|
ScienceWeek
SCIENCE-WEEK - January 11, 2002 - Vol. 6 Number 2
An Email Research Digest Published Weekly Since 1997
-------------------------------------------
There is no national science just as there is
no national multiplication table; what is national
is no longer science.
-- Anton Chekhov (1860-1904)
-------------------------------------------
=-=-=-=-=-=-=-=-=
Section 1
=-=-=-=-=-=-=-=-=
Contents of this Issue (Full reports in Section 2):
1. A Cross-Bred Transparent Adult Fish
2. Mimicry in Bacterial Pathogens
3. Cellular Signaling and Protein Kinases
4. On RNA Silencing
5. The Vertebrate Circadian Clock System
6. Control of Sex Ratios in Ant Colonies
7. On the Asteroid Eros
8. Solid-Solid Organic Reactions
9. Long-Lived Entanglement of Macroscopic Objects
10. Catastrophe Theory and Potential Energy Surfaces
11. On the Physics of Friction
12. The Bragg Glass Phase
13. PostDoctoral Fellowship Profile:
Laboratory of David H. Gutmann, Washington University St. Louis
14. In Focus: On the Mathematician Arthur Cayley (1821-1895)
15. From PRAXIS: On Lead as an Environmental Pollutant
16. This Week in PRAXIS
=-=-=-=-=-=-=-=-=
Section 2
=-=-=-=-=-=-=-=-=
1. A CROSS-BRED TRANSPARENT ADULT FISH
Y. Wakamatsu et al (Nagoya University, JP) discuss a cross-
breeding-engineered transparent adult fish. The bodies of most
vertebrates are opaque, and internal organs are not visible from
the outside. This makes non-invasive studies of internal organs
difficult in vertebrate models. Small laboratory fish, such as
the medaka (Oryzias latipes) and the zebrafish (Brachydanio
rerio), are recognized as excellent vehicles for studies of the
embryonic development of vertebrates because of the transparency
of their bodies. However, this advantage is lost after the
hatching stage because of the development of pigment cells in the
skin, peritoneum, and some other tissues. Pigment cells in some
fish species, including medaka, are classified into four main
types based on their color: melanophores, iridophores,
leucophores, and xanthophores. Melanophores contain melanin in
melanosomes, the melanin-containing organelles, and show black or
brown color. Iridophores exhibit various structural colors and
iridescence through the reflection of light from the surface of
orderly distributed organelles called "reflecting platelets".
These organelles contain guanine as the main component, although
guanine is not a true pigment. Leucophores are considered an
extreme type of iridophore and show a structural white color.
Xanthophores are pale yellow to bright red cells containing
pteridines and carotenoids in pterinosomes and carotenoid
vesicles, respectively, in the cytoplasm. Approximately 50
natural color mutants of medaka are maintained in the laboratory
of the authors, and some of these color mutants exhibit
deficiency in pigmentation. By crossing selected mutants, the
authors report they genetically removed the pigment from the
entire body of the fish, and have thereby generated a transparent
fish, the "see-through medaka". In this fish stock, the main
internal organs are visible through the body wall of the living
adult. A transgenic variety containing green fluorescent protein
has also been produced. The authors suggest the see-through
medaka will provide an opportunity for non-invasive studies of
morphological and molecular events that occur in internal organs
in the later stages of life.
-----------
Proc. Natl. Acad. Sci. 2001 98:10046
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
2. MIMICRY IN BACTERIAL PATHOGENS
C.E. Stebbins and J.E. Galan (Yale University, US) discuss
bacterial mimicry. The pressures of survival have engendered a
wide spectrum of adaptations in organisms, and various organisms
have evolved sophisticated methods to exploit both the
surrounding environment and each other. An important mechanism
that frequently occurs in adaptation is that of mimicry: many
organisms both large and small have found a selective advantage
in imitating the appearance or function associated with an
otherwise distinct creature or aspect of the natural environment.
In many of these cases, organisms imitate or copy the appearance
of something else, either for the purpose of concealment (e.g.,
the African praying mantis and chameleons) or to send a message
(e.g., coloration in non-poisonous species to mimic poisonous
species). Some of the most interesting examples of mimicry,
however, may occur in the microbial world. Recent studies have
revealed that many bacterial pathogens mimic the function of host
proteins in order to manipulate host physiology and cellular
functions for the benefit of the microbe. This is in contrast
with strategies used by some pathogens that involve microbial
products with activities lacking clear counterparts in eukaryotic
cells. In particular, recent structural work has provided unique
insights into the mechanisms of host mimicry by bacterial
virulence factors. In some cases, these factors are homologous to
host proteins whose genes have been incorporated into the genome
of the pathogen and subverted for its benefit. In other cases,
convergent evolution has produced new effectors that have no
obvious relationship to host factors. However, although hidden at
the sequence level, determination of the crystal structures of
several bacterial factors and bacterial-host protein complexes
has revealed the presence of mimicry at the molecular level, and
examination of such factors is providing important insights into
the interplay between host and pathogen, into the mechanisms
underlying eukaryotic functional homologues, and into the nature
of the evolutionary dynamics shaping these complex ecologies.
-----------
Nature 2001 412:701
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
3. CELLULAR SIGNALING AND PROTEIN KINASES
X-L. Zhan et al (University of California Berkeley, US) discuss
cell signaling and protein kinases. Biological cells continuously
monitor changes in their extracellular and intracellular
environments and produce appropriate adaptive responses. Nearly
every aspect of cell life is controlled by signal transduction
processes, processes by which extracellular perturbations are
perceived, converted into intracellular signals, and conveyed to
effectors whose activities are necessary to generate cellular
responses. Many of these intracellular processes are controlled
by signal transduction pathways that regulate protein
phosphorylation. In particular, reversible tyrosine
phosphorylation is one of the fundamental mechanisms for
controlling cell proliferation, cell differentiation, and tissue
and organ development. The extent of tyrosine phosphorylation is
determined by the opposing actions of protein tyrosine kinases
and protein tyrosine phosphatases. During the past two decades,
protein kinases have become the focus of intense interest as
tyrosine phosphorylation has emerged as a central theme in the
field of signal transduction. In the past, in research
strategies, studies of tyrosine dephosphorylation were relegated
to a "back seat" until the first protein tyrosine phosphatase was
biochemically characterized. However, in the past decade, much
progress has been made toward understanding the catalytic
mechanism, cellular regulation, and physiological functions of
protein tyrosine phosphatases, and a large number of non-receptor
tyrosine phosphatases have now been demonstrated to specifically
regulate mitogen-activated protein kinase signaling.
-----------
Chem Revs. 2001 101:2477
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. ON RNA SILENCING
M. Matzke et al (Austrian Academy of Sciences, AT) discuss gene
silencing. RNA silencing is a new field of research that has
coalesced during the last decade from independent studies on
various organisms. Researchers who study plants and fungi have
known since the late 1980s that interactions between homologous
DNA and/or RNA sequences can silence genes and induce DNA
methylation. The discovery of RNA interference (RNAi) in
Caenorhabditis elegans in 1998 focused attention on double-
stranded RNA as an elicitor of gene silencing, and indeed many
gene silencing effects in plants are now known to be mediated by
double-stranded RNA. RNA interference is usually described as a
post-transcriptional gene-silencing phenomenon in which double-
stranded RNA triggers degradation of homologous messenger RNA
(mRNA) in the cytoplasm. However, the potential for nuclear
double-stranded RNA to enter a pathway leading to epigenetic
modifications of homologous DNA sequences and silencing at the
transcriptional level -- that potential does exist. Although the
nuclear aspects of RNA silencing have been studied primarily in
plants, there are hints that similar RNA-directed DNA or
chromatin modifications might occur in other organisms as well.
In general, RNA silencing is proving to be useful for the study
of functional genomics in invertebrates and plants, but routine
RNA interference might also be possible in mammalian cells, where
double-stranded RNA normally elicits a global shutdown of protein
synthesis, by direct injection of small interfering RNAs.
-----------
Science 2001 293:1080
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. ON THE VERTEBRATE CIRCADIAN CLOCK SYSTEM
M.P. Pando et al (Louis Pasteur University Strasbourg, FR)
discuss the vertebrate circadian clock system. In recent years a
new dimension has been added to our knowledge of the vertebrate
circadian clock system. The classical view of the circadian
system describes it as diverse physiological rhythms regulated by
a centralized clock structure. However, during the past few
years, the idea that the clock consists exclusively of a few
centralized structures has been challenged. Data coming from both
vertebrate and invertebrate systems have demonstrated that the
circadian timing system is dispersed throughout the animal, and
that possibly every cell contains a functional circadian clock.
In these studies, it has been revealed that a variety of tissues
and cells contain functional autonomous clocks, and these clocks
are able to maintain an oscillation when placed in vitro and
removed from any external cues or signals that originate from the
classical clock structures and/or the environment. The discovery
of a number of genes involved in the generation and maintenance
of circadian oscillations, and the recent realization that the
circadian system consists of a complex network of independent
clocks that are somehow synchronized to properly regulate all
physiological rhythms has necessitated the development of new
tools and methodologies for deciphering the circadian system. An
ideal tool is an in vitro cell-based system that displays robust
circadian rhythms: cultured cells may be used to fully understand
the complex molecular mechanisms, signal coupling, and regulatory
feedback loops that are responsible for the proper timing of a
circadian oscillation.
-----------
Proc. Natl. Acad. Sci. 2001 98:10178
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
----------
Related Background:
PHYSIOLOGY: ON BIOLOGICAL CLOCKS
In biology, a "circadian rhythm" is a daily cyclical process, be
it biochemical, or physiological, or behavioral. The human sleep-
wake cycle is the most familiar example. Circadian rhythms are
often described in terms of endogenous "biological clocks", with
the thrust of research to reduce some particular behavioral or
physiological circadian rhythm to biochemical events. These
clocks are usually set by environmental cues such as the light-
dark cycle, and what is characteristic of an endogenous clock is
that if one removes the environmental cue, keeps the organism in
constant light, for example, the endogenous rhythm will continue,
but will tend to drift out of phase with the outdoors
environmental light-dark cycle. Restoring the external light-dark
cue will reset the clock to its normal intrinsic rhythm.
... ... Michael W. Young (Rockefeller University, US) presents a
review of current research on circadian rhythms, the author
making the following points:
1) In mammals, the master clock that dictates the day-night
cycle of activity known as circadian rhythm resides in a part of
the brain called the "suprachiasmatic nucleus". But cells
elsewhere also show clock activity.
2) Within individual cells of the suprachiasmatic nucleus,
specialized clock genes are switched on and off by the proteins
they encode in a feedback loop that has a 24-hour rhythm. The
molecular rhythms of clock-gene activity do not depend on the
normal 24-hour cycle of light and darkness: these rhythms are
innate and self-sustaining, and they persist in the absence of
environmental cycles of day and night.
3) Bright light absorbed by the retina during the day helps
to synchronize the rhythms of activity of the clock genes to the
prevailing environmental cycle. Exposure to bright light at night
resets circadian rhythms by changing the concentrations of
certain clock-gene products. The fluctuating proteins synthesized
by clock genes control additional genetic pathways that connect
the molecular clock to timed changes in physiology and behavior.
4) The author concludes: "An exciting prospect for the
future involves the recovery of an entire system of clock-
regulated genes in organisms such as fruit flies and mice. It is
likely that previously uncharacterized gene products with
intriguing effects on behavior will be discovered within these
networks."
-----------
Scientific American March 2000
-----------
SCIENCE-WEEK 2000 19 May
-----------
Related Background:
CIRCADIAN OSCILLATORS: RESETTING BY TEMPERATURE CHANGES
Many organisms exhibit daily (circadian) rhythms, cyclical
variations in various bodily functions, metabolisms, etc., the
variations having considerable endurance even in constant light
or constant darkness. Such biological clocks are most easily
studied in simple organisms. Recent research has resulted in the
identification of common elements in the molecular mechanisms of
circadian rhythms and in the ways that these mechanisms respond
to environmental cues such as light and temperature. Phase
resetting by light is understood in terms of rapid light-induced
responses of central clock biochemical components. However, a
description of how small, naturally occurring temperature cycles
can reset a clock is lacking. ... ... Liu et al (4 authors at
Dartmouth Medical School, US) now report a study of the clock
protein FRQ in *Neurospora. Levels of FRQ were measured at
various temperatures. At higher temperatures, the amount of FRQ
oscillated around higher levels. Absolute FRQ amounts thus
identified different times at different temperatures, so
temperature shifts corresponded to shifts in clock time without
immediate synthesis or turnover of components. Moderate
temperature changes could dominate light-to-dark shifts in the
influence of circadian timing. The authors suggest their results
provide insight into how physiologically and ecologically
relevant temperature steps and pulses act to reset a day-phase
circadian oscillator. They further suggest their results provide
another example in which highly conserved and plainly adaptive
behaviors of a circadian system can be understood in terms of the
straightforward responses of clock components to factors in the
environment of the organism.
-----------
Science 1998 281:825
ScienceWeek 1998 4 Sep
-----------
Notes:
... ... *Neurospora: (pink bread mold) A genus of fungi grown in
culture and widely used in research in genetics and biochemistry.
-----------
Related Background:
DISSEMINATED INDEPENDENT CIRCADIAN CLOCKS IN THE FRUIT FLY
Drosophila melanogaster is a fruit fly that has been extensively
used in research in genetics, development, and molecular biology.
A transgenic Drosophila is a fruit fly into which genetic
material from another organism has been transferred, the
transferred and incorporated new genes then being expressed with
the resultant production of specific proteins. Bioluminescence is
the production of light in biological organisms, the process
usually involving the protein luciferin and the enzyme
luciferase. In molecular biology, a "promoter" is a DNA sequence
essential for the initiation of the transcription of RNA from a
particular gene sequence. Many organisms exhibit daily
(circadian) rhythms, cyclical variations in various bodily
functions, metabolisms, etc., even in constant light or constant
darkness. Such biological clocks are most easily studied in
simple organisms, but several clock genes have been identified
that have apparently been conserved in evolution and are present
in both simple organisms and higher forms such as mammals. Now
Plautz et al (4 authors at 2 installations, US) report that using
transgenic Drosophila expressing either luciferase or a green
fluorescent protein derived from the promotor of the clock gene
_period_ to monitor the circadian clock in various explanted body
parts reveals that such tissues show rhythmic bioluminescence
that can be reset by light. The authors suggest their results
show that autonomous circadian oscillators are present throughout
the body of Drosophila, that individual cells in Drosophila are
capable of supporting their own independent clocks, and that the
idea of a master oscillator controlling all other oscillators may
need to be revised.
-----------
Science 1997 28 Nov
ScienceWeek 1997 19 Dec
-------------------
Related Background:
IDENTIFICATION OF A MAMMALIAN CIRCADIAN RHYTHM GENE
Circadian rhythms are *biological rhythms based on a 24-hour
cycle apparently controlled by an endogenous biochemical clock.
The essential aspect is that if the environmental cues are
removed, the rhythm will continue for some time before the
periodicity degenerates. These rhythms are observed in both
primitive organisms and in mammals including humans, and in
primitive organisms as well as in insects, several genes have
been isolated that apparently are involved in the endogenous
clock process. Cheng-Chi Li et al (Baylor College of Medicine,
Houston TX US) have now isolated a gene in mice and humans that
is apparently similar to the "period" _per_ gene of the fruit fly
Drosophila melanogaster. Previous evidence shows the fruit fly
gene is switched on and off in a daily cycle, and with several
other genes produces the oscillating mechanism responsible for
the organism's diurnal clock. This is apparently the first
evidence that clock genes have been conserved through the course
of evolution.
-----------
Cell 1997 19 Sep
ScienceWeek 1997 10 Oct
-------------------
Notes:
... ... *biological rhythms: Cyclic behaviors of various kinds
have been observed in organisms ranging from single-celled
protozoans and primitive plants to the most recently evolved
primates. In animals, an endogenous clock apparently exists in
the brain, and the light- sensitive production of the hormone
melatonin is evidently involved in this brain clock. Many
biologists believe primeval 13-hour tidal rhythms and monthly
lunar cycles influenced the clocks of primitive seashore
creatures, beginning the evolution of related biological clocks.
But of course the most significant periodicity to which the Earth
biosphere has always been exposed is the day-night light cycle,
and the term "circadian" technically applies only to this 24-hour
cycle.
----------
Related Background:
MEDICAL BIOLOGY: ENTRAINMENT OF FREE-RUNNING CIRCADIAN RHYTHMS BY
MELATONIN IN BLIND PEOPLE
... In humans, the endogenous circadian pacemaker oscillates with
a period that is slightly longer than 24 hours, and the pacemaker
therefore requires synchronization ("entrainment") to the 24-hour
day.
In mammals, including humans, the biological clock
apparently resides in a group of neuron clusters in a part of the
brain called the hypothalamus, a region that responds to many
chemical inputs, including the hormone melatonin, an indole
derived from the metabolism of serotonin. Melatonin is secreted
by another hypothalamic brain structure, the pineal gland, which
in turn is stimulated by neurons in a nearby cluster (the
suprachiasmatic nucleus) that receives input from the retina of
the eye. So this is the apparent pathway of light-induced
secretion of melatonin and action in mammals: light on the
retina, electrical activity in the retino-hypothalamic tract,
activity in a hypothalamic region called the suprachiasmatic
nucleus, electrical signals to the pineal gland, secretion of the
hormone melatonin, action of melatonin on other neural structures
in the hypothalamus and elsewhere.
In totally blind people, light cues are unavailable, and
disturbances of circadian rhythms are common.
... ... R.L. Sack et al (4 authors at Oregon Health Sciences
University Portland, US) report a study of the effects of
melatonin administration on the circadian rhythms of totally
blind people, the authors making the following points:
1) Most totally blind people have circadian rhythms that are
"free-running", i.e., that are not synchronized to environmental
time cues and that oscillate on a cycle slightly longer than 24
hours. This condition causes recurrent insomnia and daytime
sleepiness when the rhythms drift out of phase with the normal
24-hour cycle. The authors investigated whether a daily dose of
melatonin could entrain the circadian rhythms of totally blind
people to a normal 24-hour cycle.
2) A crossover study was performed on 7 totally blind
subjects who had free-running circadian rhythms. The subjects
were given 10 milligrams of melatonin or a placebo daily, one
hour before their preferred bedtime, for 3 to 9 weeks. They were
then given the other treatment. The timing of the production of
endogenous melatonin was measured as a marker of the circadian
time (phase), and sleep was monitored by *polysomnography.
3) At base line, the subjects had free-running circadian
rhythms with distinct and predictable cycles averaging 24.5 hours
(range: 24.2 to 24.9). These rhythms were unaffected by the
administration of placebo. In 6 of the 7 subjects, the rhythm was
entrained to a 24.0 hour cycle during melatonin treatment.
4) The authors point out that there are approximately 1
million blind people in the US, of whom approximately 20 percent
are totally blind, and it is estimated that at least half of this
20 percent (approximately 100,000 people) probably have free-
running circadian rhythms, with a high proportion having
circadian sleep-wake disorders. The authors suggest melatonin may
prove to be a safe and effective treatment for many of these
people.
5) The authors also suggest that the phase-shifting effects
of melatonin observed in their study of circadian rhythms in
blind people may be relevant to the treatment of sighted people
as well. "People who fly across multiple time zones or who work
nighttime or early-morning shifts routinely have symptoms of
disordered sleep as a result of circadian disturbances. Similar
pathophysiologic mechanisms have been proposed for advanced and
delayed sleep-phase syndromes as well as for winter depression."
6) The authors conclude: "Free-running circadian rhythms in
blind people can be entrained to a 24-hour cycle with a daily
dose of melatonin, thereby preventing a burdensome sleep
disorder."
-----------
New Engl. J. Med. 2000 343:1070
-----------
Notes:
... ... *polysomnography: This technique involves synchronized
recordings of electrical activity in the brain, muscles, and
eyes, as well as other physiological measures (e.g.,
electrocardiogram) during sleep.
-------------------
SCIENCE-WEEK 2000 3 Nov
-------------------
Related Background:
NEUROBIOLOGY: ON MELATONIN
Melatonin is a hormone secreted by the human pineal gland during
night-time darkness, and it is now being marketed in the US as a
nutritional supplement. The hormone is an indoleamine compound
derived from the amino acid *tryptophan, with *serotonin as an
intermediate precursor. ... ... R.L. Sack reviews the
neurobiology and medical aspects of melatonin, and makes the
following points: 1) The most important role of melatonin in all
species is to provide a hormonal signal of night-time darkness.
The secretion of the hormone is tightly controlled by the
*circadian pacemaker. 2) Melatonin is a phylogenetically ancient
hormone, found even in some single-cell organisms and in some
plants. In lower vertebrates (e.g., reptiles), the pineal body
lies close to the skin and is directly photosensitive: sunlight
falling on the overlying skin inhibits melatonin production. In
these species, the pineal body has been called a "third eye". In
mammals, the pineal gland is deep within the skull and is not
photosensitive. The timing of melatonin secretion in mammals is
controlled by neural pathways: tracts from the retina of the eye
to the *hypothalamus (retino-hypothalamic tract) and from the
hypothalamus (suprachiasmatic *nucleus) to the pineal gland. The
suprachiasmatic nucleus of the hypothalamus is the master
circadian pacemaker in mammals, controlling the timing of most
circadian rhythms, including core body temperature, *cortisol
secretion, sleepiness, and melatonin secretion. 3) At the
cellular level, melatonin receptors are members of the
superfamily of *G protein-coupled receptors, which
characteristically have 7 *transmembrane domains. Activation of
these receptors inhibits *cyclic AMP production by the enzyme
adenylyl cyclase. 4) The author suggests that as a therapeutic
agent, melatonin can be useful in the treatment of certain sleep
and mood disorders. The author suggests the basis for this is
circadian phase-shifting and the release of accumulated sleep
drive. 5) Concerning its use as a nutritional supplement, the
author says, "Melatonin appears to be remarkably safe, at least
for short-term use... The effects of long-term administration are
not defined." Concerns have been raised about possible
reproductive effects, but most studies have shown little or no
effect on reproductive hormone levels. There are reports that
melatonin modifies the *vasoconstriction response in rat
arteries.
-----------
Science & Medicine 1998 Sep/Oct
-----------
Notes:
... ... *tryptophan: A nutritionally essential amino acid that
serves as a precursor for many molecular entities of importance
in the nervous system.
... ... *serotonin: (5-hydroxytryptamine, 5-HT) Synthesized from
tryptophan. Acts as both a peripheral neurotransmitter in the gut
and a central neurotransmitter in the brain.
... ... *circadian pacemaker: Many organisms exhibit daily
(circadian) rhythms, cyclical variations in various bodily
functions, metabolisms, etc., even in constant light or constant
darkness. In simple organisms, the pacemakers are biochemical
reaction loops; in higher organisms, complex signaling structures
are involved in the rhythms.
... ... *hypothalamus: A deep brain structure with various
clusters of nerve cells controlling several important homeostatic
functions such as temperature regulation and food intake, and in
addition the sex drive, aggressive emotions, psychosomatic
effects, etc. The hypothalamus essentially integrates the
activity of the autonomic nervous system, and it acts as an
intermediary between the endocrine (hormone) system and the
nervous system, with various hypothalamic neuron types secreting
hormones themselves. In general, the term "hormones" refers to
chemical messengers which are distributed systemically via the
bloodstream.
... ... *nucleus: In this context, the term "nucleus" refers to a
cluster of nerve cells involved in a particular neurological
function.
... ... *cortisol: A corticosteroid hormone secreted by the
adrenal gland.
... ... *G protein: G-proteins are a family of signal-coupling
proteins that act as intermediaries between activated cell
receptors and effectors, for example, the transduction of
hormonal signals from the cell surface to the cell interior, and
certain G-proteins are known to interact with adenylyl cyclase.
The G-protein is apparently embedded in the cell membrane with
parts exposed on the outside surface and inside surface. The
outside moiety is activated by the first messenger, and the
inside moiety activates the second messenger, the G-protein thus
acting as a trans-membrane signal transducer.
... ... *transmembrane domains: A transmembrane domain is a
segment of protein anchored in the plasma membrane bilayer. If
one visualizes the protein as a long linear polymer, the polymer
can be looped back and forth across the plasma membrane with
different segments of the protein anchored in the membrane
according to lipid solubility characteristics of the segments of
the polymer chain.
... ... *cyclic AMP: ATP (adenosine triphosphate) is the most
important chemical energy source in all living cells, intimately
involved in various cell functions and cell metabolism, and an
entity in numerous cyclic chemical pathways involved in the
synthesis of components. One of the reaction products of ATP is
cAMP (cyclic AMP, or adenosine 3,5-monophosphate), which acts as
an intracellular hormone (i.e., a chemical messenger). Cyclic AMP
is derived from ATP in a reaction catalyzed by the enzyme
adenylyl cyclase (also called adenyl cyclase and adenylate
cyclase). Cyclic AMP is called the second messenger; the first
messenger is the hormone that interacts with its receptor on the
cell surface.
... ... *vasoconstriction response: In general, the term
vasoconstriction refers to a narrowing of blood vessels, which in
higher organisms is under physiological control via various
signaling systems. Vasoconstriction produces an increase in blood
pressure, systemic or local, depending on the distribution of
signals.
-----------
SCIENCE-WEEK 1998 30 Oct
-----------
Related Background:
CIRCADIAN RHYTHMS IN HUMANS: EXTRA-VISUAL PHOTOTRANSDUCTION
... ... Campbell and Murphy (Cornell Univ., US) report
measurements of the response of the human circadian clock to
extraocular light exposure involving light pulses presented to
the popliteal region (the area behind the knee). They report a
systematic relation between the timing of the light pulse and the
magnitude and direction of clock phase shifts. The authors
suggest their findings challenge the belief that mammals are
incapable of extraretinal circadian phototransduction, and that
the findings also have implications for the development of more
effective treatments of sleep and circadian rhythm disorders.
-----------
Science 1998 16 Jan
ScienceWeek 1998 30 Jan
-----------
Related Background:
ANALYSIS OF MELATONIN ROLE IN HUMAN CIRCADIAN RHYTHMS
... In mammals, including humans, the biological clock apparently
resides in a group of neuron clusters in a part of the brain
called the hypothalamus, a region that responds to many chemical
inputs, including the hormone melatonin, an indole derived from
the metabolism of serotonin. Melatonin is secreted by another
hypothalamic brain structure, the pineal gland, which in turn is
stimulated by neurons in a nearby cluster (the suprachiasmatic
nucleus) that receives input from the retina of the eye. So this
is the apparent pathway in mammals: light on the retina,
electrical activity in the retino-hypothalamic tract, activity in
a hypothalamic region called the suprachiasmatic nucleus,
electrical signals to the pineal gland, secretion of the hormone
melatonin, action of melatonin on other neural structures in the
hypothalamus and elsewhere. In some insects, the biological clock
may be located in the optic lobes of the brain, and biological
clocks of one sort or another have been found at nearly all
levels of organism complexity. In humans, there is some evidence
that our biological clocks can be implicated in psychiatric
entities known as mood disorders, which is not surprising, since
the hypothalamus and nearby structures are known to play a key
role in emotions. But more generally, the biological clock is
apparent to anyone who has experienced jet-lag upon travelling
long distances east or west, the jet-lag effects resulting from
the body's biological clock being out of synch with the
light-dark cycle. The key question, of course, is that if
melatonin is involved in the workings of the human biological
clock, exactly how does it function? A number of laboratories
have been concerned with this problem, and this week Steven
Reppert et al (installations in MA, CT US) report that melatonin
apparently has two receptors, a major and a minor, and that the
result of binding to the minor receptor is a lowering of activity
in the suprachiasmatic nucleus, which seems to indicate a
chemical negative feedback loop. Identification of such feed-
back loops are the key to understanding brain function. The
studies were carried out on genetically altered mice lacking the
major melatonin receptor.
-----------
Neuron 1997 25 Jul 97
SW Reports 1997 1 Aug 97
-----------
Related Background:
IDENTIFICATION OF A CLOCK GENE IN THE MOUSE
The unraveling of the molecular mechanisms of biorhythmicity
received new impetus recently. Two weeks ago Loros and Dunlap
reported the identification of two new clock genes in the bread
mold Neurospora, the genes chemically related to those already
known in Neurospora and also in the fruit fly, and now Joseph
Takahashi and his colleagues (Northwestern Univ., Evanston IL
US) report the identification of the first mammalian clock gene
in the mouse. Circadian rhythms in mice are precise, animals
kept in darkness beginning exercise on a treadmill each night
with a 23.7 hour cycle. The method used by Takahashi and his
group was to produce random chemical mutations of the entire
mouse genome until one mouse was found that had its clock
altered. Proceeding from there, the damaged gene was identified.
The gene's function was proved by repairing the clock in mutant
mice by substituting a correct piece of DNA. Of great interest
is the news that one domain of the mouse clock protein is the
same as a domain in the clock proteins of the fruit fly and
Neurospora, which supports the idea that circadian rhythms in
mammals have evolved from biochemical feedback loops in
primitive organisms.
-----------
Cell 1997 16 May
ScienceWeek 1997 22 May
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. ON THE CONTROL OF SEX RATIOS IN ANT COLONIES
L. Passera et al (Paul-Sabatier University Toulouse, FR) discuss
the control of sex ratios in fire ant colonies. Social insects
provide some of the most striking examples of elaborate
cooperative behavior, yet life within colonies also entails
conflicts. In ants, the most strident conflict concerns sex
ratio, with workers favoring a more female-biased sex investment
ratio than queens. This conflict arises because of the
hymenopteran haplodiploid system of sex determination, whereby
unfertilized eggs develop into males and fertilized eggs develop
into females. As a result, workers in colonies headed by a single
once-mated queen are three times more related to sisters than to
brothers. Because of this asymmetry in relatedness, the
population-wide sex allocation ratio should equilibrate at 3:1
(female:male) if workers control the colony's investment in
reproductive offspring. In contrast, because queens are equally
related to their daughters and sons, an equal investment in male
and female reproductives is expected if the colony's allocation
of resources is under the control of the queen. Because they
control brood rearing and food flow in the colony, workers can
bias sex allocation, for example, by selectively eliminating
males and/or preferentially feeding females, and workers have
indeed been demonstrated to manipulate colony sex ratios in this
way. The authors report a study in which they exchanged queens
between male- and female-specialist colonies of the fire ant
Solenopsis invicta. These exchanges quickly reversed the sex-
ratio biases of adopting colonies. The sex ratio of queen-laid
eggs differed strongly between male- and female-specialist
colonies. The authors suggest that these findings indicate that
queens can force workers to raise male sexuals by limiting the
number of female brood, and that the findings help to explain why
sex investment ratios lie between the queen and worker equilibria
in this and many other ant species.
-----------
Science 2001 293:1308
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
----------
Related Background:
GENETIC CONTROL OF SOCIAL ORGANIZATION IN ANTS
Biological evolution is marked by a number of major transitions,
one of which is the evolution of complex social behavior. Animal
social life can take a variety of forms, each distinguished by
features such as group size and the reproductive roles of group
members. One focus in evolutionary biology is to identify the
causes of social behavior and its conspicuous variation, and to
determine the extent to which social organization is under
genetic control. Such information is useful for reconstructing
pathways of animal social evolution. Current views on insect
social evolution stress the importance of ecological and
behavioral environments in molding what are largely plastic
social behaviors.
... ... K.G. Ross and L. Keller (2 installations, US CH) report
evidence that major variation in the social organization of fire
ant colonies is under simple genetic control, providing a
demonstration of an apparent strong genetic component to complex
social behavior. The authors report that a single genomic element
(the gene [Gp-9]) is responsible for the existence of two
distinct forms of social organization in the fire ant *Solenopsis
invicta. This genetic factor apparently influences the
reproductive *phenotypes and behavioral strategies of ant queens
and determines whether workers tolerate a single fertile queen or
multiple queens per colony. The authors suggest "these findings
reveal how a single genetic factor can have major effects on
complex social behavior and influence the nature of social
organization."
-----------
Proc. Natl. Acad. Sci. 1998 95:14232
-----------
Notes:
... ... *Solenopsis invicta: The fire ant S. invicta is an
introduced pest species in the southern US, the species existing
in two distinct social forms. The "monogyne" form features
colonies with a single fertile (egg-laying) queen, whereas the
"polygyne" form features colonies with multiple fertile queens.
The two social forms differ in other major aspects of their
reproductive biology.
... ... *phenotypes: The term "phenotype" refers to the total
appearance of an organism as determined by the interaction during
development between its genetic constitution (genotype) and the
environment.
-----------
ScienceWeek 1999 15 Jan
-----------
SCIENCE-WEEK 2 Nov 2001 http://scienceweek.com
-------------------
Related Background:
ON HONEYBEE SOCIAL BEHAVIOR, GENES, AND THE ENVIRONMENT
The so-called social insects live in societies that rival human
societies in complexity and internal cohesion. Honey bees, for
example, apparently always follow 3 rules: a) they live in
colonies with overlapping generations; b) they care cooperatively
for offspring other than their own; and, c) they maintain a
reproductive division of labor. ... ... In a review of research
(much of it from the author's own laboratory) concerning the
genetic and environmental factors responsible for honey bee
behavior, Gene E. Robinson (University of Illinois Urbana-
Champaign, US) makes the following points: 1) Genes do not play
an exclusive role in regulating behavior: biologists have long
realized that behavior is influenced by genes, the environment,
and interactions between the two. 2) Genes never act alone. They
must operate in an environment where they code for proteins that
participate in many systems in an organism, with these systems in
turn influencing the expression of genes. Consequently,
biologists must take a broad approach in assessing the impact of
any gene. 3) The research group of the author uses the Western
honey bee, Apis mellifera. Honey bees pass through different life
stages as they age, and their behavioral responses to
environmental and social stimuli change in predictable ways.
Although worker bees go through a consistent path of behavioral
development, this path is not rigidly determined. Bees can
accelerate, retard, or even reverse their behavioral development
in response to changing environmental and colony conditions. 4)
Experimental evidence indicates that juvenile hormone, one of
the most important hormones influencing insect development, helps
time the pace of behavioral maturation in honey bees. The rate of
endocrine-mediated behavioral development is influenced by
inhibitory social interactions. Older bees inhibit the behavioral
development of younger bees: the rate of behavioral development
is negatively correlated with the proportion of older bees in a
colony. Inhibitory social interactions that influence the rate of
behavioral development involve chemical communication between
colony members. 5) Evidence from the laboratory of the author in
1993 indicated the so-called mushroom bodies in the bee brain are
involved in the behavioral changes occurring during maturation,
the volume of the bodies increasing, and the volume increase
associated with an increase in synapses with neurons from brain
regions devoted to sensory input. The author suggests this was
the first report of brain plasticity in an invertebrate. 6) The
author suggests that, in general, two-way interactions between
the nervous system and the genome contribute fundamentally to the
control of social behavior. Information about social conditions
that is acquired by the nervous system is likely to induce
changes in genomic function that in turn produce adaptive
modifications of the structure and function of the nervous
system. 7) The author proposes a new research initiative called
"sociogenomics", defined as a "wide-ranging approach to identify
genes that influence social behavior, determining the influence
of these genes on underlying neural and endocrine mechanisms, and
exploring the effects of the environment -- particularly the
social environment -- on gene action."
-----------
American Scientist 1998 86:456
ScienceWeek 11 Sep 98
-----------
SCIENCE-WEEK 2 Nov 2001 http://scienceweek.com
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
7. ON THE ASTEROID EROS
Erik Asphaug (University of California Santa Cruz, US) discusses
the asteroid Eros. Planetary formation apparently began as
delicate sedimentation, with granules clinging to other granules
in the solar nebula. The process climaxed 10 million to 100
million years later with planets smashing together at velocities
of tens of kilometers per second. The Moon, for example, was
apparently created when a body the size of Mars hit the early
Earth approximately 4.5 billion years ago. In between was a fast-
paced epoch, as poorly comprehended as it was brief, which we try
to understand by studying asteroids and comets, the remnants that
wander the Solar System like ghosts from a bygone time. One such
ghost is 433 Eros, recently contacted by the NASA NEAR-Shoemaker
spacecraft (NEAR = Near Earth Asteroid Rendezvous). Eros
originated in the main asteroid belt, beyond the orbit of Mars,
where Jupiter's gravitational stirring stunted planet growth. The
belt is crowded, and the long-term fate of asteroids is to batter
each other to bits. A variety of dynamical process cause objects
to leak from the main belt into the inner Solar System, where
they typically crash into the Sun or into a planet or are ejected
from the Solar System, after approximately 10 million years.
Planet-crossing orbits tend to be chaotic, so the fates of
asteroids are expressed as probabilities. Eros has a chance of
approximately 5 percent of striking Earth, but not anytime soon.
Eros is approximately 34 x 13 x 13 kilometers in size, and is the
second largest of the known near-Earth asteroids; the largest is
1036 Ganymed, which is approximately 40 kilometers in diameter.
Eros bears enormous impact scars from its 4 billion years in the
main asteroid belt, and it may have lost much of its original
mass and shape to cratering.
-----------
Nature 2001 413:369
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
----------
Related Background:
PLANETARY SCIENCE:
IMAGING AND SPECTRAL ANALYSIS OF ASTEROID EROS
The Near Earth Asteroid Rendezvous (NEAR), a Discovery
spacecraft launched on 17 February 1996, was designed to carry
out the first detailed orbital investigation of an asteroid.
After a flyby of asteroid 253 Mathilde in June 1997, and a
swingby of Earth in January 1998, the spacecraft carried out an
unintended flyby of the asteroid Eros, approaching within 3800
kilometers on 23 December 1998. During this flyby, data important
to the detailed planning of the subsequent orbital mission, such
as mass, dimensions, and spin state, were obtained. On 3 January
1999, a burn was executed to slow the speed of the spacecraft
relative to Eros from approximately 1 kilometer per second to
only 10 meters per second, resulting in a year-long gradual
return to the asteroid. The spacecraft was successfully inserted
into an initial orbit around Eros on 14 February 2000.
... ... J. Veverka et al (33 authors at 10 installations, US)
present an analysis of data obtained from the NEAR mission to
Eros, the authors making the following points:
1) Discovered in 1898, Eros was the first asteroid found to
cross the orbit of Mars, and was the first asteroid detected to
show periodic brightness fluctuations, which were soon attributed
correctly to its elongated shape and a rapid rotation period of
5.27 hours. The orbital path of Eros takes it from 1.13
*astronomical units (AU), close to Earth's orbit, to beyond the
orbit of Mars at 1.73 AU, every 1.76 years. Spectrally, Eros is
classified as an S-type asteroid, the type common in the inner
portions of the asteroid belt, the surface mineralogy of which is
apparently dominated by silicates (pyroxene and olivine) and Fe-
metal.
2) Eros is a very elongated (34 kilometers by 11 kilometers
by 11 kilometers) asteroid, most of the surface of which is
saturated with craters smaller than 1 kilometer in diameter. The
largest crater is 5.5 kilometers across, but there is a 10-
kilometer saddle-like depression with attributes of a large
degraded crater. Surface lineations, both grooves and ridges, are
prominent on Eros; some of these probably involve planes of
weakness produced by collisions on Eros and/or its parent body.
Ejecta blocks (30 to 100 meters across) are abundant but not
uniformly distributed over the surface. *Albedo variations are
restricted to the inner walls of certain craters and may be
related to downslope movement of *regolith. On scales of 200
meters to 1 kilometer, Eros is relatively bland in terms of color
variations, and spectra are consistent with an *ordinary
chondrite composition for which the measured density of 2.67 +-
0.1 grams per cubic centimeter implies internal porosities
ranging from approximately 10 to 30 percent.
... ... In a commentary on the above report, Richard P. Binzel
(Massachusetts Institute of Technology, US) states: "No longer
just "star-like" points in the sky, asteroids have become the
focus of dedicated geological and geophysical studies aimed at
gaining insights into planetary formation and addressing
practical concerns for the long-term safety of our planet. These
studies are yielding new links to meteorites and planetary
origins."
-----------
Science 2000 289:2065,2088
-----------
Notes:
... ... *astronomical units (AU): 1 AU = the mean distance from
the Sun to the Earth = approximately 93 million miles, and
exactly 149,597,870 kilometers.
... ... *Albedo: In this context, the fraction of total sunlight
falling on an asteroid that is reflected from it. In general, the
albedo is equal to the amount of light reflected divided by the
amount of light received.
... ... *regolith: In general, surface layers of rock that overly
bedrock.
... ... *ordinary chondrite: Meteorites are divided roughly into
3 main classes according to their composition. "Iron meteorites"
consist of an alloy of iron and nickel; "stony meteorites"
consist of silicate minerals; and "iron-stony meteorites" are a
mixture of the two previous types. The stony meteorites are
further divided into "chondrites" and "achondrites". Chondrites
contain small spherules of high-temperature silicates
("chondrules") and constitute more than 85 percent of recovered
meteorites. The achondrites range in composition from rocks made
up essentially of single minerals (e.g., olivine) to rocks
resembling *basaltic lava. Each category is further subdivided on
the basis of chemical composition. "Carbonaceous chondrites" have
little or no metal but abundant carbon, and display evidence of
chemical alteration by water; they have the highest proportion of
volatile elements and are the most oxidized. "Ordinary
chondrites" (the most common type) are intermediate in volatile
element abundance and oxidation state.
... ... *basaltic lava: Basalt is a dark gray to black igneous
rock of volcanic origin that cools rapidly. "Igneous rocks" are
rocks that have congealed from a molten mass.
-----------
SCIENCE-WEEK 2000 20 Oct
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
8. ON SOLID-SOLID ORGANIC REACTIONS
G. Rothenberg et al (University of York, UK) discuss solid-solid
organic reactions. Often depending on specific crystal
modification and high temperature interactions, solid-state
chemistry is associated primarily with inorganic crystalline
compounds, where reactions are believed to occur via ion
displacement and crystal deformation mechanisms. It seems
unreasonable that a physical class of reactions should be
confined solely to "inorganic" materials, so it is not surprising
that several studies on organic reactions in the solid phase have
also been published. What is surprising is the fact that many of
these organic transformations were reported to afford high
conversions and yields after short reaction times, at moderate
and even ambient temperatures, yet were reported to occur between
two macroscopic solids. The concept of a chemical reaction
between two solids is a difficult one. Moreover, various name
tags are frequently used in this context. The authors distinguish
between "solid-phase synthesis" (the reaction of molecules from a
fluid phase with a solid substrate, e.g., the polymer-supported
peptide syntheses), "solvent-free synthesis" (any system in which
neat reagents react together in the absence of solvent), and
"solid-state synthesis" (or "solid-solid reactions") (in which
two macroscopic solids interact directly and form a third solid
product without intervention of a liquid or vapor phase). The
authors examine some so-called "solid-solid" organic
transformations and demonstrate that many of these reactions are
actually reactions in a liquid melt, with phase change to a melt
an apparent precondition for a rapid chemical reaction between
solid organic compounds under solvent-free conditions.
-----------
J. Am. Chem. Soc. 2001 123:8701
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
9. LONG-LIVED ENTANGLEMENT OF MACROSCOPIC OBJECTS
B. Julsgaard et al (University of Aarhus, DK) discuss quantum
mechanical entanglement. In 1935, Albert Einstein, Boris
Podolsky, and Nathan Rosen (EPR) formulated what they perceived
as a paradox created by quantum mechanics. Since then, the EPR
correlations and other types of entanglement have been
extensively analyzed, notably by John Bell. Entangled or
inseparable states are fundamental to the field of quantum
information, specifically to quantum teleportation of discrete
and continuous variables, and to quantum dense coding of discrete
and continuous variables, to name a few examples. The majority of
experiments on entanglement until now have dealt with entangled
states of light. Entangled states of discrete photonic variables
as well as entangled states of continuous variables of the
electromagnetic field have been generated experimentally.
Entangled states of material particles are much more difficult to
generate experimentally, but such states are vital for storage
and processing of quantum information. Recently, entangled states
of 4 trapped ions have been produced, and two atoms have been
entangled via interaction with a microwave photon field. It is
generally believed that entanglement is usually manifest in
systems consisting of a small number of microscopic particles.
The authors demonstrate experimentally the entanglement of two
macroscopic objects, each consisting of a cesium gas sample
containing approximately 10^(12) atoms. Entanglement is generated
via interaction of the samples with a pulse of light that
performs a non-local Bell measurement on the collective spins of
the samples. The entangled spin-state can be maintained for 0.5
milliseconds. The authors state that besides being of fundamental
interest, they expect the robust and long-lived entanglement of
material objects demonstrated by their experiment to be useful in
quantum information processing, including teleportation of
quantum states of matter and quantum memory.
-----------
Nature 2001 413:400
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
10. CATASTROPHE THEORY AND POTENTIAL ENERGY SURFACES
In general, "catastrophe theory" studies how the qualitative
nature of solutions to equations depends on the parameters that
appear in the equations. Specializations of catastrophe theory
include bifurcation theory, nonequilibrium thermodynamics,
singularity theory, synergetics, and topological dynamics.
... ... David J. Wales (Cambridge University, UK) discusses
potential energy surfaces and catastrophe theory. The structure,
dynamics, and thermodynamics of any system in a particular
electronic state are determined by the underlying potential
energy surface, and considerable effort has been made to discover
how the global topography of the potential energy surface enables
some systems, such as proteins and crystals, to locate low-energy
states reliably, whereas other systems readily form glasses. Of
equal importance, but much less studied, is the parallel question
of how the global characteristics of the potential energy surface
are determined by the nature of the interatomic or intermolecular
potential. A multidimensional potential energy surface can be
thought of as a mountain range, where local minima are basins,
and transition states correspond to the lowest passes linking
valleys. The lowest energy structure is the global minimum, and
this is where the system will reside at 0 kelvins unless this
point is kinetically inaccessible. Above this temperature,
thermodynamic properties depend on the different local minima
that the system samples as it explores the landscape dynamically
using thermal energy to overcome the potential energy barriers.
The author demonstrates that the appearance of a multidimensional
potential energy surface, or potential energy landscape, can be
related to the form of the interatomic or intermolecular
potential. Catastrophe theory makes possible a description of how
the geometry of the surface changes with parameters in the
potential, and the theory provides universal scaling relations
that explain, for example, the asymmetric reaction profiles
observed for systems bound by long-range forces. The principal
result is an unexpected connection between barrier heights, path
lengths, and vibrational frequencies, with applications to a wide
variety of problems in chemical physics.
-----------
Science 2001 293:2067
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
11. ON THE PHYSICS OF FRICTION
E. Gerde and M. Marder (University of Texas Austin, US) discuss
the physics of friction. The most intriguing fact about friction
is that it is proportional to the force pushing two objects
together, but independent of the area of contact. Consider a
block placed on a table and pushed sideways until it begins to
slide. G. Amontons (1663-1705) and C. Coulomb (1736-1806) found
that the force required to initiate sliding is proportional to
the weight of the block (the constant of proportionality being
the "static coefficient of friction"), but independent of the
area of contact. This is commonly explained by asserting that,
owing to the presence of asperities (roughnesses) on the two
surfaces, the actual area in physical contact is much smaller
than it seems and grows in proportion to the applied compressive
force. The authors present an alternative picture of the static
friction coefficient, their approach starting with an atomic
description of surfaces in contact and then using a multiscale
analysis technique to describe how sliding occurs for large
objects. The authors demonstrate the existence of self-healing
cracks that have been postulated to solve geophysical paradoxes
concerning heat generated by earthquakes, and the authors
demonstrate that when such cracks are present at the atomic scale
they result in solids that slip in accord with Coulomb's law of
friction. The authors state they expect that this mechanism for
friction will be found to operate at many length scales, and they
suggest their approach for connecting atomic and continuum
descriptions will enable more realistic first-principles
calculations of friction coefficients.
-----------
Nature 2001 413:285
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
12. ON THE BRAGG GLASS PHASE
T. Klein et al (CNRS, FR) discuss the Bragg glass phase. Although
crystals are usually quite stable, they are sensitive to a
disordered environment: even an infinitesimal amount of
impurities can lead to the destruction of crystalline order. The
resulting state of matter has been a long-standing puzzle, and
until recently it was believed to be an amorphous state in which
the crystal would break into "crystallites". But a different
theory predicts the existence of a novel phase of matter: the so-
called "Bragg glass", which is a glass and yet nearly as ordered
as a perfect crystal. After the Bragg glass was first proposed in
1995, its existence was supported by further analytical and
numerical calculations, but up to now experimental evidence for
this phase has been indirect. The "lattice" of vortices
containing magnetic flux in *type II superconductors provide a
convenient system to investigate these ideas, and the authors
demonstrate that neutron-diffraction data of the vortex lattice
provides unambiguous evidence for a weak power-law decay of the
crystalline order characteristic of a Bragg glass. The theory
also accurately predicts the electrical transport properties of
superconductors, and naturally explains the observed phase
transitions and the dramatic jumps in the critical current
associated with the melting of the Bragg glass. Moreover, the
model explains experiments as diverse as x-ray scattering in
disordered liquid crystals and the conductivity of electronic
crystals.
-----------
Nature 2001 413:404
-----------
Notes:
... ... *type II superconductors: Examples of materials of this
type are niobium and vanadium (the only type II superconductors
among the chemical elements) and some alloys and compounds,
including the high-temperature superconductivity compounds. The
experiments in this report were performed on a large single-phase
(K,Ba)BiO(sub3) crystal mass of 300 milligrams, superconducting
transition temperature 23 kelvins.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
13. POSTDOCTORAL FELLOWSHIP PROFILE:
Laboratory of David H. Gutmann, Washington University St. Louis
---------------------------------------------------------------
INSTALLATION: Washington University St. Louis (US)
DEPARTMENT: Neurology
GENERAL RESEARCH AREA: Neuro-oncology and Neurogenetics
HEAD OF THIS SPECIFIC LABORATORY: David H. Gutmann, MD-PhD
POSTDOCTORAL FELLOWSHIPS AVAILABLE IN THE FOLLOWING SPECIFIC
RESEARCH PROBLEMS: Molecular biology and genetics of brain
tumors, including the development of preclinical mouse models and
studies aimed at determining the mechanism of action of relevant
brain tumor suppressor genes. Additional studies are directed at
identifying novel genes involved in the molecular pathogenesis of
nervous system tumors.
PREVIOUS RESEARCH EXPERIENCE AND DEGREES REQUIRED: MD or PhD
degree with experience in molecular biology, genetics,
biochemistry, and tissue culture.
USUAL STARTING STIPEND: NIH salary structure for PDFs
SPECIAL REQUIREMENTS: None
APPROXIMATELY NUMBER OF PEOPLE CURRENTLY WORKING IN THIS
SPECIFIC LABORATORY (FACULTY, STAFF, STUDENTS, POSTDOCS): 12 (6
post-doctoral fellows, 5 technicians, and 1 student)
CONTACT FOR MORE INFORMATION: pouchera@neuro.wustl.edu
--------------------------------------------------------------
Please note: Postdoctoral Fellowship Profiles are provided to
ScienceWeek by the heads of laboratories, and ScienceWeek does
not charge for their publication. For information about
publishing a Postdoctoral Fellowship Profile, contact Claire
Haller at haller@scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
14. IN FOCUS: ON THE MATHEMATICIAN ARTHUR CAYLEY (1821-1895)
"Arthur Cayley (1821-1895), English mathematician, educated at
Cambridge University, becoming a fellow of Trinity College in
1845 but leaving after three years because he did not wish to
take holy orders. He then spent 15 years as a lawyer before being
appointed the first Sadleirian Professor of Mathematics at
Cambridge in 1863. Apart from half a year spent at Johns Hopkins
University in the US (1881-1882), he remained at Cambridge until
his death. Cayley is best known for his work with James Joseph
Sylvester (1814-1897) on invariants, algebraic expressions that
remain unchanged when their variables are transformed. The theory
of invariants, generalized to differentials of the variables,
formed the mathematical basis of Albert Einstein's General Theory
of Relativity -- the idea that gravity is a manifestation of the
curvature of a 4-dimensional space-time. Cayley was also
responsible for matrix algebra, which is now widely used in all
branches of pure and applied mathematics: A matrix is a
rectangular table of numbers and represents a transformation of
variables... Cayley was [also] one of the creators of higher-
dimensional geometry, beginning with a paper of 1845 on spaces of
(n) dimensions. The main founder of this theory was the German
mathematician Hermann Grassmann (1809-1877), in his
_Ausdehnungslehre_ (Theory of Extension) of 1844, but Cayley's
early work was done independently."
-----------
Edwin A. Abbot: _The Annotated Flatland: A Romance of Many
Dimensions_
(Annotations by Ian Stewart, Professor of Mathematics, University
of Warwick, UK)
(Perseus Publishing, Cambridge MA 2002, p.9)
(SW Note: The text of the annotations exceeds by far the text of
the original monograph (1884) by Abbott.)
http://www.amazon.com/exec/obidos/ASIN/0738205419/scienceweek
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
15. FROM PRAXIS:
ON LEAD AS AN ENVIRONMENTAL POLLUTANT
Vincent T. Breslin (Southern Connecticut State University, US)
discusses lead as an environmental pollutant, the author making
the following points:
1) Despite the ban on lead-based paints and leaded gasoline
in the US in the 1970s and 1980s, 4.4 percent of American
children aged 1 to 5 years still have blood lead levels high
enough to cause irreversible damage to the developing nervous
system. In addition, almost 12 percent of children in older
housing in large urban areas have elevated blood lead levels, and
African-American children living in the major US inner cities are
affected disproportionately (approximately 22 percent). Lead
exposure in young children results primarily from ingestion or
inhalation of soil particles, drinking water, paint, and dust
particles in and around the home and play areas.
2) Lead was used extensively as a corrosion inhibitor and
pigment in both interior and exterior oil-based paints prior to
1978, and some paints were manufactured with lead concentrations
of 50 percent by weight. Therefore, weathering of lead-based
exterior paint and deposition of paint chips and dust on soils
remains a significant source of lead to soils surrounding homes.
Soil lead concentration at or above 500 micrograms per gram will
result in a 1 to 5 percent probability that a child will have a
blood lead concentration that equals or exceeds 10 micrograms per
deciliter.
3) Drinking water is another source of ingested lead.
Household plumbing fixtures, including metal pipes, faucets, and
soldered joints, are possible sources of lead in drinking water.
The lower the pH of the water and the lower the concentration of
dissolved salts in the water, the greater is the solubility of
lead in the water. Leaching of lead from plastic pipes has also
been documented and has been attributed to the use of lead
stearate, a stabilizer used in the manufacture of polyvinyl
plastics.
-----------
J. Chem. Educ. 2001 78:1647
-----------
PRAXIS 7 Jan 2002 http://scienceweek.com/praxis
----------
Related Background:
AIR LEAD EXPOSURE AND HOMICIDE
There is ample evidence that the developing nervous system of a
child is highly sensitive to a number of toxic substances, the
effects of which are various encephalopathies (brain disorders).
One environmental toxic substance of considerable importance in
this context is lead, an urban environmental contaminant that in
the past few decades has become a focus of public awareness. The
classical sources of urban environmental lead contamination are
the gasoline exhaust fumes of motor vehicles and lead-based
paints, but recently another important source of such
contamination has become apparent, the lead contamination of
inner city soil and dust.
... ... P.B. Stretsky and M.J. Lynch (Colorado State University,
US) discuss the relationship between lead exposure and homicide
in the US. Previous studies have suggested that excessive lead
exposure is related to aggressive and violent behavior. The
authors conducted a study in order to evaluate the association
between estimated air lead concentrations and homicide rates, the
study involving all counties in the contiguous 48 states of the
US. After adjusting for sociologic confounding factors and 9
measures of air pollution, the only indicator of air pollution
found to be associated with homicide rates was air lead
concentration. Across all counties, estimated air lead
concentrations ranged from 0 to 0.17 micrograms per cubic meter.
The adjusted results suggest that the difference between the
highest and lowest level of estimated air lead is associated with
a homicide incidence rate of 4.12. The authors conclude their
results support recent findings of an association between lead
exposure and violent behavior.
-----------
Arch. Pediat. Adolescent Med. 2001 155:579
PRAXIS 31 Sep 2001 http://scienceweek.com/praxis
-----------
PRAXIS 7 Jan 2002 http://scienceweek.com/praxis
-------------------
Related Background:
CHELATION THERAPY AND CHILDREN EXPOSED TO LEAD
Thousands of children in the US, especially poor children living
in deteriorated urban housing, are exposed to enough lead to
produce neurodevelopmental damage and cognitive impairment. Blood
lead levels as low as 10 micrograms per deciliter are associated
with cognitive deficits. A new study demonstrates that treatment
with succimer, a lead chelator, lowers blood lead levels but does
not improve scores on tests of cognition, behavior, or
neuropsychological function in children with blood lead levels 20
to 44 micrograms per deciliter. Since succimer is as effective as
any lead chelator currently available, chelation therapy is
apparently useless for children with such blood lead levels.
-----------
New Engl. J. Med. 2001 344:1421
ScienceWeek 2001 13 Jul
PRAXIS 31 Sep 2001 http://scienceweek.com/praxis
-----------
PRAXIS 7 Jan 2002 http://scienceweek.com/praxis
-------------------
Related Background:
A DANGEROUS NEW SOURCE OF ENVIRONMENTAL LEAD
... ... Howard W. Mielke (Xavier University New Orleans, US)
presents a review of the problem of lead in inner cities with
emphasis on the newly recognized danger of lead contamination of
inner city soil and dust. The author makes the following points:
1) Since the 1920s, millions of US children have been quietly
poisoned by lead, and thousands of deaths are attributed to this
over the long term. 2) Although childhood lead exposure in the
US has diminished during the past 2 decades, the problem has not
been solved. Instead, the demographics has shifted. 3) Over 50
percent (and perhaps even 70 percent) of children living in the
inner city of New Orleans and Philadelphia have blood lead
levels above the current guideline of 10 micrograms per
deciliter [*Note #2]. In contrast, in the concrete "jungle" of
Manhattan, where very little of the soil is exposed and almost
all apartments and housing contain lead-based paints, only
between 5 and 7 percent of children under the age of 6 have been
reported to have blood-lead levels of 10 micrograms per
deciliter or higher. It is of significance that in Brooklyn,
across the river from Manhattan, where yards containing soil are
common, the percentage of affected children is several times
higher than in Manhattan. 4) The serious of the problem has been
recognized by the US Centers for Disease Control and Prevention
since the early 1990s, which has called pediatric lead poisoning
"entirely preventable". 5) The author suggests that effective
prevention assumes an accurate identification of the
environmental reservoirs of lead, and that current policies to
reduce lead exposure are based on the assumption that the
greatest lead hazard comes from lead-based paints [*Note #3].
Most lead-based have now been removed from the market, and
parents have been instructed to guard their children from eating
paint flakes. However, for children, paint is now neither the
most abundant nor the most accessible source of lead. The common
problem is lead dust in the environment, with the soil a giant
reservoir of tiny particles of lead. The greatest risk for
exposure of inner city children is in the yards around houses
and to a lesser extent in public playgrounds. 6) The author
suggests that an accurate and complete appreciation of the
distribution of lead in the environment can help shape policies
that more effectively protect the health of children. The author
concludes: "It took nearly 10 decades for lead to accumulate to
its current levels in urban areas. With judicious planning, the
problem can be resolved in much less time."
-----------
American Scientist 1999 87:62
-----------
Notes:
... ... *Note #1: There is much data concerning certain
syndromes, e.g., fetal alcohol syndrome, lead poisoning, etc. One
research problem is that effects of low levels of environmental
toxins on the developing nervous system can be subtle and not
detected unless specific rather than general behavioral measures
are applied.
... ... *Note #2: There is hardly a consensus concerning
acceptable levels of lead in the whole blood of children. Some
clinicians consider the danger point to be in the region of 50
micrograms per deciliter whole blood; other clinicians consider
anything above 10 micrograms per deciliter as a cause for alarm.
In terms of low-level effects on the developing central nervous
system, general concentration cut-off points are perhaps
arbitrary, since there is considerable individual variation in
toxic susceptibility.
... ... *Note #3: In the US, lead was used in residential paint
between 1884 and 1978, and leaded paint remains on the walls of
many old buildings.
-----------
ScienceWeek 1999 15 Jan
PRAXIS 31 Sep 2001 http://scienceweek.com/praxis
-----------
PRAXIS 7 Jan 2002 http://scienceweek.com/praxis
----------
Related Background:
PRENATAL LEAD EXPOSURE AND POSTNATAL LEAD TOXICITY
Lead is a highly toxic metal, especially in children, but
unfortunately the incidence of lead poisoning in metropolitan
slums remains high due to past widespread use of lead based
paints and lead water pipes. There are also other sources that
contribute to dangerous concentrations in the environment. In
children, concentrations of lead in the range 40 micrograms per
deciliter, and probably as low as 10 micrograms per deciliter,
will produce definite serious cognitive deficits. Higher blood
concentrations produce encephalopathies that are both malignant
and difficult to treat. In the past, the focus has been on the
exposure of young children to environmental lead, since children
are the most vulnerable and environmental lead the apparent
primary source. Now William H. Bowen et al (University of
Rochester, US) present evidence that in rats toxic concentrations
of lead can pass from mother to offspring when mother rats are
drinking water that produces blood lead levels of only 40
micrograms per deciliter, with transmission evidently occurring
via blood to the fetus and via milk to the postnatal rat pup.
Evidently one consequence in these lead-exposed rat pups is a
high incidence of dental caries. Bowen's group is a dental
research group, and they apparently became interested in the
problem after considering that although dental caries in children
has shown a marked drop in prevalence in the U.S., about 80 per
cent of the cases that still occur are occurring in only about
20% of the children -- those that live in inner cities, where
lead exposures can still be relatively high. Metropolitan
children are thus faced with another source of lead poisoning --
lead of maternal origin.
-----------
Nature Med. 1997 September
ScienceWeek 1997 12 Sep
PRAXIS 31 Sep 2001 http://scienceweek.com/praxis
-----------
PRAXIS 7 Jan 2002 http://scienceweek.com/praxis
-----------
SCIENCE-WEEK 11 Jan 2002 http://scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
16. THIS WEEK IN PRAXIS (7 Jan 02):
-------------------------------
1. On Lead as an Environmental Pollutant
2. Limits of Silicon Nanoelectronics
3. On Fabrication of Porous Silicon Arrays
4. On Synthesis of Monodisperse Metallic Nanoparticles
5. A New Holographic Technique for Biological Applications
6. Ultrafast Nanopatterning of Biocatalytically Formed Silica
7. On Autism
8. Biota Responses to a Warming World
9. Design of Folded Peptides
10. Applying Genomics and Biotechnology to Global Health
11. De Novo Design of Proteins
12. On Human Serum Albumin and Drug Binding
For information about PRAXIS, see:
http://www.scienceweek.com/praxis
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
In the text, the affiliation following the names of authors is
the affiliation of the lead author.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
NOTICES
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
CHANGE OF EMAIL ADDRESS:
If at any time you need to change the Email address at which you
receive SW, please send the information to:
request@scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Information concerning other subscriptions is available at:
http://www.scienceweek.com/subinfo.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
We welcome comments, suggestions, and criticisms from our
subscribers. Public letters relevant to any report are also
welcome. Editorial contact: editors@scienceweek.com
Editor/Publisher: Dan Agin
Managing Editor: Claire Haller
Associate Editor: Joan Oliner
Copyright (c) 1997-2002 SCIENCE-WEEK
All Rights Reserved
US Library of Congress ISSN 1529-1472
---------------------------------------------
ScienceWeek/Spectrum Press Inc.
3023 N. Clark Street #109
Chicago, 60657-5205 IL, USA.
---------------------------------------------
-----end file
|