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.

September 21, 2001 -- Vol. 5 Number 38

-----------------------------------------------

What constitutes proof in one generation is not
the same thing as proof in another.
-- Fred Hoyle (1915-2001)

-----------------------------------------------

=-=-=-=-=-=-=-=-=
Section 1
=-=-=-=-=-=-=-=-=

Contents of this Issue (Full reports in Section 2):

1. Neural Basis of Functional Oxygen-Level Changes
2. Caveolae in the Plasma Membrane
3. Oligonucleotides and Anti-Gene DNA Binding
4. Amyloid Fibrils from Muscle Myoglobin
5. Crystallographic Structure of Flagellin
6. Anti-Tumor Activity of a Mutant Virus
7. Carbon Dioxide and the Triassic/Jurassic Boundary
8. Possible Shapes of the Universe
9. Galactic Cannibalism
10. On Polar Wander
11. Magnetism and Superconductivity
12. Electron Transfer in Polymers
13. In Focus: Edward Teller on the History of Physics
14. SW Archive: On Supermassive Black Holes
15. Sources

=-=-=-=-=-=-=-=-=
Section 2
=-=-=-=-=-=-=-=-=

1. NEURAL BASIS OF FUNCTIONAL OXYGEN-LEVEL CHANGES
Marcus E. Raichle (Washington University St. Louis, US) discusses
the neural basis for blood oxygen level changes in brain recorded
by *functional magnetic resonance imaging. Over the past decade,
research in the field of cognitive neuroscience has grown
exponentially, with experimental strategies combining psychology
and techniques that examine brain activity during mental
processes. The two most important techniques for imaging brain
function are *positron emission tomography (PET) and functional
magnetic resonance imaging (fMRI). Functional magnetic resonance
imaging effectively measures the level of oxygen in brain
localities, the oxygen-level varying because changes in brain
activity in a locality are usually associated with changes in
blood flow in the locality. The empirical relationship between
brain activity and blood flow has been known for more than a
century, but its neural basis has been unknown, so that
contemporary researchers have been assuming that the signals they
obtain from fMRI are related to actual changes in neuronal
activity. N.K. Logothetis et al (2001) now confirm this in 
experiments relating the fMRI signal to underlying neural
activity. But the major determinants of the fMRI signal are
apparently *local electric field potentials rather than *action
potentials. Thus, activation of an area of the brain, as seen by
fMRI, predominantly reflects the input to that area and
corresponding changes in information processing, rather than
output from the area, which consists of action potentials.
Earlier work, using *autoradiographic measurements of glucose
consumption by different brain areas in rats, anticipated exactly
this. Cognitive neuroscientists using fMRI must now take into
account that the absence of an fMRI signal does not necessarily
mean that no information processing is occurring in a particular
area, since fMRI is apparently not sensitive to changes in action
potential activity. Similarly, neurophysiologists must now take
into account that the absence of action potential activity in an
area does not indicate the absence of information processing,
which may be occurring via other neural mechanisms (e.g., slow
electric field integration).
-----------
NAT 2001 412:128,150
-----------
Notes:
... ... *functional magnetic resonance imaging: (fMRI) We must
first distinguish between magnetic resonance imaging (MRI) and
"functional" magnetic resonance imaging (fMRI) as applied to the
brain. The former is essentially a technique for examining
morphology, while the latter is a technique for examining
activity of brain tissue. Both techniques involve computerized
analysis of data. 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 current magnetic field
strengths of 1.5 tesla. Functional magnetic resonance imaging
(fMRI), the variant of MRI discussed here, is based on the fact
that oxyhemoglobin, the oxygen-carrying form of hemoglobin, has a
different magnetic resonance signal than deoxyhemoglobin, the
oxygen-depleted form of hemoglobin. Activated brain areas utilize
more oxygen, which transiently decreases the levels of
oxyhemoglobin and increases the levels of deoxyhemoglobin, and
within seconds the brain microvasculature responds to the local
change by increasing the flow of oxygen-rich blood into the
active area. This local response thus leads to an increase in the
oxyhemoglobin-deoxyhemoglobin ratio, which forms the basis for
the fMRI signal in this technique. Because of its high spatial
resolution (millimeters) and high temporal resolution (seconds)
compared to other imaging techniques, fMRI is now the technology
of choice for studies of the functional architecture of the human
brain.
... ... *positron emission tomography (PET): Positron emission
tomography is a technique for producing cross-sectional images of
the body after ingestion and systemic distribution of safely
metabolized positron-emitting agents. The images are essentially
functional or metabolic, since the ingested agents are
metabolized in various tissues. Fluoro-deoxyglucose and
H(sub2)O(sup15) are common agents used for cerebral applications,
and in cerebral applications of central importance to the
technique is the fact that changes in the cellular activity of
the brains of normal, awake humans and unanesthetized laboratory
animals are invariably accompanied by changes in local blood flow
and also changes in oxygen consumption.
... ... *local electric field potentials: This refers to slowly
varying electric potential differences produced by slow membrane
potential changes that in turn are produced by synaptic inputs to
dendrites and neuron cell bodies. Slow electric field changes in
dendritic trees (see next note) can be substantial.
... ... *dendrites: The general input extensions of nerve cells
are called "dendrites", and they may be extensively branched. In
general, dendrites are considered to receive input and axons to
propagate output, but the electrical architecture of most neurons
is complicated, and in many types of nerve cells activation of
the axon produces electrical activity that not only propagates
down the axon but also propagates backward through the cell body
and dendrites.
... ... *action potentials: In contrast to slow local electric
field changes, action potentials are short high-amplitude pulses
of membrane potential change of the order of 1 millisecond. The
output of a nerve cell consists primarily of action potentials
propagated along its axon and axon branches,
... ... *autoradiographic measurements: In general,
autoradiography is a technique by which radioactive material can
be localized, with the sample containing radioactive material put
in direct contact with a layer of photographic emulsion. The
radiation emitted in the decay of the radionuclide(s) activates
individual silver halide grains in the emulsion, rendering the
grains susceptible to conversion to metallic silver grains by a
photographic developer. After fixation, the silver grains can be
localized either visually or by microscopy.
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

2. CAVEOLAE IN THE PLASMA MEMBRANE
J-S. Shin and S.N. Abraham (Duke University, US) discuss
invagination in the plasma membrane. Caveolae -- flask-shaped
invaginations in the plasma membrane -- are present on the
surfaces of many types of mammalian cells, including *endothelial
cells, *smooth muscle cells, and *adipocytes. Like *lipid rafts
(their close relatives), caveolae are plasma membrane assemblies
of glycosphingolipids and cholesterol that are associated with
specific molecules, including *signaling proteins. Caveolae often
contain the protein *caveolin and have proteins anchored to
glycosylphosphatidylinositol, a plasma membrane phospholipid. But
exactly why cells have caveolae or lipid rafts has been a puzzle.
They are believed to be involved in cholesterol transport, the
transport of solutes across endothelial cells, tumor suppression,
and signal transduction through immune and *growth factor
receptors. In addition, it seems they have been commandeered by
various species of viruses, parasites, bacteria (and even
bacterial toxins) to enable these pathogens to enter host cells.
Classic endocytosis (cellular uptake of extracellular agents)
depends on *clathrin-coated pits and involves an intracellular
pathway in which *lysosomes fuse with internalized vesicles,
degrading their contents. In contrast, and clearly of benefit to
pathogens, caveolae-dependent endocytosis does not feed into the
lysosome pathway and does not result in degradation of the
contents of caveolar vesicles.
-----------
SCI 2001 293:1447
-----------
Notes:
... ... *endothelial cells: Endothelial cells are flat cells
forming a layer lining blood vessels, lymphatic vessels, the
heart, etc.
... ... *smooth muscle cells: Smooth muscle was originally
differentiated from striated muscle on the basis of microscopic
appearance, but there are important other differences both
functional and molecular. In general smooth muscle is
specialized for slow sustained contractions such as those
involved in the control of the diameters of blood vessels.
... ... *adipocytes: These are fat cells in connective tissue,
each cell containing one or more fat globules that compress the
cytoplasm of the cell into a thin envelope. The fat globules are
essentially energy storage bins.
... ... *lipid rafts: Lateral heterogeneities in the classical
fluid-mosaic model of cell membranes are now thought of as
domains or "rafts" comprising clusterings of various lipids and
*anchored proteins, these rafts dictating the sorting of
associated proteins and/or providing sites for assembling
cytoplasmic signaling molecules.
... ... *signaling proteins: In general, extracellular or
intracellular proteins involved in directed pathways that result
in specific cellular responses.
... ... *caveolin: Dog caveolin has 178 amino acids and a
molecular weight of 20.6 kilodaltons.
... ... *growth factor: In general, in this context, a "growth
factor" is any specific extracellular substance involved in the
replication of cells.
... ... *clathrin: A 180 kilodalton protein (see related
background material below).
... ... *lysosomes: A lysosome is a cytoplasmic membrane-bound
vesicle 5 to 8 nanometers in diameter and containing a variety of
glycoprotein hydrolytic enzymes used to digest foreign material
or defective organelles.
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
STRUCTURAL MECHANISMS OF ENDOCYTOSIS
The term "endocytosis" refers in general to any process in which
materials are taken into a biological cell by membrane-bound
vesicles that pinch off from the plasma membrane. When the
material taken up consists of large fragments or whole organisms,
the process is called "phagocytosis". "Receptor-mediated
endocytosis" is a specialized type of endocytosis that brings
specific macromolecules into the cell. Many hormones, *growth
factors, *lymphokines, and nutrients, enter the cell in this
manner. During receptor-mediated endocytosis, the external ligand
first binds to its corresponding plasma membrane receptor; the
receptor-ligand complex then becomes concentrated in specific
regions of the plasma membrane, regions called "coated pits".
Each coated pit is an infolding of the plasma membrane whose
cytoplasmic surface is coated with a polyhedral lattice
constructed from the protein clathrin. The clathrin molecule
consists of 3 large polypeptide chains and 3 small polypeptide
chains organized into a 3-pronged structure, a "triskeleton", and
clathrin triskeletons polymerize with one another to form the
polyhedral lattice. After ligand-receptor complexes have become
clustered within a coated pit, the invaginated membrane pinches
off and becomes internalized as a coated vesicle. This coated
vesicle is initially surrounded by a cage of clathrin molecules,
but this clathrin coat is quickly shed, and vesicles soon
accumulate in what is known as the "endosome compartment" of the
cell. Nearly all the details of the above brief description of
receptor-mediated endocytosis were completely unknown several
decades ago; our current picture is a result of intensive
research in many laboratories, research combining electron
microscopy, biochemistry, and molecular biology.
... ... M. Marsh and H.T. McMahon (2 installations, UK) present a
review of recent research on endocytosis, with a focus on
developments in the clathrin-mediated endocytic pathway. The
authors make the following points:
     1) The uptake, or endocytosis, of extracellular material
into cells in membrane-bound vesicles has been of great interest
to cell biologists for most of this century. The many functions
in which endocytosis plays a role include *antigen presentation,
nutrient acquisition, *clearance of apoptotic cells, pathogen
entry, receptor regulation, and *synaptic transmission.
     2) Concerning clathrin-mediated endocytosis, a high-
resolution 3-dimensional view of the clathrin coat is beginning
to emerge. Clathrin-coated vesicle formation is a complex process
dependent on, and regulated by, the activities of a set of
intracellular proteins that are recruited through various
protein-protein and protein-lipid interactions. *Phosphorylation
and dephosphorylation are apparently key regulators of these
interactions and of the activities of the involved proteins, but
the precise order in which the different components act at each
step of the process remains to be solved.
     3) Of all the coat-mediated transport events characterized
so far in cell biology, endocytic clathrin-coated vesicles are
unique in their degree of complexity. This may reflect a need for
a higher order of control to coordinate clathrin-mediated
endocytosis in various important processes: e.g., the rapid
recovery of synaptic vesicles membranes, or cellular responses to
environmental stimuli.
     4) The authors conclude: "The developments seen over the
past couple of years will continue; new insights and structures
will be published soon. The challenge for the new century will be
to understand how these structures interact to drive endocytosis.
-----------
SCI 1999 285:215
-----------
Notes:
... ... *growth factors: Growth factors are peptide hormones that
regulate the growth of cells and tissues.
... ... *lymphokines: (interleukins) Hormones secreted by certain
antigen-processing cells of the immune system, the hormones
causing immune cells specific for the antigen to proliferate.
... ... *antigen presentation: In general, "antigen presentation"
refers to the presentation of antigens on the surfaces of
antigen-presenting cells of the immune system. In order for an
antigen to be presented on the surface of such a cell, the
antigen must first by taken up by the cell via endocytosis. [See
report #5, this issue of SW.]
... ... *synaptic transmission: This is a general term referring
to the events mediating the membrane-to-membrane interaction
between a neuron and another neuron, or a neuron and a muscle or
gland cell, or a neuron and a sensory receptor cell. The junction
is called a "synapse", and in many cases junction transmission
involves release and uptake of "transmitter" substances.
... ... *Phosphorylation: In general, the process of introducing
a phosphoric acid group into a molecule. Biochemical
phosphorylation reactions are of importance in the trapping of
energy, in the formation of biosynthetic intermediates during
metabolic processes, and in the control of the activity of many
enzymes and other proteins.
-----------
SW 1999 6 Aug
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

3. OLIGONUCLEOTIDES AND ANTI-GENE DNA BINDING
T. Carlomagno et al (University of Frankfurt, DE) discuss
oligonucleotide-DNA binding. The most general way to interfere
with cellular processes is the inhibition of enzyme activity by
small molecules. Alternatives are the binding of oligonucleotides
to *messenger RNA ("*anti-sense" approach) or to DNA ("anti-gene"
approach). In the anti-gene technology, a particular *promotor
region of transcription factors can be targeted, thereby
activating or repressing the transcription of specific genes.
Moreover, chemically modified oligonucleotides linked to the DNA-
phototoxin *psoralen can precisely place point mutations at
endogenous chromosomal loci, and they represent a new tool for
gene knockout and sequence manipulation. In case the DNA contains
a polypurine or polypyrimidine sequence, a triple helix can be
formed with the anti-gene oligonucleotide. The authors have
previously investigated various modified oligonucleotides with
the aim of designing a particularly stable triple helix motif,
and they have found that 2'-aminoethoxy substituents at the
ribose rings form extraordinarily stable triplexes. They have
investigated the structural basis for this stability with NMR,
and their results suggest that the enhanced stability of the
modified triplex in comparison to the unmodified DNA triplex of
the same sequence is due to strong interactions of the
aminoethoxy groups of the 3rd strand with the phosphate groups of
the purine strand. While the interactions that drive protein-DNA
recognition are often considered to be unspecific, these results
demonstrate that specific interactions are possible and may have
an impact on the stability and specificity of the complex.
-----------
JACS 2001 123:7364
-----------
Notes:
... ... *messenger RNA (mRNA): In this context, "transcription"
is the process by which genetic information in DNA is converted
into RNA, with the RNA ultimately "translated" into protein.
Messenger RNA (mRNA) is the ribonucleic acid molecule transcribed
from DNA that carries the coded information specifying the
sequence of amino acids in a protein.
... ... *anti-sense: In this context, an "anti-sense RNA" or
anti-sense nucleotide is a nucleotide complementary to a specific
messenger RNA, binding to that RNA and thus interfering with
expression of the encoded protein.
... ... *promotor: A DNA sequence near a gene that indicates the
site for the initiation of transcription.
... ... *psoralen: A furocoumarin compound present in many
plants, with photosensitizing and phototoxic effects in humans
and animals, and which is used in photochemistry. It intercalates
into DNA and forms crosslinks on irradiation.
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

4. AMYLOID FIBRILS FROM MUSCLE MYOGLOBIN
     The term "amyloid" ("starch-like") refers to a variety of
polypeptide molecules defined by their properties, particularly
by their tendency to arrange in a twisted beta-pleated fibrillar
structure. Amyloid is in general a proteinaceous material,
deposits of which have been classically noted to occur in the
brains of Alzheimer's disease and older Down syndrome patients,
and to a much lesser degree, in association with normal aging.
Amyloid material consists primarily of a highly aggregated
42-amino acid polypeptide called "beta-amyloid".
     The term "beta sheet" (beta-pleated sheet) refers to an
array of two or more "beta strands", with each beta strand
consisting of two polypeptide chains in a so-called "beta
configuration", which in turn is a stable configuration of a
polypeptide chain in which the chain is almost fully extended and
hydrogen-bonded to an adjacent polypeptide chain.
     In general, the term "heme" (ferroprotoporphyrin) refers to
any iron-porphyrin complex irrespective of the valence state of
the iron atom, and the term "heme protein" (hemoprotein) refers
to any protein to which an iron-porphyrin compound is linked in a
stoichiometric manner [*Note #1]. The heme proteins include
hemoglobin, myoglobin, various cytochromes, catalase, and some
peroxidases. The term "hemoglobin" refers to any of a group of
red, iron-containing, oxygen-carrying pigments of the blood of
vertebrates and some invertebrates; the substance also occurs in
the root nodules of leguminous plants. All vertebrate (including
human) hemoglobins consist of two pairs of associated protein
polypeptide chains ("*globins"), each polypeptide chain carrying
a heme *prosthetic group in non-covalent bonding, the iron atom
of which is in the ferrous state and forms a coordination complex
with the pyrrole nitrogens. Myoglobin is a similar macromolecule,
occurring in muscle fibers, and consisting of a single
polypeptide chain of 153 amino-acid residues to which a single
ferroheme prosthetic group is non-covalently bound. (In a
vertebrate physiological context, in muscle fibers, hemoglobin
essentially transports oxygen via the blood to cellular
myoglobin, and myoglobin then stores the oxygen for subsequent
release to intracellular enzymes when the local oxygen supply
becomes limiting.)
... ... M. Fandrich et al (University of Oxford, UK) discuss the
formation of amyloid fibrils by muscle myoglobin. The sequence of
amino acids in a natural protein determines the way in which it
folds up into its unique biologically active "native"
conformation. The authors report that myoglobin, the archetypical
globular protein, can convert into an alternative and radically
different structure that closely resembles the amyloid and prion
aggregates seen in pathological conditions such as Alzheimer's
and Creutzfeldt-Jakob diseases. The authors suggest this most
striking example of such a structural transition provides
compelling evidence for the idea that amyloid represents a
generic form of polypeptide conformation, although one that
evolution has ensured is not normally formed in living systems.
Myoglobin is a compact and highly soluble protein without any
native-state properties that suggest it has a predisposition to
form amyloid fibrils. Whereas amyloid fibrils are rich in beta-
sheets, native myoglobin lacks any elements of such structure,
and has most of its sequence contained in well-defined alpha-
helices. The authors suggest that evolution has provided a number
of molecular mechanisms to suppress amyloid formation of various
proteins in vivo, and that conditions compromising such
protective mechanisms, including ageing or mutational changes,
may allow even a highly selected natural sequence to revert to
its alternative conformation.
-----------
NAT 2001 410:165
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

5. CRYSTALLOGRAPHIC STRUCTURE OF FLAGELLIN
F.A. Samatey et al (Protonic NanoMachine Project ERATO JST, JP)
discuss the structure of bacterial flagella. Bacteria swim by
rotating helical flagellar filaments, which are up to 15 microns
long, but only 120 to 250 angstroms in diameter. The rotary motor
at the base of the filament drives the rotation of this helical
propeller at hundreds of revolutions per second. For chemotaxis
and thermotaxis, the swimming pattern of bacteria such as
Salmonella and Escherichia coli alternates between "run" and
"tumble": a run lasts for a few seconds and a tumble for a
fraction of a second. During a run, the motor rotates
counterclockwise (as it is viewed from outside the cell), and
several flagellar filaments with a left-handed helical shape form
a bundle and propel the cell. A tumble is caused by quick
reversal of the motor to clockwise rotation, which produces a
twisting force that transforms the left-handed helical form of
the filament into a right-handed one, causing the bundle to
rapidly fall apart. The separated filaments act in an
uncoordinated way to generate forces that change the orientation
of the bacterial cell. Thus, the structure of the flagellar
filament and its dynamic properties have an essential role in
bacterial taxis. The flagellar filament is constructed from 11
protofilaments of a single protein, flagellin. The filament
switches between left- and right-handed supercoiled forms when
bacteria switch their swimming mode between running and tumbling.
Supercoiling is produced by two different packing interactions of
flagellin called L and R. In switching from L to R, the
intersubunit distance (approximately 52 angstroms) along the
protofilament decreases by 0.8 angstroms. Changes in the number
of L and R protofilaments govern supercoiling of the filament.
The authors report crystallographic evidence for identification
of possible switch regions responsible for the bi-stable
mechanical switch that generates the 0.8 angstrom difference in
repeat distance.
-----------
NAT 2001 410:331
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

6. ANTI-TUMOR ACTIVITY OF A MUTANT VIRUS
S. Taneja et al (New York University, US) discuss anti-tumor
viruses. Throughout the past century, there have been sporadic
reports of malignancies regressing in response to coincidental
infections. These reports are supported in principle by the
observation that viral infections can regress experimentally
induced tumors in animal models of human cancer. To implement a
biological agent as a potential therapeutic agent in the
treatment of human cancer, however, requires that the ability of
the agent to cause disease be effectively curtailed. The advent
of genetic engineering techniques has made it possible to
selectively ablate viral virulence genes in the hope of creating
a safe virus that can selectively replicate in and destroy tumor
cells. Such a virus would be able to propagate an infection
throughout a tumor mass and directly kill the cancer cells, but
be unable to inflict substantial damage to normal cells. Although
safe viruses have been created by such methodology, the
attenuation process often has an overall deleterious effect on
viral replication, and this replication defect prevents the virus
from completely destroying the tumor mass, so that the surviving
cancer cells can simply regrow. The authors report that a mutant
herpes simplex virus, when genetically selected in cancer cells
for enhanced replicative ability, can be used as a tumor-
suppressor virus, and is a particularly potent inhibitor of tumor
growth in an animal model of human prostate cancer.
-----------
PNAS 2001 98:8804
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
TREATMENT OF CANCER WITH A VIRUS
R.J. Wong et al (Memorial Sloan-Kettering Cancer Center, US)
discuss the treatment of a carcinoma by an oncolytic virus. The
prognosis of patients with advanced or recurrent head and neck
squamous cell carcinoma remains poor despite refinements in
various therapeutic procedures. The authors report a study to
evaluate the efficacy of a replicative-competent attenuated
oncolytic herpes simplex virus (type NV1020) as a new agent in
the treatment of human head and neck squamous cell carcinoma. The
authors report this virus displays efficient replication and
destruction of cancer cells (oncolysis) in human head and neck
squamous cell carcinoma lines cultured in vitro, and that
injection of the virus into mouse flank tumors grown from the
human cell lines resulted in effective tumor regression. The
authors suggest treatment with this virus is a promising form of
therapy with potential clinical applicability in humans.
-----------
Journal of the American College of Surgeons 2001 193:12
-----------
PRAXIS 3 Sep 2001 http://scienceweek.com/praxis
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
MEDICAL BIOLOGY: USE OF POLIOVIRUS TO TREAT A CANCER
     In general, "glial cells" are cells of the central and
peripheral nervous system that metabolically support neurons,
with certain of such cells also producing the multiple membrane
layers called myelin and enfolding nerve cell axons with it. The
glial cells are found everywhere in the brain and spinal cord,
and they are usually categorized according to morphology.
"Astrocytes" (astroglia) are the largest glial cells, with many
extensions radiating outward like a starburst, and at least one
of their functions is apparently to maintain the so-called
"blood-brain barrier" effectively separating neural tissue from
blood.
     A malignant glioma, in general, is a cancer of any type of
glial cell, and in humans, malignant gliomas are the most common
primary tumors of the central nervous system. Such tumors are
usually diffuse and spread rapidly throughout the brain;
treatment is usually limited in effectiveness, and death usually
occurs within 1 or 2 years after first symptoms. The resistance
of malignant gliomas to conventional therapies has produced a
search for novel strategies, and recently these strategies have
involved animal viruses, either as delivery vehicles for foreign
genetic material intended to result in the self-destruction of
tumor cells, or as attenuated variants of pathogenic viral
species that can directly and specifically invade and destroy
tumor cells during the viral replication process.
     Poliovirus is an RNA virus that is the causative agent of
the human disease paralytic poliomyelitis. The majority of
poliovirus infections remain asymptomatic, but 1 to 2 percent of
such infections result in neurological complications within the
spinal cord and brainstem, the complications producing a
characteristic clinical syndrome dominated by flaccid paralysis.
Selective targeting of neurons that innervate muscle
(motorneurons) in the spinal cord by poliovirus apparently
involves a specific motorneuron cell-surface receptor (CD155),
with a contribution of certain favorable intracellular
conditions.
... ... M. Gromeier et al (5 authors at 3 installations, US) now
report that genetically engineered poliovirus (PV1RIPO) can
specifically infect and propagate in cell lines derived from
malignant gliomas. The authors make the following points:
     1) The study involved a genetically engineered (recombinant)
polio virus containing genome components of a human common cold
virus (rhinovirus type 2), the resultant genome hybrid (chimera)
characterized by exceedingly poor growth in normal neuron tissue
cultures, and the chimera virus nonpathogenic in both mice and
monkeys.
     2) The authors report that treatment of mice bearing
subcutaneous or intracerebral human glioma *xenografts with the
indicated genetically engineered polio virus halted tumor
progression and resulted in tumor elimination.
     3) The authors conclude: "This study provides evidence that
highly attenuated poliovirus/human rhinovirus type 2 chimeras
possess strong *oncolytic activity against malignant gliomas. The
neuropathogenic properties inherent to poliovirus are greatly
reduced in the recombinant poliovirus described. This *phenotype,
combined with a natural *tropism for malignant cells that express
CD155, suggests that polio/human rhinovirus type 2 chimeras may
be suitable for the treatment of malignant disease of the central
nervous system."
... ... In a commentary on the above work, Eric C. Holland (M.D.
Anderson Cancer Center Houston, US) states: "The paper by
Gromeier et al... [describes] a hybrid virus that infects and
kills clonal human glioma cell lines, in culture and *athymic
mice, without affecting nonneoplastic cells within the brain. For
those viewing this battle from a distance, the continued
unsuccessful attempts at novel therapies for this disease
[specifically, glioblastoma multiforme, the most aggressive of
the malignant gliomas] may be difficult to understand. However,
for those treating these patients, and certainly for the patients
themselves, the importance and urgency of each attempt is clear."
-----------
PNAS 2000 97:6242,6803
-----------
Text Notes:
... ... *xenografts: In general, a graft of tissue from one
species into the body of another species.
... ... *oncolytic activity: In general, activity that destroys
tumor cells. 
... ... *phenotype: In this context, the term "phenotype" refers
to the specific individuality of a cell line as determined by the
interaction between its genetic constitution (genotype) and the
environment.
... ... *tropism: In this context, an affinity for specific
locations in the body.
... ... *athymic mice: An "athymic mouse" is a mouse with an
absent thymus gland. This study involved so-called "nude mice", a
mutant strain of mice lacking the thymus gland. The reported
experiments involved xenografts, in this case the grafting of
human glioma cell lines into mice, and such grafts ordinarily do
not endure long enough for experimental study because of
rejection by the host immune system. Use of mice without a thymus
gland, the maturation site of a major component of immune system
cells (immune system T lymphocytes), is one way to achieve
relatively long-lasting xenografts.
-----------
SW 2000 11 Aug
-------------------
Related Background:
NEW TECHNIQUE USES COMMON COLD VIRUS TO KILL TUMOR CELLS
Human cells have a suicide program that is triggered by a protein
(p53) when the cell's genetic machinery is damaged. Some viruses
possess a gene that inactivates the protein p53, which enables
them to use the cell's genetic apparatus to reproduce, with the
eventual death of the host cell. Adenovirus, a common respiratory
virus, is one of these viruses. That is fact #1. Fact #2 is that
in many kinds of tumor cells, the protein p53 has become
intrinsically inactivated. Fact #3 is that a mutant strain of
adenovirus exists that has lost the p53 jamming gene. This mutant
adenovirus will therefore be lethal to tumor cells with already
inactivated p53, but not to ordinary cells. So Frank McCormack
and his colleagues (Onyx Pharmaceuticals, US) injected the mutant
strain of adenovirus into head and neck tumors in patients who
had failed to respond to surgery, radiation, or chemotherapy, and
they found significant destruction of tumors in 25% of the
patients. The mutant adenovirus killed the tumor cells in these
large, refractory cancers. These are preliminary trials, but the
beginning of what may be an extremely important approach to
cancer therapy -- the use of pathogens with a specificity for
tumor cells. The results were reported at the recent meeting of
the American Society of Clinical Oncology.
-----------
NYT 1997 20 May
SW 1997 22 May
-----------
PRAXIS 3 Sep 2001 http://scienceweek.com/praxis
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

7. CARBON DIOXIDE AND THE TRIASSIC/JURASSIC BOUNDARY
L.H. Tanner et al (Bloomsburg University, US) discuss the
*Triassic/Jurassic extinctions. The Triassic/Jurassic temporal
boundary, 208 million years ago, is associated with widespread
extinctions of both marine and terrestrial biological forms, with
the cause of these extinctions widely attributed to the volcanic
eruption of *flood basalts in the region called the Central
Atlantic Magmatic Province. This volcanic event is thought to
have released significant amounts of carbon dioxide into the
atmosphere, which could have led to catastrophic greenhouse
warming and consequent extinctions of biological systems. The
evidence, however, for these carbon dioxide-induced extinctions
remains equivocal. The authors report an analysis of carbon-
isotope compositions of paleo-soil formations spanning a 20-
million year period across the Triassic/Jurassic temporal
boundary. Using a standard diffusion model, the authors interpret
the isotopic data to represent a rise in atmospheric carbon
dioxide concentrations of approximately 250 ppm across the
boundary, as compared with previous estimates of a 2000 to 4000
ppm increase. The authors suggest the relative stability of
atmospheric carbon dioxide across the Triassic/Jurassic boundary
suggests that environmental degradation and extinction during the
Early Jurassic were not caused by volcanic outgassing of carbon
dioxide. The authors suggest that other volcanic effects, such as
the release of atmospheric aerosols or *tectonically driven sea-
level change, may have been responsible for this extinction
event.
-----------
NAT 2001 411:675
-----------
Notes:
... ... *Triassic/Jurassic: "Triassic Period" refers to the time-
frame 245 to 208 million years ago; "Jurassic Period" refers to
the time-frame 208 to 145.6 million years ago.
... ... *flood basalts: Massive volcanic lava flows. 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.
... ... *tectonically driven: The term "tectonics" refers in
general to changes in the Earth's crust.
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

8. POSSIBLE SHAPES OF THE UNIVERSE
C. Adams and J. Shapiro (Williams College, US) discuss
possibilities for the shape of the Universe. Recent observations
in astronomy are beginning to hint at the shape of the spatial
Universe -- or at least limit the range of possibilities. One
type of shape, called a Euclidean 3-manifold, has arisen as a
prime candidate, and mathematicians have previously demonstrated
that there are only 18 Euclidean 3-manifolds and of these only 10
are probably candidates for the Universe. Cosmologists believe
that at any point in the Universe, at a given distance from that
point a 3-dimensional ball of points exists, and it is that
special property that defines the Universe mathematically as a 3-
manifold. But which 3-manifold is it? In general, there are an
infinite number of possible 3-manifolds. Certain observed
physical properties of the Universe can help limit the
possibilities. One of these physical properties, curvature, has
major implications for the topology of the Universe. The
uniformity of the cosmic microwave background radiation should
only occur in a universe whose curvature does not vary with
either position or direction. Therefore, the spatial Universe is
believed to have one of three possible geometries: spherical
geometry with positive curvature; Euclidean geometry with zero
curvature; or hyperbolic geometry with negative curvature. These
three geometries have very different properties. If one believes 
the Universe is a Euclidean 3-manifold, there are the 18
possibilities demonstrated by Werner Nowacki in 1934.
-----------
AS 2001 89:443
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
COSMOLOGY: OPEN, CLOSED, OR FLAT UNIVERSE?
Marc Kamionkowski (Columbia University, US) reviews current
research in cosmology, making the following points: 1)
Determination of the geometry of the universe has been a central
goal of cosmology ever since Hubble discovered its expansion 75
years ago. 2) The central question is whether the universe is a
multi-dimensional equivalent of a 2-dimensional surface
("flat"), a sphere ("closed"), or a saddle ("open"). The
geometry, in the context of current theory and observations,
determines whether the universe will expand forever or eventually
collapse. 3) Until now, most astronomers have pursued the
geometry by attempting to measure the mass density of the
universe. According to general relativity, if the density is
equal to, larger than, or smaller than a critical density fixed
by the expansion rate, then the universe is flat, open, or
closed, respectively. 4) Another possibility is to look directly
at the predicted observational effects of a curved (open or
closed) universe versus a flat universe, and in particular at the
angular power spectrum of the cosmic microwave background. The
authors suggest that in the near future a new generation of
experiments will provide substantial advances in these
observations, enabling more definitive statements about the
geometry of the universe, and that these results will in turn
provide clues to the new particle physics required to understand
the inflation phase following the Big Bang origin of the
universe.
-----------
SCI 1998 280:1397
SW 19 Jun 98
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
COSMIC BACKGROUND RADIATION AND COSMOLOGICAL MODELS
     The central problem of cosmology is to provide a self-
consistent view of the past, present, and future of the Universe
that takes into account not only concepts and observations in
astrophysics but also concepts and observations in all of
physics, particularly those in particle physics and quantum
physics. Thus, a cosmological model is a conceptual edifice built
with the bricks of fundamental physics, and as physics changes,
and observations change, so do the acceptable models. Can there
be a "final theory" in cosmology? Probably not until there is a
"final physics", a physics in which all new observations are
without surprises. We are certainly not there yet; in fact, the
objective is so distant, there is perhaps not even a glimmer of a
shadow in the murky mist. But as the astrophysicist Edwin Hubble
said, "The search will continue. The urge is older than history.
It is not satisfied and it will not be oppressed."
     Contemporary cosmology distinguishes two kinds of matter,
"ordinary matter" and "dark matter". In general, a baryon is a
nuclear particle (e.g., a proton) built from 3 quarks
(fundamental particles that combine to make up protons, neutrons,
and mesons), and so-called "ordinary matter" is baryonic. In this
context, the term "dark matter" refers to material whose presence
can be inferred from its effects on the motions of stars and
galaxies, but which cannot be seen directly because it emits
little or no radiation. It is believed that as much as 90 percent
of the mass in the Universe may exist as some form or dark
matter, although the proposed percentage of dark matter varies
widely with different cosmological models.
     The major current cosmological models propose a "Big Bang",
an initial explosive origin of the Universe at a time zero. The
inflationary model, first proposed by Alan Guth in 1980, proposes
that quantum fluctuations in the time period 10^(-35) to 10^(-32)
seconds after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame, and that these density variations
eventually led to the formation of galaxies and clusters of
galaxies.
     What is known as the "cosmic microwave background radiation"
was discovered accidentally in 1964, when A.A. Penzias and R.
Wilson, measuring noise that might interfere with satellite
communications, noted a mysterious signal that was soon
interpreted to be the microwave background radiation originating
in the Big Bang. In 1978, Penzias and Wilson received the Nobel
Prize in Physics for this discovery. The cosmic microwave
background is black-body radiation (the emission radiation of a
perfect absorber of radiation) at a present temperature of 2.73
degrees Kelvin, and has an almost equal intensity in all
directions in space. The deviations from isotropic intensity,
however, are of extreme importance in theoretical cosmology.
     Central to current cosmological considerations are the
distinctions between the geometries of a "flat" (uncurved;
infinite in both extent and lifetime), "closed" (spherical;
finite in both extent and lifetime), and "open" (*hyperbolic;
infinite and expanding forever) Universe. An important quantity
is the Omega parameter, defined as the ratio of the density of
matter (or energy) in the Universe to the theoretical density
required for flatness. An Omega with a value of greater than 1
implies a closed Universe; a value less than 1 implies an open
Universe; a value equal to 1 implies a flat Universe. The problem
for the past 60 years has thus been to obtain an estimate of the
mass density of the Universe from observations. The current
standard conception is that the geometry of the Universe is flat.
... ... Craig J. Hogan (University of Washington Seattle, US)
presents a commentary on current research in cosmology, the
author making the following points:
     1) Recent measurements of temperature variation in the
cosmic microwave background reveal distinctive patterns in these
fluctuations, patterns which depend on the details and
composition of the Universe, and cosmologists are beginning to
interpret these patterns through detailed statistical studies.
The results are in accord with the expectations of inflation -- a
nearly "flat" Universe, which can be described as a small piece
of an enormous *hypersphere -- and in accord with independent
estimates of various quantities such as the density of dark
matter.
     2) Nevertheless, there are several unexpected and possibly
important discrepancies. The sharpest and most interesting
discrepancy is the estimate of the density of ordinary (baryonic)
matter in the Universe: the new data suggest that the mean number
of neutrons and protons per unit volume is greater than was
thought.
     3) The author concludes: "Exploration of this discrepancy
might lead to something really new -- perhaps a simple
reinterpretation of data on abundances, or perhaps a new
ingredient not yet included in the standard cosmological model."
-----------
NAT 2000 408:47
-----------
Notes:
... ... *hypersphere: If one considers a circle (2-dimensional),
a sphere (3-dimensional), the term "hypersphere" refers to the
subsequent members of the series, where the number of dimensions
is >= 4. A hypersphere is thus defined as the set of n-tuples of
points [x(sub1), x(sub2), ..., x(subn)] such that
x(sub1)^(2) + x(sub2)^(2) + ... + x(subn)^(2) = R^(2) where R is
the radius of the hypersphere.
-----------
SW 2000 24 Nov
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
COSMOLOGY: EXPECTATIONS IN THE NEXT CENTURY OF RESEARCH
Cosmology is one of the grand sciences, a domain of research
whose results have enormous intellectual consequences, at least
for people who care about what they are and where they are.
Martin Rees (Cambridge University, UK) presents an essay on the
near-future research expectations of cosmologists, the author
making the following points:
     1) Astronomers still do not know what the Universe is made
of. Observable radiation-emitting objects -- such as stars,
*quasars, and galaxies -- apparently constitute only a small
fraction of the matter in the Universe. The vast bulk of matter
is dark and unaccounted for, and most cosmologists believe this
dark matter is composed of weakly interacting particles left over
from the *Big Bang. But dark matter could be something more
exotic. "Whatever the case, it is clear that galaxies, stars and
planets are a mere afterthought in a Cosmos dominated by quite
different stuff." The author suggests that intensive searches for
dark matter, mainly via sensitive underground experiments
designed to detect elusive subatomic particles, will continue in
the coming decade, and that within the next decade both the
amount and nature of dark matter will be clarified.
     2) The author suggests that research in the near-future is
also likely to focus on the evolution of the large-scale
structure of the Universe. The current view is that ever since
the Big Bang, gravity has been amplifying inhomogeneities,
building up structures and enhancing temperature contrasts -- "a
prerequisite for the emergence of the complexity that lies around
us now and of which we're a part." The author suggests that
astronomers are now learning more about the 10 billion year
process of Cosmic evolution by creating virtual universes on
computers, and that in the coming years researchers will be able
to simulate the history of the Universe with ever improving
realism and then compare the results with astronomical
observations.
     3) The author suggests that the great mystery for
cosmologists is the series of events that occurred less than 1
millisecond after the Big Bang, when the Universe was
extraordinarily small, hot, and dense. "The laws of physics with
which we are familiar offer little firm guidance for explaining
what happened during this critical period." To solve this
problem, it will necessary to improve and refine current
observations in order to understand the characteristics of the
Universe when it was only one second old: its expansion rate, the
size of its density fluctuations, and its proportions of ordinary
atoms, dark matter, and radiation.
     4) The author suggests the following Cosmic timeline for the
evolution of the Universe from the Big Bang to the present:
... ... a) 10^(-43) seconds after the Big Bang: the *Quantum
Gravity Era.
... ... b) 10^(-36) seconds after the Big Bang: Probable *Era of
Inflation.
... ... c) 10^(-5) seconds after the Big Bang: Formation of
protons and neutrons from *quarks.
... ... d) 3 minutes after the Big Bang: Synthesis of atomic
nuclei.
... ... e) 300,000 years after the Big Bang: First atoms form.
... ... f) 1 billion years after the Big Bang: Appearance of
first stars, galaxies, and quasars.
... ... g) 10 to 15 billion years after the Big Bang: Appearance
of modern galaxies.
     5) The author concludes: "How did a hot amorphous fireball
evolve, over 10 to 15 billion years, into our complex Cosmos of
galaxies, stars, and planets? How did atoms assemble -- here on
Earth and perhaps on other worlds -- into living beings intricate
enough to ponder their own origins? These questions are a
challenge for the new millennium. Answering them may well be an
unending quest."
-----------
SA December 1999
-----------
Notes:
... ... *quasars: (quasi-stellar objects). Extremely luminous
sources radiating energy over the entire spectrum from x-rays to
radio waves, and which are apparently the oldest and most distant
objects in the universe. They are believed to involve massive
black holes.
... ... *Big Bang: The Big Bang theory is the general
cosmological model that proposes that all matter and radiation in
the universe originated in an explosion at a finite time in the
past.
... ... *Quantum Gravity Era: Quantum field theory is the
mathematical fusion of quantum mechanics with special relativity
theory, and the term "quantum gravity" refers to the fusion of
quantum mechanics with general relativity theory. The essential
basis for these fusions is the so-called "equivalence principle",
which identifies the mass involved in the gravitational force
equation with the inertial mass in the equation that relates any
force to the product of inertial mass and acceleration. The
"quantum gravity era" is the time-frame during which both quantum
effects and gravity determined the behavior of particles.
... ... *Era of Inflation: The inflationary model, first
proposed by Alan Guth in 1980, proposes that quantum
fluctuations in the time period 10^(-35) to 10^(-32) seconds
after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame.
... ... *quarks: A quark is a hypothetical fundamental particle,
having charges whose magnitudes are one-third or two-thirds of
the electron charge, and from which the elementary particles may
in theory be constructed.
-----------
SW 2000 28 Jan
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

9. GALACTIC CANNIBALISM
A "galactic halo", such as that associated with our own Galaxy,
is a spheroidal distribution of old stars and globular clusters
of old stars surrounding the galaxy. In the case of our own
Galaxy, the galactic halo has a radius of approximately 50,000
light years.
... ... R. Ibata et al (Strasbourg Observatory, FR) discuss
galactic cannibalism. Within the framework of the current theory
of hierarchical galaxy structure formation, large spiral galaxies
like the Milky Way or Andromeda arose from the merger of many
small galaxies and protogalaxies. Later in their evolution,
spiral galaxies became the dominant component in such mergers,
cannibalizing smaller systems that fell within their sphere of
influence. Recent observations have revealed streams of gas and
stars in the halo of the Milky Way that are the debris from
interactions between our Galaxy and some of its dwarf companion
galaxies, e.g., the Sagittarius dwarf galaxy and the Magellanic
clouds. Analysis of the material has demonstrated that much of
the Galactic halo consists of cannibalized satellite galaxies,
and that dark matter is distributed nearly spherically in the
Milky Way. It remains unclear, however, whether cannibalized
substructures are as common in the haloes of galaxies as
predicted by galaxy formation theory. The authors report the
discovery of a giant stream of metal-rich stars within the halo
of the nearest large galaxy, M31 (the Andromeda galaxy). The
source of this stream could be the dwarf galaxies M32 and NGC205,
which are close companions of M31 and which may have lost a
substantial number of stars as a consequence of tidal
interactions. The authors suggest these results demonstrate that
the epoch of galaxy building still continues, albeit at a modest
rate, and that tidal streams may be a generic feature of galaxy
haloes.
-----------
NAT 2001 412:49
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

10. ON POLAR WANDER
The term "polar wander(ing)" refers to a true or apparent motion
of the north magnetic pole over the surface of the Earth.
"Apparent polar wander" is believed to be caused by slow
continental drift with a time-scale of hundreds of millions of
years. In contrast, "true polar wander" is believed to be caused
by relatively rapid global shifts of mass derived from the
interaction of sudden new mantle-convection/crust-mass parameters
and the spinning of the Earth, with a time-scale of millions of
years.
... ... Robert Irion (NS) discusses true polar wander. The
essential idea of true polar wander is that there are times when
the spinning Earth goes out of balance, forcing continents and
the underlying mantle to slide over large distances en masse. As
a result, whole continents can be relocated to new climate zones
as the land masses of the Earth are rearranged. Although this
phenomenon would require millions of years, the time-frame is
short compared to the hundreds of millions of years required for
significant translocation of continents by the mechanism of
continental drift. True polar wander has been proposed as the
origin of the "*Cambrian explosion" of animal species that
occurred approximately 530 million years ago. The idea is that
the continents slid across one-quarter of the surface of Earth
within 15 million years, with a consequent rearrangement of ocean
currents, alterations of sea levels by 100 meters or more, and
the forcing of ecosystems to adapt to rapidly changing climate
conditions. The idea, proposed in 1997 by J. Kirschvink and D.
Evans (California Institute of Technology, US), has apparently
caused much controversy among paleogeologists and
paleobiologists.
-----------
NS 2001 18 Aug
-----------
Notes:
... ... *Cambrian explosion: The geological period known as the
Cambrian is the time frame from about 510 million years ago to
570 million years ago. Its most outstanding aspect is the rather
sudden appearance of numerous invertebrate fossils, so numerous
that some have termed it an explosion of evolutionary processes.
Many of the life forms that existed during the Cambrian are long
extinct, but their fossils are numerous, and through their
fossils the various Cambrian species have been the subject of
much study by paleobiologists. The Cambrian explosion of life
forms has been a long-standing puzzle for paleobiologists, and at
present there is apparently no single generally accepted
explanation. Among the ideas proposed have been, 1) that the
explosion of new forms resulted from a sudden increase in
atmospheric oxygen; 2) that the explosion is only apparent, and
the Precambrian, the period previous to the Cambrian, lacks
fossils because of heat and pressure associated with important
geological changes; 3) that living forms evolved mostly in
freshwater areas, and are therefore absent in Precambrian
sediments, which are primarily marine; 4) that changes in the
shape and extent of shorelines produced by continental drift
dramatically transformed climate and environment; 5) that the
previous evolution of DNA recombination and regulatory genes
culminated in and sparked the diversity and anatomical complexity
manifested in the explosion; 6) that an exponential increase of
species could become significant only after attaining a threshold
value at the start of the Cambrian; and, 7) that once
multicellular organisms appeared, the intrinsic possibilities for
variation increased enormously with a resultant explosion of
evolved forms. Unfortunately, there is no evidence to suggest a
selection of one of these proposals, although some of them are
less convincing than others. And of course the truth may be that
more than one factor was involved. No matter the origin, the
Cambrian explosion is apparently accepted by most paleobiologists
as a real discontinuity, a period that saw the sudden emergence
of dozens of new orders and phyla.
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

11. MAGNETISM AND SUPERCONDUCTIVITY
Michael E. Flatte (University of Iowa, US) discusses the
interaction of magnetism and superconductivity. The unusual
properties of superconductors arise from the coherent behavior of
electrons when they flow together in pairs ("Cooper pairs"). Two
key properties for electrons in a superconductor are their
negative charge, which normally keeps them apart, and their spin,
which can be thought of as a tiny bar magnet pointing either up
or down. If electrons of opposite spin overcome their mutual
repulsion, they can form Cooper pairs and flow without resistance
-- the essence of superconductivity. One useful way of exploring
the properties of a superconductor is to alter the electron
pairing coherence. Applying a magnetic field disrupts the pairing
of electrons, and as a result diminishes the energy required to
break Cooper pairs, which reduces the "transition temperature" at
which the material becomes superconducting. A sufficiently large
magnetic field can even disrupt pairing to the point where the
material ceases to be superconducting. Adding a single magnetic
atom to a superconductor creates a similar effect, although the
perturbation to the superconductor is localized near the magnetic
atom. E.W. Hudson et al (2001) describe the effect of adding a
magnetic nickel atom to a high-temperature superconductor. Nickel
weakly perturbs its local environment in the superconductor,
suggesting that this impurity could be used as a non-invasive
probe of superconducting behavior on the nanoscale.
-----------
NAT 2001 411:901,920
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

12. ELECTRON TRANSFER IN POLYMERS
W.B. Davis et al (Northwestern University, US) discuss electron
transfer reactions in polymers. Progress toward a fundamental
understanding of electron transfer reactions has come from an
intricate tapestry of theoretical tools and techniques, new
methods in synthetic chemistry, and the development of
spectroscopic apparatus capable of spanning the time range from
femtoseconds to hours. Early theoretical treatments of electron
transfer come from the work of Marcus (1956) and Hush (1958),
this work an extension of transition state theory. Later
theories, such as those developed by Jortner (1976) and others,
incorporate the quantum-mechanical nature of high-frequency
solvent and intramolecular vibrational modes that are coupled to
the electron transfer reaction coordinate. Long conjugated
molecules appear at first glance to be ideally suited for
incorporation into donor-bridge-acceptor assemblies designed to
carry out long-distance charge transport. However, an important
aspect of these long pi-electron bridges that is often neglected
is their propensity for significant conformational flexibility
due to low-frequency vibrational modes. The authors report
experimental and theoretical studies that indicate that long-
distance electron transfer, and therefore wire-like behavior
within conjugated bridge molecules, depend critically on low-
frequency torsional motions, and the authors suggest that
molecular device designs that utilize such bridges will need to
address these issues.
-----------
JACS 2001 123:7877
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

13. IN FOCUS: EDWARD TELLER ON THE HISTORY OF PHYSICS
"In the history of physics, there have been three great
revolutions in thought that first seemed absurd yet proved to be
true. The first proposed that the Earth, instead of being
stationary, was moving around at a great and variable speed in a
Universe that is much bigger than it appear to our immediate
perception. That proposal, I believe, was first made by
Aristarchos two millennia ago at the Greek center of knowledge in
Alexandria. It took more than a millennium and a half before
Copernicus offered a substantial proof, Galileo and Kepler
further developed and enlarged the theory, and Newton turned it
into a scientific fact. It has changed our outlook on the
Universe in a manner that is deep and thorough yet not deep
enough. The next two revolutions in thought occurred during my
lifetime. In the early part of the 20th century, the theory of
relativity and the science of quantum mechanics came into
existence. Relativity seems absurd because it challenges our idea
of time; it points out that we can't talk about time
independently of space. This concept goes far beyond our ability
of immediate perception. The other novel development, quantum
mechanics, disproves the mechanistic and predictive structure of
our Universe that was assumed true and concludes that in
predicting the future, we can make statements only about
probabilities. Relativity and quantum mechanics have introduced
the need for great revisions in human thought. Thoroughly
changing one's mind about the nature of the physical world
requires considerable time and effort. Widespread recognition
that our physical world is organized along the principles of
relativity and uncertainty rather than by absolutes and
determinism would, I believe, contribute a great deal to the
prospects for the future. But for most people, even for many
intellectuals, these novel ideas remain incompletely absorbed, a
source of uneasiness."
-----------
Edward Teller: _Memoirs: A Twentieth-Century Journey in Science
and Politics_
(Perseus Publishing, Cambridge MA, 2001, p.562)
(to be published November 2001)
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

14. SW ARCHIVE: ON SUPERMASSIVE BLACK HOLES
     The death of supermassive stars must result in collapse,
since no known force can resist gravity in such stars once their
nuclear energy sources are exhausted. For the most massive stars,
the result of this inevitable collapse is the "*black hole", at
the present time the most exotic astronomical object in the
Universe. Although existence of black holes is predictable from
Einstein's theory of general relativity, the possibility of
something like a black hole exists even within the Newtonian
theory of gravity, if one considers the Newtonian escape velocity
for supermassive objects: when the mass of an object is so great
that the velocity required to escape the object exceeds the speed
of light, the object turns "black", since no light can escape.
     In recent years, astrophysicists have come to distinguish
ordinary black holes and supermassive black holes, and these
latter objects, of unknown origin, now reign as the supreme
cosmic exotica.
... ... John Kormandy (University of Texas Austin, US) presents
an account of supermassive black hole research presented at the 6
June 2000 meeting of the American Astronomical Society, the
author making the following points:
     1) Black holes 10^(6) to 10^(9.5) times as massive as the
Sun were first invoked in the 1960s to explain the enormous
energy output of *active galactic nuclei (AGN) such as *quasars.
These supermassive black holes are in contrast to ordinary black
holes, which have masses of only a few solar masses and which are
well known to form when massive stars die.
     2) The origin of supermassive black holes is unknown, and
their existence long remained a hypothesis. By the mid-1980s,
however, black hole "engines" had become part of the theoretical
framework for understanding the activity of active galactic
nuclei, but evidence for their existence was still lacking. In
the 1990s, much effort was made to look for dynamical evidence of
dark objects in galactic nuclei, and that evidence is now strong.
In two objects, our Galaxy and the galaxy NGC 4258, the evident
dark mass must live inside such a small radius that plausible
alternatives to a supermassive black hole can be excluded.
     3) The author points out that although until recently
supermassive black holes were studied to understand the
spectacular but restricted phenomena of active galactic nuclei,
the situation is changing rapidly. Surveys with the Hubble Space
Telescope are finding evidence of supermassive black holes in
every galaxy that has an *elliptical-galaxy-like central "bulge"
component. These observations strengthen hints from ground-based
spectroscopy that supermassive black holes are standard features
of galaxy bulges. The observations indicate that supermassive
black hole growth and galaxy formation are closely linked, and
these results have profoundly changed the view of supermassive
black holes: they are more than just exotica needed to explain
rare active galactic nuclei: "Supermassive black holes are
becoming an integral part of our understanding of galaxy
formation."
-----------
SCI 2000 289:1484
-----------
Notes:
... ... *black hole: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable.
... ... *active galactic nuclei (AGN): Some galaxies are known to
have very "active" central regions from which enormous amounts of
energy are emitted each second. These "active galactic nuclei"
are probably powered by accretion of matter into a supermassive
black hole of 10^(6) to 10^(9) solar-masses.
... ... *quasars: (quasi-stellar object). An extremely luminous
source radiating energy over the entire spectrum from x-rays to
radio waves. Quasars are apparently the oldest and most distant
objects in the universe.
... ... *elliptical-galaxy: These are galaxies that have no
disc component, the shape varying from almost circular to narrow
ellipses. The stars within elliptical galaxies are predominantly
old stars. Elliptical galaxies display the greatest variation in
mass, ranging down to extreme dwarfs (approximately 10^(6) solar-
masses.
-----------
SW 2000 29 Sep
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com
-------------------
Related Background:
ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY
Recent observations have led to the conclusion that at the center
of many galaxies there is an object producing effects
characteristic of a supermassive *black hole. Alexei V.
Filippenko (University of California Berkeley, US) reviews
current research on black holes, the author making the following
points concerning the apparent massive black hole at the center
of our own Galaxy:
     1) Some galaxies are known to have very "active" central
regions from which enormous amounts of energy are emitted each
second. These "active galactic nuclei" are probably powered by
accretion of matter into a supermassive black hole of 10^(6) to
10^(9) solar-masses. The center of our own Galaxy exhibits mild
activity, especially at radio wavelengths: so-called "nonthermal
radiation" characteristic of high-energy electrons spiraling in
magnetic fields is emitted by a compact object at the Galactic
center known as *Sagittarius A*. Does the center harbor a
supermassive black hole? One approach is to determine whether
stars in the central region are moving very rapidly, as would be
expected if a large mass were present. During the past 5 years,
two teams have obtained high-resolution images of our Galactic
center, each team on several occasions, so that temporal changes
in the positions of stars could be detected. The observations
were conducted at infrared wavelengths, which penetrate the gas
and dust between Earth and the Galactic center (a distance of
approximately 25,000 light years) much more readily than optical
light. In summary, the data are in excellent agreement with the
conclusion that the gravitational potential of the central region
of our Galaxy is dominated by a single object. The derived mass
of this object is (2.6 +- 0.2) x 10^(6) solar-masses, and the
mass density within a radius of 0.05 light-years is at least 6 x
10^(9) solar-masses per cubic light-year, effectively eliminating
all possibilities other than a black hole.
     2) Although our Galaxy provides the most convincing case for
the existence of supermassive black holes, observations of the
centers of a few other galaxies bolster the conclusion. For
example, very precise measurements of the galaxy NGC 4258 reveal
a central compact object with a derived mass 3.6 x 10^(7) solar-
masses. On somewhat larger scales, spectra obtained with the
Hubble Space Telescope show gas and stars rapidly moving in a
manner consistent with the presence of a supermassive black hole.
The most massive existing case, that of the giant elliptical
galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover,
x-ray observations of some active galactic nuclei reveal emission
from a hot disk of gas apparently very close to a black hole,
since extreme relativistic effects are detected. In general, it
now seems that a supermassive black hole is found in nearly every
large galaxy amenable to such observations.
     3) The author concludes: "In the last decade of the 20th
century, black holes have moved firmly from the arena of science
fiction to that of science fact. Their existence in some *binary
star systems, and at the centers of massive galaxies, is nearly
irrefutable. They provide marvelous laboratories in which the
strong-field predictions of Einstein's general theory of
relativity can be tested."
-----------
PNAS 1999 96:9993
-----------
Notes:
... ... *black hole: See notes in main report.
... ... *Sagittarius A*: Sagittarius A is a prominent radio
source in the constellation Sagittarius, coincident with or close
to the center of our Galaxy. It is a highly complex region
consisting of a central core approximately 50 light-years in
diameter. Sagittarius A* is a compact component at the heart of
the central core of Sagittarius A. Sagittarius A* is an intense
source of radio waves, and is apparently unique in our Galaxy:
while everything else in our Galaxy is on the move as they follow
their orbits, Sagittarius A* is absolutely stationary and must
therefore lie exactly at the Galaxy's center. Many astronomers,
in fact, use Sagittarius A* as the "Greenwich Meridian" of the
Galaxy.
... ... *binary star systems: Binary stars are a pair of stars
revolving around a common center of mass under the influence of
their mutual gravitational attraction, and apparently the
majority of stars in the Universe are binaries and not singlets.
In some cases the binary system is resolvable into two
components, and in other cases the presence of a second star is
inferred by perturbations in the motion or emitted radiation of
the first star. If the binaries are close enough, they may share
stellar material, and this results in a particular kind of
stellar evolution. In the black hole-binary systems mentioned in
this report, matter transfers from a relatively normal star
(known as the "secondary star") to a dark compact object (the
"primary"). Recent comparisons of x-ray and optical brightness
suggest that in many cases the dark primary in such a binary
system is a black hole.
-----------
SW 1999 15 Oct
-----------
SCIENCE-WEEK 21 Sep 2001 http://scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

15. SOURCES:
AS: Amer. Scientist; CEN: Chem. & Eng. News; GD: Genes & Dev.;
GR: Genome Res.; JACS: J. Amer. Chem. Soc.; JAMA: J. Amer. Med.
Assoc.; JCE: J. Chem. Educ.; MMWR: CDC Morbidity and Mortality
Weekly Report; NAT: Nature; NATM: Nature Medicine; NEJM: New
Engl. J. Med.; NS: New Scientist; NYT: New York Times; NYR: New
York Review; PNAS: Proc. Natl. Acad. Sci.; PRL: Phys. Rev.
Lett.; PT: Physics Today; SA: Scientific American; SCI: Science;
SW: ScienceWeek; TS: The Scientist.

In the text, the affiliation following the author's name is the
affiliation of the lead author. The indication (na) signifies no
known research affiliation.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

SCIENCE-WEEK SUBSCRIPTIONS:
Complete subscription information is available at:
http://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) 1997-2001 SCIENCE-WEEK/Spectrum Press Inc.
All Rights Reserved
US Library of Congress ISSN 1529-1472

---------------------------------------------
ScienceWeek is published by Spectrum Press Inc.,
3023 N. Clark Street #109, Chicago, 60657-5205 IL, USA.
---------------------------------------------
-----end file