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.

November 5, 1999 -- Vol. 3 Number 45

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The most modest research worker at his bench, pushing a probe
into a neuron to measure the electric response when a light is
flashed, is enmeshed in a large and intertwined network of
theories that he carries into his work from the whole field of
science, all the way from Ohm's law to Avogadro's number. He is
not alone; he is sustained and held and in some sense imprisoned
by the state of scientific theory in every branch. And what he
finds is not a single fact either: It adds a thread to the
network, ties a knot here and another there, and by these
connections at once binds and enlarges the whole system.
-- Jacob Bronowski (1908-1974)

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Contents of This Issue:

1. Science and Society: Physicist I.I. Rabi (1898-1988)
2. Molecular Biology: On Model Systems
3. Neurobiology: A Decade of Research on Long-Term Potentiation
4. Epidemiology: On the AIDS Epidemic
5. Medical Biology: Reduction of Side Effects of Cancer Therapy
6. Astrophysics: Perspectives in General Relativity

In Focus: On Watson and Crick

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1. SCIENCE AND SOCIETY: PHYSICIST I.I. RABI (1898-1988)
The physicist I.I. Rabi (pronounced rah'bee) obtained his first
degree in chemistry, worked as a chemist a number of years, and
then obtained a PhD in physics from Columbia University (US) in
1927. Rabi then did the customary sojourn in Europe, working
short stints in the laboratories of various prominent European
physicists (e.g., Bohr, Sommerfeld, Pauli, Heisenberg, Stern). It
was the research of Otto Stern (1888-1969) on *molecular beams
that interested Rabi most, and when Rabi returned to the US he
began his own research in that field. He obtained a faculty
position at Columbia University in 1929, and in 1944 he won the
Nobel Prize in Physics for his molecular beam work, particularly
the resonance method of recording the magnetic properties of
atomic nuclei. Rabi remained at Columbia University throughout
his career, with a presence in fields other than his specialty.
He contributed to the development of radar and the nuclear bomb,
and to the development of the laser and the atomic clock. He was
one of the founders of the Brookhaven National Laboratory (US), a
member of UNESCO, and he originated the movement that established
CERN (Conseil Europeen pour la Recherche Nucleaire) in Geneva
(CH). As a prominent physicist during the decades following the
Second World War, Rabi had much to say about physics and science
and their roles in society. ... ... Gerald Holton (Harvard
University, US) presents a biographical essay on I.I. Rabi as a
scientist-statesman, the author making the following points:
     1) Rabi discussed the apparent gulf between the sciences and
the humanities several years before C.P. Snow made the concept
famous in his "Two Cultures" lecture. C.P. Snow, in fact,
acknowledged privately that while Rabi was in London in 1957,
Rabi had been "the man who gave me the idea for the two
cultures."
     2) After enrolling at Cornell University (US) in 1916 as a
chemistry major, Rabi proceeded to acquire as broad an education
as possible by reading widely outside the sciences. Later, while
reminiscing about his undergraduate education, Rabi said: "If you
decide you don't have to get A's, you can learn an enormous
amount in college."
     3) During his two years in Europe (1927-1929), Rabi became
annoyed at the general contempt of European physicists toward
American physicists. With some other American physicists (e.g.,
E. Condon and R. Robertson) who were also irked by the contempt,
Rabi made a compact to put an end to the second-class status of
American physics. These young physicists in Europe were
determined that when they returned to America they would not only
do important physics in the proper style, but they would also
undertake to become leaders of the field.
     4) During his years at Columbia University, Rabi was
considered an awful lecturer and a great teacher. "In terms of
giving perspective to the significance of a particular discovery,
the state of the art, the humanistic aspects of science, a sense
of excitement -- he was without peer." His lectures, however,
were famous at Columbia for being improvised and disorganized.
     5) After receiving the Nobel Prize in 1944, Rabi became a
public person, a "civic scientist". The role apparently came to
him quite naturally. He felt that intellectuals should have a
life beyond their research specialty, and that scientists had a
particular ethical responsibility. Rabi believed that now that
technological advances "have made it absurdedly easy to kill
human beings," the disinterested search for objective truth...
requires also "the shouldering of responsibility." Author Holton
states: "He became one of the most effective statesmen of science
that America has ever had."
     6) Concerning science and culture, Rabi had a passionate
commitment to safeguarding a rightful place for science as a
necessary and liberating aspect of the total culture, and he
promulgated specific reasons why this had to be so:
... ... a) Science is an international enterprise and it can act
as a unifying force for all humanity, and thereby counteract the
divisive and destructive tendencies of almost all other human
enterprises.
... ... b) Science can act as a personally ennobling activity.
... ... c) Science is a counterforce to the "anti-Galileans", the
"anti-rational attitude which has been the "curse of our
century."
... ... d) Science teaches how to think objectively, rationally,
and therefore productively for solving the problems of society --
especially in a democracy.
... ... e) Science can be a good preparation for a useful life
outside science. Said Rabi: "If science were taught more
humanistically in the schools and in the universities, it could
become a foundation for any career, not only teaching or
research."
     7) Rabi's ideal whole man was Benjamin Franklin, who was
initially a craftsman, and who became one of the greatest
scientists of his time, and also a successful diplomat and
statesman. Rabi asked: "Where are the life-size, or even the
pint-size Benjamin Franklins of today?"
-----------
Gerald Holton: I.I. Rabi as educator and science warrior.
(Physics Today September 1999)
QY: Gerald Holton, Dept. of Physics, Harvard Univ. 617-495-1000
-----------
Text Notes:
... ... *molecular beams: The essential idea in classical
molecular beam research was to allow a gas to escape from a very
small hole into a high vacuum. Since the escaping molecules
experience virtually no scattering in the vacuum, they form a
straight "beam" of moving particles. Although the molecules may
be neutral overall, they do possess small magnetic moments, and
these magnetic moments can be quantitatively investigated by
studying the behavior of the beam in applied magnetic fields. The
magnetic resonance method developed by Rabi and others involved
the application of oscillating magnetic fields to molecular
beams.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99


2. MOLECULAR BIOLOGY: ON MODEL SYSTEMS
During the past several decades, primarily as a result of genome
research in molecular biology, the range of model systems
considered relevant for human biology has significantly
increased. Thirty years ago, basic research on non-pathogenic
primitive organisms such as yeast was barely recognized as having
medical importance. At present, the use of such model systems to
increase our understanding of human disease and the underlying
biology of human disease is routine in biological research.
... ... Laurie Goodman (Cold Spring Harbor Laboratories, US), in
the context of a summary of a recent international meeting
devoted to yeast genetics and human disease (Vancouver, CA 24-27
June 1999), presents a review of current trends in the use of
model systems as the basis for understanding the molecular
biology of pathological processes. The author makes the following
points:
     1) The growing availability of genomic data from a variety
of animals and plants, and the increasingly advanced methods for
probing, classifying, and characterizing these resources, has
resulted in an enormous increase in the power of model systems as
tools in fundamental biological research.
     2) *Genome sequence homologies, and consequent potential
functional homologies of genes and proteins, are the primary
reasons why model systems are useful in research. The power of
model systems derives from the possibility of defining both
similarities and differences in sequences, and the use of these
similarities and differences to direct research strategies.
     3) But there are recognized limitations. Functional
similarity indeed has numerous applications, but the relevance of
a particular similarity from one organism to another may be far
from complete. In addition, when considering the overall
relevance of a particular model to humans, the consensus view
among researchers is that the ease with which an organism can be
worked with in the laboratory is often inversely proportional to
its relevance for considerations of human biology.
     4) At the present time, the genome of *yeast (Saccharomyces
cerevisiae) has been completed. This data, in conjunction with
data from a number of other model organisms, some also with
complete genomes (the *nematode worm C. elegans and numerous
bacteria and *archaea), as well as growing sequence data for
mouse, human, *fruit fly (Drosophila), the plant *Arabidopsis,
etc., suggests that future advances will continue to cross
species and disciplinary boundaries, with a resultant increase in
our understanding of general biological systems and the place of
humans within these systems.
     5) The identification of human disease genes continues to
become more and more straightforward as more sequence data,
higher quality genome maps, and better resources and tools become
available. However, the identification of a gene involved in a
disease is really only the starting point for understanding the
etiology of that disease. Obviously, studying diseases that cause
blindness or brain disease in organisms such as yeast or the worm
C. elegans may seem to lack the main components for research in
such diseases -- the presence of eyes or brains. Comparative
studies, however, provide one of the best tools for delineating
the mechanisms underlying disease at the cellular level, and
although such studies are unlikely to provide the entire story,
they can provide vital clues for new research to understand the
role of a human genetic homolog.
-----------
Laurie Goodman: Celebrating similarities -- embracing
differences.
(Genome Research September 1999 9:797)
QY: Laurie Goodman [goodman@cshl.org]
-----------
Text Notes:
... ... *Genome sequence homologies: In this context, the term
"homology" refers to a similarity between two nucleotide base
sequences in two different genomes.
... ... *yeast (Saccharomyces cerevisiae): Yeast are unicellular
fungi that reproduce by budding. The most important yeast species
in research is the common bread and beer yeast Saccharomyces
cerevisiae.
... ... *nematode worm C. elegans: Nematodes are an abundant and
ubiquitous phylum of unsegmented roundworms. Caenorhabditis
elegans is a small (1 mm) nematode worm. It is transparent,
hermaphroditic, free-living, and found in soil. It has a
relatively small genome (approximately 3000 genes), and only a
few types of cells in its body. It has a 16-hr embryogenesis that
can be achieved in a petri dish, and is thus highly suitable for
the study of developmental and behavioral genetics.
... ... *archaea: The archaebacteria (also called the Archaea)
are a subkingdom of bacteria considered to be ancient compared to
other bacterial kingdoms, and possibly the most ancient life
forms and the ancestors of all eukaryotes (cells and organisms
with intracellular membrane-bound organelles). They typically
exist in extreme environments, and include the methane-producing
bacteria (methanogens), the "salt-loving" bacteria (halophilic
bacteria), and the sulfur-acid tolerant thermoacidophilic
bacteria.
... ... *fruit fly (Drosophila; Drosophila melanogaster) A major
advantage of this experimental system is the presence of giant
chromosomes in the insect's salivary glands. (In cells with
chromosomes, the chromosomes are the physical structure into
which DNA is organized and on which genes are carried.)
Drosophila also has a short reproductive cycle (approximately 10
days), and it produces 100 to 400 progeny per mating.
... ... *Arabidopsis: (Arabidopsis thaliana; thale cress) A weed
of the mustard family with a small genome of 120 million base
pairs. Arabidopsis is now an important laboratory species, and it
is presently the model for physiological, biochemical, cell
biological, and developmental studies of over 250,000 plant
species.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]


3. NEUROBIOLOGY: A DECADE OF RESEARCH ON LONG-TERM POTENTIATION
One of the chief characteristics of the nervous systems of humans
and animals is the ability to change both structure and function
in response to new conditions. One aspect of this, most evident
in the human species, is learning and memory behavior. At the
present time, the consensus among neurobiologists is that the
cellular basis of learning and memory is a set of events
occurring at the junctions (*synapses) between nerve cells, and
in recent years a major focus of research in this area has been
the experimental phenomenon of "long-term potentiation" (LTP).
The mechanism of long-term potentiation is believed by many
researchers to provide the basis for memories or learned
behaviors that persist for weeks, months, or years. Long-term
potentiation was discovered in the early 1970s, when T. Bliss and
his colleagues observed that a few seconds of high-frequency
electrical stimulation of a *nerve fiber pathway in the rabbit
*hippocampus enhances synaptic transmission between the
stimulated axons and postsynaptic cells, and that this change
persists for weeks. Although long-term potentiation was first
observed in intact experimental animals, intensive subsequent
investigations have relied on *in vitro brain slice preparations.
It was not until the mid-1980s that the molecular basis of long-
term potentiation began to be delineated, a number of
laboratories demonstrating that the unique properties of a
specific type of cell-membrane glutamate receptor, the NMDA
receptor, were involved in the phenomenon. This receptor is named
for the *neurotransmitter glutamate analogue, N-methyl-D-
aspartate, that selectively activates it.
... ... R.C. Malenka and R.A. Nicoll (2 installations, US)
present a review on research during the past decade on long-term
potentiation, the authors making the following points:
     1) Despite the enormous interest in long-term potentiation,
it has proven difficult to elucidate the detailed cellular and
molecular changes responsible for the phenomenon. In fact, for
over a decade, there has been a vigorous and highly visible
debate about whether the changes that occur soon after the
generation of long-term potentiation occur on the presynaptic or
postsynaptic side of the synapse.
     2) The fact that long-term potentiation can be most reliably
generated in brain regions demonstrated to be involved in
learning and memory is often used as evidence for its functional
relevance. The authors suggest, however, that long-term
potentiation is a fundamental property of the majority of
*excitatory synapses in the mammalian brain, and that it is
likely to subserve many functions in addition to functions
underlying some forms of learning and memory.
     3) It is now well accepted that the triggering of long-term
potentiation requires synaptic activation of postsynaptic NMDA
receptors. This activation apparently requires depolarization of
the postsynaptic cell, which is usually accomplished
experimentally by repetitive *tetanic stimulation of synapses or
by directly depolarizing the cell with electrical input. The
consensus view is that when the postsynaptic cell is depolarized
during the induction of long-term potentiation, magnesium ions
dissociate from binding sites within NMDA receptor channels,
allowing calcium ions as well as sodium ions to enter the
postsynaptic cell. This consequent rise of intracellular calcium
ion concentration is the apparent critical trigger for long-term
potentiation, and this local source of calcium ions accounts for
the input specificity of long-term potentiation. In the context
of a *dendritic tree, association occurs because strong
activation of one set of synapses depolarizes adjacent regions of
the tree.
     4) No question concerning long-term potentiation has
generated more debate and confusion over the last two decades
than the seemingly simple question of whether the increase in
synaptic strength is due primarily to a presynaptic or a
postsynaptic modification. Great technical difficulties are
inherent in examining the changes at individual synapses embedded
in a network in which each individual neuron receives
approximately 10,000 to 30,000 synapses. Most neurobiologists
agree that the simplest postsynaptic change that could cause
long-term potentiation would be a modification in postsynaptic
receptor function or number or both, whereas the simplest
presynaptic change would be an increase in the probability of
neurotransmitter release.
     5) The authors suggest that the evidence is now sufficiently
strong to indicate that the initial increase in synaptic strength
during long-term potentiation involves postsynaptic modifications
of receptor function and localization. However, this conclusion
does not preclude the occurrence of substantial presynaptic
changes. The authors state: "The synapse is a structural unit
and, as do many in the field, we would predict that if long-
lasting synaptic modifications are in fact a mechanism by which
experiences are translated into memories, then [both] pre- and
postsynaptic structural alterations are likely to occur."
-----------
R.C. Malenka and R.A. Nicoll: Long-term potentiation: A decade of
progress.
(Science 17 Sep 99 285:1870)
QY: Robert C. Malenka [malenka@stanford.edu]
-----------
Text Notes:
... ... *synapses: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. The term "synapse" refers to the
junction between the terminal of a neuron's axon and another
neuron. When studying the synapse, the first neuron is called the
"presynaptic" neuron, and the second neuron is called the
"postsynaptic" neuron.
... ... *nerve fiber pathway: In the context of the central
nervous system (e.g., the brain), a "nerve fiber pathway" is a
collection of nerve fibers (axons) all running in the same
direction.
... ... *hippocampus: The hippocampus is a deep region of the
brain involved with many fundamental activities, including memory
storage.
... ... *in vitro brain slice preparations: "Brain slices" are
exactly that, the brain removed from the animal and a thin slice
of a particular region prepared in an appropriate solution for
electrophysiological recording of nerve cell activity.
... ... *neurotransmitter: The term "neurotransmission" refers
to all the events at a synapse, particularly the release of
"neurotransmitters" and their action on the postsynaptic neuron.
Neurotransmitters are chemical substances released at the
terminals of nerve axons in response to the propagation of an
impulse to the end of that axon. The neurotransmitter substance
diffuses into the synapse, the junction between the presynaptic
nerve ending and the postsynaptic neuron, and at the membrane of
the postsynaptic neuron the transmitter substance interacts with
a receptor. Depending on the type of receptor, the result may be 
an excitatory or an inhibitory effect on the postsynaptic nerve
cell.
... ... *excitatory synapses: A synapse which when activated
produces excitation of the postsynaptic nerve cell.
... ... *tetanic stimulation: The term "tetanic stimulation"
refers to repetitive stimulation. A "tetanus" is a sustained
muscular contraction caused by a rapid and repeated nerve
stimulation. Tetanus is also a disease marked by such muscle
contractions, the nerve activity produced by a neurotoxin.
... ... *dendritic tree: Dendrites are highly branched extensions
of certain types of nerve cells, the extensions literally covered
with synapses with thousands of other neurons. In certain
neurons, when appropriately stained for microscopic observation,
the dendritic region of the nerve cell has the generally
morphology of a "tree", a dense arborization of dendrites, with
thousands of branches, the volume of the tree orders of magnitude
greater than the volume of the mother neuron. The classic
paradigm is that dendrites receive and integrate input, which
then produces output in the single axon of the neuron. Nerve
cells, however, are extremely varied in morphology and behavior,
and the classic paradigm is often not applicable.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ACTIVITY-INDUCED POTENTIATION OF DEVELOPING SYNAPSES
In general, the nervous system of an animal consists of a large
number of active entities (neurons) receiving input from a
variety of sensory cells, the active entities integrating and
performing various operations on this input via interactions
between the entities, and the entities ultimately producing an
output that controls groups of "effector cells" (e.g., muscle
cells). Of critical importance in this scheme are the junctions
between neurons, between receptor cells and neurons, and between
neurons and effector cells, the so-called "synapses". For many
decades now, the consensus view among neurobiologists has been
that the psychological functions that we call "learning and
memory" are somehow dependent on short-term or long-term changes
in these synapses, but progress in elucidating precisely what is
occurring at synapses that relates to learning and memory has
been difficult and frustrating. At the present time, two key
experimental concepts at the cellular level in this field are
"long-term potentiation" (LTP) and "long-term depression" (LTD). 
Long-term potentiation is an experimentally observed persistent
strengthening (facilitation of activity) of synapses based on
past patterns of activity, the strengthening lasting hours or
days or weeks. Long-term depression is the converse, a persistent
weakening (damping of activity) of synapses based on past
patterns of activity, the weakening also lasting hours or days or
weeks. These are experimentally observed phenomena in a wide
variety of nervous systems, both relatively simple and complex,
and the central questions are what are the cellular controls, the
molecular controls, and the relevance of these phenomena to
learning and memory behaviors?
... ... J. Wan and M. Poo (University of California San Diego,
US) now report that in African clawed toad (Xenopus) nerve-muscle
cultures, a brief burst of *action potentials in the presynaptic
neuron induced a persistent potentiation of neuromuscular
synapses that exhibit immature synaptic functions. Until now,
long-term potentiation has not been reported in any neuromuscular
system, although long-term depression in such systems is well
known. In the experiments of the authors, induction of
potentiation required an elevation of *postsynaptic calcium ion
concentration and expression of potentiation appeared to involve
an increased probability of *transmitter secretion from the
*presynaptic terminal. The authors suggest that activity-
dependent persistent synaptic enhancement may reflect properties
characteristic of immature synaptic connections, and that
bursting activity in developing spinal neurons may promote
functional maturation of the neuromuscular synapse.
-----------
J. Wan and M. Poo: Activity-induced potentiation of developing
neuromuscular synapses.
(Science 10 Sep 99 285:1725)
QY: Mu-ming Poo [mpoo@ucsd.edu]
-----------
Text Notes:
... ... *action potentials: (nerve impulses) In general,
transient pulses (e.g., 1 millisecond) of reversed membrane
potential propagated over the long extensions of neurons (axons),
in some cases over relatively large distances (e.g., 1 meter
between spinal motorneurons and peripheral muscle cells). The
physical characteristics of the action potentials in the nervous
systems of diverse animal forms are often quite similar. Also
often similar across diverse animal forms are events at various
synapses. The nervous system of the African clawed toad (Xenopus
laevis) has been a common laboratory model in neurophysiology for
the past half century.
... ... *postsynaptic: Considering a 2-unit junction (synapse)
between two cellular entities, entity A transmitting activity to
entity B, events on the entity-B side of the junction are called
"postsynaptic", and events on the entity-A side of the junction
are called "presynaptic".
... ... *transmitter secretion: (see main report)
... ... *presynaptic terminal: See above: *postsynaptic.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 29Oct99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
TARGET-SPECIFIC PRESYNAPTIC PLASTICITY IN NEURONS
The term "mossy fibers" refers to a type of nerve fiber in the
brain with large axon terminal endings, and their synapses
(connections to other nerve cells) are among the largest in the
mammalian central nervous system... The term "interneurons"
refers to nerve cells localized in a succinct region (population
of nerve cells), with the primary function of these nerve cells
an involvement with information processing in the region, rather
than with input to the region or output from the region:
interneurons are therefore involved in "local circuitry"... In
this context, the term "depression" refers to a long-term
reduction in efficacy of synaptic transmission. Cyclic adenosine
monophosphate (cAMP) is an important postsynaptic intracellular
substance activated by incoming synaptic activity, a "messenger"
involved in various aspects of cell regulation and protein
synthesis. In general, the term "afferent pathway" refers to any
input pathway, as opposed to an output pathway (efferent).
... ... Maccaferri et al (3 authors at National Institutes of
Health, US) report a comparison of mossy fiber synaptic
transmission at hippocampal pyramidal cells and interneurons in
rat brain slices, finding that tetanic stimulation of mossy
fibers induces long-term potentiation in pyramidal neurons, but
is either without effect or induces depression at synapses with
interneurons. Furthermore, unlike transmission onto pyramidal
neurons, transmission onto interneurons was not potentiated after
cAMP activation. The authors suggest their results indicate that
synaptic terminals arising from a common afferent pathway do not
behave as a single computational unit, but are functionally
specialized with effects depending on the postsynaptic target.
QY: Chris J. McBain [crismcb@codon.nih.gov]
(Science 27 Feb 98) (Science-Week 13 Mar 98)
-------------------
Related Background:
A NEW TYPE OF SYNAPTIC PLASTICITY OF NEOCORTICAL NEURONS
The term "synaptic plasticity" refers to a changeability of
synaptic connections and/or the efficacy of particular connect-
ions. "Cultured neurons" are embryological neurons separated from
the animal and growing and making connections in a suitable
experimental chamber. Glutamate is a major excitatory amino acid
neurotransmitter (transmitter substance at synapses) in the
brain, involved in about 40% of all brain activity. The term
"Hebbian modification" (named after the neuropsychologist Donald
Hebb) refers to the Hebbian "rule" that essentially states that
when one nerve cell repeatedly activates another nerve cell,
changes involving growth or metabolism occur in one or both nerve
cells that increase the efficiency of the activation.
... ... Turrigiano et al (5 authors at Brandeis University, US)
report a new form of synaptic plasticity in cultured neurons that
increases or decreases the strength of all of a neuron's synaptic
inputs as a function of activity, the changes partly due to
postsynaptic alterations in the response to glutamate. The
authors suggest that such "synaptic scaling" may help prevent
saturation of firing rates during developmental changes in the
number and strength of synaptic inputs, may stabilize synaptic
strengths during Hebbian modification, and may facilitate
competition between synapses and associated elimination of
synapses during development.
QY: Gina G. Turrigiano [turrigiano@binah.cc.brandeis.edu]
(Nature 26 Feb 98) (Science-Week 13 Mar 98)
-------------------
Related Background:
POSTSYNAPTIC MEMBRANE FUSION AND LONG-TERM POTENTIATION
The anatomical connections between nerve cells are called
"synapses", and in mammalian nervous systems the most important
type of connection involves the axon terminals of one neuron
(presynaptic endings) terminating on various parts of the second
neuron (postsynaptic loci). In general, electrical activity of
the first neuron causes secretion of a "neurotransmitter"
substance at its presynaptic terminals, and this neurotransmitter
may excite or inhibit the electrical activity of the second
neuron, the effect often mediated by inputs to the second neuron
from hundreds of other neurons. Prior to its release, the neuro-
transmitter substance is contained in vesicles in the presynaptic
terminals, and one of the significant events associated with
release of the neurotransmitter substance is the fusion of the
vesicle with the presynaptic membrane -- a "membrane" fusion,
since the shell of the vesicle itself is also a bilayer membrane.
In general, long-lasting enhancement of the postsynaptic response
of a neuron as a result of repetitive incoming synaptic activity
is called "long-term potentiation", and there are many varieties
of this phenomenon observed in different kinds of nerve cells,
and it is considered an example of synaptic plasticity. Since
long-term potentiation involves the behavior of a neuron relating
to its past history, the phenomenon is naturally of great
interest to neurobiologists seeking to understand neuronal
information storage. ... ... Lledo et al (5 authors at 2
installations, US) report that introducing substances that block
membrane fusion into the postsynaptic cell reduces long term
potentiation. Introducing a protein (SNAP) that promotes membrane
fusion enhances synaptic transmission, but at a reduced level in
already potentiated synapses. The authors suggest that fusion
events, in addition to being important for the presynaptic
release of neurotransmitters, are also involved in some mechanism
at the postsynaptic membrane, and thus contribute to long term
potentiation.
QY: Roger A. Nicoll [nicoll@phy.ucsf.edu]
(Science 16 Jan 98) (Science-Week 30 Jan 98)
[For more information: http://scienceweek.com/search/search.htm]


4. EPIDEMIOLOGY: ON THE AIDS EPIDEMIC
Although in the US the AIDS epidemic long ago lost its starring
role as a major media event, the epidemic is still a major
medical event in the US and elsewhere, and one with enormous
potential and actual consequences for entire populations of
people on the planet.
... ... Anthony S. Fauci (National Institutes of Health, US)
reviews the current situation concerning the international AIDS
epidemic, the author making the following points:
     1) Concerning infectious diseases, this century has
witnessed two unexpected cataclysmic events. The first, the
*influenza A *pandemic of 1918, was due to an old but reemerging
microbe. Influenza had been a problem for centuries, but in that
one winter of 1918-1919 it was responsible for the deaths of
approximately 25 million people worldwide and 550,000 people in
the US. The other pandemic, the acquired immunodeficiency
syndrome (AIDS), is due to a newly recognized microbe, the human
immunodeficiency virus (HIV). The world first became aware of
this new disease in the summer of 1981, and it has exploded in
successive waves in various regions of the world.
     2) Recent molecular epidemiological data have clearly
indicated that *HIV type 1 (HIV-1) evolved with the Pan
troglodytes troglodytes subspecies of chimpanzee and was present
in that subspecies for centuries. The virus apparently does not
readily cause disease in the chimpanzee.
     3) The most likely mechanism of transmission of HIV-1 from
chimpanzees to humans was by contamination of a person's open
wound with the infected blood of a chimpanzee, probably when the
chimpanzee was being butchered for the purposes of consumption.
Chimpanzees have traditionally served as a source of nutrition
for humans in certain parts of sub-Saharan Africa.
     4) AIDS continues to exact an enormous toll throughout the
world in both human and economic terms. In the US, an estimated
650,000 to 900,000 people are infected with HIV, of whom more
than 200,000 are unaware of their infection. Through 1998,
688,200 cumulative cases of AIDS and 410,800 AIDS related deaths
had been reported to the US Centers for Disease Control and
Prevention (CDC).
     5) The magnitude of the world epidemic is huge. As of the
end of 1998, there were more than 33 million people worldwide
with HIV infection or AIDS, with an estimated 43 percent of them
female. An estimated 5.8 million new HIV infections occurred
worldwide during 1998 -- approximately 16,000 each day. More than
95 percent of these new infections occurred in developing
countries. In 1998, HIV infection or AIDS was the 4th leading
cause of death worldwide, resulting in an estimated 2.3 million
deaths. If the current trend in the incidence of HIV infection
continues, more than 40 million people will be infected with HIV
as we enter the new millennium.
     6) In addition to the enormous human tragedy associated with
HIV and AIDS, the economic costs of the epidemic are staggering,
posing a serious impediment to the growth and economic stability
of many developing countries. It is also clear that this epidemic
will produce political instability in some nations and in
communities within these nations.
     7) The author concludes: "The HIV pandemic has posed a
formidable challenge to the biomedical research and public health
communities of the world. What began as a handful of recognized
cases among homosexual men in the United States has become a
global pandemic of such proportions that it clearly ranks as one
of the most destructive microbial scourges in history. We are at
pivotal point in the evolution of this historic event as we enter
the new millennium. Biomedical research has provided the tools
for the development of treatments as well as a still elusive
vaccine. It has become apparent over the past few years that
minimizing the destructive impact of this epidemic will require
partnerships between the public and private sectors as well as a
stronger political will among the nations of the world. Unless
methods of prevention, with or without a vaccine, are successful,
the worst of the global pandemic will occur in the 21st century."
-----------
Anthony S. Fauci: The AIDS epidemic: Considerations for the 21st
century.
(New England J. Med. 30 Sep 99 341:1046)
QY: Anthony S. Fauci [afauci@niaid.nih.gov]
-----------
Text Notes:
... ... *influenza A: The influenza viruses are of the group of
orthomyxoviruses, and all known orthomyxoviruses are influenza
viruses. There are 3 immunological types, known as A, B, and C.
The "myxovirus" name implies an affinity for certain glycoprotein
(mucin) cell surface receptors. Influenza virus particles are
usually spherical and approximately 100 nanometers in diameter.
The influenza A virus has a single-stranded RNA genome.
... ... *pandemic: In general, the term "pandemic" refers to any
extensive epidemic occurring over a wide geographic area and
affecting a relatively large proportion of the population.
... ... *HIV type 1 (HIV-1): HIV-1 is the subtype of HIV (human
immune-deficiency virus) that causes most cases of AIDS in the
Western Hemisphere, Europe, and Central, South, and East Africa.
HIV is a retrovirus (subclass lentivirus), and retroviruses are
single-stranded RNA viruses that have an enzyme called reverse
transcriptase. With this enzyme the viral RNA is used as a
template to produce viral DNA from cellular material. This DNA is
then incorporated into the host cell's genome, where it codes for
the synthesis of viral components. HIV should be distinguished
from AIDS. Acquired immunodeficiency syndrome (AIDS) is a
secondary immunodeficiency syndrome resulting from HIV infection
and characterized by opportunistic infections, malignancies,
neurologic dysfunction, and a variety of other syndromes.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON AIDS, DRUGS, PATENTS, AND RESPONSIBILITIES
The general philosophical question concerning epidemics and drugs
and profits is simply stated: If 25 million people in
underdeveloped countries are being ravaged by an epidemic of a
lethal disease, and pharmaceutical firms in developed countries
have in their hands one or more drugs established as effective in
the prevention or treatment of that disease, and if the current
prices for these drugs make them unaffordable in the
underdeveloped countries, what is to be done? There is perhaps no
clearer entanglement of basic research, applied science, and
commercial interests than the above question as it concerns the
current international AIDS epidemic. An important conflict in the
pharmacotherapeutic arena is developing, one which may set the
pattern for future interactions between pharmaceutical companies
and governments. The essentials of the conflict are as follows:
The pharmaceutical industry, with the assistance of the US
government, is fighting efforts to make cheaper generic versions
of AIDS drugs available to the underdeveloped countries currently
being ravaged by the AIDS epidemic. The drug firms wish to
maintain exclusive control of the manufacture and marketing of
their patented AIDS drugs, but some countries are issuing to
local firms licenses that allow production of affordable generic
versions of these drugs. ... ... Merrill Goezner (Chicago
Tribune, US) presents a review of the conflict, the author making
the following points: 1) Although the World Trade Organization
guards intellectual property rights among its member nations, its
rules of trade do allow for compulsory licensing in the case of
national emergencies. Thus South Africa, with more than 3 million
HIV cases, and one-fourth of pregnant mothers in the poorest
provinces HIV-positive, recently passed a compulsory licensing
law that apparently meets the World Trade Organization national
emergency guidelines, the law allowing licenses to local firms to
manufacture low-cost generic versions of patented anti-AIDS
drugs. 2) The South African law, however, has evidently angered
the US pharmaceutical industry, which apparently fears the
widespread licensing of its products will lead to a global "gray
market" in low-priced drugs, a gray market that will undermine
its profits and incentive to invest in costly research. 3) The US
pharmaceutical industry has therefore pressured the US government
to take action against the South African law. Legislation has
been introduced in the US Congress, and US trade representatives
are actively applying pressure in South Africa to have the
compulsory licensing law repealed. So far, South Africa has
refused. 4) Meanwhile, the pharmaceutical industry in both the US
and Europe has taken a firm position: Thomas Bombelles of the
Pharmaceutical Research and Manufacturers Association says
compulsory licensing is "a form of patent piracy... It's
stealing." 5) When used in a treatment regimen, the drug AZT,
manufactured by Glaxo-Wellcome, a drug that has proved effective
in inhibiting transmission of HIV from pregnant women to their
fetuses, costs approximately US$240 a month in South Africa when
purchased from Glaxo-Wellcome. Indian drug firms manufacture a
generic version of the drug that costs US$48 a month. The issues,
it seems, are clear.
-----------
Merrill Goezner: Third World battles for AIDS drugs.
(Chicago Tribune 28 Apr 99)
QY: Merrill Goezner, Chicago Tribune, 435 N. Michigan Ave.,
Chicago, IL 60611 US.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 7May99
[For more information: http://scienceweek.com/search/search.htm]


5. MEDICAL BIOLOGY: REDUCTION OF SIDE EFFECTS OF CANCER THERAPY
The protein p53 is a 53 kilodalton nuclear *phosphoprotein. It is
a probable human *cell cycle regulator that acts to regulate cell
division by controlling a set of genes required for the process.
p53 also acts as a tumor suppressor in some tissues, and
mutations in the p53 gene are the most common genetic alterations
in human cancers. In the laboratory, *wild-type p53 can suppress
or inhibit the transformation of cells in culture by viral or
cellular *oncogenes. A DNA-binding domain in the p53 protein has
been identified, and mutations in the p53 gene that are
associated with human cancers generally produce changes in this
DNA-binding domain. In general, as a "tumor suppressor", p53
apparently functions as a key component of a cellular emergency
response mechanism. In response to a variety of stress signals,
p53 induces growth arrest or *apoptosis (programmed cell death),
thereby eliminating damaged and potentially dangerous cells from
the organism. Experimentally induced lack of functional p53 is
accompanied by high rates of genomic instability, rapid tumor
progression, resistance to anticancer therapy, and increased
*angiogenesis. p53 deficiency in mice is associated with a high
frequency of spontaneous cancers. Chemotherapy and radiation
therapy for cancer often have severe side effects that limit
their efficacy, and these side effects are apparently in part
determined by p53 mediated apoptosis in non-cancerous tissues.
Therefore, temporary suppression of p53 in normal tissues has
been suggested as a therapeutic strategy to prevent damage of
normal tissues during treatment of p53-deficient tumors.
... ... P.G. Komarov et al (7 authors at 4 installations, US) now
report a study to test the effects of p53 suppression on cancer
therapy side effects. The authors report that a small molecule
was isolated for its ability to reversibly block p53-dependent
*transcriptional activation and p53-dependent apoptosis. The
authors report that this compound, *pifithrin-alpha, protected
mice from the lethal *genotoxic stress associated with anticancer
treatment, and that the protection occurred without promoting the
formation of tumors. The authors suggest that inhibitors of p53
may be useful drugs for reducing the side effects of cancer
therapy and other types of stress associated with p53 induction.
-----------
P.G. Komarov et al: A chemical inhibitor of p53 that protects
mice from the side effects of cancer therapy.
(Science 10 Sep 99 285:1733)
QY: Andrei V. Gudkov [gudkov@uiuc.edu]
-----------
Text Notes:
... ... *phosphoprotein: In general, any protein containing one
or more phosphoric residues directly attached (usually by an
ester linkage) to amino-acid residues.
... ... *cell cycle: In this context, the term "cell cycle"
refers to the entire life history of a single cell from mitosis
to mitosis, including the sequence of intervening phases.
... ... *wild-type p53: A "wild-type" gene is the non-mutated
gene that normally appears in a population.
... ... *oncogenes: There are two general meanings of this term
in current use. The first meaning refers to any of a family of
cellular genes that normally code for proteins involved in cell
growth or regulation, but which may produce malignant processes
when mutated or activated by viruses. The second meaning of the
term "oncogene" refers to viral genes found in certain DNA tumor
viruses, genes that are required for viral replication, but whose
activation produces malignant transformations.
... ... *apoptosis (programmed cell death): In general, the term
"apoptosis" refers to programmed cell death, whether as a part of
normal tissue differentiation and development, or as a program
activated in a defective cell. In the molecular biology of
cancer, apoptosis is the name given to the programmed cell death
provoked by the proteins expressed by tumor suppressor genes.
Thus, malignant cells are defective cells with a deactivated
apoptosis program, and this allows malignant cells to survive and
replicate.
... ... *angiogenesis: The origin and development of blood
vessels. Angiogenesis is an important consideration in the growth
of cancerous tumors, since the tumor provokes directed
angiogenesis into itself with the end result that the tumor is
supplied with oxygen and nutrients. Without angiogenesis, tumors
can attain only a small size before becoming self-inhibiting.
... ... *transcriptional activation: Transcription is the process
by which the genetic information in DNA is converted into RNA --
the transcription of DNA code into RNA code.
... ... *pifithrin-alpha: This is a synthetic water-soluble
compound with a molecular weight of 367. The chemical name is
[2-(2-imino-4.5,6,7-tetrahydrobenzothiazol-3-yl)-1-p-
tolylethanone].
... ... *genotoxic: In general, a "genotoxic" substance is any
substance that exerts its effect by damaging or otherwise
interfering with the action of a gene.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
MECHANISMS OF TUMOR SUPPRESSOR GENES
Cancer is fundamentally a genetic disease in which damage to
cellular DNA leads to disruption of the normal mechanisms that
control cellular proliferation. ... ... Ellisen and Haber (2
installations, US) review current knowledge concerning the genes
targeted in human cancer, and they make the following points: 1)
In general, cancer genes have been divided into 2 classes, proto-
oncogenes and tumor suppressor genes. 2) Proto-oncogenes are
genes that sustain activating changes in human cancer. These
changes may take the form of point mutations or gene
rearrangements that lead to increased or uncontrolled activity of
the encoded protein, or they make take the form of gene
amplification, which results in increased levels of protein
expression. 3) Tumor suppressor genes are characterized by
inactivating changes in human cancer, typically point mutations
that result in truncation or functional inactivation of the
encoded protein, or gross deletions of chromosomal fragments
carrying these genes. 4) Tumor suppressor genes are of particular
interest in cancer genetics because they are the genes most
commonly associated with hereditary predisposition to cancer. In
cases where familial cancer is linked to inheritance of a mutant
allele of a tumor suppressor gene, inactivation of the remaining
wild-type allele of that gene constitutes the critical genetic
event that initiates the development of cancer. 5) Although much
attention has focused on the initial tumor suppressor gene
mutation that initiates malignant transformation, cancer actually
results from the accumulation of a large number of genetic
events, both in tumor suppressor genes and in proto-oncogenes.
The authors tabulate 14 selected different tumor suppressor
genes, indicating the related familial syndrome, the types of
sporadic tumors containing mutations of these genes, and the
presumed normal function (mechanism of action) of these genes.
Mutations of the tumor suppressor gene  have been found in
approximately 50 percent of all cancers. The authors conclude:
"As the understanding of genetic heterogeneity evolves,
population studies are likely to uncover the contribution of
common subtle genetic variations to the risk of developing
different types of cancer. Together with societal and ethical
guidelines on the use of such genetic information, the study of
mutations in tumor suppressor genes and of their role in cancer
predisposition may provide important clues to the clinical
management of human cancer."
QY: Leif W. Ellisen, Dana-Farber Cancer Institute, Boston, MA US.
(Science & Medicine Jul/Aug 1998) (Science-Week 17 Jul 98)
-------------------
Related Background:
EVIDENCE THAT BREAST CANCER GENE IS APOPTOSIS COACTIVATOR
In molecular biology, the term "transcription" refers to the
sequence of biochemical events producing the conversion of DNA
code to RNA code. Apoptosis is programmed cell death produced by
control mechanisms designed to destroy defective cells or cells
that must be discarded in the process of tissue differentiation.
Mutations of the gene {BRCA1} have been linked to 45% of the
cases of familial breast cancer, and to 80% to 90% of families
with both breast and ovarian cancer. ... ... Now Ouchi et al (5
authors at 4 installations, US) report that {BRCA1} stimulates
artificial and genomic entities that contain elements responsive
to the tumor suppressor protein p53, which is known to be
involved in apoptosis. The authors suggest their findings
indicate the gene {BRCA1} is involved in transcriptional
regulation and has a function as a p53 coactivator.
QY: Hidesaburo Hanafusa (saburo@rockvax.rockefeller.edu)
(Proc. Natl. Acad. Sci. US 3 Mar 98)
(Science-Week 2 Apr 98)
[For more information: http://scienceweek.com/search/search.htm]


6. ASTROPHYSICS: PERSPECTIVES IN GENERAL RELATIVITY
First proposed in 1915 by Albert Einstein (1879-1955), the
General Theory of Relativity describes the effects of the
gravitational fields of matter on space and time [*Note #1]. One
of the major conclusions of the theory is that gravitational
fields alter the geometry of space and time, causing this
geometry to become curved. According to the theory, when gravity
is weak and the resultant space-time curvature small, the motion
of matter is close to that predicted by Newton's laws. In recent
years, the predicted small deviations from Newton's laws have
been indirectly confirmed by studies of *millisecond
radiopulsars. However, no direct measurements have ever been
performed to measure the gross deviations from Newtonian
mechanics predicted in strongly-curved space-time. Thus, these
strong-field predictions of general relativity theory have not
yet been tested.
... ... Michiel van der Klis (University of Amsterdam, NL)
presents a review of current research on general relativistic
effects produced by massive astronomical objects, the author
making the following points:
     1) The strongest space-time curvatures that are accessible
to observations are those near collapsed massive stars. These
objects concentrate within a few kilometers the mass of a star
one to several times the mass of the Sun (1 solar-mass = 2 x
10^(30) kg). Such objects have the strongest gravity, and hence
curve space-time the tightest of all known objects in the
Universe. Some of these objects are so massive that they can only
be black holes [*Note #2].
     2) X-ray binaries, discovered in the early 1960s, are double
star systems in which a compact object (a neutron star or a black
hole) orbits a normal star and pulls a flow of *plasma out of the
normal star's atmosphere onto itself. Such binary systems have
long been considered promising natural laboratories for studying
orbital motion in strongly curved space-time, and since the
launch of the *Rossi Explorer 3 in 1996, these systems have
finally begun to make good on their promise.
     3) The process of "accretion", the flow of matter from the
normal star onto the neutron star or into the black hole,
produces a differentially rotating disk around the compact
object: the closer material is to the compact object, the faster
the material rotates around the object. Thus, the plasma orbits
at ever-increasing speed as it spirals down toward the center. In
the strong gravity region near the compact object, speeds
approach that of light and orbital radii of the plasma are only a
few kilometers, so that the orbit of a particle around the center
takes less than a millisecond to complete.
     4) The Rossi x-ray satellite has made it possible to detect
such extremely tight orbits. The fastest signals, seen in neutron
stars, have periods as short as 0.75 milliseconds, corresponding
to orbital motion at a 12-kilometer radius, which is deep within
the strong-gravity region and close to the marginal orbital
radius within which no stable orbits are possible. Such phenomena
have now been seen in approximately 20 neutron stars. Three black
holes have also shown periods as short as 3 milliseconds, which,
given their higher masses, implies that the signals come from a
similar depth in the strong-gravity region around a black hole.
     5) The author suggests that the success of the Rossi
satellite has demonstrated that when probing dynamics in strongly
curved space-time, there is no substitute for size. It is the
huge effective area of 0.7 square meters of the main x-ray
instrument onboard the satellite that gave Rossi the sensitivity
to make the first direct measurements of orbital motion near
collapsed stars.
-----------
Michiel van der Klis: The buzz of general relativity.
(Science 3 Sep 99 285:1499)
QY: Michiel van der Klis [ michiel@astro.uva.nl]
-----------
Text Notes:
... ... *Note #1: The essential basis of general relativity
theory 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. (In general
relativity, the equivalence principle states that the observable
local effects of a gravitational field are indistinguishable from
those arising from acceleration of the frame of reference.)
... ... *millisecond pulsars: (see Note #2 below)
... ... *Note #2: During the life of a star, two opposing forces
control the star's equilibrium: the gravitational force, which
drives the collapse of the star's mass inward to the center of
gravity, and the counteracting outward pressure derived from the
nuclear fusion reactions in the star's core. When the nuclear
fuel burns out, the star begins its death and gravitational
collapse occurs. 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. Another view of a black hole
is that it is a mass that has collapsed to such a small volume
that its gravity prevents the escape of all radiation. Space and
time essentially have no meaning in a black hole. The boundary of
the black hole is called the "event horizon", because any event
within the boundary is invisible outside, the invisibility
resulting from the fact that no radiation can escape to be
detected. The radius of the black hole depends upon how much
matter has fallen into the region; it is called the "Schwarzchild
radius", and it is usually a few kilometers. However, massive
black holes are possible and are thought to be the source of
quasars (quasi-stellar objects), which are 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. If quasars indeed involve black holes,
the radiation is from material just outside the black hole, and
not from anything within it. Nothing inside a black hole can get
out of it. If, following its terminal stages, the remnant mass of
a star is between 1.4 and 2 to 3 solar masses, the star will
collapse into a neutron star, a body with a radius of 10 to 15
kilometers, with a core so dense that its component protons and
electrons have merged into neutrons. The average density of a
neutron star is 10^(15) grams per cubic centimeter, and the
weight of an object on the surface of a neutron star would be
10^(11) its weight on the surface of the Earth. Neutron stars
apparently have an outer shell of iron, but it is iron like no
Earth iron, an iron of 4 orders of magnitude greater density.
Theory predicts that a neutron star should rotate very rapidly,
be extremely hot, and have an intense magnetic field. Pulsars,
sources of pulsed radio energy, are evidently spinning neutron
stars which emit beams of radiation from their magnetic poles. A
few pulsars have been found in binary systems, and the empirical
estimated masses of the pulsars are consistent with the masses
predicted by neutron star models. Pulsars were originally
discovered at radio wavelengths, but they have also been detected
at optical and gamma-ray wavelengths. They can be powerful
gamma-ray emitters (gamma-ray pulsars), and there is also a class
of x-ray pulsars. The periods of pulsars range from approximately
1.5 milliseconds to 4 seconds and can typically be measured to
accuracies of one part in 10^(10). Pulsars with periods shorter
than approximately 0.01 seconds constitute the distinct class of
millisecond pulsars. Most pulsars are single stars, but binary
pulsars are known, about half of which are millisecond pulsars.
The millisecond-pulsar neutron star is believed to be rotating
hundreds of times per second, and a large number of millisecond
binary pulsars have been discovered in globular clusters. The
origin and evolution of pulsars has not been clear, but it has
been thought that millisecond radio pulsars, which are often
found in binary systems, start as ordinary pulsars, then lose
most of their magnetic field and "spin up" to millisecond periods
by the accretion of matter (and transfer of angular momentum)
from a companion star in an x-ray binary system. Until now, there
... ... *plasma: In this context, a "plasma" is a state of matter
consisting of ions and electrons moving freely. Stars, for
example, consist of plasma. Because a plasma is highly ionized,
it can be affected by external electrical and magnetic fields,
and the charged particles of the plasma interact electrically and
magnetically.
... ... *Rossi Explorer 3: The Rossi X-Ray Timing Explorer (RXTE)
is a NASA satellite launched in December 1995. It carries the
largest array of proportional counters ever flown, nearly a meter
square. The purpose is to study the variability of the brightest
x-ray sources at time-scales down to 10 microseconds. The
satellite is named after the astronomer Bruno Benedetto Rossi
(1905-1993).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]


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

IN FOCUS: ON WATSON AND CRICK
"We already knew because of Mendel that genetics was digital and
not analogue. That is to say, genes do not blend, they are either
there or they are not, it is like beads on a string. What Crick
and Watson did was to show that even within a gene it is digital.
Even down to the finest minute structure of a gene, everything is
digital code. It is just like computer language. The only
significant difference is that it is not binary but it is
quaternary and that is a trivial difference. So, whereas before
Crick and Watson came along it was still possible for people to
wax a bit mystical about genes and to say there is something
deeply mysterious, a sort of life force about them, that turns
out to be total nonsense. The gene is just like a piece of human
computer tape. You can transcribe a gene, it is being done at
this moment for the Human Genome Project, into print on to
computer tape. You could get the whole human genome and put it
into a library in the form of ink on paper and then at some
future date, some future century, take down the books from the
library, transcribe that ink on paper information back into DNA,
and in principle you could regrow an identical twin of the
individual whose data you originally used. So that is a supreme
revolution in our view of life and it has clearly been the basis
for an immense amount of work in medicine, in biochemistry. The
whole of biology has been completely turned upside down by Watson
and Crick."
-----------
Richard Dawkins: in Melvyn Bragg: _On Giants' Shoulders_
(John Wiley & Sons, New York 1998, p. 317)


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