEEK 7 Dec 2001 http://scienceweek.com
-------------------------------
This week in PRAXIS (3 Dec 01):
-------------------------------
1. Anticircumvention Rules: A Threat to Science
2. On Childhood Sunburn and Adult Melanoma
3. Estrogen, Pregnancy, and Breast Cancer
4. Prevention of Pneumococcal Disease in Children
5. Mortality Among Patients Admitted to Hospitals on Weekends
6. On Differences Among Hospital Death Rates
7. On Cutaneous Anthrax
8. Inhibition of Prostate Carcinogenesis by Green Tea Polyphenols
9. On Near-Field Scanning Optical Microscopy
10. Dendrimers in Langmuir Films
11. On Self-Assembly of Chemical Building Blocks
12. On Semiconductor Thermocouples
For information about PRAXIS, see:
http://www.scienceweek.com/praxis
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