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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.
March 10, 2000 -- Vol. 4 Number 10
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I would live to study, and not study to live.
--- Francis Bacon (1561-1626)
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Contents of This Issue:
1. Astronomy:
On the History of Planetary Science
Planetary science remains one of the most dynamic and important
disciplines, a field whose greatest discoveries have had enormous
impact on the human intellect.
2. Astrophysics:
Discovery of a Planet Orbiting a Binary Star System
An international team reports both the first discovery of a
Jupiter-size planet via gravitational microlensing and the first
discovery of a planet orbiting a binary star system. (Includes
related background material.)
3. Applied Physics:
Nanotube Nanotweezers
A nanoscale electromechanical system, based on carbon nanotubes,
has been developed for the manipulation and investigation of
nanoscale structures. The advantage of the system is that it can
grab and reposition nanoscale objects, and make two-probe
electrical measurements on such objects.
4. Molecular Biology:
DNA Loss as a Determinant of Genome Size
One of the riddles of molecular biology is the wide variation in
genome size within phyla (the C-value paradox). New observations
suggest these differences may result from variation in the rate
of spontaneous loss of nonessential DNA. (Includes related
background material.)
5. Neurobiology:
Long-Term Potentiation and the Formation of Synapses
A new observational method reveals that long-term potentiation
may be associated with the formation of new synapses between
neurons that duplicate already activated synapses. (Includes
extensive related background material.)
6. Science Policy:
On Public Health Disparities
Although immense gains in public health were made during the 20th
century, the distribution of these gains has been far from
uniform both globally and in the US. Overall, disparities in life
expectancy between different parts of the US are greater than for
any other nation in the world, and the reasons for this are not
clear.
In Focus: On Reality
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1. ASTRONOMY:
ON THE HISTORY OF PLANETARY SCIENCE
The first great epoch of discovery in planetary science began in
the 16th century with the startling proposal by Copernicus of
heliocentrism. The second great epoch of discovery began only 50
years ago with the advent of the Space Age.
... ... David J. Stevenson (California Institute of Technology,
US) reviews the history of planetary science, the review noting
the following developments:
Ancient times: Observers notice that although most of the
stars seen in the sky at night move en masse, certain stars
appear to wander. These stars were called "the wanderers", which
yields from the Greek the term "planets".
1519: Ferdinand Magellan circumnavigates the globe.
1543: In his _De Revolutionibus Orbium Coelestium_, Nicolaus
Copernicus proposes that Earth and the other planets travel
around the Sun.
1600: The Inquisition burns Giordano Bruno at the stake,
perhaps partly for his belief that the Earth revolves around the
Sun, and partly for his belief in an infinite number of inhabited
worlds.
1608: Hans Lippershey invents the telescope.
1609: Johannes Kepler publishes _Astronomia Nova_ containing
the first two laws of planetary motion.
1610: Galileo Galilei discovers the moons of Jupiter, the
"handles" (rings) of Saturn, the phases of Venus, and sunspots.
1671: Jean Dominique Cassini calculates distances from the
Sun of all planets then known.
1687: Isaac Newton publishes the _Principia Mathematica_,
establishing the three laws of motion and the law of universal
gravitation.
1755: Immanuel Kant proposes the Solar System arose from a
vast nebula of material.
1781: William Herschel discovers the planet Uranus.
1795: James Hutton publishes _Theory of the Earth_, in which
he argues that all apparent geological features emerge from
observable changes unfolding over great expanses of time. The
theory is called "uniformitarianism".
1797: James Hall demonstrates that igneous rock forms
crystalline rock upon cooling.
1798: Henry Cavendish determines the mass of the Earth as
6.6 x 10^(21) tons.
1830: Charles Lyell publishes the first volume of his
uniformitarianism work _The Principles of Geology_.
1837: Louis Agassiz proposes the idea of an ice age, that at
one time glaciers covered Europe.
1846: Johann Galle discovers the planet Neptune, the
discovery based on earlier calculated predictions by others.
1857: James Clerk Maxwell demonstrates theoretically that
the rings of Saturn consist of small particles that do not
coalesce.
1859: Gustav Kirchhoff and Robert Bunsen introduce
spectroscopy to chemistry and use it to infer the chemistry of
the Sun.
1907: Bertram Boltwood combines information on the half-life
of uranium and the proportion of lead found within uranium
deposits to estimate the age of the Earth at 2.2 billion years.
1912: Alfred Wegener proposes the idea of "continental
drift".
1919: Joseph Larmor develops the idea of self-exciting
dynamos inside the Earth and the Sun to account for their
magnetic fields.
1930: Clyde Tombaugh discovers the planet Pluto.
1931: Harold Urey deduces that hydrogen probably has
isotopes, and then discovers deuterium with a spectroscopic
technique that becomes important for cosmochemical studies.
1937: Grote Reber constructs the first radio telescope (9.4
meters in diameter).
1950: Jan Oort suggests that a distant shell of comets
surrounds the Solar System.
1958: James van Allen demonstrates the value of satellite-
based studies when he uses data from a particle counter on
Explorer IV to discover Earth's magnetosphere.
1960s: The US and the Soviet Union begin an epoch of
planetary exploration using satellites that eventually reaches
every object in the Solar System larger than the Moon.
1969: Human beings land on the Moon.
1973: First images of Jupiter transmitted from close
vicinity by Pioneer 10.
1974: First images of Mercury transmitted from close
vicinity by Mariner 10.
1976: Viking space probes land on Mars.
1979: Voyagers 1 and 2 relay flyby data from Jupiter and
some of its moons, and from Saturn, Uranus, Neptune, and some of
their moons.
1986: Space shuttle Challenger explodes soon after launch,
killing all 7 crew members.
1990: Hubble Space Telescope is placed in orbit, and soon
begins to produce "a relentless stream of spectacular imagery."
1992: Alexander Wolszczan and Dale Frail discover two Earth-
sized planets orbiting a pulsar.
1994: Comet Shoemaker-Levy crashes into Jupiter.
1995: Michel Mayor and Didier Queloz discover the first
planet around a Sun-like star -- 51 Pegasi. An era of extrasolar
planetary discovery begins.
1999: Evidence for extrasolar planets accumulates.
The author suggests that the future of planetary science is
likely to emerge from three intertwined trends: 1) the search for
extrasolar planets; 2) the search for life elsewhere, and for the
origin of life; and 3) the search for a fully integrated view of
planets in general and our planet in particular.
-----------
David J. Stevenson: Planetary science: A space odyssey.
(Science 11 Feb 00 287:997)
QY: David J. Stevenson, California Institute of Technology 818-
395-6811.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 10Mar00
[For more information: http://scienceweek.com/search/search.htm]
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2. ASTROPHYSICS:
DISCOVERY OF A PLANET ORBITING A BINARY STAR SYSTEM
Since the discovery by M. Mayor and D. Queloz in 1995 of a
planet orbiting the star 51 Pegasi, approximately 20 other
planets have been detected through their influence on the radial
velocities of lines in the spectra of their parent stars. The
orbital motion of the planet can be detected by perturbations of
the motion of the parent star ("reflex motion"), and these
perturbations can be measured using high-precision spectroscopy.
The currently observed properties of extrasolar planets were not
anticipated by theoretical work on the formation of planetary
systems, primarily because such theories were based on our own
Solar System, and current techniques are not sensitive enough to
detect planetary systems like our own.
Binary stars are a pair of stars revolving around a common
center of mass under the influence of their mutual gravitational
attraction, and apparently the majority of stars in the Universe
are binaries and not singlets. In some cases the binary system is
resolvable into two components, and in other cases the presence
of a second star is inferred by perturbations in the motion or
emitted radiation of the first star.
Gravitational lensing is the bending of light and other
radiation by a massive gravitational entity such as a star, a
black hole, a galaxy, or a cluster of galaxies. The effect is
predicted by Einstein's theory of relativity and was first
detected during a total solar eclipse by Eddington in 1919.
Large-scale gravitational lensing causes multiple images of an
object, the type and arrangement of the images determined by the
specifics of the lensing entity. Gravitational "microlensing" is
a small-scale lensing effect, the gravitational field of the
lensing object not strong enough to form distinct images of the
background source, but instead causing an apparent brightening of
the source.
... ... D.P. Bennett et al (15 authors at 6 installations, US IL
AU) now report observations and modeling of the gravitational
microlensing event known as MACHO-97-BLG-41. The authors conclude
the lens system consists of a planet of approximately 3 Jupiter-
masses orbiting a binary star system consisting of a *main-
sequence star *type K and a main-sequence star *type M. The stars
are separated by 1.8 AU (1 AU = Earth-Sun distance), and the
planet is orbiting the stars at a distance of approximately 7 AU.
The authors suggest that their apparent detection of a planet
orbiting a binary star system represents, to their knowledge,
both the first discovery of a Jupiter-size planet via
gravitational microlensing and the first discovery of a planet
orbiting a binary star system. The authors further suggest that
their present result indicates that Jupiter-size planets may be
more common in *short-period binary systems (such as the system
investigated by the authors) than in single star systems.
-----------
D.P. Bennett et al: Discovery of a planet orbiting a binary star
system from gravitational microlensing.
(Nature 4 Nov 99 402:57)
QY: D.P. Bennett [bennett@nd.edu]
-----------
Text Notes:
... ... *main-sequence star: The Hertzsprung-Russell diagram is a
plot of stellar absolute magnitude against spectral type, and is
perhaps the most useful diagrammatic aid in astrophysics. It
allows the portrayal of the evolution of a star as occurring
along various paths in the diagram. The "Main Sequence" is a
region on the Hertzsprung-Russell diagram where most stars lie,
including our own Sun. The evolution of a star can be diagrammed
as a movement along the Main Sequence and an eventual branching
off the Main Sequence to regions associated with various types of
old stars.
... ... *type K, *type M [stars]: These are stars showing two
different types of spectra, denoted according to a specific
spectral classification scheme currently used by astronomers. In
general, type K stars are cool orange stars, and type M stars are
the coolest red stars. (In this context, "cool" is approximately
4900 degrees kelvin, and "coolest" is 3000 to 3500 degrees
kelvin.)
... ... *short-period binary systems: See background material
below.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Mar00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON THE BIRTH OF BINARY STARS
... Alan P. Boss (Carnegie Institution of Washington, US),
presents a review of current research concerning the origin of
binary star systems, the author making the following points:
1) Astrophysicists have reached a point where both
observations and theory apparently yield the same answer: binary
star systems appear to form at the very same time that the stars
themselves form -- during the gravitational collapse of
*interstellar clouds. Thus, stars are paired from the outset, and
this has profound consequences for the long-term viability of any
planets created in these same systems.
2) Binary stars have been discovered with orbital periods
that range from a few hours for the closest pairs to millions of
years for the most distant and barely bound systems. Separations
thus vary from a few stellar radii to almost a light year, and
different observational methods are required to find binary stars
over this extended range.
3) Because of continuing advances in instrumentation,
extremely small companion objects -- either planets or *brown
dwarf stars -- can now be detected around *main-sequence stars.
Brown dwarfs are particularly difficult to detect, so many nearby
stars previously thought to be single stars may actually be
binary. There is indeed evidence that low mass objects accompany
approximately 10 percent of main-sequence stars.
4) There is no longer any doubt that binary *protostars are
prevalent and that pairing occur with approximately the same
frequency for old stars, clusters of young stars, and newly
formed stars. The formation of a binary system apparently occurs
prior to the protostar phase, during the gravitational
contraction of dense interstellar clouds. The culmination of this
process is a rapid collapse lasting approximately 100,000 years
or less.
5) The breakup or division that can occur during the
collapse of a cloud is called "fragmentation", and various models
of dense-cloud-core fragmentation have been designed. The success
of any of these theoretical models depends on an understanding of
the properties of dense interstellar clouds on the verge of
collapse. According to current views, such clouds rotate at
different rates, are centrally condensed, and have prolate
shapes. The structure of the cloud is determined in part by
magnetic fields and in part by pressure from hot gases. Magnetic
fields control the early phases of the contraction of a cloud,
but once the fields begin to leak out of the cloud by *ambipolar
diffusion, a rapid collapse follows. Given these initial
conditions, theoretical models of collapsing magnetic clouds
indicate that fragmentation into binary protostars is possible
provided that the systems rotate relatively fast, with a rotation
period of approximately 2 million years or less. Slower rotating
clouds lead only to single protostars. Approximately half of the
observed dense cloud cores rotate faster than the theoretical
critical rate, and so will eventually form binary protostars. In
addition, current models indicate that binary protostars formed
by fragmentation inevitably begin their lives in eccentric
orbits, and indeed observations confirm orbital eccentricities in
binary systems.
-----------
Alan P. Boss: The birth of binary stars.
(Sky & Telescope June 1999)
QY: Alan P. Boss [boss@dtm.ciw.edu]
-----------
Text Notes:
... ... *interstellar clouds: These are mainly hydrogen gas, some
helium, and approximately 1 percent dust grains. The temperatures
of these clouds vary, with hot clouds having temperatures of
10^(6) degrees kelvin.
... ... *brown dwarf stars: Brown dwarf stars are formed by the
contraction of a lump of gas with a mass too small for nuclear
reactions to begin in the core. Such a star has a relatively
short-lived luminosity (approximately 100 million years) as the
result of conversion of gravitational energy to radiation. The
surface temperature of a brown dwarf is below 2500 degrees
kelvin. As recently as 1994, brown dwarfs were only "theoretical"
stars, with no brown dwarfs considered to be unambiguously
identified.
... ... *main-sequence stars: See notes in main report above.
... ... *protostars: A star in the earliest phase of its life,
before the onset of nuclear burning. Protostars are visible at
infrared wavelengths, but not at visible wavelengths due to
obscuration by infalling material.
... ... *ambipolar diffusion: In general, this refers to a
diffusion regime in which more than one type of gradient drives
diffusion, e.g., the diffusion of charged particles. In a charged
particle diffusion regime, both particle density gradients and
electric charge density gradients interact and drive the
diffusion process. Interstellar clouds are mostly plasmas (i.e.,
ionized gases) exhibiting ambipolar diffusion.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 23Jul99
[For more information: http://scienceweek.com/search/search.htm]
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3. APPLIED PHYSICS:
NANOTUBE NANOTWEEZERS
The development of new tools for manipulating and probing matter
at nanoscale lengths is considered critical to advances in
nanoscale science and technology. *Scanning probe microscopes,
such as the scanning tunneling microscope and the atomic force
microscope, are now widely used for these purposes and have the
capability of working at length scales as small as a single atom.
However, the single probe tips used in scanning probe microscopy
limit the ability of these tools to manipulate objects and
measure physical properties. For example, one tip cannot grab an
object, and electrical measurements cannot be made without a
second contact to structures. Two probes in the form of tweezers
could overcome these limitations of scanning probe microscopes,
and thus might enable new types of fabrication and easy
electrical measurements on nanostructures.
... ... P. Kim and C.M. Lieber (Harvard University, US) now
report the development of nanoscale electromechanical systems --
nanotweezers -- based on carbon nanotubes, the systems to be used
for the manipulation and "interrogation" of nanostructures. The
authors report that electrically conducting and mechanically
robust carbon nanotubes were attached to independent electrodes
fabricated on pulled glass micropipettes. Voltages applied to the
electrodes closed and opened the free ends of the nanotubes, and
this electromechanical response was simulated quantitatively
using known nanotweezer structure and nanotube properties. The
mechanical capabilities of the nanotweezers were demonstrated by
grabbing and manipulating submicron clusters and nanowires. The
conducting nanotube arms of the tweezers were also used for
measuring the electrical properties of silicon carbide
nanoclusters and gallium arsenide nanowires. The authors
conclude: "At their current size, the nanotube nanotweezers also
open up exciting opportunities for manipulation and modification
of biological systems such as structures within a cell."
-----------
P. Kim and C.M. Lieber: Nanotube nanotweezers.
(Science 10 Dec 99 286:2148)
QY: Charles M. Lieber [cml@cmliris.harvard.edu]
-----------
Text Notes:
... ... *Scanning probe microscopes: 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.
... ... *tunneling: "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.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Mar00
[For more information: http://scienceweek.com/search/search.htm]
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4. MOLECULAR BIOLOGY:
DNA LOSS AS A DETERMINANT OF GENOME SIZE
One of the current riddles in molecular biology is the "C-value
Paradox", the essential of which are as follows: The total amount
of DNA in the (*haploid) genome is a characteristic of each
living species and is known as the "C-value". There is enormous
variation in the range of C-values, from less than 10^(6)
nucleotide base pairs to as much as 10^(11) nucleotide base pairs
for certain plants and amphibians. In general, there is an
increase in the minimum genome size found in each phylum as the
complexity of the phylum increases [*Note #1]. But along with
this general increase in the absolute amounts of DNA in the
higher phyla, a number of wide variations in genome size within
the same phylum occur. Birds, reptiles, and mammals all show
little variation of C-value within the phylum, with a range of
genome size in each case of approximately 2-fold, but in insects,
amphibians, and plants there is wide range of C-values, often
more than 10-fold. For example, the common housefly (Musca
domestica) has a genome 6 times larger than the common fruit fly
(Drosophila melanogaster). Within protozoa, the genome size
variation can be as large as 5800-fold; within arthropods 250-
fold; within fish 550-fold; within algae 5000-fold. In
amphibians, the smallest genomes are just below 10^(9) nucleotide
base pairs, while the largest genomes are almost 10^(11)
nucleotide base pairs, and it is difficult to believe this could
reflect a 100-fold difference in the number of genes needed to
specify different amphibians. Thus the problem: How do we account
for these differences within phyla?
... ... D.A. Petrov et al (5 authors at 3 installations, US) now
report a study of the C-value paradox, the authors making the
following points:
1) The Drosophila fruit fly species, which have relatively
small genomes, spontaneously lose DNA at a much higher rate than
mammalian species, which have large genomes. Although many
mechanisms can affect genome size, studies of Drosophila suggest
that some differences in haploid genome size may result from
variation in the rate of spontaneous loss of *nonessential DNA.
2) In connection with this hypothesis, the authors examined
the insertion-deletion mutation spectrum in Hawaiian crickets
(Laupala), which have a large genome size -- 11-fold larger than
that of Drosophila, in order to test the prediction of a lower
rate of DNA loss in Laupala than in Drosophila. The basis for the
method of the authors is that certain DNA sequences apparently
unconstrained by natural selection exhibit patterns of
substitution that reflect the underlying spectra of spontaneous
mutations. The authors report their results indicate that
consistent with their hypothesis, DNA loss is more than 40 times
slower in Laupala than in Drosophila, with a higher rate of DNA
loss resulting in a lower steady-state number of *pseudogenes in
relatively small genomes.
3) The authors suggest that the key question that remains is
empirical and quantitative: How much of the variation in genome
size can be explained by variation in the insertion-deletion
spectra? The authors suggest their method can be used to answer
this question in a wide variety of eukaryotes and "thus to test
the mutational hypothesis for the C-value paradox in a
comprehensive fashion."
-----------
D.A. Petrov et al: Evidence for DNA loss as a determinant of
genome size.
(Science 11 Feb 00 287:1060)
QY: Dmitri A. Petrov [dpetrov@oeb.harvard.edu]
-----------
Text Notes:
... ... *haploid: In general, germ cells (egg cells and sperm
cells) and somatic cells (non-germ cells) carry different numbers
of chromosomes, with germ cells carrying exactly half the number
(haploid number) of somatic cell chromosomes (diploid number).
... ... *Note #1: There are important exceptions to the general
rule that genome size increases with complexity of the phylum.
Many species of plants, for example, have a genome 2 orders of
magnitude larger than the human genome.
... ... *nonessential DNA: (junk DNA; selfish DNA) In eukaryotes
(cells with a nucleus), the bulk of nuclear DNA is apparently
noncoding, i.e., it does not get translated into protein
polypeptide chains, and it does not code for RNA used in cell
function. This sort of DNA, of unknown function, is
called "junk DNA", and much of it is highly repetitive and
present in many copies scattered or clustered in the chromosomes.
... ... insertion-deletion mutation spectrum: In this context,
the insertions and deletions are random insertions and deletions
of nucleotide bases in the genome.
... ... *pseudogenes: The term "pseudogene" refers to a gene
bearing close resemblance to a known gene at a different locus,
but rendered nonfunctional by additions or deletions in its
structure that prevent normal expression of the gene.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Mar00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON DNA MICROSATELLITES
In general, "satellite DNA" is a type of DNA consisting mostly of
repeated sequences not transcribed into RNA. In an operational
sense, satellite DNA was originally any fraction of DNA that
differs sufficiently in its base composition from that of the
majority of the DNA fragments that separate into bands during
*cesium chloride gradient centrifugation. The satellite DNA
fraction produces a small "satellite" peak on the shoulder of the
main DNA distribution peak, and thus the terminology. Satellite
DNA is now defined as arrays of tandemly repeated sequences, in
some cases involving hundreds of nucleotide bases. In the human
genome, satellite DNA apparently comprises as much as 30 percent
of genomic DNA. "Minisatellite DNA" has been defined as arrays of
tandemly repeated sequences of at least 10 base pairs
interspersed with genomic DNA. "Microsatellite DNA" has been
defined as tandem arrays of mono-, di-, tri-, and
tetranucleotides interspersed with other genomic DNA. In
*eukaryotes, the long-repeat-sequence satellite DNA tends to be
localized near the *chromosome centromeres; minisatellite DNA
tends to be localized at chromosome ends; microsatellite DNA is
in general randomly dispersed in the genome. Satellite,
minisatellite, and microsatellite DNA have all been categorized
as part of the larger rubric of "junk DNA" -- apparently
functionless segments of DNA that are replicated along with the
rest of the genome. ... ... E.R. Moxon and C. Wills (2
installations, UK US) present a review of recent work on
microsatellite DNA, with a focus on the evidence that
microsatellite DNA may be of considerable importance for the
adaptation of organisms to changing environments. The authors
make the following points: 1) Of the approximately 3 billion
nucleotide bases in the human genome, only 10 to 15 percent
comprise genes. Some of the non-gene base sequences have apparent
control and structural functions, but most non-gene DNA has no
apparent purpose ("junk DNA"). 2) During the past several years,
there has been evidence that microsatellite DNA, at least in
bacteria, may be involved in a variety of important functions. In
certain disease-causing bacteria, for example, the microsatellite
DNA repeat sequences promote the emergence of new properties that
can enable the microbes to survive potentially lethal changes in
the environment. This role of microsatellite DNA in bacteria is
apparently dependent on the promotion of enormous evolutionary
diversity by the repeat sequences. The key is that the
microsatellite DNA repeats are especially prone to DNA-
replication errors, resulting in mutated genes that in turn
produce new proteins. In general, such diversity ensures that at
least one bacterium in a population can survive the immune system
of its host (or other defenses) and then can replicate to produce
a new and thriving colony. 3) In humans, the only function so far
assigned to microsatellite DNA is negative: microsatellite DNA
has been related to a variety of neurological diseases. There is
also evidence that microsatellite DNA sequences change in length
early in the development of some cancers, making the repeat
sequences useful markers for early cancer detection. Also in
humans, because the length of microsatellite DNA sequences may
vary from one individual to the next, they are used in the
identification procedure known as DNA profiling or DNA
"fingerprinting". 4) In general, what makes microsatellite DNA so
important for evolution is its extremely high mutation rate:
microsatellite DNA is 10^(4) times more likely to gain or lose a
repeat sequence from one generation to the next than an ordinary
gene is to undergo the single-base mutation that may produce a
disease. In addition, although it is quite rare for a single-base
mutation to mutate back again to its benign state, microsatellite
DNA repeat sequences can apparently readily return to their
former lengths, often within a few generations.
-----------
E.R. Moxon and C. Wills: DNA microsatellites: Agents of
Evolution?
(Scientific American January 1999)
QY: E. Richard Moxon, University of Oxford, UK
-----------
Text Notes:
... ... *cesium chloride gradient centrifugation: The salt cesium
chloride forms dense solutions in water, and so is used in
"isopycnic centrifugation" to separate DNA molecules of different
densities. Isopycnic centrifugation (band centrifugation) is a
separation technique in which macromolecules are subjected to a
centrifugal field in a solvent containing a density gradient. In
the solvent column, the macromolecules arrange themselves in
bands (isopycnic bands) where the density of the solvent is the
same as that of the particular macromolecule.
... ... *eukaryotes: Eukaryotes are cells (or organisms
consisting of such cells) with internal membrane-bound organelles
such as a nucleus.
... ... *chromosome centromeres: In cells with chromosomes, the
chromosomes are the physical structures into which DNA is
organized and on which genes are carried. The "centromere" is a
region of the chromosome to which traction fibers are attached
during replication.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Feb99
[For more information: http://scienceweek.com/search/search.htm]
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5. NEUROBIOLOGY:
LONG-TERM POTENTIATION AND THE FORMATION OF SYNAPSES
The various functions of the brains of animals and humans depend
on connections between neurons. The archetype of such a
connection consists of a long extension (axon) of one neuron, an
extension which propagates electrical activity (action
potentials), making contact at its terminal with a receiving
extension (dendrite) of a second neuron, the junction itself
called a "synapse". The general consensus view in neurobiology
for many decades has been that memory and learning depend on the
selective formation of new synapses and the selective
strengthening of already existing synapses. In general, the term
"long-term potentiation" (LTP) refers to an experimentally
observed persistent strengthening (facilitation of activity) of
synapses based on past patterns of activity, the strengthening
lasting hours or days or weeks (see background material below).
... ... N. Toni et al (5 authors at 2 installations, CH) now
report the use of electron microscopy to analyze the morphology
of synapses activated by high frequency electrical stimulation
and identified by accumulated calcium in dendritic
microprojections (dendritic spines). The authors report that 3-
dimensional reconstruction revealed that these spines arose from
the same dendrite. Experiments were performed on rat brain
(*hippocampus) slices maintained for 12 days in culture. Since
pharmacological blockade of long-term potentiation prevented
these morphological changes, the authors conclude that long-term
potentiation is associated with the formation of new, mature, and
probably functional synapses contacting the same presynaptic
terminal and thereby duplicating already activated synapses. The
authors further suggest that this mechanism of synapse
duplication "maintains specificity in information processing and
provides a framework for understanding the numerous in vivo
experiments that have demonstrated an increase in synapse number
and complexity of dendrite arborization following exposure to an
enriched environment or learning paradigms."
-----------
N. Toni et al: LTP promotes formation of multiple spine synapses
between a single axon terminal and a dendrite.
(Nature 25 Nov 99 402: 421)
QY: Dominique Muller [Dominique.Muller@medicine.unige.ch]
-----------
Text Notes:
... ... *hippocampus: A brain cortex structure in the medial part
of the temporal lobe. In humans, among other functions, the
hippocampus is apparently involved in short-term memory. Analysis
of the neurological correlates of learning behavior in the rat
indicates that the hippocampus is also involved in memory in that
species (see background material below).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Mar00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
NEUROBIOLOGY: A DECADE OF RESEARCH ON LONG-TERM POTENTIATION
One of the chief characteristics of the nervous systems of humans
and animals is the ability to change both structure and function
in response to new conditions. One aspect of this, most evident
in the human species, is learning and memory behavior. At the
present time, the consensus among neurobiologists is that the
cellular basis of learning and memory is a set of events
occurring at the junctions (*synapses) between nerve cells, and
in recent years a major focus of research in this area has been
the experimental phenomenon of "long-term potentiation" (LTP).
The mechanism of long-term potentiation is believed by many
researchers to provide the basis for memories or learned
behaviors that persist for weeks, months, or years. Long-term
potentiation was discovered in the early 1970s, when T. Bliss and
his colleagues observed that a few seconds of high-frequency
electrical stimulation of a *nerve fiber pathway in the rabbit
*hippocampus enhances synaptic transmission between the
stimulated axons and postsynaptic cells, and that this change
persists for weeks. Although long-term potentiation was first
observed in intact experimental animals, intensive subsequent
investigations have relied on *in vitro brain slice preparations.
It was not until the mid-1980s that the molecular basis of long-
term potentiation began to be delineated, a number of
laboratories demonstrating that the unique properties of a
specific type of cell-membrane glutamate receptor, the NMDA
receptor, were involved in the phenomenon. This receptor is named
for the *neurotransmitter glutamate analogue, N-methyl-D-
aspartate, that selectively activates it.
... ... R.C. Malenka and R.A. Nicoll (2 installations, US)
present a review on research during the past decade on long-term
potentiation, the authors making the following points:
1) Despite the enormous interest in long-term potentiation,
it has proven difficult to elucidate the detailed cellular and
molecular changes responsible for the phenomenon. In fact, for
over a decade, there has been a vigorous and highly visible
debate about whether the changes that occur soon after the
generation of long-term potentiation occur on the presynaptic or
postsynaptic side of the synapse.
2) The fact that long-term potentiation can be most reliably
generated in brain regions demonstrated to be involved in
learning and memory is often used as evidence for its functional
relevance. The authors suggest, however, that long-term
potentiation is a fundamental property of the majority of
*excitatory synapses in the mammalian brain, and that it is
likely to subserve many functions in addition to functions
underlying some forms of learning and memory.
3) It is now well accepted that the triggering of long-term
potentiation requires synaptic activation of postsynaptic NMDA
receptors. This activation apparently requires depolarization of
the postsynaptic cell, which is usually accomplished
experimentally by repetitive *tetanic stimulation of synapses or
by directly depolarizing the cell with electrical input. The
consensus view is that when the postsynaptic cell is depolarized
during the induction of long-term potentiation, magnesium ions
dissociate from binding sites within NMDA receptor channels,
allowing calcium ions as well as sodium ions to enter the
postsynaptic cell. This consequent rise of intracellular calcium
ion concentration is the apparent critical trigger for long-term
potentiation, and this local source of calcium ions accounts for
the input specificity of long-term potentiation. In the context
of a *dendritic tree, association occurs because strong
activation of one set of synapses depolarizes adjacent regions of
the tree.
4) No question concerning long-term potentiation has
generated more debate and confusion over the last two decades
than the seemingly simple question of whether the increase in
synaptic strength is due primarily to a presynaptic or a
postsynaptic modification. Great technical difficulties are
inherent in examining the changes at individual synapses embedded
in a network in which each individual neuron receives
approximately 10,000 to 30,000 synapses. Most neurobiologists
agree that the simplest postsynaptic change that could cause
long-term potentiation would be a modification in postsynaptic
receptor function or number or both, whereas the simplest
presynaptic change would be an increase in the probability of
neurotransmitter release.
5) The authors suggest that the evidence is now sufficiently
strong to indicate that the initial increase in synaptic strength
during long-term potentiation involves postsynaptic modifications
of receptor function and localization. However, this conclusion
does not preclude the occurrence of substantial presynaptic
changes. The authors state: "The synapse is a structural unit
and, as do many in the field, we would predict that if long-
lasting synaptic modifications are in fact a mechanism by which
experiences are translated into memories, then [both] pre- and
postsynaptic structural alterations are likely to occur."
-----------
R.C. Malenka and R.A. Nicoll: Long-term potentiation: A decade of
progress.
(Science 17 Sep 99 285:1870)
QY: Robert C. Malenka [malenka@stanford.edu]
-----------
Text Notes:
... ... *synapses: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. The term "synapse" refers to the
junction between the terminal of a neuron's axon and another
neuron. When studying the synapse, the first neuron is called the
"presynaptic" neuron, and the second neuron is called the
"postsynaptic" neuron.
... ... *nerve fiber pathway: In the context of the central
nervous system (e.g., the brain), a "nerve fiber pathway" is a
collection of nerve fibers (axons) all running in the same
direction.
... ... *hippocampus: The hippocampus is a deep region of the
brain involved with many fundamental activities, including memory
storage.
... ... *in vitro brain slice preparations: "Brain slices" are
exactly that, the brain removed from the animal and a thin slice
of a particular region prepared in an appropriate solution for
electrophysiological recording of nerve cell activity.
... ... *neurotransmitter: The term "neurotransmission" refers
to all the events at a synapse, particularly the release of
"neurotransmitters" and their action on the postsynaptic neuron.
Neurotransmitters are chemical substances released at the
terminals of nerve axons in response to the propagation of an
impulse to the end of that axon. The neurotransmitter substance
diffuses into the synapse, the junction between the presynaptic
nerve ending and the postsynaptic neuron, and at the membrane of
the postsynaptic neuron the transmitter substance interacts with
a receptor. Depending on the type of receptor, the result may be
an excitatory or an inhibitory effect on the postsynaptic nerve
cell.
... ... *excitatory synapses: A synapse which when activated
produces excitation of the postsynaptic nerve cell.
... ... *tetanic stimulation: The term "tetanic stimulation"
refers to repetitive stimulation. A "tetanus" is a sustained
muscular contraction caused by a rapid and repeated nerve
stimulation. Tetanus is also a disease marked by such muscle
contractions, the nerve activity produced by a neurotoxin.
... ... *dendritic tree: Dendrites are highly branched extensions
of certain types of nerve cells, the extensions literally covered
with synapses with thousands of other neurons. In certain
neurons, when appropriately stained for microscopic observation,
the dendritic region of the nerve cell has the generally
morphology of a "tree", a dense arborization of dendrites, with
thousands of branches, the volume of the tree orders of magnitude
greater than the volume of the mother neuron. The classic
paradigm is that dendrites receive and integrate input, which
then produces output in the single axon of the neuron. Nerve
cells, however, are extremely varied in morphology and behavior,
and the classic paradigm is often not applicable.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ACTIVITY-INDUCED POTENTIATION OF DEVELOPING SYNAPSES
In general, the nervous system of an animal consists of a large
number of active entities (neurons) receiving input from a
variety of sensory cells, the active entities integrating and
performing various operations on this input via interactions
between the entities, and the entities ultimately producing an
output that controls groups of "effector cells" (e.g., muscle
cells). Of critical importance in this scheme are the junctions
between neurons, between receptor cells and neurons, and between
neurons and effector cells, the so-called "synapses". For many
decades now, the consensus view among neurobiologists has been
that the psychological functions that we call "learning and
memory" are somehow dependent on short-term or long-term changes
in these synapses, but progress in elucidating precisely what is
occurring at synapses that relates to learning and memory has
been difficult and frustrating. At the present time, two key
experimental concepts at the cellular level in this field are
"long-term potentiation" (LTP) and "long-term depression" (LTD).
Long-term potentiation is an experimentally observed persistent
strengthening (facilitation of activity) of synapses based on
past patterns of activity, the strengthening lasting hours or
days or weeks. Long-term depression is the converse, a persistent
weakening (damping of activity) of synapses based on past
patterns of activity, the weakening also lasting hours or days or
weeks. These are experimentally observed phenomena in a wide
variety of nervous systems, both relatively simple and complex,
and the central questions are what are the cellular controls, the
molecular controls, and the relevance of these phenomena to
learning and memory behaviors?
... ... J. Wan and M. Poo (University of California San Diego,
US) now report that in African clawed toad (Xenopus) nerve-muscle
cultures, a brief burst of *action potentials in the presynaptic
neuron induced a persistent potentiation of neuromuscular
synapses that exhibit immature synaptic functions. Until now,
long-term potentiation has not been reported in any neuromuscular
system, although long-term depression in such systems is well
known. In the experiments of the authors, induction of
potentiation required an elevation of *postsynaptic calcium ion
concentration and expression of potentiation appeared to involve
an increased probability of *transmitter secretion from the
*presynaptic terminal. The authors suggest that activity-
dependent persistent synaptic enhancement may reflect properties
characteristic of immature synaptic connections, and that
bursting activity in developing spinal neurons may promote
functional maturation of the neuromuscular synapse.
-----------
J. Wan and M. Poo: Activity-induced potentiation of developing
neuromuscular synapses.
(Science 10 Sep 99 285:1725)
QY: Mu-ming Poo [mpoo@ucsd.edu]
-----------
Text Notes:
... ... *action potentials: (nerve impulses) In general,
transient pulses (e.g., 1 millisecond) of reversed membrane
potential propagated over the long extensions of neurons (axons),
in some cases over relatively large distances (e.g., 1 meter
between spinal motorneurons and peripheral muscle cells). The
physical characteristics of the action potentials in the nervous
systems of diverse animal forms are often quite similar. Also
often similar across diverse animal forms are events at various
synapses. The nervous system of the African clawed toad (Xenopus
laevis) has been a common laboratory model in neurophysiology for
the past half century.
... ... *postsynaptic: Considering a 2-unit junction (synapse)
between two cellular entities, entity A transmitting activity to
entity B, events on the entity-B side of the junction are called
"postsynaptic", and events on the entity-A side of the junction
are called "presynaptic".
... ... *transmitter secretion: (see main report)
... ... *presynaptic terminal: See above: *postsynaptic.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 29Oct99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
TARGET-SPECIFIC PRESYNAPTIC PLASTICITY IN NEURONS
The term "mossy fibers" refers to a type of nerve fiber in the
brain with large axon terminal endings, and their synapses
(connections to other nerve cells) are among the largest in the
mammalian central nervous system... The term "interneurons"
refers to nerve cells localized in a succinct region (population
of nerve cells), with the primary function of these nerve cells
an involvement with information processing in the region, rather
than with input to the region or output from the region:
interneurons are therefore involved in "local circuitry"... In
this context, the term "depression" refers to a long-term
reduction in efficacy of synaptic transmission. Cyclic adenosine
monophosphate (cAMP) is an important postsynaptic intracellular
substance activated by incoming synaptic activity, a "messenger"
involved in various aspects of cell regulation and protein
synthesis. In general, the term "afferent pathway" refers to any
input pathway, as opposed to an output pathway (efferent).
... ... Maccaferri et al (3 authors at National Institutes of
Health, US) report a comparison of mossy fiber synaptic
transmission at hippocampal pyramidal cells and interneurons in
rat brain slices, finding that tetanic stimulation of mossy
fibers induces long-term potentiation in pyramidal neurons, but
is either without effect or induces depression at synapses with
interneurons. Furthermore, unlike transmission onto pyramidal
neurons, transmission onto interneurons was not potentiated after
cAMP activation. The authors suggest their results indicate that
synaptic terminals arising from a common afferent pathway do not
behave as a single computational unit, but are functionally
specialized with effects depending on the postsynaptic target.
QY: Chris J. McBain [crismcb@codon.nih.gov]
(Science 27 Feb 98) (Science-Week 13 Mar 98)
-------------------
Related Background:
A NEW TYPE OF SYNAPTIC PLASTICITY OF NEOCORTICAL NEURONS
The term "synaptic plasticity" refers to a changeability of
synaptic connections and/or the efficacy of particular connect-
ions. "Cultured neurons" are embryological neurons separated from
the animal and growing and making connections in a suitable
experimental chamber. Glutamate is a major excitatory amino acid
neurotransmitter (transmitter substance at synapses) in the
brain, involved in about 40% of all brain activity. The term
"Hebbian modification" (named after the neuropsychologist Donald
Hebb) refers to the Hebbian "rule" that essentially states that
when one nerve cell repeatedly activates another nerve cell,
changes involving growth or metabolism occur in one or both nerve
cells that increase the efficiency of the activation.
... ... Turrigiano et al (5 authors at Brandeis University, US)
report a new form of synaptic plasticity in cultured neurons that
increases or decreases the strength of all of a neuron's synaptic
inputs as a function of activity, the changes partly due to
postsynaptic alterations in the response to glutamate. The
authors suggest that such "synaptic scaling" may help prevent
saturation of firing rates during developmental changes in the
number and strength of synaptic inputs, may stabilize synaptic
strengths during Hebbian modification, and may facilitate
competition between synapses and associated elimination of
synapses during development.
QY: Gina G. Turrigiano [turrigiano@binah.cc.brandeis.edu]
(Nature 26 Feb 98) (Science-Week 13 Mar 98)
-------------------
Related Background:
POSTSYNAPTIC MEMBRANE FUSION AND LONG-TERM POTENTIATION
The anatomical connections between nerve cells are called
"synapses", and in mammalian nervous systems the most important
type of connection involves the axon terminals of one neuron
(presynaptic endings) terminating on various parts of the second
neuron (postsynaptic loci). In general, electrical activity of
the first neuron causes secretion of a "neurotransmitter"
substance at its presynaptic terminals, and this neurotransmitter
may excite or inhibit the electrical activity of the second
neuron, the effect often mediated by inputs to the second neuron
from hundreds of other neurons. Prior to its release, the neuro-
transmitter substance is contained in vesicles in the presynaptic
terminals, and one of the significant events associated with
release of the neurotransmitter substance is the fusion of the
vesicle with the presynaptic membrane -- a "membrane" fusion,
since the shell of the vesicle itself is also a bilayer membrane.
In general, long-lasting enhancement of the postsynaptic response
of a neuron as a result of repetitive incoming synaptic activity
is called "long-term potentiation", and there are many varieties
of this phenomenon observed in different kinds of nerve cells,
and it is considered an example of synaptic plasticity. Since
long-term potentiation involves the behavior of a neuron relating
to its past history, the phenomenon is naturally of great
interest to neurobiologists seeking to understand neuronal
information storage. ... ... Lledo et al (5 authors at 2
installations, US) report that introducing substances that block
membrane fusion into the postsynaptic cell reduces long term
potentiation. Introducing a protein (SNAP) that promotes membrane
fusion enhances synaptic transmission, but at a reduced level in
already potentiated synapses. The authors suggest that fusion
events, in addition to being important for the presynaptic
release of neurotransmitters, are also involved in some mechanism
at the postsynaptic membrane, and thus contribute to long term
potentiation.
QY: Roger A. Nicoll [nicoll@phy.ucsf.edu]
(Science 16 Jan 98) (Science-Week 30 Jan 98)
[For more information: http://scienceweek.com/search/search.htm]
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. SCIENCE POLICY:
ON PUBLIC HEALTH DISPARITIES
Public health is best distinguished from clinical medicine by its
emphasis on preventing disease rather than curing it, and by the
focus of public health on populations and communities rather than
on the individual patient.
... ... Barry R. Bloom (Harvard School of Public Health, US)
presents a commentary on recent and current public health with a
focus on disparities both globally and in the US. The author
makes the following points:
1) Half of all the increases in life expectancy in recorded
history occurred within the 20th century, and most increases
occurred within the first half of the 20th century and before the
introduction of modern drugs and vaccines. The major gains in
health in the past century are attributable largely to the impact
of public health and disease prevention, rather than to medical
interventions.
2) In the past 20 years, deaths from heart attacks and
strokes in the US have dropped by 30 to 50 percent, in part as a
result of behavior changes, in part as a result of primary
prevention with medications. Smoking, estimated to be responsible
for approximately 20 percent of all deaths in the US, has
declined from 42 percent to 25 percent in adults over 30 years of
age (although it has increased to 36 percent among US teenagers
and is still rising). In addition, the use of the drug tamoxifen
has reduced the incidence of breast cancer by 45 percent in women
at high risk.
3) Perhaps the most dramatic effect of all is the impact of
immunization. Vaccines have eliminated smallpox from the world
and polio from the Northern Hemisphere, and have reduced the
incidence of measles, rubella, tetanus, diphtheria, and
meningitis in many countries to a handful of cases each year,
saving millions of lives and billions of dollars. Vaccines remain
the most cost-effective intervention known for preventing death
and disease. In 1999, for the first time, infectious diseases
were no longer the largest cause of death worldwide.
4) Unfortunately, the benefits of biomedical science and
public health are not uniformly distributed, and the disparities
are striking. Japan, with the highest life expectancy of 80
years, contrasts with Sierra Leone and its life expectancy of 37
years in 1998. What is most striking are the strong disparities
within single countries. It is generally believed that in
industrialized countries life expectancy is high for everyone and
that the major health issues center on the quality of life and
health care rather than on life expectancy. But the
contradictions to this idea are striking:
... ... a) People born in particular rural counties of Minnesota,
Colorado, Iowa, or Wisconsin on average will live 25 years longer
than those born in 4 counties of South Dakota, 23 years longer
than in 12 counties in Mississippi and Alabama, and 22 years
longer than people born in Washington, DC or Baltimore, MD.
... ... b) The variance in life expectancy in the US between
women of Japanese extraction in Bergen County, New Jersey, and
Bennett County, South Dakota is 41 years.
... ... c) Overall, disparities in life expectancy between
different parts of the US are greater than for any other nation
in the world, and the reasons for this are not clear. As in many
countries, the correlation in the US between per capita income
and life expectancy is not particularly good. For example, per
capita income is significantly higher in the county of
Washington, DC than in several counties along the Texas-Mexican
border, but life expectancy is significantly lower -- by
approximately 15 years -- in Washington, DC.
5) The author concludes: "We know that cardiovascular
disease, psychiatric disease, and physical injuries represent the
major global burdens of disease and disability in industrialized
and developing countries alike. The challenge for biomedical
science and public health in the coming century is to develop the
population-based interventions needed to reduce these burdens."
-----------
Barry R. Bloom: The future of public health.
(Nature 2 Dec 99 402supp:C63)
QY: Barry R. Bloom [bbloom@hsph.harvard.edu]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 10Mar00
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
IN FOCUS: ON REALITY
"Some of my cousins who had the great advantage of university
education used to tease me with arguments to prove that nothing
has any existence except what we think of it... These amusing
mental acrobatics are all right to play with. They are perfectly
harmless and perfectly useless... I always rested on the
following argument... We look up to the sky and see the sun. Our
eyes are dazzled and our senses record the fact. So here is this
great sun standing apparently on no better foundation than our
physical senses. But happily there is a method, apart altogether
from our physical senses, of testing the reality of the sun. It
is by mathematics. By means of prolonged processes of
mathematics, entirely separate from the senses, astronomers are
able to calculate when an eclipse will occur. They predict by
pure reason that a black spot will pass across the sun on a
certain day. You go and look, and your sense of sight immediately
tells you that their calculations are vindicated. So here you
have the evidence of the senses reinforced by the entirely
separate evidence of a vast independent process of mathematical
reasoning. We have taken what is called in military map-making "a
cross bearing"... When my metaphysical friends tell me that the
data on which the astronomers made their calculations, were
necessarily obtained originally through the evidence of the
senses, I say, "No." They might, in theory at any rate, be
obtained by automatic calculating-machines set in motion by the
light falling upon them without admixture of the human senses at
any stage. When it is persisted that we should have to be told
about the calculations and use our ears for that purpose, I reply
that the mathematical process has a reality and virtue in itself,
and that once discovered it constitutes a new and independent
factor. I am also at this point accustomed to reaffirm with
emphasis my conviction that the sun is real, and also that it is
hot--in fact hot as Hell, and that if the metaphysicians doubt it
they should go there and see."
-----------
Winston S. Churchill (1874-1965): _My Early Life_
(Fontana, London, 1972, pp 123-124.)
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