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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.
December 15, 2000 -- Vol. 4 Number 50
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At the present time it is of course quite customary
for physicists to trespass on chemical ground, for
mathematicians to do excellent work in physics, and
for physicists to develop new mathematical procedures.
Trespassing is one of the most successful techniques
in science.
-- Wolfgang Koehler (1887-1967)
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. PALEOBIOLOGY: ICHTHYOSAURS
Since their discovery, it has become evident that the various
adaptations of ichthyosaurs for life in water made them quite
successful, their fossils revealing that these creatures ruled
the oceans from approximately 245 million until 90 million years
ago -- which is roughly the entire era that dinosaurs dominated
the continents. One interesting feature of ichthyosaurs is the
size of their eyes -- the largest eyes relative to body size of
any adult vertebrate, living or extinct. For example, the
ichthyosaur known as Temnodontosaurus had eyes 26 centimeters in
diameter. (Scientific American December 2000)
2. MEDICAL BIOLOGY: EMERGING VIRAL DISEASES
RNA viruses can quickly adapt to and exploit varying
environmental conditions because of the high error rates of the
virus enzymes (polymerases) that replicate their genomes. It is
not surprising, therefore, that several recent prominent examples
of emerging or re-emerging diseases are caused by RNA viruses.
However, a complex interplay of factors can influence disease
emergence. In addition to virus genetic variation (mutation,
*recombination, and *reassortment), environmental factors
(including ecological, social, health care, and behavioral
influences) can play important roles.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12411)
3. MEDICAL BIOLOGY: GENETIC COMPONENTS IN PARKINSON'S DISEASE
The causes of Parkinson's disease are largely unknown, but there
is evidence that the disease has a genetic component. In a few
large families with early onset Parkinson's disease or juvenile
Parkinson's disease, the disease is transmitted as an autosomal
dominant or recessive trait resulting from mutations in the genes
encoding the proteins alpha-synuclein and parkin, respectively.
However, in the majority of families affected by Parkinson's
disease, the disease appears to skip generations, irrespective of
the age of onset. Therefore, this disease is considered a
complex, multifactorial disease resulting from interaction
between one or more genes and the environment. Results of a new
study in a genealogically well-documented population suggest that
late-onset Parkinson's disease has a genetic component as well as
an environmental component.
(New England J. Med. 14 Dec 00 343:1765)
4. CHEMISTRY:
TRANSITION METAL CATALYSIS AND THE CARBON-CARBON BOND
One of the most important objectives in synthetic and industrial
chemistry involves the rapid cleavage of a carbon-carbon bond by
inserting a metal atom between two carbons (the process called
"oxidative addition). This process "activates" the bond and
allows potentially more useful molecules to be made from
relatively inactive unreactive starting materials. A full
characterization of this catalytic process has now been reported,
the authors investigating cleavage of the carbon-carbon bond with
a rhodium complex. Although previous carbon-carbon bond breaking
reactions have generally required temperatures of at least 100
degrees centigrade, this new method breaks such bonds at
temperatures as low as -70 degrees centigrade.
(Nature 23 Nov 00 408:415)
5. ASTROPHYSICS: ON WATER AND THE BIRTH OF STARS
Water may be present in the star-forming region of a molecular
cloud either as a gas or as ice on the surface of dust particles.
The relative abundance of gas to ice depends on the physical and
chemical characteristics of the environment and can thus be used
as a diagnostic parameter to probe different stages of stellar
evolution -- but only if the water molecules can be detected.
Such detection is difficult with ground-based instruments because
of water vapor in Earth's atmosphere. In recent years, this
problem has been overcome by two space missions, the Infrared
Space Observatory (ISO), which ended its activity in 1998, and
the Submillimeter Wave Astronomy Satellite, which has been
operating since the beginning of 1999.
(Science 24 Nov 00 290:1513)
6. ASTROPHYSICS: ON EXTRASOLAR PLANETS
Extrapolating from the small and biased sample of planets that
have been detected to a model of the variety of planetary systems
that may be present elsewhere in the Galaxy is a daunting
challenge fraught with pitfalls. Detailed predictions are almost
certain to be erroneous. However, the substantial progress made
over the past few decades toward understanding the origins and
dynamical stability of planetary systems makes it possible to
assess hypothesized common attributes and scaling relations of
planetary systems in a quantitative manner.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12405)
7. IN BRIEF: FINE PARTICLE AIR POLLUTION AND URBAN MORTALITY
8. IN FOCUS: ON THE ANCIENT GREEKS AND THE FIGURE OF THE EARTH
9. FROM THE SCIENCEWEEK ARCHIVE:
DEVELOPMENT OF NEURAL PATHWAYS IN CHILDREN AND ADULTS
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Section 2
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1. PALEOBIOLOGY: ICHTHYOSAURS
In the late 1400s, Leonardo da Vinci (1452-1519) observed
marine shells far inland where the ocean clearly was absent, and
he correctly concluded that where he stood had once been
inundated with ocean waters. By the early 1800s, the anatomist
Georges Cuvier (1769-1832), studying strata around Paris, noted
that higher strata had a greater proportion of fossil shells with
living counterparts than did lower strata. The increasingly
modern aspect of the fauna is due to the fact that the highest
rocks are closest in time to the present, and thus the faunas
which they contain are most like those of today. Within a
generation of Cuvier, a revolution had occurred in geological
thinking. The Phanerozoic time interval (570 million years ago to
the present), representing that interval of Earth's history
during which there have been organisms with skeletons or hard
shells present, became identified. With a method of relative
dating that uses fossil organisms (biostratigraphy), a variety of
rock outcrops in northeastern Europe were designated as "type
sections", or original locations, where a particular interval of
time is represented. The names of the "Eras", the largest blocks
of time within the Phanerozoic, came from a description of the
life contained within each era. The Eras are, from the oldest to
the youngest:
... ... Paleozoic (570 to 245 million years ago)
... ... Mesozoic (245 to 65 million years ago)
... ... Cenozoic (65 million years ago to the present)
Within each of these eras are smaller subdivisions
(consisting of tens of millions of years each) called "Periods",
and within the periods, in turn, are yet smaller subdivisions of
time called "Epochs" (consisting of several millions of years
each). The Mesozoic Era, of concern in this report, thus consists
of three periods: Triassic Period (245 to 208 million years ago);
Jurassic Period (208 to 144 million years ago); Cretaceous Period
(144 to 65 million years ago).
In paleontology, a "clade" is a group of organisms in which
all members are more closely related to each other than they are
to anything else. All members of a clade apparently share a most
recent common ancestor that is itself the most basal member of
that clade.
During the Mesozoic Era, there burgeoned a clade called
"diapsids", characterized by a number of shared derived features,
including the presence of two temporal openings in the skull
roof. Diapsids include crocodiles, dinosaurs, pterosaurs, and
birds, and more particularly, a group called "ichthyosaurs", a
clade of fierce marine reptiles ("fish-lizards") that dominated
the Mesozoic seas for more than 150 million years. Ichthyosaurs
were shaped much like dolphins, except that the tail flukes are
horizontal in dolphins and vertical in ichthyosaurs. Impressive
ichthyosaur fossils have been known to paleontologists for 200
years, and they were involved in many early discussions of
evolutionary theory because their adaptations for life in the
water were readily visible. The largest ichthyosaurs discovered
have been as long as 15 meters, i.e., the size of a small whale.
Recently, as a consequence of newly available fossils from
China and Japan, there have been important new insights into the
ichthyosaurs and their probable evolution and habitats.
... ... Ryosuke Motani (Royal Ontario Museum Toronto, CA)
presents a review of recent research on ichthyosaur fossils, the
author making the following points:
1) The author points out that what has been evident since
their discovery is that the various adaptations of ichthyosaurs
for life in water made them quite successful, their fossils
revealing that these creatures ruled the oceans from
approximately 245 million until 90 million years ago -- which is
roughly the entire era that dinosaurs dominated the continents.
2) Ichthyosaur fossils have been discovered all over the
world, an indication that they migrated extensively, just as
whales do today. Despite their fish-like appearance, ichthyosaurs
were obviously air-breathing reptiles: they did not have gills,
and the configuration of their skull and jawbones was undeniably
reptilian. In addition, they had two pairs of limbs (fish have
none), which implies that their ancestors once lived on land.
3) The origins of ichthyosaurs baffled paleontologists for
nearly two centuries. They were at times thought to be closely
related to everything from fish to salamanders to mammals, but
they are now known to belong to the diapsid group, diverging at
approximately the same time as the *lepidosaurs and *archosaurs.
It is not yet known whether ichthyosaurs first appeared shortly
before the lepidosaur-archosaur divergence or shortly afterward.
4) One interesting feature of ichthyosaurs is the size of
their eyes -- the largest eyes relative to body size of any adult
vertebrate, living or extinct. For example, the ichthyosaur known
as Temnodontosaurus had eyes 26 centimeters in diameter. Many
characteristics of ichthyosaurs -- including the shape of their
bodies and backbones, the size of their eyes, their aerobic
capacity (as indicated by the size of their rib-cage), and their
habitat and diet -- seem to have changed in a connected manner
during their evolution, and such adaptations enabled ichthyosaurs
to reign for 155 million years. New fossils of the earliest of
these sea dwellers are now making it clear just how they evolved
so successfully for aquatic life, but it is still not known why
ichthyosaurs went extinct. The period of their disappearance
approximately corresponds to the appearance of advanced sharks in
the oceans, but there is no direct evidence of competition
between the two groups.
-----------
Ryosuke Motani: Rulers of the Jurassic seas.
(Scientific American December 2000)
QY: Ryosuke Motani: Royal Ontario Museum Toronto, CA.
-----------
Text Notes:
... ... *lepidosaurs: A major clade of diapsid reptiles that
includes lizards, snakes, and the tuatara of New Zealand.
... ... *archosaurs: A major clade of diapsid reptiles that
includes dinosaurs, pterosaurs, crocodiles, and birds.
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
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2. MEDICAL BIOLOGY: EMERGING VIRAL DISEASES
An argument can be made that after the genome of the living
cell, the most important cellular entity is the ribosome, or more
exactly the ribosomes of the cell, since although each cell
contains only one genome, each cell contains many thousands of
ribosomes. The reason for the importance of ribosomes is simple:
the genome contains programs for the ultimate synthesis of
proteins, the molecules that do the work of the cell, but these
programs are translated into chemical action in the ribosomes --
it is the ribosomes that do the actual synthesis of proteins.
Genetic information flow in a biological cell can be
described as "transcription" to "translation". In general,
transcription is the process whereby genome DNA code is
transcribed into RNA code, ultimately in the form of "messenger
RNA" (mRNA), and messenger RNA in turn is essentially a tape that
feeds into ribosomes one triplet nucleotide base unit (codon) at
a time to program the sequence of amino acids of a polypeptide,
the protein synthesis process called "translation": the messenger
RNA polymer tape goes into the ribosome codon by codon, and the
synthesized protein polymer product comes out of the ribosome
amino acid by amino acid.
In the context of viruses, what is important is that viruses
do not contain the protein catalysts (enzymes) required for their
own replication, and they must therefore parasitize host cells
and by one of various means get the ribosomes of such host cells
to manufacture viral proteins, including the enzymes necessary to
catalyze the replication of the viral genome. The final complete
virus (virion) is thus assembled from components provided by the
host cell, the virus essentially programming the synthesis of its
own parts.
Concerning the genomes of viruses, there are two general
types, the DNA viral genome and the RNA viral genome. Only in
certain viruses do we find genomes consisting of RNA; all
biological cells (and they are all potential hosts for viruses)
contain DNA genomes. There are more than 2500 groups of different
viruses now recognized and at least partially characterized, and
a variety of logical classifications of viruses exist, but
certainly one useful broad classification scheme differentiates
viruses into various DNA viruses and various RNA viruses, with
each type of virus having a more or less different challenge once
it enters a host cell. But in each case, for both DNA and RNA
viruses, the general challenge is the same: directly or
indirectly the viral genome must bring about the production of
the messenger RNAs necessary to get the host ribosomes to produce
the proteins necessary for viral replication.
In general, with some types of RNA viruses, the RNA genome
("plus-sense"; "positive-strand") can itself act as messenger RNA
for host ribosomes; while other types of RNA viruses, the RNA
genome ("minus-sense"; negative-strand) must first produce a
complementary RNA, which then acts as messenger RNA for the host
ribosomes. The replication process in minus-sense RNA viruses is
complex, since host cells do not carry enzymes that can
polymerize complementary RNA from an RNA template, and such
viruses therefore must carry their own special enzymes ("RNA-
dependent transcriptases") to achieve this synthesis.
In this context, the term "segmented genome" refers to a
genome that consists of more than one nucleic acid molecule.
Depending on the type of virus, some viral genomes are segmented,
while others are unsegmented.
... ... S.T. Nichol et al (3 authors at 3 installations, US JP)
present a short review of some emerging diseases caused by RNA
viruses, the authors making the following points:
1) The authors point out that RNA viruses can quickly adapt
to and exploit varying environmental conditions because of the
high error rates of the virus enzymes (polymerases) that
replicate their genomes. It is not surprising, therefore, that
several recent prominent examples of emerging or re-emerging
diseases are caused by RNA viruses. However, a complex interplay
of factors can influence disease emergence. In addition to virus
genetic variation (mutation, *recombination, and *reassortment),
environmental factors (including ecological, social, health care,
and behavioral influences) can play important roles.
2) Influenza virus strains that cause worldwide outbreaks
(pandemics) are classic examples of emerging viruses that are
maintained in non-human animal hosts before transmission to
humans. Influenza viruses are isolated from a variety of animals,
including humans, pigs, horses, wild and domestic birds, and even
sea mammals. The most devastating viral infection of the past 100
years was not that caused by human immunodeficiency virus (HIV),
but by Spanish influenza (1918-1919), which killed more than 20
million people worldwide. Genetic studies suggest that the
Spanish influenza virus originally was derived from birds.
Furthermore, the causative virus for the 1957 and 1968 influenza
pandemics were hybrids between human and avian influenza viruses.
Because humans did not have immunity to avian influenza viruses,
the hybrid viruses produced devastating consequences: 70,000 and
46,500 deaths worldwide in the 1957 and 1958 pandemics,
respectively.
3) Hantaviruses are segmented RNA viruses belonging to the
genus Hantavirus in the family Bunyaviridae (*Note #1).
Hantaviruses are maintained in various rodent reservoirs in which
the hosts are persistently infected without disease symptoms.
Specific hantaviruses transmitted from the contaminated urine and
feces of infected rodents cause two important human diseases,
*hemorrhagic fever with renal syndrome and hantavirus pulmonary
syndrome. Annually, hundreds of thousands of cases of the former
are reported throughout Euro-Asia, whereas hundreds of cases of
the latter are reported in North and South America. Because
rodents act as the natural reservoirs for hantaviruses and human-
to-human infections are rare, understanding the ecology of
hantaviruses within their natural reservoir is important for
preventing and controlling the emergence of such diseases. The
comparison of many hantavirus genomes from different rodent
species has shown a clear correlation between the rodent species
and the virus genotype, suggesting that hantaviruses have co-
evolved with their natural hosts for more than 20 million years,
since before the first humans evolved.
4) Ebola virus is a nonsegmented RNA virus, which together
with Marburg virus makes up the "filovirus" family. This now
notorious group of viruses was discovered in 1967 when Marburg
virus was identified as the etiologic agent of a hemorrhagic
fever outbreak in research facilities in Europe that handled
tissues from African green monkeys imported from Uganda.
Subsequently, Ebola viruses were shown to be the cause of
simultaneously occurring hemorrhagic fever outbreaks in 1976 in
the Democratic Republic of Congo (formerly Zaire) and Sudan.
These outbreaks were shown to be caused by two different subtypes
of Ebola virus, which became known as the Zaire and Sudan
subtypes. Mortality rates of up to 80 percent occurred in these
and more recent outbreaks in 1995-1996. Despite considerable
efforts to identify the natural reservoir for Ebola and Marburg
viruses, the host species remains unknown.
5) Nipah virus is a newly discovered member of the
paramyxovirus family of nonsegmented RNA viruses. This virus was
responsible for a viral encephalitis outbreak in Malaysia that
was first recognized in October 1998 and ended in midsummer 1999.
This outbreak resulted in almost 300 confirmed infections, with a
mortality rate for hospitalized infections of approximately 35
percent. Fruit bats of the genus Pteropus have been implicated as
the likely virus reservoir. The virus was apparently first
introduced into pigs, where close contact caused by intensive
farming practices led to efficient pig-to-pig transmission, and
subsequently pig-to-human transmission. Virtually all human cases
were in close proximity to the infected pigs.
6) The authors conclude: "These examples highlight the
subtle balance of environmental and genetic factors that can mold
the diverse evolutionary patterns observed for RNA viruses and
illustrate the complexity of these systems, which makes it
difficult to predict future viral disease emergences."
-----------
S.T. Nichol et al: Emerging viral diseases.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12411)
QY: Stuart T. Nichol: stn@cdc.gov
-----------
Text Notes:
... ... *recombination: In this context, in general, the term
"recombination" refers to the mixing of genes from two or more
types, strains, etc: i.e., a process that produces a mixed genome
that differs from the original genome by containing parts of the
genome(s) of one or more other types of viruses.
... ... *reassortment: In this context, in general, the term
"reassortment" refers to the mixing of genome and protein coat in
a virus: i.e., a process that produces a virus of one type that
contains the protein coat of a virus of another type. Since the
immune system is usually responding to the protein coat of a
virus, a reassortment, for example, that provides a formerly
docile virus with a protein coat not initially recognized by the
immune system can make that virus suddenly extremely dangerous.
... ... *Note #1: Although many authors insist on labeling
viruses with the standard nomenclature used for animals and
plants, it can be argued that since it not clear that viruses
have a common origin, a classification scheme involving family,
genus, species, etc. (Linnaean classification) is not justified.
... ... *hemorrhagic fever: The hemorrhagic fevers, of which
there are a number of types, are marked by a cluster of severely
debilitating symptoms, and especially by capillary bleeding
(hemorrhaging).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CLONAL INTERFERENCE AND THE EVOLUTION OF RNA VIRUSES
Studies of the dynamics of evolution indicate that
populations adapt through the appearance and subsequent fixation
of beneficial mutations. In large populations, beneficial
mutations may arise frequently enough so that two or more are
simultaneously present in independent lineages. Once beneficial
mutations have arisen, there is a certain probability of losing
them by "*genetic drift" while their frequency is low, but after
an early period dominated by genetic drift, beneficial mutations
reach a substantial frequency in the population. In a sexual
system, these beneficial mutations will eventually combine,
ensuring their fixation together. But if the system is asexual,
the lineages created by beneficial mutations will compete, and
only the mutation with the largest effect will be fixed. This
competition of beneficial mutations in asexual populations is
called "*clonal interference", and it is a phenomenon that
ensures that beneficial mutations that do achieve fixation are of
large effect.
Excluding *prions, whose categorization is still uncertain,
viruses are the smallest infectious agents, ranging from
approximately 20 to 300 nanometers in diameter, and containing
only one kind of nucleic acid (RNA or DNA) as their genome. In
general, the nucleic acid is encased in a protein shell, which
may be surrounded by a lipid-containing membrane, and the entire
infectious unit is termed a "virion". Viruses are inert in the
extracellular environment, replicating only in living cells, and
they are essentially genetic-level parasites: the viral nucleic
acid contains information necessary for programming the infected
host cell to synthesize various virus-specific macromolecules
required for the production of viral progeny [*Note #1].
RNA viruses are viruses whose genome consists of RNA, and
they are involved in a number of serious human diseases,
including hemorrhagic fevers and human immune deficiency syndrome
(AIDS). One important characteristic of RNA viruses is that as a
group they show the highest mutation rates in nature. This,
together with their potentially large effective population sizes,
and the fact that their reproduction can be asexual, suggests
that clonal interference may play an important role in their
adaptive evolution.
... ... R. Miralles et al (4 authors at 2 installations, ES US)
report a study of clonal interference in an RNA virus population
(*vesicular stomatitis virus), the study involving two variants
differing only in their ability to grow in the presence of a
*monoclonal antibody. The authors report their results provide
evidence that clonal interference does indeed occur in viral
populations, and that this evidence along with models of clonal
interference allows certain properties of the adaptive evolution
of RNA viruses to be inferred. The authors suggest the following:
1) Adaptive substitutions appear as discrete rare events,
regardless of mutation rate or population size. They often do not
occur simply as the result of a single mutation but instead
represent the best of several competing mutations. The authors
suggest this fact has consequences for the dynamics of drug
resistance and the search for resistance mutations.
2) In medium to large populations, the rate of fitness
increase is hardly affected by changes in either mutation rate or
population size.
3) Resident populations are protected from invaders simply
because of their numerical advantage. A high-fitness vesicular
stomatitis viral clone seeded at low frequency into a resident
population of low-fitness variants was displaced by the low-
fitness competitors. When its initial frequency was above a
certain threshold, however, the high-fitness clone always
outcompeted the low-fitness variants in the resident population.
The authors suggest that the existence of a frequency threshold
for dominance imposes an element of uncertainty in virus sampling
during disease outbreaks.
-----------
R. Miralles et al: Clonal interference and the evolution of RNA
viruses.
(Science 10 Sep 99 285:1745)
QY: Santiago F. Elena: santiago.elena@uv.es
-----------
Text Notes:
... ... *genetic drift: The term "genetic drift" refers to the
random fluctuations of gene frequencies due to sampling errors.
Genetic drift occurs in all populations, but its effects are most
evident in populations that are small.
... ... *clonal interference: In this context, a "clone" is a
lineage derived from a single ancestor.
... ... *prions: Prions are a class of poorly understood proteins
implicated in a number of exotic human neurological diseases and
in some common animal diseases such as sheep scrapie and bovine
spongiform encephalopathy in cattle ("mad cow disease").
... ... *Note #1: Over 4000 animal and plant viruses have been
identified (as of 1995), these entities categorized into 71
families, 11 subfamilies, and 164 genera, with hundreds of
viruses still unassigned. 24 families contain viruses that infect
humans and animals. Classical categorizations of viruses were
based on the diseases they produces, but modern categorizations
are based on molecular biological parameters.
... ... *vesicular stomatitis virus: This virus causes a disease
of cattle. The viral entity is bullet-shaped, approximately 75
nanometers in diameter, 180 nanometers in length, the genome
single-stranded RNA, 13-16 kilobases in size. The virus
replicates in cell cytoplasm, with viral assembly involving
budding from the cell membrane.
... ... *monoclonal antibody: In general, a monoclonal antibody
is an immunoglobulin protein derived from a single clone of
plasma cells. Such antibodies are chemically and structurally
identical and constitute a pure population with highly specific
antigen-binding properties. In general, an "antigen" is any
chemical entity that activates an immune response, especially an
entity originating outside the body, and an "antibody" is a
specific immunoglobulin protein produced by an immune cell, the
protein specifically binding a particular antigen. In the context
of this report, a typical antigen would be a moiety in the
surface coat of the virus.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Nov99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
3. MEDICAL BIOLOGY: GENETIC COMPONENTS IN PARKINSON'S DISEASE
Parkinson's disease (also called Parkinson disease) is a
slowly progressive degenerative central nervous system disorder
characterized by decreased movement, muscular rigidity, resting
tremor, and postural instability. The disease was first described
by James Parkinson (1755-1824) in 1817 and is now known to be
associated with degeneration of one or more specific regions of
the brain (dopaminergic neuron groups) and resultant loss of
neural connections (projections) from these groups to several
important brain centers. Dopaminergic neurons are nerve cells
that use dopamine as a *neurotransmitter substance. Dopamine is
found in several major areas of the brain, and it is the
degeneration of so-called dopamine neurons that is apparently
involved in Parkinson's disease. The disease has an early-onset
form (age =< 50 years), and a childhood form (juvenile
Parkinson's disease), but most cases are adult onset, with
incidence increasing markedly past age 50 years.
The causes of Parkinson's disease are largely unknown, but
there is evidence that the disease has a genetic component. In a
few large families with early onset Parkinson's disease or
juvenile Parkinson's disease, the disease is transmitted as an
autosomal dominant or recessive trait resulting from mutations in
the genes encoding the proteins alpha-synuclein and parkin,
respectively. However, in the majority of families affected by
Parkinson's disease, the disease appears to skip generations,
irrespective of the age of onset. Therefore, this disease is
considered a complex, multifactorial disease resulting from
interaction between one or more genes and the environment.
... ... S. Sveinbjoernsdoettir et al (9 authors at 2
installations, IS US) report a study of familial aggregation of
Parkinson's disease in Iceland, the authors making the following
points:
1) The authors reviewed the medical records and confirmed
the diagnosis of Parkinson's disease in 772 living and deceased
patients in whom the disease had been diagnosed during the
previous 50 years in Iceland. With the use of an extensive
computerized database containing genealogical information on
610,920 people in Iceland during the past 11 centuries, several
analyses were conducted to determine whether the patients were
more related to each other than random members of the population
(the control subjects).
2) Patients with Parkinson's disease, including a subgroup
of 560 patients with late-onset disease, were significantly more
related to each other than were subjects in matched groups of
controls, and this relatedness extended beyond the nuclear
family. The risk ratio for Parkinson's disease was 6.7 for
siblings, 3.2 for offspring, and 2.7 for nephews and nieces of
patients with late-onset Parkinson's disease.
3) In summary, the authors suggest late-onset Parkinson's
disease has a genetic component as well as an environmental
component. The authors conclude: "There has been a recent trend
to discount the possibility that genetic factors contribute to
the late-onset form of the disease, which represents the majority
of cases of Parkinson's disease. Although the search for
environmental factors contributing to late-onset Parkinson's
disease is important and should continue, our data suggest that
the search to discover its genetic basis should also continue."
-----------
S. Sveinbjoernsdoettir et al: Familial aggregation of Parkinson's
disease in Iceland.
(New England J. Med. 14 Dec 00 343:1765)
QY: Kari Stefansson: kstefans@decode.is
-----------
Text Notes:
... ... *neurotransmitter substance: See notes to background
report below.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MEDICAL BIOLOGY:
LINK BETWEEN HOME PESTICIDE EXPOSURE AND PARKINSON'S DISEASE
... One must distinguish "parkinsonism" from Parkinson's
disease. Parkinsonism is a syndrome (a complex of symptoms; in
this context, a complex of various movement symptoms) that may be
caused by Parkinson's disease, but which may also be caused by
infectious, vascular, pharmacological, toxic, metabolic,
structural, and various degenerative disorders. In other words,
not every individual with parkinsonism has Parkinson's disease.
The major differentiating characteristic is the response to the
drug "*levodopa", which is converted by the body into dopamine.
Individuals with parkinsonism who respond to levodopa treatment
receive a diagnosis of Parkinson's disease. At the present time,
Parkinson's disease is the 4th most common neurodegenerative
disease of the elderly. It affects approximately 1 percent of
people older than 65 years, and 0.4 percent of people between 40
and 65 years.
... ... Joan Stephenson (J. Amer. Med. Assoc., US) reviews a
presentation by Lorene Nelson (Stanford University) at a recent
meeting of the American Academy of Neurology, at which meeting
Nelson presented evidence of a link between home pesticide use
and Parkinson's disease. Stephenson makes the following points:
1) The study involved 496 patients diagnosed with
Parkinson's disease within the Kaiser Permanente Medical Care
Program of Northern California during the years 1994-1995, and
541 age- and sex-matched controls from the same population. Using
in-person structured interviews, a research team collected
information about lifetime history of exposure to home pesticides
(herbicides, insecticides, and fungicides) prior to diagnosis.
2) After controlling for known risk factors such as family
history of the disorder, occupational exposure to pesticides and
herbicides, and cigarette smoking, the investigators found that
home exposure to insecticides and herbicides were associated with
an increased risk of Parkinson's disease. Fungicide exposure was
not linked with an increased risk of the disorder.
3) Individuals with high-level herbicide exposure had a 70
percent increased risk compared with those who were not exposed.
People who used insecticides in the garden showed a 50 percent
increased risk compared to those who had never been exposed to
home pesticides of any type. In-home use of insect-killing
chemicals was associated with a 70 percent increased risk of
Parkinson's disease compared with no use of pesticide.
4) In her report to the American Academy of Neurology,
Lorene Nelson, a neuroepidemiologist, pointed out that the idea
that pesticides might be linked with Parkinson's disease is
biologically plausible, since many pesticides are neurotoxic and
may affect various aspects of central nervous system function,
possibly even resulting in the death of specific nerve cells.
Previous studies have found a substantially increased rate of
Parkinson's disease among city dwellers who gardened for a hobby.
-----------
Joan Stephenson: Exposure to home pesticides linked to Parkinson
disease.
(J. Amer. Med. Assoc. 21 Jun 00 283:3055
QY: Joan Stephenson editors@jama.com
-----------
Text Notes:
... ... *neurotransmitter substance: 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.
... ... *levodopa: (L-dopa) The biologically active form of
"dopa", which is converted into dopamine. Dopamine = 3,4-
dihydroxyphenylethylamine. Dopa = 3,4-dihydroxypheynylalanine.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Aug00
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. CHEMISTRY: TRANSITION METAL CATALYSIS
AND THE CARBON-CARBON BOND
In general, a "catalyst" is a substance which when added to
a reaction changes the rate at which equilibrium is attained. A
positive catalyst increases the rate; a negative catalyst
decreases the rate. In many cases, little or no permanent
chemical change occurs in the catalyst. Most catalysts of
interest are positive catalysts, i.e., substances that markedly
increase the rate of a chemical reaction, and in many instances
they are of enormous industrial importance [see related
background material below].
Certain metals are well known as catalysts in biochemistry,
synthetic chemistry, and industrial chemistry, and this is
particularly true of the transition metals, i.e., metals of
atomic numbers 22 to 28, 40 to 46, and 72 to 78. An important
goal in the chemistry of the transition metals is to understand
why these metals so effectively catalyze such a wide variety of
reactions. Typical transition metal catalysts consist of a
transition metal atom surrounded by a set of ligands that
determine the catalytic behavior of the central metal. In this
context, the most significant property of transition metals
(transition elements) is apparently that they have a partially
filled d shell, ranging from 1 d electron to 10 d electrons.
Concerning catalysis, one of the most important objectives
in synthetic and industrial chemistry involves the rapid cleavage
of a carbon-carbon bond by inserting a metal atom between two
carbons (the process called "oxidative addition). This process
"activates" the bond and allows potentially more useful molecules
to be made from relatively inactive unreactive starting
materials. A full characterization of this catalytic process has
now been reported by M. Gandelman et al (J. Amer. Chem. Soc.
122:9848 2000), the authors investigating cleavage of the carbon-
carbon bond with a *rhodium complex, and amplifying work first
reported in 1997. Although previous carbon-carbon bond breaking
reactions have generally required temperatures of at least 100
degrees centigrade, the authors report their method breaks such
bonds at temperatures as low as -70 degrees centigrade.
In a commentary on this work, Robert H. Crabtree (Yale
University, US) states that the results of Gandelman et al
"indicate that if the metal reagent can be brought close enough
to the carbon-carbon bond, the actual barrier to the oxidative
addition is relatively low. The next step is to investigate
whether this concept can be developed for use in industrial
processes."
-----------
Robert H. Crabtree: Clipping the carbon-carbon bond.
(Nature 23 Nov 00 408:415)
QY: Robert H. Crabtree: robert.crabtree@yale.edu
-----------
Text Notes:
... ... *rhodium complex: The complex of rhodium used was
[(C(sub2)H(sub4))(sub2)RhCl](sub2)
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
POLYMER CHEMISTRY: NEW CATALYSTS IN OLEFIN POLYMERIZATION
Although plastic materials (e.g., celluloid and bakelite) were
known in the 19th and early 20th centuries, it was not until the
second half of the 20th century that new plastics to replace wood
and metal became ubiquitous in daily life. One of the signal
events in this introduction of new plastics was the discovery in
1953 of the so-called Ziegler catalysts in olefin polymerization.
Olefins are hydrocarbons containing at least one double bond
between adjacent carbon atoms, and polyolefins are polymers
constructed of olefin monomers. Polyethylene is a typical
polyolefin and the foundation for a large variety of common
plastics. A Ziegler catalyst is derived from a transition-metal
halide and a metal hydride or metal alkyl, and such a catalyst
can be used to produce stereospecific (stereoregular) olefin
polymers, which are olefin polymers with a specific or definite
ordering of molecules in space (e.g., isotactic polypropylene),
with a consequent close packing of molecules that leads to a high
degree of crystallinity, strength, and durability. Karl Ziegler
(1898-1973) and Giulio Natta (1903-1979) shared the Nobel Prize
in Chemistry in 1963 for their work in discovering and developing
the class of Ziegler catalysts in the synthesis of stereospecific
polymers.
... ... T.R. Younkin et al (6 authors at California Institute of
Technology, US) now report a new class of olefin polymerization
catalysts, the authors making the following points:
1) More than half of the 170 million metric tons of polymers
produced each year are polyolefins. Current technology uses
highly active cationic catalysts (Ziegler catalysts and
metallocene catalysts), which suffer from an inability to
tolerate *heteroatoms such as oxygen, nitrogen, and sulfur. These
systems require both scrupulously clean starting materials and
activating cocatalysts.
2) The authors report the development by rational design of
a family of nickel-complex organic catalysts whose members are
tolerant of both heteroatoms and less-pure starting materials.
These heteroatom-tolerant neutral *late transition metal
complexes are highly active systems that produce high-molecular-
weight polyethylene, polymerize functionalized olefins, and
require no cocatalyst.
3) The authors suggest that the catalytic properties of the
family of neutral, single-component, late-transition metal olefin
polymerization catalysts which they have synthesized indicates
that a cationic metal center is not required to achieve high
polymerization activity. The authors conclude: "The promise of
these catalysts is realized in their ability to polymerize
ethylene in the presence of functional additives such as ethers,
ketones, esters, alcohols, amines, and water, and in their
incorporation of polar monomers into the polymer backbone in
variable quantities."
-----------
T.R. Younkin et al: Neutral, single-component nickel (II)
polyolefin catalysts that tolerate heteroatoms.
(Science 21 Jan 00 287:460)
QY: Robert H. Grubbs, California Inst. of Technology 818-395-6811
-----------
Text Notes:
... ... *heteroatoms: In general, "heteroatoms" are any atoms
other than carbon and hydrogen in an organic compound.
... ... *late transition metal: The "early transition metals" are
transitions metals on the left side of the periodic table; the
"late transition metals" are transition metals on the right side
of the periodic table.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 28Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON CATALYSIS AND SURFACE SCIENCE
The term "catalysis" was invented in 1835 by the chemist J.J.
Berzelius (1779-1848) to describe chemical reactions in which the
progress of the reaction is affected by a substance that is not
consumed in the reaction and thus apparently not involved in the
reaction. But this definition, proposing no interaction by the
catalyst with reactants, is not useful, and it was later amended
by W. Ostwald (1853-1952), who proposed the more modern
definition: "A catalyst is a substance that accelerates the rate
of a chemical reaction without being part of its final products."
Essentially, as is now recognized, the catalyst acts by forming
intermediate compounds with the molecules involved in the
reaction, providing an alternate and more rapid path to the final
products. Catalysis is of vital importance: In biological
systems, enzymes are essential catalysts for various biosynthetic
pathways; in the chemical and petroleum industries, key processes
are based on catalysis; in environmental chemistry, catalysts are
essential to breaking down pollutants such as automobile and
industrial exhausts. ... ... G. Ertl and H-J. Freund (Friz Haber
Institute Max Planck Society Berlin, DE) present a review of the
basics of catalysis, the authors making the following points: 1)
If the catalyst and the reacting species are in the same phase
(e.g., in a liquid), then the process is known as "homogeneous
catalysis". More relevant in industrial processes is
"heterogeneous catalysis", where the catalyst is a solid and the
reacting molecules interact with the surface of the solid from
the gaseous or liquid phases. 2) The economic significance of
heterogeneous catalysis is reflected in the fact that the world
market for solid catalysts in the automotive, petroleum, and
other industries is of the order of US$100 billion per year and
growing rapidly. 3) In heterogeneous catalysis, the chemical
transformation typically occurs in a flow reactor through which
the reacting species pass. Atoms in the surface of the catalyst
may form chemical bonds with atoms in impinging molecules, a
phenomenon known as "chemisorption". If existing bonds in the
impinging molecule break, the process is known as "dissociative
chemisorption". The chemisorbed species are mobile on the surface
and may bond to other particles, thus leading to new molecules,
which eventually leave the surface (desorb) as the desired
reaction products. 4) Detailed identification and
characterization of these elementary processes of heterogeneous
catalysis are hampered by several fundamental problems: a) The
reacting systems exist merely as 2-dimensional phases for which
most of the usual methods of investigation are not well suited.
b) The surfaces of real catalysts are typically inhomogeneous as
a result of methods to increase catalytic efficiency. For
example, because in heterogeneous catalysis efficiency, in
general, increases with total surface area of the solid catalyst,
finely divided particles are usually applied to a support
material which is only relatively inert. Also, catalytic activity
is often further enhanced by the addition of compounds called
"promoters". At the present time, analysis of the fundamentals of
heterogenous catalysis is largely dependent on the use of surface
science models, real but simple systems such as single crystal
surfaces whose structure may be varied by choosing different
surface orientations.
-----------
G. Ertl and H-J. Freund: Catalysis and surface science.
(Physics Today January 1999)
QY: Gerhard Ertl, Fritz Haber Institute MPG, Berlin, DE.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Apr99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. ASTROPHYSICS: ON WATER AND THE BIRTH OF STARS
One of the great achievements of 20th century science was
the detailed quantitative understanding of star formation. In
general, a star is born when a massive clump of gas contracts
under its own gravity, an idea first introduced by the
mathematician and physicist Pierre Simon de Laplace (1749-1827).
Laplace proposed that a rotating cloud of gas, as it pulled
itself together, would flatten into a disk, and the central
portion of the disk would gather itself into a ball to form a
star, while the outlying regions of the disk coalesced into
planets. Although physics and mathematics were not advanced
enough during the time of Laplace to develop the details of this
model, the general idea is still a good qualitative description
of the process that creates new stars.
Today we know that the most likely stellar nurseries are
gigantic molecular clouds, huge aggregations of cold gas, these
clouds containing a significant number of molecules, rather than
merely solitary atoms. As each would-be star collapses due to its
self-gravity, the gas retains its spin (angular momentum), and
forms a disk similar to that imagined by Laplace. The collapse
compresses the gas, causing the gas to increase in temperature.
Some of the rotational energy of the gas is carried away,
possibly by magnetic fields threading the cloud, allowing further
collapse and compression at the center. Eventually, most of the
matter accumulates at the center, while the rest remains in an
encircling disk. The central sphere of condensed gas, now a
"protostar", continues to contract and heat. As its temperature
rises, more and more of its hydrogen ionizes. Free electrons
scatter and absorb photons very effectively, so the more
electrons that are liberated, the more opaque the protostar
becomes. If photons cannot escape from the gas, their energy is
trapped within the protostar, causing the temperature to rise
even further. Finally, the temperature within the core of the
protostar rises to a sufficient level to ignite nuclear fusion,
and the energy generated from this process provides the newborn
star with the pressure required to prevent further collapse.
Thus, a star is born. The entire gravitational condensation
followed by nuclear fusion ignition may take 100 million years;
if the star is of "ordinary" mass (like our Sun), it will then
survive approximately 10 billion years before its nuclear fuel is
exhausted and it begins the phase of star death.
... ... Brunella Nisini (Rome Astronomical Observatory, IT)
presents a commentary on recent research on the role of water in
star formation, the author making the following points:
1) The author points out that the water molecule plays a
fundamental role during the first stages of star formation. Water
is an important oxygen reservoir in the warm environments of
star-forming regions and is believed to contribute substantially
to the cooling of the circumstellar gas disk, thereby helping to
remove the excess energy accumulating during protostellar
collapse.
2) Water may be present in the star-forming region of a
molecular cloud either as a gas or as ice on the surface of dust
particles. The relative abundance of gas to ice depends on the
physical and chemical characteristics of the environment and can
thus be used as a diagnostic parameter to probe different stages
of stellar evolution -- but only if the water molecules can be
detected. Such detection is difficult with ground-based
instruments because of water vapor in Earth's atmosphere. In
recent years, this problem has been overcome by two space
missions, the Infrared Space Observatory (ISO), which ended its
activity in 1998, and the Submillimeter Wave Astronomy Satellite,
which has been operating since the beginning of 1999.
3) According to current models, as gravitational collapse
proceeds in a molecular cloud, the radiation energy released by
the accretion of matter onto the newly born protostar is absorbed
by the dense and thick circumstellar envelope, which quickly
heats up as a result. At the same time, protostars also violently
eject mass in the form of bipolar jets, which transport the
excess energy and angular momentum away from the central object.
When the jets encounter the dense ambient medium, they produce
strong shock waves that can increase the gas temperature up to a
few thousand degrees kelvin.
4) Concerning water, the Infrared Space Observatory was able
to confirm, for the first time, the expectations of current
models of star formation. The observatory detected a large amount
of warm gaseous water in active star-forming regions such as the
cluster of protostars inside the Orion BN region, which shows a
very strong and complex water spectrum originating both from
shocked gas and from the hot core surrounding the cluster. The
infrared water abundance exceeds 10^(-4) relative to molecular
hydrogen, thus confirming that large amounts of gas phase water
are produced once the protostar is formed.
-----------
Brunella Nisini: Water's role in making stars.
(Science 24 Nov 00 290:1513)
QY: Brunella Nisini: bruni@coma.mporzio.astro.it
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. ASTROPHYSICS: ON EXTRASOLAR PLANETS
Currently, the identification and study of extrasolar planets
depends for the most part on indirect methods such as those
involving the measurement of perturbations of the observed
brightness or motions of their parent stars. The ideal method
would be direct imaging of extrasolar planets, and this would
considerably enhance the possibilities for understanding their
nature. A major problem in direct imaging of extrasolar planets
is that the bright light from the parent star (more particularly,
its diffracted halo in the imaging apparatus) can easily
overwhelm nearby faint light sources such as orbiting planets.
... ... J.J. Lissauer et al (3 authors at 3 installations, US JP)
present a review of current research on extrasolar planets, the
authors making the following points:
1) The authors point out that more than two dozen extrasolar
planets have been detected during the 1990s by observations of
the wobble that results from their gravitational tugs on the
stars to which they are bound. These extrasolar planets
demonstrate the large diversity of planetary systems, and current
research aims at detecting an even greater variety of extrasolar
planetary systems, and also aims at systematically explaining the
origins of extrasolar planets and the origin of our Solar System.
2) Several research groups have successfully pursued an
indirect method of detecting extrasolar planets, the method
making use of Newton's Second Law, i.e., "for every action, there
is an equal and opposite reaction." Thus, it is stellar wobble
that betrays the existence of an invisible orbiting planet. The
greater the wobble, the more massive the planet, and the time to
complete one wobble cycle is the orbital period of the planet.
The Doppler effect has been used to detect these small stellar
movements: As a star travels toward the observer, the light waves
are shortened toward the blue; conversely, as a star moves away
from Earth, the wavelengths are lengthened toward the red. These
Doppler shifts are extremely small. In our Solar System, the Sun
wobbles by only 12.5 meters per second because of the presence of
Jupiter; Saturn induces variations of 2.7 meters per second on a
longer time scale, and the effect of other planets is
substantially less. A reliable detection of this wobble requires
precision of 3 meters per second, which is equivalent to
detecting changes in the wavelengths of starlight by 1 part in
10^(8). The periodic wobble of a star, analyzed with Newton's
laws, provides the planet's orbital period, the orbital distance,
and the mass of the planet multiplied by the unknown parameter
sin(i), where (i) is the inclination of the planet's orbital
plane to the line of sight.
3) After a century of hopeful but dubious claims, evidence
for planets around other stars finally appears robust. Surveys of
normal stars indicate that 5 percent of such stars harbor
planetary companions having masses 0.5 to 8 times that of
Jupiter, and orbital periods of a few years or less. Within that
mass range, low-mass planets are more common. To date, 28
extrasolar planet candidates are known. Their orbits are either
very small or quite elliptical, both properties being different
from those of planets within our Solar System.
4) The authors conclude: "Extrapolating from the small and
biased sample of planets that have been detected to a model of
the variety of planetary systems that may be present elsewhere in
the Galaxy is a daunting challenge surely fraught with pitfalls.
Detailed predictions are almost certain to be erroneous. However,
the substantial progress made over the past few decades toward
understanding the origins and dynamical stability of planetary
systems makes it possible to assess hypothesized common
attributes and scaling relations of planetary systems in a
quantitative manner."
-----------
J.J. Lissauer et al: Extrasolar planets.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12405)
QY: J.J. Lissauer: lissauer@ringside.arc.nasa.gov
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
... ... A.C. Cameron et al (4 authors at 2 installations, UK) now
report detection and measurement of starlight reflected from an
extrasolar planet, the authors making the following points:
1) In the 4 years following the discovery (by M. Mayor and
D. Queloz [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. This indirect technique cannot
investigate the radius or composition of the planet, and can
place only a lower limit on the mass of the planet.
2) The authors report the probable detection of *Doppler-
shifted starlight reflected from the planet known to orbit tau-
Bootis with a period of just 3.3 days. The authors find that the
*orbital inclination of the planet is approximately 29 degrees,
from which the authors infer that the mass is approximately 8
times that of Jupiter, and that the planet has the size and
reflectivity expected for a "*gas-giant planet".
3) The authors point out that if this is indeed a giant
planet, with a Jupiter size and *albedo, the scattered starlight
will be 10,000 to 20,000 times fainter than the parent star even
under the most favorable planet-illumination conditions.
4) The authors conclude: "Our candidate detection of
starlight scattered from the atmosphere of an extrasolar planet
strengthens the case for the existence of the giant, close-
orbiting planets whose presence has so far been inferred only
indirectly from the reflex motions of their parent stars... The
close-orbiting planets of other systems, including both 51 Pegasi
and the nu-Andromedae triple-planet system, should be amenable to
similar studies in the near future."
-----------
A.C. Cameron: Probable detection of starlight reflected from the
giant planet orbiting tau-Bootis.
(Nature 16 Dec 99 402:751)
QY: Andrew Collier Cameron: andrew.cameron@st-and.ac.uk
-----------
Text Notes:
... ... *Doppler-shifted starlight: In general, the term "Doppler
shift" refers to the change in wavelength of electromagnetic
radiation as a result of relative movement between the source and
the observer.
... ... *orbital inclination: In general, the angle between the
orbital plane of a body and a reference plane centered on the
object about which the body is revolving. In this context, the
orbital inclination of the planet is not directly measured, but
is implied by a "best-fitting" orbital velocity.
... ... *gas-giant planet: In general, a planet of much larger
mass and diameter than the Earth, and which consists mostly of
gas. In our own Solar System, Jupiter, Saturn, Uranus, and
Neptune are gas-giant planets.
... ... *albedo: In this context, the fraction of total starlight
falling on a planet that is reflected from it. In general, the
albedo is equal to the amount of light reflected divided by the
amount of light received.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 11Feb00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
GIANT PLANETS VS. BROWN DWARFS
Filipe D. Santos (Centro de Fisica da Universidade de Lisboa, PT)
presents a short review of current ideas concerning giant
extrasolar planets and *brown dwarf stars. The author makes the
following points: 1) The recent discoveries of planets orbiting
nearby Sun-like stars have revealed that planetary systems can be
surprisingly diverse. The initial discovery in 1995 of the planet
around the star 51 Pegasi was a surprise because it is apparently
a planet with mass about that of Jupiter (at least 0.44 Jupiter-
mass) and an orbital period of only 4.2 days, which implies that
the planet is 20 times closer to its star than Earth is to the
sun. 2) Seven additional planets around solar-type stars have
since been discovered, with Jupiter-mass values ranging from 0.44
to 6.84. 3) Two critical questions are, a) Where should we set
the dividing line that distinguishes massive planets from brown
dwarfs? and, b) What are the mechanisms leading to the formation
of massive planets and brown dwarfs? 4) Brown dwarfs are expected
to have masses smaller than the hydrogen-burning limit of
approximately 0.075 solar-mass (approximately 75 Jupiter-mass),
but probably larger than the deuterium-burning limit of 0.013
solar-mass (approximately 13 Jupiter-mass). 5) Like the companion
massive planets mentioned, several companion brown dwarfs to
solar-type stars have also been identified. One method of
investigating brown dwarfs involves *astrometric measurements,
and in all cases of brown dwarfs investigated by the astrometric
method, the masses are above or very close to the hydrogen-
burning limit. The extant data thus suggest that the distribution
of mass of brown dwarfs does not extend to masses as small as
giant planets. Also, the new measurements indicate that brown
dwarfs orbiting solar-type stars are very rare. 6) The discovery
of Jupiter-mass planets with orbits very close to their stars
poses a considerable problem, because it is difficult to
understand how such planets could form in place. (Five known
Jupiter-mass planets have orbital radii smaller than the distance
from Mercury to the Sun.) The suggestion has been made that these
planets formed at larger distances and migrated inward, but the
proposed migration mechanisms are not yet empirically
distinguishable. The author concludes: "Clearly the discovery of
planetary systems outside our solar system has opened a Pandora's
box of startling phenomena and new questions."
QY: Filipe D. Santos fdsantos@milkyway.cii.fc.ul.pt
(Science 17 Jul 98 281:359) (ScienceWeek 31 Jul 98)
-------------------
Related Background:
... ... *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 "theoretical"
stars, with no brown dwarfs considered to be unambiguously
identified.
... ... *astrometric measurements: This method of detection
infers the presence of a companion to a star by measuring the
position of the star as it orbits the center of mass of the
entire system. From the orbital inclination, the real mass of the
companion can be derived.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 31Jul98
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
7. IN BRIEF: FINE PARTICLE AIR POLLUTION AND URBAN MORTALITY
J.M. Samet et al (5 authors Johns Hopkins University, US)
assessed the effects of 5 major outdoor-air pollutants on daily
mortality rates in 20 of the largest cities and metropolitan
areas in the US from 1987 to 1994. The pollutants were
particulate matter less than 10 microns in *aerodynamic diameter,
ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The
authors report they found consistent evidence that the level of
particulate matter less than 10 microns in aerodynamic diameter
is associated with the rate of death from all causes and from
cardiovascular and respiratory illnesses. Weaker evidence
indicated that increases in ozone levels increased the relative
rates of death during the summer, when ozone levels are highest,
but not during the winter. Levels of the other pollutants were
not significantly related to the mortality rate. The authors
conclude: "There is consistent evidence that the levels of fine
particulate matter in the air are associated with the risk of
death from all causes and from cardiovascular and respiratory
illnesses. These findings strengthen the rationale for
controlling the levels of respirable particles in outdoor air."
-----------
J.M. Samet et al: Fine particulate air pollution and mortality in
20 US cities 1987-1994.
(New England J. Med. 14 Dec 00 343:1742)
QY: Jonathan M. Samet: jsamet@jhsph.edu
-----------
Text Notes:
... ... *aerodynamic diameter: In this context, the term
"aerodynamic diameter" is defined as the diameter of a unit-
density sphere that has the same settling velocity in gas as the
particle of interest.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Dec00
For more information: http://scienceweek.com/swfr.htm
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8. IN FOCUS: ON THE ANCIENT GREEKS AND THE FIGURE OF THE EARTH
"Pythagoras (582-507 B.C.) and his followers were apparently the
first to speculate that the Earth was a sphere. This idea was
further propounded by the influential philosopher Aristotle (384-
322 B.C.). Although he taught the scientific principle that
theory must follow fact, Aristotle is responsible for the logical
device called syllogism, which can explain correct observations
by apparently logical accounts that are based on false premises.
His influence on scientific methodology was finally banished by
the scientific revolution in the 17th century. The first
scientifically sound estimate of the size of the terrestrial
sphere was made by Eratosthenes (275-195 B.C.), who was the head
librarian at Alexandria, a Greek colony in Egypt during the 3rd
century B.C. Eratosthenes had been told that in the city of Syene
(modern Aswan) the Sun's noon rays on midsummer day shone
vertically and were able to illuminate the bottoms of wells,
whereas on the same day in Alexandria shadows were cast. Using a
sun-dial, Eratosthenes observed that at the summer solstice the
Sun's rays made an angle of one-fiftieth of a circle (7.2
degrees) with the vertical in Alexandria. Eratosthenes believed
that Syene and Alexandria were on the same Meridian. In fact they
are slightly displaced... Syene is actually about half a degree
north of the tropic of Cancer. Eratosthenes knew that the
approximate distance from Alexandria to Syene was 5000 stadia,
possibly estimated by travellers from the number of days ('10
camel days') taken to travel between the two cities. From these
observations, Eratosthenes estimated that the circumference of
the global sphere was 250,000 stadia. The Greek _stadium_ was the
length (about 185 meters) of the U-shaped racecourse on which
foot races and other athletic events were carried out.
Eratosthenes' estimate of the Earth's circumference is equivalent
to 46,250 kilometers, about 15 percent higher than the modern
value of 40,030 kilometers."
-----------
William Lowrie: _Fundamentals of Geophysics_
(Cambridge University Press, Cambridge UK 1997, p.29)
-------------------
SCIENCE-WEEK http://scienceweek.com 15Dec00
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9. FROM THE SCIENCEWEEK ARCHIVE:
DEVELOPMENT OF NEURAL PATHWAYS IN CHILDREN AND ADULTS
The human brain is not a finished organ at birth -- in fact,
another 10 or 12 years are needed before even a general
development is completed. Structural maturation of individual
brain regions and their connecting pathways is required for the
successful development of cognitive, motor, and sensory
functions. This maturation eventually provides for a smooth flow
of neural impulses throughout the brain, which allows for
information to be integrated across the many spatially segregated
brain regions involved in these functions. The speed of neural
transmission is an important factor, and this depends not only on
the junctions between nerve cells (synapses), but also on the
structural properties of the connecting fibers (axons). Critical
axon structural properties include their diameters and the
thickness of the special insulation (myelin) around many fibers
[*Note #1]. Large groups of myelinated axons, which connect
various regions in the brain, appear visibly as "white matter".
Axons of the major pathways in the human brain, such as those of
the corpus callosum (which connects the two halves of the brain)
or the corticospinal tract (which connects the brain to the
spinal cord and the rest of the body), continue to develop
throughout childhood and adolescence. Postmortem studies suggest
that axon diameters and myelin sheaths undergo conspicuous growth
during the first 2 years of life, but may not be fully mature
before adolescence or even late adulthood. The scarcity of human
brain specimens for postmortem analysis has made it difficult to
draw definite conclusions about the timetable of myelinization
during childhood and adolescence. ... ... T. Paus et al (8
authors at 2 installations, CA US) now report a computational
analysis of structural *magnetic resonance images obtained in 111
living children and adolescents. The authors report the analysis
reveals age-related increases in white-matter density in fiber
tracts constituting apparent corticospinal and *frontotemporal
pathways. The maturation of the corticospinal tract was
bilateral, but that of the frontotemporal pathway was found
predominantly in the left (speech-dominant) hemisphere. The
authors suggest these findings provide evidence for a gradual
maturation, during late childhood and adolescence, of fiber
pathways presumably supporting motor and speech functions. The
authors also suggest their finding may provide guidance for
further investigations of neurodevelopmental disorders such as
schizophrenia: "the abnormal rate of myelinization during
childhood or adolescence may very well underlie the emergence of
psychotic symptomatology." Finally, the authors suggest that the
demonstrated possibility of detecting subtle structural
variations in white matter in the living human brain opens up new
avenues of research on normal and abnormal cognitive development
and in the evaluation of the long-term effects of various
treatment strategies.
-----------
T. Paus et al: Structural maturation of neural pathways in
children and adolescents: In vivo study.
(Science 19 Mar 99 283:1908)
QY: Tomas Paus [tomas@bic.mni.mcgill.ca]
-----------
Text Notes:
... ... *Note #1: Our understanding of the propagation of nerve
impulses represents an interesting convergence of physics and
biology. The nerve impulse is a rapid propagating
wave (approximately 1 millisecond in duration) of depolarization
followed by repolarization. In the language of physics, the
neuron axon behaves as an electrical transmission line with a
transverse time-variant and voltage-dependent negative
conductance element in parallel with a high capacitance. In fact,
the equations describing the propagation of neuron action
potentials derive from the classical equations for wave
propagation along electrical transmission lines developed by
Maxwell and Kelvin. As expected from these equations, the cross-
sectional diameter of an axon is an important determinant of
impulse propagation velocity: the larger the diameter, the
greater the velocity of propagation. The myelin sheath that
surrounds certain types of axons is a periodically interrupted
electrical insulation, and on physical grounds it can be
demonstrated that the effect of this type of insulation,
considering the known electrical properties of the axon, is a
substantial increase in pulse propagation velocity over that of a
bare axon of the same diameter. Myelinization is thus a major
aspect of the workings of neural circuits.
... ... *magnetic resonance images: Magnetic resonance imaging
(MRI) is essentially a technique for examining morphology (as
opposed to _functional_ magnetic resonance imaging, which is a
technique for examining anatomical correlates of function). In
general, MRI involves magnetic coils producing a static magnetic
field parallel to the long axis of the patient or subject,
combined with inner concentric magnetic coils producing a static
magnetic field perpendicular to the long axis. A radio-frequency
coil specifically designed for the head perturbs the static
fields to generate a magnetic resonance image. The interaction
physics in this technique is that between the magnetic fields and
atomic nuclei in brain tissue. "Sliced" views can be obtained
from any angle, and the resolution is quite high and on the order
of millimeters for magnetic field strengths of 1.5 tesla.
... ... *frontotemporal: In this context, this term refers to
connections between the frontal lobes and temporal lobes of the
brain.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Jun99
For more information: http://scienceweek.com/swfr.htm
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