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ScienceWeek
SCIENCE-WEEK - December 14, 2001 -- Vol. 5 Number 50
An Email Research Digest Published Weekly Since 1997
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The current method is essentially robotic: Noticing that
a phrase about to be written can be coded by a new
acronym (NA), like a robot, the researcher-writer (RW)
hurries to create the NA -- and necessity be damned.
Has the creation of new acronyms become another highway
to fame and riches (FAR)? Or is this merely a playful
children's game (PCG), an attempt to recover lost youth
(ATRLY)? Or are we slowly going mad (SGM) in the face of
increasing complexity (IC)? Where are the brave souls who
will forsake FAR and rise up to banish these hordes of NAs
that now bedevil us? Where is the needed Movement Against
Games and Madness (MAGAM)?
-- Unknown
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. Humans in the Arctic 40,000 Years Ago
2. On Cell Signaling
3. On Rab Proteins and Intracellular Transport
4. On Genes, Biological Complexity, and the Transcriptome
5. RNA World: RNA-Catalyzed RNA Polymerization
6. On the Cytoskeleton of Neurons
7. Estrogen and Brain Synapses
8. Vertebrate Suspended Animation Produced by Oxygen Deprivation
9. Another Critique of the Nobel Prizes
10. On the Processes That Trigger Melting
11. Global Warming: Short-Term vs. Long-Term Predictions
12. On the Physics of Clouds
13. PostDoctoral Fellowship Profiles:
Laboratory of M.M. Hussain at SUNY Downstate Medical Cntr, US
14. In Focus: On Janet Rowley
15. From PRAXIS: On the Dynamics of Aging Muscle
16. This Week in PRAXIS
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Section 2
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1. HUMANS IN THE ARCTIC 40,000 YEARS AGO
The human group we call the "Neanderthals" lived in much of
Europe, part of Asia, and the Middle East between 150,000 to
probably less than 30,000 years ago. Neanderthals were the first
fossil humans to be discovered, and they have long been the focus
of anthropological investigation. More bones of Neanderthals are
known than for any other human-related (hominine) fossil group,
including 30 nearly complete skeletons.
... ... P. Pavlov et al (Russian Academy of Sciences, RU) discuss
evidence for a human presence in the Paleolithic Arctic. The
transition from the Middle to the Upper Paleolithic, dated at
approximately 40,000 to 35,000 radiocarbon years ago, marks a
turning point in the history of human evolution in Europe. Many
changes in the archeological and fossil record at this time have
been associated with the appearance of anatomically modern
humans. Before this transition, the Neanderthals roamed the
continent, but their remains have not been found in the
northernmost part of Eurasia. It is generally believed that this
vast region was not colonized by humans until the final stage of
the last Ice Age approximately 13,000 to 14,000 years ago. The
authors report the discovery of traces of human occupation nearly
40,000 years old at Mamontovaya Kurya, a Paleolithic site
situated in the European part of the Russian Arctic. The riverbed
at this site has been known as a place for finding mammoth tusks
and bones since the end of the 18th century, but finds of
artefacts have not been reported. The authors report they have
recovered stone artefacts at this site, animal bones, and a
mammoth tusk with human made marks, from strata covered by thick
Quaternary deposits. This is the oldest documented evidence for
human presence at this high latitude (66 deg 34' North). The
authors suggest their find implies that either the Neanderthals
expanded much further north than previously believed, or that
modern humans were present in the Arctic only a few thousand
years after their first appearance in Europe.
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Nature 2001 413:64
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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2. ON CELL SIGNALING
Julian Downward (Imperial Cancer Research Fund, UK) discusses
cell signaling. All biological cells continually sense their
surrounding environment and respond on the basis of that
information. Single-celled organisms can determine which
nutrients are nearby and regulate their metabolic processes
accordingly. Cells in multicellular organisms such as humans
sense the presence of neighboring cells and hormones when
responding, effectively deciding whether to proliferate, move, or
die. These processes all require the transfer of information from
detection systems called "receptors" through intermediate
molecules within the cell, the information transfer resulting in
the expression of various genes and changes in the activity of
various enzymes. The terms "signal transduction", "cell
signaling", or simply "signaling" refer to the mechanisms by
which the transfer of biological information is accomplished.
Signaling can be studied at the level of the individual cell or
at the level of the whole organism. For individual cells,
signaling is crucial to responses involving cell division,
specialization, metabolic control, and death. In more specialized
cells, signaling is central to immunity and the transmission of
nerve impulses, to name but two examples. At the level of whole
multicellular organisms, signaling controls growth and
development, as well as aspects of metabolism and behavior. It is
therefore not surprising that malfunctions in signaling underlie
many human diseases.
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Nature 2001 411:759
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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Related Background:
TRANSCRIPTION FACTORS AND CELL SIGNALING
"Transcription" is the process by which the genetic information
in DNA is converted into RNA, and transcription factors are a
class of DNA-binding proteins that regulate RNA transcription.
In this context, a "growth factor" is any specific substance
that must be present in a culture medium for multiplication of
the cultured cells to occur. Certain growth factors have been
identified as cytokine proteins (peptide hormones) that stimulate
the growth and division of target cells by binding to cell
membrane receptors.
G-proteins are a family of signal-coupling proteins that act
as intermediaries between activated cell receptors and effectors,
for example, the transduction of hormonal signals from the cell
surface to the cell interior. The G-protein is apparently
embedded in the cell membrane with parts exposed on the outside
surface and inside surface. The outside moiety is activated by
the first messenger, and the inside moiety activates the second
messenger, the G-protein thus acting as a trans-membrane signal
transducer.
... ... Lewis C. Cantley (Harvard University, US) discusses
transcription factors in intracellular signaling pathways. Once
receptors in the plasma membrane of cells become activated
through binding with their ligands (e.g., hormones or growth
factors), the binding initiates a cascade of signals that are
transmitted to the nucleus of the cell where they switch on the
expression of specific genes ("target genes"). In many of these
signal transduction pathways, there is a key transcription factor
ordinarily "trapped" in the cell cytoplasm that becomes modified
as a consequence of the activation of the hormone or growth
factor receptor. The modified transcription factor is then free
to escape from the cytoplasm and enter the cell nucleus, where it
activates target genes. The signaling events that lead from
receptor activation of the cell surface to the release of the
transcription factor into the nucleus are still poorly understood
for certain families of transcription factors. Santagata et al
(2001) now present evidence for a new signaling pathway in which
the apparent transcription factor "tubby" is released from its
association with plasma membrane phosphatidylinositol and moves
to
the cell nucleus. A defective version of tubby has been
implicated in mature-onset obesity. Tubby is apparently clipped
from the plasma membrane by the enzyme phospholipase C-beta,
which hydrolyzes phosphatidylinositol lipids following activation
of G-protein coupled hormone receptors.
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Science 2001 292:2019,2041
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SCIENCE-WEEK 24 Aug 2001 http://scienceweek.com
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Related Background:
CELL BIOLOGY: ON INTERNAL SIGNALING IN BIOLOGICAL CELLS
Every multicellular organism is essentially a colony of
biological cells, and in all cases the viability of the colony
(and the viability of the "organism") is critically dependent on
direct or indirect organized communication between the cells
constituting the colony population.
In some cases, certain intercellular communication pathways
in an organism are anatomically explicit (e.g., cell
communication via a nervous system), but in most cases
intercellular communication involves the release and reception of
chemical signals (e.g., hormones) that are transported either by
simple diffusion or by hydraulics (e.g., a circulating blood or
other fluid system).
Although these general principles have been known for more
than a century, it is only during the past few decades that the
molecular details of the mechanisms of communication between
various cells in an organism have become apparent. Research in
this area is not only important for the understanding of the
biology of organisms in general, but it is also of great
consequence for clinical medicine: If any important communication
pathway between the various cells and tissues that constitute the
colony we call the human body fails, or if any mechanism
responsible for the appropriate response of cells to incoming
signals fails, that failure produces disease, often with
reverberations to other communication pathways, and the result
can be severe debilitation or even death of the organism,
followed by rapid disintegration of all the cells comprising the
colony. In essence, intercellular communication (and appropriate
responses by cells to received signals) is what holds the body
together as a viable entity.
... ... J.D. Scott and T. Pawson (2 installations, US CA) present
a review of current research in cell communication, with a focus
on what happens inside cells when cells receive externally-
derived signals. The authors make the following points:
1) Internal signal transmission in cells usually begins
after a chemical messenger responsible for carrying information
between cells (e.g., a hormone) docks temporarily in lock and key
fashion with a specific receptor on the recipient cell surface.
Such receptors are able to relay chemical information into a cell
because they are physically connected to the cell interior
(cytoplasm). The typical receptor is a protein that includes at
least 3 domains: a) an external docking region for a hormone or
other messenger; b) a component that spans the plasma membrane of
the cell; c) a tail that extends a distance into the cytoplasm.
When a messenger binds to the external site, this binding induces
a change in the shape of the cytoplasmic tail, and this change in
shape facilitates the interaction of the tail with one or more
information-relaying molecules in the cytoplasm. These
interactions in turn initiate cascades of further intracellular
signaling.
2) During the last decade, researchers discovered that many
of the proteins involved in internal cell communication consist
of strings of modules, some of which serve primarily to connect
one protein to another. At times, whole proteins in intracellular
signaling pathways apparently contain nothing but such linker
modules. Also, within the past few decades, it has become more
apparent that the cytoplasm of cells is not amorphous, but is
instead densely packed with organelles and proteins, and that
high-fidelity signaling within cells depends profoundly on
configurational interlocking of selected proteins via dedicated
linker modules and adapter proteins. These complexes assure that
enzymes or DNA-binding modules and their targets are brought
together promptly and in the correct sequence as soon as a
receptor on the cell surface is activated.
3) Certain intracellular signaling networks apparently rely
on relatively small adapter proteins, and in some cases such
proteins contain a large number of linker domains. These proteins
are often called "scaffolding molecules", since they permanently
hold groups of signaling proteins together in one location. The
existence of such scaffolds means that certain signaling networks
are hard-wired into cells, with such hard-wiring enhancing the
speed and accuracy of intracellular information transfer.
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Scientific American June 2000
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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Related Background:
CELL BIOLOGY: ON THE SIGNIFICANCE OF INTRACELLULAR CIRCULATION
The prevailing idea in biology is that many physiological and
molecular functions are the sum of individual processes linked in
sequence, although when studied in isolation, many such
individual processes are without apparent function. How such
systems evolve and become regulated continues to be one of the
most important puzzles confronting both biochemists and cell
physiologists.
... ... P.W. Hochachka (University of British Columbia, CA)
presents a review of current theoretical approaches to cell
metabolism and regulation, the author making the following
points:
1) Two views currently dominate research into cell function
and regulation: The first view (Model #1) assumes that cell
behavior is quite similar to that expected in a watery bag of
enzymes and ligands. The second view (Model #2) assumes that
3-dimensional order and structure constrain and determine
metabolite behavior.
2) A major puzzle in the study of cell metabolism is that
essentially all metabolite concentrations are remarkably stable
(are *homeostatic) over large changes in pathway fluxes. The
author calls this the "stability paradox". For muscle cells, for
example, *adenosine triphosphate (ATP) and oxygen are the most
perfectly homeostatic, even though oxygen delivery and metabolic
rate correlate in a 1:1 fashion. In total, more than 60
metabolites are known to be remarkably homeostatic in differing
metabolic states.
3) Several explanations of stability are usually given by
traditional Model #1 studies -- none of which apply to all
enzymes in a pathway, and all of which require diffusion as the
means for changing enzyme-substrate encounter rates. In contrast,
recent developments in our understanding of intracellular
*myosin, kinesin, and dynein motors running on *actin and tubulin
tracks or cables provide a mechanistic basis for regulated
intracellular circulation systems with *cytoplasmic streaming
rates varying over an approximate 80-fold range (from 1 to more
than 80 microns per second).
4) These new studies suggest a Model #2 hypothesis of
intracellular perfusion or convection as a primary means for
bringing enzymes and substrates together under variable metabolic
conditions. In this view, changes in intracellular perfusion
rates causes changes in enzyme-substrate encounter rates and thus
changes in pathway fluxes -- with no requirement for large
simultaneous changes in substrate concentrations. The author
concludes: "The ease with which this hypothesis explains the
stability paradox is one of its most compelling features."
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Proc. Natl. Acad. Sci. 1999 96:12233
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Notes:
... ... *homeostatic: The term "homeostasis" refers to a
physiological equilibrium necessary in general for the viability
of an organism, and in particular for the operation of many
cellular functions. Homeostatic mechanisms in biological systems
usually involve an element of negative feedback signaling. In
vertebrates, for example, when blood temperature is too high,
temperature receptors provoke a sequence of events involving many
pathways that ultimately results in a lowering of body
temperature. Similar homeostatic mechanisms operate at cellular
levels.
... ... *adenosine triphosphate (ATP): ATP is the most important
chemical energy source in all living cells, intimately involved
in various cell functions and cell metabolism, and an entity in
numerous cyclic chemical pathways involved in the synthesis of
various cell components.
... ... *myosin, kinesin, and dynein motors: It is now recognized
that the interiors of biological cells are structurally complex,
and that this structure is dynamic. Microtubules are part of the
cytoskeleton of biological cells, the quasi-rigid matrix that
among other things determines cell shape. The microtubules are 25
nanometers in diameter, and composed of the protein tubulin. They
occur in regular arrays in various cell organelles, and in the
cytoplasm in general, and they contribute not only to cell shape,
but also to cell motility. Microfilaments are 4 to 6 nanometers
in diameter, highly variable in length, and are found in all
eukaryotic cells. They are composed of a protein called "actin"
and several other accessory proteins, and they are important in
cell locomotion and in the molecular dynamics of muscle cells.
"Motor proteins" are mechanico-chemical enzymes involved in
locomotion or transport, and there are three families of such
proteins: kinesins, dyneins, and myosins. Kinesins and dyneins
are microtubule based motor proteins, while myosin is a
microfilament based motor protein. In general, as mechanico-
chemical enzymes, motor proteins convert energy from hydrolysis
of nucleotides to mechanical force, and since they are involved
in many important cellular events, the molecular details are
currently the focus of intensive research.
... ... *actin and tubulin tracks or cables: See previous note.
... ... *cytoplasmic streaming: Supremely evident to biologists
who study living cells with the light microscope is the fact that
the interiors of cells are often in visible motion, the
cytoplasmic contents circulating in various streams. The
classical name for these movements was "cytoplasmic streaming",
and for nearly a century, such movements remained a profound
puzzle in biology. With the advent of molecular biology,
intracellular streaming motion was recognized as a phenomenon
related to dynamic motor proteins.
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ScienceWeek 2000 7 Jan
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Related Background:
ANALYSIS OF INTRACELLULAR SIGNALING MECHANISMS
In multicellular organisms, chemical messengers of various types
are important entities in the communication between cells
necessary for the function and viability of cells, tissues, and
the organism itself. These messengers usually interact with the
surfaces of cells, particularly with specific receptors on cell
surfaces. Such an interaction is called an "extracellular
signal", and what happens next is a cascade of internal signal
events that effectively transmit the external signal from the
cell membrane to one or more places inside the cell, especially
to the cell nucleus. This sequence in internal signal events
apparently involves specific protein-protein and protein-
phospholipid interactions, with the interactions mediated by
protein domains (regions) of tertiary structure (higher order
configuration) that have evidently been conserved through
evolution. The details of these biochemical interactions are
becoming apparent, at least in some types of somatic cells, so
that molecular biologists are now characterizing the involved
proteins as anchoring (docking) proteins, adaptor proteins,
scaffold proteins, and so on, according to the role played by the
particular protein in the spatial location and translocation and
signal events that eventually produce important reactive or
regulatory responses of the cell. Pawson and Scott (2
installations, CA US), in a review of how extracellular signals
are relayed from the plasma membrane to specific intracellular
sites, discuss the role of scaffold, anchoring, and adaptor
proteins that contribute to signal transduction by recruiting
active enzymes into signaling networks or by placing enzymes
close to their substrates. The authors suggest that the challenge
ahead is to understand both the physiological functions and
regulation of such signaling networks.
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Science 1997 19 Dec
ScienceWeek 1998 9 Jan
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3. ON RAB PROTEINS AND INTRACELLULAR TRANSPORT
The term "endoplasmic reticulum" refers to a complex system
of flattened sacs in all biological cells that have a nucleus
(eukaryotes). The endoplasmic reticulum is the site of many
important syntheses, including the production of new surface
membrane and the intracellular transport of various biochemical
entities.
The term "Golgi apparatus" refers to a compound membranous
cytoplasmic organelle of eukaryotic cells, the system consisting
of flattened ribosome-free vesicles arranged in a more or less
regular stack. In general, the Golgi apparatus processes proteins
produced by the ribosomes of the rough endoplasmic reticulum,
such processing including modification of the oligosaccharides of
glycoproteins, and the sorting and packaging of proteins for
transport to a variety of cellular locations. The Golgi apparatus
is also a major site of synthesis of polysaccharides.
... ... Nava Segev (University of Illinois Chicago, US) discusses
Rab proteins. Eukaryotic cells are filled with membrane-bound
compartments, such as the endoplasmic reticulum and Golgi
apparatus, that form a transport network for newly synthesized
proteins. In the exocytic pathway, "cargo proteins" destined for
secretion are inserted into the endoplasmic reticulum and are
transported through the various cisternae of the Golgi apparatus.
The cargo proteins are then sorted into secretory vesicles in the
trans-Golgi network, the proteins sorted into budding vesicles by
cargo receptors that span the compartment membrane. Once loaded,
the vesicles then fuse with the plasma membrane and release their
cargo at the cell surface. In the endocytic pathway, proteins in
or at the surface of the plasma membrane are internalized into
"early endosomes", and then are transported in "late endosomes"
to enzyme-filled sacs called "lysosomes", where they are
degraded. Like traffic police at key intersections, a family of
small proteins called the "Rab guanosine triphosphatases"
regulate the sorting and transport of cargo proteins. These
molecular switches ensure the specific and efficient targeting of
vesicles that move cargo between various cellular compartments.
In addition, Rabs may be required for the formation and movement
of transport vesicles, the remodeling of vesicle membranes, the
coupling of individual transport steps, and the coordination of
protein transport with other cellular processes.
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Science 2001 292:1313
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4. ON GENES, BIOLOGICAL COMPLEXITY, AND THE TRANSCRIPTOME
Eors Szathmary et al (Institute for Advanced Study Budapest, HU)
discuss genes and biological complexity. Although natural
selection does not guarantee that organisms will increase in
complexity as they evolve, it is apparent that the complexity of
certain lineages, such as our own, has increased during
evolution. But despite our intuitive notion of biological
complexity -- in terms of morphological or behavioral complexity,
or the variety of cell types in an organism -- the term itself is
notoriously difficult to define. Is the number of genes in the
genome of an organism an appropriate measure of biological
complexity? It has been assumed that eukaryotes have more genes
than bacteria, that animals have more genes than plants, and that
vertebrates have more genes than invertebrates -- which fits with
the traditional idea of a "scala naturae". But recent completed
genome sequences indicate this is not necessarily the case:
surprisingly, it turns out that the nematode worm C. elegans has
18,424 genes in its genome, the fruit fly. Drosophila has 13,601
genes, the plant Arabidopsis approximately 25,498, and humans
approximately 35,000 genes. This suggests there must be other and
more sensible genomic measures of complexity than the mere number
of genes. Transcription factors are DNA binding proteins that
switch target genes on and off. For all transcription factor
families, their members increase in number in the order yeast,
nematode, fruit fly, human. The diversity of cell types in these
organisms also increases in that order. This makes sense, given
that maintaining the differential state of increasingly diverse
cell types requires the presence of more and more molecular
switches. Claverie (2001) has suggested that we define biological
complexity in terms of the number of "transcriptome" states that
the genome of an organism can achieve, with a transcriptome
defined as the complete set of RNA transcripts. The authors
(Szathmary et al) propose that biological complexity might be
better explained by considering networks of transcription factors
and the genes they regulate, rather than by simply counting the
number of genes or the number of interactions among genes.
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Science 2001 292:1315
Science 2001 291:1255
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5. RNA WORLD: RNA-CATALYZED RNA POLYMERIZATION
W.K. Johnston et al (Massachusetts Institute of Technology, US)
discuss RNA-catalyzed RNA polymerization. The "RNA world
hypothesis" states that early life forms lacked protein enzymes
and depended instead on enzymes composed of RNA. Much of the
appeal of this hypothesis arises from the realization that
RNA-enzymes (ribozymes) would have been far easier to duplicate
than proteinaceous enzymes. Whereas coded protein replication
requires numerous macromolecular components (including messenger
RNAs, transfer RNAs, the ribosome, etc.), replication of a
ribozyme requires only a single macromolecular activity: an RNA-
dependent RNA polymerase that synthesizes first a complement, and
then a copy of the ribozyme. If this RNA polymerase were itself a
ribozyme, then a simple ensemble of molecules might be capable of
self-replication and eventually, in the course of evolution, give
rise to the protein-nucleic acid world of contemporary biology.
Finding a ribozyme that can efficiently catalyze general RNA
polymerization would support the idea of the RNA world and would
provide a key component for the laboratory synthesis of minimal
life forms based on RNA. The authors report the generation of an
RNA molecule that catalyzes the type of polymerization needed for
RNA replication. The ribozyme uses nucleotide triphosphates and
the coding information of an RNA template to extend RNA primer by
the successive addition of up to 14 nucleotides -- more than a
complete turn of an RNA helix.
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Science 2001 292:1319
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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Related Background:
ORIGIN OF LIFE:
DIFFERENTIAL ADSORPTION OF NUCLEIC ACID BASES
The idea that the adsorption of organic substances onto
inorganic surfaces might have been involved in the origin of life
on Earth was discussed 50 years ago by the crystallographer J.D.
Bernal (1901-1971). More recently, in 1982, the chemist A.G.
Cairns-Smith suggested that life developed from crystals, with
life originating from replication of clay crystals. In the
Cairns-Smith model, life began through the influence of natural
selection on the growth of inorganic crystals, with clay
structures the first carriers of genetic information. Replication
occurred by the accidental detachment of layers in the clay
lattice, the layers serving as nuclei for the growth of new
daughter molecules. Later, organic chemicals were incorporated
into the structure of the replicating crystallites, and
competition favored those systems that were more adaptable
because they used organic molecules to carry out their functions.
Nucleic acids (RNA and DNA) then evolved, taking over as the
basic information repository making up the genes of the organism,
and under the pressure of natural selection, the original clay-
mineral component was dispensed with entirely.
In general, the term "RNA world hypothesis" refers to the
concept that RNA nucleotide sequences with catalytic and
self-replicating capabilities predated catalytic protein systems
in prebiological epochs. It is believed, however, that the
nucleotides constituting RNA were scarce on early Earth, so that
RNA-based life must have acquired the ability to synthesize RNA
nucleotides from simpler and more readily available precursors
such as sugars and nucleic acid bases. Apparently plausible
prebiotic synthesis routes have been proposed for sugars, sugar
phosphates, and the four RNA bases, but the coupling of these
molecules into nucleotides, specifically pyrimidine nucleotides,
poses a challenge to the RNA world hypothesis.
... ... S.J. Sowerby et al (4 authors at 2 installations, SE DE)
present a study of the adsorption of nucleic acid bases on
crystalline graphite, the authors making the following points:
1) The authors point out that the purine and pyrimidine
bases, the coding elements of nucleic acids, are products of
supposed prebiotic chemistries that invoked cyanide and have been
synthesized in reactions that also yield amino acids. The
apparent prebiotic availability of these compounds supports the
RNA World hypothesis for the origin of life on Earth, which
proposes that the first living system consisted of one or more
polymers of catalytic RNA capable of self-replication that
subsequently evolved the ability to encode more versatile peptide
catalysts. RNA can act as both information carrier and catalyst,
and in the laboratory, RNA can be coerced into various catalytic
functions through directed Darwinian evolution (directed natural
selection).
2) The authors point out that the adsorption of organic
molecules has long been considered a relevant prebiotic process.
The purine and pyrimidine bases adsorb spontaneously from aqueous
media onto inorganic solids and have been observed on the
surfaces of graphite, MoS(sub2), crystalline gold, and clays.
Scanning probe microscopy studies have shown that the nucleotide
bases are planar-arranged on these surfaces like jigsaw puzzle
pieces, and are stabilized by van der Waals interactions with the
underlying surface and by hydrogen bonds between adjacent
molecules, a configuration originally postulated in 1987 on the
basis of thermodynamic measurements made at the mercury-water
interface. The hydrogen bonds between the bases are drawn from a
discrete set of possible interactions, including those of Watson-
Crick pairing found in nucleic acids. The monolayers can be
likened to nucleic acid molecules because the sugar-phosphate
scaffold also supports the arrangement of bases. Each position in
the scaffold can be mapped to the next by a simple translation
operation, which resembles a major property of the underlying
crystal.
3) The authors report they have determined the equilibrium
adsorption isotherms for the nucleic acid purine and pyrimidine
bases dissolved in water on the surface of crystalline graphite.
The markedly different adsorption behavior of the bases comprises
an elution series: guanine > adenine > hypoxanthine > thymine >
cytosine > uracil. The authors propose that such differential
properties were relevant to the prebiotic chemistry of the bases,
and may have influenced the composition of the primordial genetic
architecture.
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Proc. Nat. Acad. Sci. 2001 98:820
ScienceWeek 2001 6 Apr
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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Related Background:
ORIGIN OF LIFE: PRODUCTION OF PEPTIDES ON INORGANIC SURFACES
The primordial process responsible for the activation of amino
acids and the formation of peptides under primordial conditions
is one of the great riddles of the origin of life. ... ... Huber
and Wachterschauser (Technische Universitat Munchen, DE) now
report that in experiments modeling volcanic or hydrothermal
settings, amino acids were converted into their peptides by use
of coprecipitated (Ni,Fe)S and CO in conjunction with H(sub2)S
(or CH(sub3)SH) as a catalyst and condensation agent at 100
degrees centigrade and pH 7 to 10 under anaerobic aqueous
conditions. The amino acids involved in the experiments were
phenylalanine, tyrosine, and glycine. The authors suggest their
results demonstrate that amino acids can be activated under
geochemically relevant conditions, and that the results support a
thermophilic origin of life with a primordial surface metabolism
on transition metal sulfide minerals. They further suggest that a
continuously recycling library of peptides was generated on the
surfaces of a library of (Fe,Ni)S structures, and that the
results raise the possibility that CO and Ni had a much greater
role in the primordial metabolism than in any of the known extant
metabolisms. They point out that all known extant organisms are
found in habitats with low activities of CO and Ni, and they
suggest this could explain why organisms resorted to the
formation of CO from CO(sub2) and to the elimination of nickel
from many enzymes.
----------
Science 1998 281:670
ScienceWeek 1998 28 Aug
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Related Background:
ORIGIN OF LIFE: THE PRESENT STATUS OF CHEMICAL THEORY
The essential question involved in the origin of what we call
life is how can order arise from disorder? At the present time,
this question is approached on two fronts: 1) study of the
principal features of self-organizing systems, systems in which
order does arise from disorder, systems in which order is indeed
demanded from disorder on thermodynamic grounds; and 2) study of
the detailed chemistry of such systems, the chemistry of
organization and the chemistry of components. In the case of
components, it is essential that appropriate self-organizing
components exist in the first place if they are to become self-
organized, and such candidate components are thus the focus of
much chemical research in this area. In 1953, the chemist Stanley
Miller reported what soon became a famous experiment. To water
under a gas mixture of methane, ammonia, and hydrogen, he added
an electrical discharge. After one week of continuous electrical
discharge, he found that a number of important biological
molecules, including amino acids, had been formed. Miller
proposed his experiment as a model for the conditions under which
the essential compounds necessary for life originated . The
Miller experiment was a watershed, and it began a new era of
experimentation and analysis of possible primordial components.
Coupled with this, were the new important discoveries by
astrophysicists of the presence of organic molecules in the
interstellar medium and in meteorites. In a review of origin of
life theories, P. Radetsky (Univ. of California Santa Cruz, US)
points out that the Miller theory is no longer the consensus
theory, that contemporary geologists believe the primordial
atmosphere consisted primarily of carbon dioxide and nitrogen,
which are less reactive than the gases in the Miller experiment,
and that the field is currently embroiled in controversy fueled
for the most part by an absence of hard fact.
----------
Earth February 1988
ScienceWeek 1998 2 Jan
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Related Background:
BIOCHEMICAL EVOLUTION: POLYMERIZATION ON MINERAL SURFACES
J. Smith (University of Chicago, US) proposes a conceptual
framework for consideration of the origins of replicating
biopolymers. Although extended Darwinian natural selection
coupled with Mendel-Watson-Crick genetic inheritance/mutation
provides a plausible framework for integrating the patchy
paleontological record with the increasingly complex biochemical
zoo of the present Earth, the actual chemical beginning of "life"
still poses major challenges. How could the first replicating and
energy-supplying molecules have been assembled from simpler
materials that were undoubtedly available on the early proto-
continents? Catalysis at mineral surfaces might generate
replicating biopolymers from simple chemicals supplied by
meteorites, volcanic gases, and photochemical gas reactions. But
many ideas are implausible in detail because the proposed mineral
surfaces strongly prefer water and other ionic species to organic
ones. The molecular sieve silicalite (Union Carbide; = Al-free
Mobil ZSM-5 zeolite) has a 3-dimensional 10-ring channel system
whose electrically neutral silicon-oxide surface strongly adsorbs
organic species over water, and the ZSM-5 type zeolite mutinaite
has recently been found in Antarctica. The author proposes that
zeolites with similar structures may have existed on the surface
of Earth during its life-origin phase, and that polymer migration
along weathered silicic surfaces of micrometer-wide channels of
feldspars might have led to assembly of replicating catalytic
biomolecules and perhaps primitive cellular organisms. The author
suggests that weakly metamorphosed Archaean rocks might retain
microscopic clues to the proposed mineral adsorbent/catalysts,
and that other frameworks are also possible, including ones with
laevo/dextro one-dimensional channels.
----------
Proc. Nat. Acad. Sci. 1998 95:3370
ScienceWeek 1998 8 May
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6. ON THE CYTOSKELETON OF NEURONS
The structural framework of eukaryotic cells (cells with
nuclei and other membrane-bound internal structures), called the
"cytoskeleton", consists of an arrangement of macromolecular
structures: microtubules, intermediate filaments, and
microfilaments. The microtubules are hollow cylinders about 24
nanometers in diameter, many microns in length, and consist of
heterodimers of alpha- and beta-tubulin proteins plus a variable
set of other proteins. They form the scaffolding of the mitotic
spindle (an important structure in cell division), organize other
cytoplasmic structures, and are the structural core of various
organelles involved in cell movement (cilia and flagella). The
intermediate filaments are about 10 nanometers in diameter, many
microns in length, are specific for various cell types, and are
not found in all cell types. Microfilaments are 4 to 6 nanometers
in diameter, highly variable in length, and are found in all
eukaryotic cells. They are composed of a protein called "actin"
and several other accessory proteins, and they are important in
cell locomotion and in the molecular dynamics of muscle cells.
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.
During the past several decades, the detailed anatomy of
dendrites has been a focus of much research, in particular the
often-present parts of dendrites called "dendritic spines". These
spines are small (1 to 2 microns) thorn-like protuberances along
the length of a dendrite, and there is evidence that such spines
may be important components in many kinds of neural
microcircuits.
In this context, 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. An "excitatory synapse" is a synapse which
when activated produces excitation of the postsynaptic nerve
cell.
... ... S. Kaech et al (Friedrich Miescher Institute Basel, CH)
discuss the cytoskeleton of neurons. Neuronal circuits need to
maintain a delicate balance between stability and plasticity. On
the one hand, the synaptic connections in neuronal circuits must
be stable enough to support reliable signal transmission, while
on the other hand, these circuits must be sufficiently plastic to
accommodate changes in connectivity that are necessary for the
long-duration adaptation of behavior to sensory experience. How
is neuronal structure organized and regulated to accommodate
these diverse needs? Increasingly, experimental evidence
implicates the neuronal cytoskeleton in regulating morphological
plasticity in adult as well as in developing tissue. More than
any other cell type, neurons depend for their distinctive
morphology on the cytoskeleton, the intracellular system whose
protein components are organized in a set of microdifferentiated
compartments that mirror the polarized form of the cell and play
a significant role in determining its development. Microfilaments
and microtubules act together to guide and support the growth and
differentiation of axons and dendrites. Whereas dynamic actin
filaments drive the exploratory activity of growth cones as they
respond to external guidance cues, microtubules stabilize the
structure of the newly established process. Recent results
suggest that a similar division of labor between the two
cytoskeletal filament systems may persist in dendrites beyond the
developmental period. In adult brain, for example, the highest
concentrations of actin are associated with dendritic spines that
form the postsynaptic component of most excitatory synapses.
-----------
Proc. Nat. Acad. Sci. 2001 98:7086
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7. ESTROGEN AND BRAIN SYNAPSES
Estrogen is a collective term for the female hormones, the most
powerful of which is estradiol. They control female secondary
sexual characteristics, and prepare and maintain the uterine
lining.
... ... B. McEwen et al (Rockefeller University, US) discuss
estrogen receptors in neurons. The brain is widely responsive to
gonadal hormones. Not only is the hypothalamus regulated by these
hormones in relation to reproductive behavior and neuroendocrine
physiology, but also structures like the hippocampus and midbrain
serotonin system undergo sexual differentiation during perinatal
development and are hormone responsive in maturity. One of the
processes regulated by ovarian hormones is the cyclic formation
and breakdown of excitatory synapses on dendritic spines in the
hippocampus. This finding was surprising because until recently
the hippocampus was known as a brain region in which cell nuclear
estrogen receptors are present in scattered inhibitory
interneurons but not in principal neurons where spine formation
occurs. Yet the effects of ovarian hormones on synaptic turnover
are apparently as impressive in the hippocampus as those in the
ventromedial hypothalamus, a classic estrogen target area of the
brain for female sexual behavior. Moreover, effects of estrogen
on hippocampal-dependent cognitive function are now recognized in
rodents and humans. Recent electron microscopic studies have
revealed that estrogen receptors are expressed in hippocampus in
non-nuclear locations within principal cells. This fact, along
with the discovery that estrogen receptors can couple to second
messenger systems, has raised the possibility that estrogen
receptors may be involved in local signaling within neurons as
well as in regulating expression of genes via nuclear receptors
in interneurons.
-----------
Proc. Nat. Acad. Sci. 2001 98:7093
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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Related Background:
EFFECT OF ESTROGEN ON HUMAN BRAIN ACTIVATION PATTERNS
... Perhaps the single most important biological event for most
middle-aged women is menopause, which results in decreased levels
of circulating estrogen. The declining estrogen levels
characteristic of menopause affect a range of systems, including,
in addition to the reproductive system, the cardiovascular and
skeletal systems. There is also evidence that estrogen affects
basic neural processes in mature animals, and estrogen has been
shown to affect cognitive function in animals. There have been
studies of the effect of estrogen on cognitive function in
postmenopausal women, but the results have been inconsistent.
Recent advances in technology now permit the noninvasive
measurement of brain function as individuals perform memory and
other cognitive tasks. This technique, called "*functional
magnetic resonance imaging", exploits the differences in the
magnetic properties of oxygenated compared with deoxygenated
blood. In performing a cognitive task, such as *working memory,
blood flow and oxygen concentration are altered in those brain
regions engaged in the task. ... ... S.E. Shaywitz et al (16
authors at 2 installations, US) report a study to investigate the
effects of estrogen on brain activation patterns in
postmenopausal women as they performed verbal and nonverbal
working memory tasks. The study involved 46 right-handed
postmenopausal women age 33 to 61 years in a randomized, *double-
blind, placebo-controlled, *crossover trial from 1996 through
1998. The intervention consisted of a 21 day treatment with
*conjugated equine estrogens (1.25 milligrams/day), randomly
crossed over with identical placebo, and a 14-day washout between
treatments. The authors measured brain activation patterns using
functional magnetic resonance imaging, and the authors report
that treatment with estrogen increased activation in the
*inferior parietal lobe during storage of verbal material, and
decreased activation in the inferior parietal lobe during storage
of nonverbal material. Estrogen also increased activation in the
*right superior frontal gyrus during retrieval tasks, accompanied
by greater left-hemisphere activation during *encoding. The
authors conclude that estrogen in a therapeutic dosage alters
brain activation patterns in postmenopausal women in specific
brain regions during the performance of the sorts of memory
function that are called upon frequently during any given day.
The authors suggest these results indicate that estrogen affects
brain organization for memory in postmenopausal women.
-----------
J. Am. Med. Assoc. 1999 281:1197
-----------
Notes:
... ... *functional magnetic resonance imaging: First, we
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.
... ... *working memory: In this context, the term "working
memory" refers to a particular type of short-term memory
involving the ability to hold things in mind long enough to carry
out sequential actions.
... ... *double-blind: In general, a "double-blind" experimental
procedure is one in which neither the subjects nor the
experimenters know the makeup of the test and control group
during the actual course of the experiments.
... ... *crossover trial: In general, a "crossover trial" is an
experimental or clinical procedure in which subjects are divided
randomly into at least as many groups as there are kinds of
treatment to be given, and then the groups are interchanged until
every subject has received each treatment.
... ... *conjugated equine estrogens: This is an amorphous
preparation of naturally occurring water-soluble conjugated forms
of mixed estrogens obtained from the urine of pregnant mares. The
principal estrogen present is sodium estrone sulfate.
... ... *inferior parietal lobe: the parietal lobe is
approximately the middle portion of each cerebral hemisphere seen
from the side. In this context, the term "inferior" refers to the
lower part.
... ... *right superior frontal gyrus: The term "gyrus" refers to
any of the visible convoluted ridges of the cerebral hemispheres.
Seen from the side, the superior frontal gyrus is the foremost
ridge of the frontal lobe.
... ... *encoding: In this context, the term "encoding" refers to
the first stage of the memory process, prior to storage and
retrieval, and associated with receiving stimuli through one or
more of the senses.
-------------------
ScienceWeek 1999 30 Jul
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8. VERTEBRATE SUSPENDED ANIMATION PRODUCED BY OXYGEN DEPRIVATION
P.A. Padilla and M.B. Roth (Hutchinson Cancer Research Center
Seattle, US) discuss oxygen deprivation in animals. Most animals
are very sensitive to reduced levels of oxygen. Known vertebrate
responses to low oxygen concentration (hypoxia) include changes
in carbohydrate metabolism, an increase in nitric oxide, and
stimulation of red blood cell and hemoglobin production. Hypoxia
can also induce the expression of a select set of genes, which
include the genes for glycolytic enzymes, the glycoprotein
hormone erythropoietin, and the inducible form of nitric oxide
synthase. Extreme hypoxia is central to the pathology of several
diseases involving cardiac and pulmonary dysfunction. It has been
demonstrated that some invertebrates (e.g., nematode worms, brine
shrimp, fruit flies) have the ability to survive in the absence
of molecular oxygen (anoxia). The brine shrimp A. franciscana has
been shown to survive 4 years of continuous anoxia, exhibiting an
arrest of development, a decrease in intracellular pH, a
reduction in protein synthesis, and an accumulation of heat shock
proteins. It has been demonstrated that both nematode worms (C.
elegans) and fruit flies (D. melanogaster) can survive at least 1
day of anoxia exposure by arresting development until oxygen
supply is reestablished. The authors report that embryos of the
zebrafish Danio rerio can survive for 24 hours in the absence of
oxygen, and that the evidence indicates these embryos enter into
a state of suspended animation where all microscopically
observable movement ceases, including cell division,
developmental progression, and motility. Animals that had
developed a heartbeat before anoxic exposure showed no evidence
of a heartbeat until return to terrestrial atmosphere.
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Proc. Nat. Acad. Sci. 2001 98:7331
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9. ANOTHER CRITIQUE OF THE NOBEL PRIZES
Trisha Gura (NAT) discusses the Nobel prizes. The Nobels perform
an impressive public relations job for research -- or at least
for those disciplines that the prizes honor. They create an
inspiring image of lone pioneers receiving the ultimate accolade
for their individual brilliance. But now that science has evolved
into a more collaborative process in which projects can command
billion-dollar price tags and involve thousands of scientists, is
this image really appropriate? And how do the few key individuals
to be rewarded get selected? The details remain mysterious, as
the deliberations of the Nobel committees are kept secret for 50
years after each prize is awarded. But as historians of science
trawl through earlier archives, they have found that the
selection process has, on occasion, been eccentric. Einstein's
general theory of relativity was dismissed as mere speculation by
the Nobel selection committee. And in 1906, the Royal Swedish
Academy of Sciences rejected the majority decision of the
chemistry committee to award the prize to Dmitri Mendeleev for
his periodic table of the elements. This intervention was a
result of the powerful influence exerted by Swedish chemist
Svante Arrhenius, who won the chemistry prize in 1903 for his
theory of electrolytic dissociation -- of which Mendeleev had
been a prominent critic. Mendeleev died the next year and never
won a Nobel prize.
-----------
Nature 2001 413:560
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10. ON THE PROCESSES THAT TRIGGER MELTING
Robert W. Cahn (University of Cambridge, UK) discusses the
melting of solids. Melting has been a focus of physical theory
for almost a century, the problem to understand how and why a
crystalline solid melts, and what determines the temperature at
which this occurs. Many theoretical criteria for melting have
been proposed, of which two stand out, those by Frederick
Lindemann (1910) and Max Born (1939). Lindemann proposed that
melting is caused by vibrational instability in the crystal
lattice. Born, on the other hand, proposed that a "rigidity
catastrophe" occurs, the catastrophe determining the melting
temperature within the bulk crystal: the crystal no longer has
sufficient rigidity to withstand melting, so this process is
often called "mechanical melting". These two distinct theories
have each accumulated an extensive literature. It has been
established experimentally that melting begins preferentially at
a surface, and that superheating a crystal beyond the melting
point set by the surface melting requires this process to be
impeded. For example, coating the surface with a metallic layer
that has a higher melting point can suppress surface melting and
retain the solid phase to bulk temperatures well above the
equilibrium melting point. Now Z-H. Jin et al (2001) report a
molecular dynamics simulation that demonstrates that as a crystal
is heated, melting is triggered by instabilities governed
simultaneously by the Lindemann and Born criteria.
-----------
Nature 2001 413:582
Phys. Rev. Lett. 2001 87:055703
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11. GLOBAL WARMING: SHORT TERM VS. LONG TERM PREDICTIONS
Lindsey Rustad (Dept. of Agriculture Forest Service, US)
discusses effects of global warming. The Earth is warming, and
given that carbon dioxide seems to be the main determinant of
global temperature, predictions of climate conditions in the
future depend in part on gauging the response of the carbon cycle
to warming. Over the past century, the mean surface temperature
of the Earth has increased by 0.6 degrees Celsius. During the
next 100 years, it is predicted to increase by a further 1.4 to
5.8 degrees Celsius, and by even more at higher latitudes. This
predicted rate of change is unprecedented in at least the past
10,000 years, and is largely attributed to increases in the
greenhouse gases, most notably carbon dioxide, resulting from the
burning of fossil fuels and changes in land use. Confidence in
these predictions is increasing, but considerable uncertainties
remain. We cannot even be sure whether terrestrial ecosystems
will take up atmospheric carbon dioxide (and so moderate further
increases in temperature), or be a source of it (and so drive
temperature even higher). In predictions of these climate-
ecosystem interactions, it is often assumed that, if moisture and
nutrients are not limiting factors, rates of both photosynthesis
(which removes carbon dioxide from the atmosphere) and
respiration (which releases carbon dioxide to the atmosphere)
will increase with increasing temperature in a predictable way
that will remain constant over time. Recent evidence demonstrates
that caution should be used in extrapolating results from short-
term experiments to predict longer-term responses to
environmental perturbations such as warming. The question of how
ecosystems might or might not acclimatize to a warmer world bears
serious consideration.
-----------
Nature 2001 413:
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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Related Background:
GEOCHEMISTRY: ON RIVER CARBON AND THE CARBON CYCLE
In its most general outline, the term "carbon cycle", in
geochemistry and Earth science, refers to the movement of carbon
from an atmospheric inorganic state to a biospheric organic state
and then back to an atmospheric inorganic state. In detail, there
are several pathways from biospheric organic carbon to
atmospheric inorganic carbon, one of which, of great importance,
is the movement of organic carbon into the hydrosphere,
principally via rivers that empty into oceans, with oceanic
dissolved organic carbon a reservoir for movement of organic
carbon to an atmospheric inorganic state. Concerning the transfer
of dissolved organic carbon from rivers to oceans, there are
puzzles that have not yet been solved.
... ... Wolfgang Ludwig (University of Perpignon, FR) presents a
commentary on new data concerning river carbon (P.A. Raymond and
J.E. Bauer: Nature 25 Jan 01 409:497), the author (Ludwig) making
the following points:
1) The author points out that dissolved organic carbon in
the oceans is one of the largest reservoirs in the global carbon
cycle, this reservoir comparable in size to all of the carbon in
terrestrial plants, or to all of the carbon in form of carbon
dioxide in the atmosphere. The input of terrestrial organic
carbon from rivers, the main source of most constituents of sea
water, could fill the oceanic reservoir in only a few thousand
years, which (according to radiocarbon dating) is apparently the
average age of oceanic organic carbon. But although there ought
to be a great amount of terrestrial-derived organic carbon in the
oceans, geochemical studies indicate there is apparently very
little, and the fate of river-transported carbon ("riverine
carbon") once it enters the oceans is unclear.
2) Almost all of the organic carbon on Earth is created via
photosynthesis, whether on land or in water, but on land the
process produces characteristic markers, so that terrestrial
carbon should be traceable after it has entered the oceans. For
example, many land plants synthesize certain compounds, such as
*lignin or *tannin, which are absent in marine *phytoplankton. In
principle, therefore, detecting these biomarkers in the oceans
can reveal if carbon had a terrestrial origin. The other widely
used method involves measuring the ratio between the two stable
carbon isotopes, C-13 and C-12, in the bulk organic matter. Most
land plants produce carbon that is more depleted in C-13 than
carbon produced by marine phytoplankton, which results in higher
isotopic ratios in marine than in terrestrial carbon.
3) Raymond and Bauer (2001) now present an analysis of
organic materials in four rivers (Amazon [BR], Hudson [New York,
US], (York [Virginia, US], Parker [Massachusetts, US] by
radiocarbon dating (carbon-14, carbon-13 measurements), and they
report the organic carbon in these rivers is up to several
thousand years old [*Note #1]. This is in sharp contrast with the
general belief that most of the organic carbon in rivers should
be relatively "fresh". The particulate organic carbon (i.e., the
fraction retained on a filter) was especially old (C-14-
depleted). From these results, and laboratory evidence that
suggests selective degradation of young (C-14-rich) dissolved
organic carbon over the residence times of river and coastal
waters, Raymond and Bauer conclude that pre-aging and degradation
may alter significantly the structure, distribution, and
quantities of terrestrial organic matter before its delivery to
the oceans. The implication is that the absence of riverine
carbon in the oceans is only apparent and due to the fact that we
have not been able to distinguish riverine carbon from marine-
generated carbon.
-----------
Nature 2001 409:466
-----------
Notes:
... ... *Note #1: Carbon-14 dating depends on the decay of
carbon-14 to nitrogen. Carbon-14 is continually formed in nature
by the interaction of neutrons with nitrogen-14 in the Earth's
atmosphere, the required neutrons produced by cosmic rays
interacting with the atmosphere. The carbon-14 from this reaction
is converted to carbon dioxide by reaction with atmospheric
oxygen and mixed and uniformly distributed with the atmospheric
carbon dioxide containing stable carbon-12. Since living
organisms use atmospheric carbon dioxide either directly or
indirectly, their systems contain the constant ratio of carbon-12
to carbon-14 that exists in the atmosphere. Death of an organism
terminates the equilibrium process: no fresh carbon dioxide is
added to the dead substance, and the carbon-14 present in the
dead substance decays with a half-life of 5730 years, while
carbon-12 in the dead substance remains what it was at death.
Measurement of the carbon-14 activity at a given time thus allows
calculation of the time elapsed after the death of the organism.
... ... *lignin: A complex organic polymer and major component of
wood.
... ... *tannin: A complex astringent substance occurring widely
in plants, particularly in leaves, unripe fruits, and tree bark.
... ... *phytoplankton: Small, usually microscopic, aquatic
plants capable of photosynthesis; e.g., unicellular algae.
Phytoplankton and plankton are not equivalent. The term
"plankton" is a general designation for various drifting
microscopic aquatic organisms in the upper regions of the oceans,
both photosynthetic and non-photosynthetic.
-------------------
ScienceWeek 2001 23 Feb
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Related Background:
ON THE CHEMISTRY AND BIOLOGY OF THE OCEANS
The combination of vast areas of liquid water on its surface
together with a high concentration of free molecular oxygen in
its atmosphere is unique to Earth in this solar system.
Calculations based on *ultraviolet absorption cross sections
indicate that whereas direct photolysis of water could have
produced small amounts of O(sub2), almost all of the gas was
produced by biological systems through the photobiologically
catalyzed oxidation of the liquid. ... ... Falkowski et al (3
authors at 3 installations, US DE) review the controls and
feedbacks between oceanic *phytoplankton and geochemical
processes with an emphasis on factors that cause a deviation from
the steady state. The authors make the following points: 1)
Changes in oceanic primary production, linked to changes in the
network of global biogeochemical cycles, have profoundly
influenced the geochemistry of Earth for over 3 billion years. 2)
In the contemporary ocean, photosynthetic *carbon fixation by
marine phytoplankton leads to the formation of approximately 45
gigatons of organic carbon per year, of which 16 gigatons are
exported to the ocean interior. 3) Changes in the magnitude of
total and export production can strongly influence atmospheric
CO(sub2) levels (and hence climate) on geological time scales, as
well as set upper bounds for sustainable fisheries harvest. 4)
Because the average turnover time of phytoplankton carbon in the
ocean is on the order of a week or less, total and export
production are extremely sensitive to external forcing, and
consequently are seldom in steady state. 5) Elucidating the
biogeochemical controls and feedbacks on primary production is
essential to understanding how oceanic biota responded to and
affected natural climate variability in the geological past, and
to understanding how oceanic biota will respond in the coming
decades to changes influenced by human activities.
----------
Science 1998 281:200
ScienceWeek 1998 31 Jul
-------------------
Notes:
... ... *ultraviolet absorption cross sections: The ratios of the
amount of energy removed from incident UV by absorption to the
total energy of incident UV. In other words, in this context, a
measure of how much energy is (was) actually available for direct
photolysis of liquid water.
... ... *phytoplankton: See main report.
... ... *carbon fixation: Refers to the process of converting the
carbon in a substance into a form usable by an organism. For
example, the conversion of the carbon in CO(sub2) into organic
carbon (the carbon in organic compounds).
----------
ScienceWeek 1998 31 Jul
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12. ON THE PHYSICS OF CLOUDS
Marcia Baker (University of Washington Seattle, US) discusses
cloud physics. Clouds in the troposphere, the atmospheric layer
between the ground and the stratosphere, have profound influences
on air chemistry, weather, and global climate. Many of the
particles in these clouds originate as small droplets that grow
by condensation of water vapor as the droplets rise in the
atmosphere. Some of the ice that forms in the clouds originates
in the clear air outside the droplets. But most of the ice forms
as the droplets freeze during their ascent, and predicting the
conditions under which droplet freezing occurs is one of the main
goals of cloud physics. In the atmosphere, ice is the
thermodynamically stable state of water below 0 degrees Celsius.
However, water can remain liquid in a metastable ("supercooled")
state down to much lower temperatures. Pure water droplets do not
freeze spontaneously at temperatures above approximately -37
degrees Celsius, and the freezing temperatures of droplets of
aqueous solution are even lower. In the absence of external
surfaces, a volume of water or aqueous solution is homogeneous,
and freezing is equally likely to occur at any point within it --
freezing under these conditions is therefore called "homogeneous
freezing". In contrast, if a sold surface is immersed in or in
contact with the droplet, liquid freezing can be catalyzed by
that surface at relatively high temperatures; this is called
"heterogeneous freezing".
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Nature 2001 413:586
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13. POSTDOCTORAL FELLOWSHIP PROFILES:
Laboratory of M.M. Hussain at SUNY Downstate Medical Cntr, US
-------------------------------------------------------------
Department: Anatomy and Cell Biology
General Research Areas: lipoprotein biosynthesis, cell
biology, molecular biology
Head of this laboratory: Dr. M. Mahmood Hussain
Postdoctoral fellowships are available in the following
specific research problems: mechanisms of chylomicron assembly;
identification of amino acids involved in protein-protein
interactions between apoB and MTP; identification of genes
induced after biliopancreatic bypass surgery; transcriptional
regulation of apoB expression by cytokines; developmental
regulation of apoB and MTP.
Requirements: Ph.D. or equivalent in biochemistry, cell
biology, molecular biology or related fields.
Usual starting stipend: $30,000
Special requirements concerning citizenship,
visas, etc.: None
Approximate number of people currently working in this
laboratory (faculty, staff, students, postdocs: 10
More information: Email: mahmoodhussain@netmail.hscbklyn.edu
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14. IN FOCUS: ON JANET ROWLEY
"It is... difficult to launch a significant scientific career
later in life. One can draw inspiration from the unusual career
of Janet Rowley, who in 1998 received the National Medal of
Science, the nation's highest scientific award. Trained as a
physician, Rowley first took time off to raise her three
children. In the early 1960s, in her late thirties, Rowley
recommenced studies in cytogenetics at Oxford University.
Thereafter, with her children then in school, Rowley approached
Leon Jacobson, a distinguished medical researcher at the
University of Chicago, and requested a microscope, a desk, and a
modest salary for part-time work. Jacobson was able to grant
Rowley's requests, and under his guidance, she began an in-depth
of the causes of leukemia. This work was crowned with success
when, in the early 1970s, Rowley discovered that certain types of
leukemia were connected to chromosomal abnormalities called
translocation (where chromosomes are broken and the ends of the
chromosomes are exchanged). A decade later she was able to clone
the translocation breakpoint and identify the controlling genes.
Mechanisms of translocation have been observed in many forms of
cancer and are now firmly established. Rowley's success was
abetted by several factors. She singles out the support of her
husband ('the most important person') and her supervisors,
chiefly Jacobson. She was allowed to rise through the
professorial ranks while keeping a part-time position, a most
unusual pattern at the University of Chicago and elsewhere. She
began to work full-time only when her youngest child had gone
away to school. Rowley applied for grants during her fifties and
sixties, putting in 80-hour workweeks and gradually, if
belatedly, joining the ranks of the scientific elite. Now in her
mid-seventies, she continues to conduct research, finding time to
inspire young people, and especially women, to enter the field of
medical-scientific research."
----------
H. Gardner et al: _Good Work: When Excellence and Ethics Meet_
(Basic Books, New York 2001, p.67)
http://www.amazon.com/exec/obidos/ASIN/0465026079/scienceweek
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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15. FROM PRAXIS:
ON THE DYNAMICS OF AGING MUSCLE
R. Roubenoff and C. Castenada (Tufts University, US) discuss the
dynamics of aging muscle. Sacropenia is not a disease but rather
refers specifically to the universal and involuntary decline in
lean body mass that occurs with age, primarily due to the loss of
skeletal muscle. Sarcopenia has important consequences. The loss
of lean body mass reduces function, and loss of approximately 40
percent of lean body mass is fatal. Sarcopenia is distinct from
wasting -- involuntary weight loss due to inadequate intake,
which is seen in starvation, advanced cancer, or acquired
immunodeficiency syndrome. Sarcopenia also differs from cachexia,
a cytokine-driven loss of lean body mass that occurs despite
maintenance of weight, which is seen in patients with rheumatoid
arthritis, congestive heart failure, or renal failure. However,
sarcopenia is the backdrop against which the drama of disease is
played out: a body already depleted of protein because of aging
is less able to withstand the protein catabolism that comes with
acute illness or inadequate protein intake. Protein stores in
humans have at least 2 important functions: a) Unlike fat, which
is truly stored in the sense that it is in reserve for times of
starvation, body proteins are in use as contractile proteins in
muscle, antibodies, enzymes, etc. Thus, loss of protein means
loss of function. b) During illness, nitrogen must be mobilized
from muscle to provide amino acids to the immune system, liver,
and other organs. If adequate nitrogen cannot be provided, either
exogenously from diet or endogenously from muscle, the body's
capacity to withstand an acute insult declines, and -- at about
60 percent of baseline nitrogen throughout -- the body ceases to
function. Thus, it is likely that some of the explanation for the
poorer outcomes observed with nearly all diseases in older
persons relates to their lower body protein stores.
-----------
JAMA 2001 286:1230
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PRAXIS 10 Dec 2001 http://scienceweek.com/praxis
-------------------
Related Background:
GENE THERAPY FOR AGING-RELATED LOSS OF MUSCLE FUNCTION
One of the primary consequences of aging, a consequence which
leads to significantly impaired function in the elderly
population, is the loss of *skeletal muscle strength and mass.
Both of these decrease up to one-third in humans between the ages
of 30 and 80 years. In addition, loss of the fastest and most
powerful muscle fiber types has been documented. Similar aging-
related muscle alterations have been observed in rats and mice,
indicating that the trend is maintained in other mammalian
species. The mechanisms underlying this aging-related muscle loss
have remained unclear, but there is some evidence that so-called
"insulin-like growth factor-I" may be involved. The term
"insulin-like growth factor" refers to a group of polypeptides
structurally homologous to *insulin, and which share many of the
biological activities of insulin, but which are apparently
biochemically distinct from it. These substances are "mitogens",
i.e., they enhance or induce cell division (mitosis). Insulin-
like growth factor-I (insulin-like growth factor type I) is a
monomer of 70 amino acids. ... ... E.R. Barton-Davis et al (5
authors at 2 installations, US) now report an attempt to moderate
the aging-related loss of muscle in mice by increasing the
regenerative capacity of muscle. The study involved the injection
of a genetically engineered virus to direct overexpression (i.e.,
genome-based protein overproduction) of insulin-like growth
factor-I in adult muscle. The authors report that insulin-like
growth factor-I expression promotes an average increase of 15
percent in muscle mass and a 14 percent increase in strength in
young adult mice, and prevents aging-related muscle changes in
old adult mice. In old adult mice, muscle mass and fiber type
distributions were maintained at levels similar to those in young
adults. The authors propose that these effects are primarily due
to stimulation of muscle regeneration via the activation of
*satellite cells by insulin-like growth factor-I. The authors
suggest this supports the hypothesis that the primary cause of
aging-related impairment of muscle function is a cumulative
failure to repair damage sustained during muscle utilization. The
authors further suggest that gene transfer of insulin-like growth
factor-I into muscle could form the basis of a human gene therapy
for preventing the loss of muscle function associated with aging,
and may be of benefit in diseases where the rate of damage to
skeletal muscle is pathologically accelerated.
-----------
Proc. Nat. Acad. Sci. 1998 95:15603
-----------
Notes:
... ... *skeletal muscle: (striated muscle, voluntary muscle)
Muscle in which cross striations occur in the fibers as a result
of regular overlapping of thick and thin filament structures.
Although cardiac muscle is not "voluntary" muscle, it is also
striated in appearance.
... ... *insulin: A protein hormone that promotes uptake by body
cells of free glucose and/or amino acids, depending on target
cell type.
... ... *satellite cells: The satellite cells of skeletal muscle
are cells associated with muscle fibers that are believed to play
a role in muscle repair and regeneration.
-----------
SW 1999 26 Mar
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PRAXIS 10 Dec 2001 http://scienceweek.com/praxis
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SCIENCE-WEEK 14 Dec 2001 http://scienceweek.com
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This week in PRAXIS (10 Dec 01):
-------------------------------
1. On Aggregation of Gold Nanoparticles
2. Selective Assembly of Supramolecular Aggregates
3. Methods of Photosensitization of DNA
4. Ring Closure of Carbon Nanotubes
5. Analysis of the UK Foot-and-Mouth Disease Epidemic
6. Dynamics of Aging Muscle
7. Problems of Delivery of DNA in Gene Therapy
8. On New Anti-Cancer Drug Targets
9. On the Interplay of Biology and Technology
10. Drug-Induced Hyperglycemia
11. On the Transfer of Maternal Immune Protection
12. Targeted Therapies in Cancer
For information about PRAXIS, see:
http://www.scienceweek.com/praxis
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the affiliation of the lead author.
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