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ScienceWeek
SCIENCE-WEEK
A Weekly Digest of the News of Science
August 7, 1998
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Science keeps moving us away from the Apes.
Of course, if one wants to be an ape, one
objects to the movement.
-- Anonymous
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Contents of This Issue:
1. Women on Chemistry Faculties: Evidence of Slow Progress
2. New Evidence for High-Redshift Star Formation
3. Electrical Measurements on Single Metal Atoms
4. Reversible Hydrogels from Self-Assembling Artificial Proteins
5. Complete Genome Sequence of Syphilis Spirochete
6. Nanobacteria and Pathogenic Extracellular Calcification
7. Electrophysiological Development of Auditory Hair Cells
8. Release of Two Different Transmitters by the Same Neuron
9. Rheumatoid Arthritis: A Human Parvovirus as a Causative Agent
10. Paleolithic Human Population Expansion in Africa
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1. WOMEN ON CHEMISTRY FACULTIES: EVIDENCE OF SLOW PROGRESS
Data compiled by the American Chemical Society for 1996, the most
recent year for which data are available, indicate that in the US
30.7 percent of PhDs in chemistry were awarded to women, up from
12.5 percent in 1976. But on chemistry faculties at the major US
research universities, women are barely represented. Here are
figures for some of the major US academic research installations:
-----
(Total tenure or tenure-track chemistry faculty/Women on tenure
or tenure-track chemistry faculty 1996-1997)
University of California Berkeley 53/5
California Institute of Technology 25/3
Harvard University 20/1
Stanford University 21/1
Massachusetts Institute of Technology 31/2
Cornell University 30/2
Columbia University 19/1
University of Illinois Urbana-Champaign 47/2
University of Wisconsin Madison 39/2
University of Chicago 24/1
University of California Los Angeles 37/6
-----
QY: Mairin B. Brennan
(Chem. & Eng. News 20 Jul 98) (Science-Week 7 Aug 98)
-------------------
Related Background:
US WOMEN IN SCIENCE LESS ACCEPTED THAN WOMEN IN BUSINESS
A meeting last month at the New York Academy of Sciences (US)
focused on the past 25 years of progress for women scientists and
engineers in the US. The apparent consensus at the conference was
that the climate for women in both industry and government has
improved much faster than in academia, with the changes in
industry mostly due to a drive by industry for diversity. The
Motorola Corporation (US) now has 43 women vice-presidents. In
1996, women were 51% of the US population, 46% of the labor
force, but only 22% of scientists and engineers. Nobel Laureate
Gertrude Elion advised women in science to follow Farragut's
order: "Damn the torpedoes -- full speed ahead."
(Chem. & Eng. News 6 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
WOMEN NOW SUBSTANTIAL PORTION OF ALL NEW US CHEMISTS
The latest survey of the American Chemical Society, covering
chemists and chemical engineers who graduated between July 1996
and June 1997, shows the following statistics for new women
graduates (percentage of total graduates who are women):
Chemistry Bachelor's Degree: 48.2%
Chemistry Master's Degree: 46.2%
Chemistry PhD Degree: 31.6
Chemical Engineering Bachelor's Degree: 35.4%
Chemical Engineering Master's Degree: 29.3%
Chemical Engineering PhD Degree: 22.9%
QY: Michael Heylin
(Chem. & Eng. News 9 Mar 98)
-------------------
SWEDISH STUDY SHOWS SEX BIAS AFFECTS SCIENCE EMPLOYMENT
Two female Swedish scientists have published a study which
indicates that women must publish more often than men to compete
successfully for scientific jobs in Sweden. This is the first
scientific study of sex discrimination in the awarding of a
large number of research positions, and was financed by a
Swedish government grant. The researchers, Christine Wenneras
and Agnes Wold, are both from the University of Goteborg. The
journal Nature, which published the study, states that
the "results severely undermine the credibility of the
peer-review system, not just in Sweden but elsewhere in the
world. (Nature 22 May 97)
2. NEW EVIDENCE FOR HIGH-REDSHIFT STAR FORMATION
In the local Universe, most galaxies are dominated by stars, with
less than 10 percent of their visible mass in the form of gas.
Determining when most of these stars formed is one of the central
issues of observational cosmology. Optical and ultraviolet
observations of *high-redshift galaxies (particularly those in
the *Hubble Deep Field) have been interpreted as indicating that
the peak of star formation occurred between redshifts of 1 and
1.5. But it is known that star formation occurs in dense clouds,
and that star formation is often hidden at optical wavelengths
because of radiation extinction by dust in the clouds.
... ... Hughes et al (15 authors at 5 installations, UK US) now
report a deep *submillimeter-wavelength survey of the Hubble Deep
Field. Submillimeter wavelengths trace directly the emission
from
dust that has been warmed by massive star-formation activity. The
authors report that the combined radiation of the 5 most
significant detections accounts for 30 to 50 percent of the
previously unresolved background emission in this area. Four of
these sources appear to be galaxies in the redshift range
2 < z < 4. The authors suggest that assuming these objects have
properties comparable to local dust-enshrouded *starburst
galaxies, this implies a star-formation rate during that period
approximately 5 times higher than that inferred from the optical
and ultraviolet observations. The authors also suggest that it is
possible that some of these submillimeter sources lie at z > 5,
but the demonstration of this will require improved measurement
technology.
QY: David H. Hughes
(Nature 16 Jul 98) (Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *high-redshift: Redshift (symbol: z) is a lengthening of
the wavelengths of electromagnetic radiation from a source caused
either by the movement of the source (Doppler effect) or by the
expansion of the universe (cosmological redshift). Redshift is
defined as the change in wavelength of a particular spectral line
divided by the unshifted wavelength of that line. Large redshifts
imply large radial velocities (which imply large distances,
according to current cosmological theory), but at redshifts
greater than about 0.2 there is a relativistic divergence from a
linear relation. A redshift of 4.0 corresponds to an object
receding with a radial velocity 92% that of the velocity of
light. The largest astrophysical redshifts so far observed are of
the order of z = 4.9. The furthest galaxy on record is at a
redshift z=4.92), which implies a distance of approximately 13
billion light years.
... ... *Hubble Deep Field: The orbiting Hubble Space Telescope
has the ability to discern images of galaxies too faint to be
picked up by planet-based and other orbital telescopes. After the
Hubble Space Telescope was repaired in 1993, one of the projects
begun with this instrument was the Hubble Deep Field Project,
which involves long-exposure detection of radiation received from
a relatively small area of the sky, the long-exposure providing
information concerning "deep" objects, i.e., objects extremely
distant from the instrument (in this case, beyond our local
Universe).
... ... *submillimeter-wavelength: The submillimeter band lies
between the microwave band (above 1 millimeter) and the far-
infrared band (at about 0.1 millimeter). The essential idea of
the report by Hughes et al is that warm-dust infrared radiation
from high-z objects is shifted into the submillimeter part of the
spectrum and is therefore not detected at smaller wavelengths.
... ... *starburst galaxies: A starburst galaxy is a galaxy in
which a massive burst of star formation is taking place, and such
galaxies are characterized by high infrared luminosities.
3. ELECTRICAL MEASUREMENTS ON SINGLE METAL ATOMS
Fabrication of structures at the atomic scale is now possible
using techniques for manipulation of individual atoms, and there
is thus a prospect of designing electrical circuits atom by atom.
A prerequisite for successful design of such circuits is
knowledge of the relationship between the macroscopic electrical
characteristics of a circuit and the quantum properties of the
individual atoms used as building blocks. ... ... Scheer et al (9
authors at 4 installations, FR ES NL) report that chemical
valence determines the conduction properties of the simplest
imaginable circuit -- a one-atom contact between two metallic
banks. The authors report that the extended quantum states that
carry the current from one bank to the other proceed through the
valence orbitals of the constriction atom. They considered the
idea that the number of current-carrying modes or channels of a
one-atom contact is determined by the number of valence orbitals,
and so should strongly differ for metallic elements in different
series of the periodic table. The authors tested this conjecture
using *scanning tunneling microscopy and mechanically
controllable *break-junction techniques to obtain atomic size
constrictions for four different metallic elements (Pb, Al, Nb,
Au), covering a broad range of valences and orbital structures.
The authors suggest their results demonstrate unambiguously a
direct link between valence orbitals and the number of conduction
channels in one-atom contacts, and that this cannot be understood
within a free-electron model. They suggest that only a
microscopic model that takes into account the atomic orbital
structure as well as the local atomic geometry can fully explain
the conduction properties of these contacts. The authors further
suggest that these concepts will be applicable in slightly
modified form to conduction through clusters of atoms and
molecules.
QY: Elke Scheer
(Nature 9 Jul 98 394:154) (Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *scanning tunneling microscopy: This is essentially an
instrument for producing surface images with atomic-scale lateral
resolution using a fine-probe tip raster-scanned over the surface
at a distance of 0.5 to 1 nanometer, with the resulting
*tunneling current (or position of the tip required to maintain a
constant tunneling current) monitored. The technique can be used
to manipulate single atoms or small groups of atoms, given
atomically smooth substrates in ultra-high vacuum and at low
temperature.
... ... *break-junction: A "break" junction is an extremely
narrow separation of two conductors, narrow enough so that an
electron tunneling gap is established. One way to achieve such a
separation is to literally break a conductor to achieve the
required gap.
... ... *tunneling current: In general, a tunnel junction is a
junction (potential barrier) involving a thin separation between
two conductors, with the width of the junction equal to or less
than the wavelength of the conductance entity (e.g., the
wavelength of the electron). The significant result at a tunnel
junction is that the conductance entity can penetrate the barrier
by means of quantum mechanical "tunneling", this tunneling effect
essentially arising from the fact that with the wavelength of the
particle larger than the width of the potential barrier, the
barrier is effectively transparent to the particle.
-------------------
Related Background:
A SINGLE-CARBON NANOTUBE ROOM-TEMPERATURE TRANSISTOR
Carbon nanotubes are similar to fullerenes, except their shape is
tubular. They were first discovered by Sumio Iijima (NEC
Laboratories, JP) in 1991, they come in both multi-walled and
single-walled versions, and they have diameters of the order of
10 to 30 nanometers. The use of individual molecules as
functional electronic devices was first proposed by Aviram and
Ratner in 1974. Since then, molecular electronics has attracted
much interest, particularly because it could lead to conceptually
new miniaturization strategies in the electronics and computer
industry. But the realization of single-molecule devices has
remained a challenge, largely owing to difficulties in achieving
electrical contact to individual molecules. Recent advances in
nanotechnology, however, have resulted in electrical measurements
on single nano-scale molecules. ... ... Tans et al (3 authors at
Delft University of Technology, NL) report the fabrication of a
field-effect transistor -- a 3-terminal switching device -- that
consists of one semiconducting single-wall carbon nanotube
connected to 2 metal electrodes. By applying a voltage to a gate
electrode, the nanotube can be switched from a conducting to an
insulating state. The device operates at room temperature,
thereby meeting an important requirement for potential practical
applications. Electrical measurements on the nanotube transistor
indicate that its operation characteristics can be qualitatively
described by the semi-classical band-bending models currently
used for traditional semiconductor devices, an unexpected result.
The authors suggest the fabrication of this 3-terminal switching
device at the level of a single molecule represents an important
step towards molecular electronics.
QY: Cees Dekker (dekker@qt.tn.tudelft.nl)
(Nature 7 May 98 393:49) (Science-Week 29 May 98)
-------------------
Related Background:
EXPERIMENTAL STUDY OF CONDUCTANCE AT A MOLECULAR JUNCTION
... In recent years, a number of laboratories have used [break]
junctions to examine the electrical properties of various organic
substances sequestered in the junction gap. The idea is to test
the validity of charge transport approximations at the molecular
level. Charge trans- port, or charge transfer, in the general
sense, is simply the movement of electric charge (electrons) from
one place to another, which means it includes effective movement
from one part of a single molecule to another part of the same
molecule. Since the movement of electric charge is electric
current, if means can be found to control the process at the
molecular level, one has the beginnings of molecular-electronics.
... ... M. A. Reed et al (Yale University, US; University of
South Carolina, US) report the formation of a mechanically
controllable break junction involving gold electrodes coated with
benzene-1,4-dithiol. The statically stable gold-sulfur-aryl-
sulfur-gold junction allowed direct observation of charge
transfer through the molecules. The authors suggest this study
provides a quantitative measure of the conductance of a junction
containing a single molecule, making it a fundamental step in
molecular-scale electronics.
QY: M. A. Reed, Yale Univ., Elect. Engin. (203) 432-4771.
(Science 10 Oct 1997) (Science-Week 31 Oct 97)
4. REVERSIBLE HYDROGELS FROM SELF-ASSEMBLING ARTIFICIAL PROTEINS
Thermally reversible network formation in polymeric systems is a
well-known phenomenon. In aqueous solutions, synthetic polymers
such as poly(vinyl alcohol), proteins such as gelatin, and
polysaccharides such as carrageenan exhibit reversible gelation
within prescribed limits of concentration and temperature.
Despite the utility of this behavior, the molecular origins of
gel formation in such systems are poorly understood, and
opportunities for systematic engineering of gel properties are
therefore limited. Gel formation demands two seemingly
contradictory kinds of behavior: Interchain interactions must be
strong enough to form junction points in the molecular network,
yet at the same time the chain cannot exclude solvent, or it will
precipitate from solution rather than forming a swollen gel.
Assignment of these two roles to different portions of a
repetitive polymer such as poly(vinyl alcohol) is far from
straightforward, and the ability to tune the gel-solution (gel-
sol) transition conditions is limited in such systems.
... ... Petka et al (5 authors at 3 installations, US) report an
approach to gel design that circumvents the above problem through
the creation of multidomain artificial proteins in which the
interchain binding and solvent retention functions are engineered
independently. *Recombinant DNA methods were used to create
artificial proteins that undergo reversible gelation in response
to changes in pH or temperature. The proteins consist of terminal
leucine *zipper domains flanking a central, flexible, water-
soluble polyelectrolyte segment. The authors suggest that the
mild conditions under which gel formation can be controlled
(near-neutral pH and near-ambient temperature) indicate these
materials have potential in bioengineering applications requiring
encapsulation or controlled release of molecular and cellular
species.
QY: David A. Tirrell, Univ. of Massachusetts Amherst 413-545-0222
(Science 17 Jul 98 281:389) (Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *recombinant DNA methods: Genetic engineering is the
general term used for recombinant DNA technology, a set of
methods for introducing foreign DNA into a host organism. It
usually but not always involves gene cloning. In the context of
this report, the essential idea is to introduce DNA coding for a
particular protein into a host organism genome and subsequently
use the organism as a "factory" to produce the protein.
... ... *zipper domains: These are domains capable of forming
coil-coil reversible aggregates (helix-helix).
-------------------
Related Background:
A QUADRUPLE H-BONDING METHOD FOR REVERSIBLE POLYMER FORMATION
... A dimer in the general sense is a double molecule, and in
polymer chemistry it is a compound formed by the addition
polymerization of two molecules of a monomer. A reversible
polymer is a polymer whose structure can be reversibly altered by
small changes in state variables such as temperature, pressure,
electric potential, etc., and rheology is the study of the flow
and alterations of matter in response to applied forces. In
general, a gel is an easily deformable pseudo-solid (for example,
a jelly). ... ... Sijbesma et al (8 authors at 2 installations,
NL) report a method of forming reversible polymers using special
associating end groups (2-ureido-4-pyrimidone) that dimerize with
four cooperative hydrogen bonds, the resulting linearized polymer
formation preventing uncontrolled multidirectional association or
gelation. The authors suggest such polymer networks with
thermodynamically controlled architectures and a strongly
temperature dependent rheology can be useful as coatings and hot
melts.
QY: E. W. Meijer
(Science 28 Nov 97) (Science-Week 19 Dec 97)
5. COMPLETE GENOME SEQUENCE OF SYPHILIS SPIROCHETE
Syphilis is a contagious systemic disease caused by the
*spirochete microbe Treponema pallidum, the disease characterized
by sequential clinical stages and by years of latency without
symptoms. It can affect any tissue or vascular organ of the body
and can be passed from mother to fetus. One hundred years ago,
syphilis was a scourge of the industrialized nations. Since the
introduction of penicillin and the organization of a national
control program in the 1940s, the prevalence of syphilis has been
markedly reduced, and the prevalence currently ranges from
approximately 20 cases per 100,000 in 1990 to 3.2 cases per
100,000 in 1997, with maxima and minima following what appears to
be 7 to 10 year cycles, and with more than 50 percent of the
cases in the US occurring in only 31 counties. ... ... Fraser et
al (33 authors at 2 installations, US) now report they have
determined the complete genome sequence of T. pallidum, the
microbe's genome comprising 1,138,006 *base pairs containing 1041
predicted coding sequences (*open reading frames). Systems for
DNA replication, *transcription, *translation, and repair are
intact, but *catabolic and biosynthetic activities are minimized.
Potential virulence factors include a family of 12 possible
membrane proteins and several assumed hemolysins. Comparison of
the T. pallidum genome sequence with that of another pathogenic
spirochete, Borrelia burgdorferi, the agent of Lyme disease,
reveals unique and common genes, and substantiates the
considerable diversity observed among pathogenic spirochetes. The
authors point out that T. pallidum has been a difficult organism
to study experimentally because of its absolute dependence on a
mammalian host for sustained growth and viability, and that
knowledge of the genomic sequence of the organism may provide a
foundation for the development of a culture medium and open the
possibility of future genetic studies.
QY: Claire M. Fraser
(Science 17 Jul 98 281:375) (Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *spirochete: In general, this refers to various bacteria
species that assume a spiral form and range in size from a few
microns to 500 microns in length. T. pallidum, the agent of
syphilis, is 3 to 8 microns in length, with acute regular or
irregular spirals.
... ... *base pairs: The term "base pair" refers to the bases
(nucleotides) always found chemically bonded together in the DNA
double helix (adenine, for example, always bonds with thymine,
and guanine with cytosine).
... ... *open reading frames: The term "reading frame" refers to
a specific permutation of nucleotide triplets in DNA as "framed"
by a preceding start triplet (start codon), and an open reading
frame is any DNA sequence of triplets that potentially encodes a
protein.
... ... *transcription: *Transcription is the process by which
genetic information in DNA is converted into RNA.
... ... *translation: Translation is protein synthesis, the
process during which polypeptides are synthesized on ribosomes in
accordance with RNA code.
... ... *catabolic: This refers to catabolism, the enzyme-
mediated breakdown of complex organic substances into simple
substances, with conversion of the energy stored in chemical
bonds to the bond energy of ATP (adenosine triphosphate). A
companion term is "anabolism", which refers to the opposite
process -- the enzyme synthesis of complex molecules from simple
molecules. The term "metabolism" refers to the totality of
physical and chemical processes occurring in a living organism,
and comprises both anabolism and catabolism.
6. NANOBACTERIA AND PATHOGENIC EXTRACELLULAR CALCIFICATION
The formation of discrete and organized inorganic crystalline
structures within macromolecular extracellular matrices is a
widespread biological phenomenon generally referred to as
biomineralization. Mammalian bone and dental enamel are examples
of biomineralization involving *apatite minerals, but the
molecular basis of such mineralization remains largely unknown.
Recently, bacteria have been implicated as factors in
biogeochemical cycles for mineral formation in aqueous sediments.
The principle constituent of modern *authigenic phosphate
minerals in marine sediments is carbonate apatite. Microorganisms
are capable of depositing apatite in sea water, and they can
segregate Ca from Mg and actively nucleate carbonate apatite by
means of specific *oligopeptides under conditions of pH < 8.5 and
an Mg/Ca concentration ratio of greater than 0.1. Such conditions
are also present in the human body. ... ... Kajander and
Ciftcioglu (University of Kuopio, FI) report a study of biogenic
apatite production by "nanobacteria", identified by the authors
as "the *smallest cell-walled bacteria, only recently discovered
in human and cow blood and commercial cell culture serum." The
authors report that nanobacteria can act as crystallization
centers for the formation of biogenic apatite structures, and
that nanobacteria can produce apatite in media mimicking tissue
fluids and *glomerular filtrate, and provide a unique model for
in vitro studies of calcification. The authors suggest that
nanobacteria may play a key role in the formation of all kidney
stones, and they report they have found nanobacteria in all 30
human kidney stones that they have screened. The authors suggest
their findings are of concern in medicine because nanobacterial
*bacteremia occurs in humans, and nanobacterial crystallization
centers might initiate pathological calcification.
QY: E. Olavi Kajander
(Proc. Natl. Acad. Sci. US 7 Jul 98 95:8274)
(Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *apatite minerals: A group of phosphate-containing
minerals.
... ... *authigenic phosphate minerals: Authigenic minerals
(authigenes) are minerals that came into existence with or after
the formation of the rock of which they constitute a part. The
principal constituent of modern authigenic phosphate minerals in
marine sediments is carbonate (hydroxy)fluorapatite:
Ca(sub10)(PO(sub4))(sub6-x)(CO(sub3))(subx)(F,OH)(sub2+x).
... ... *oligopeptides: A peptide composed of no more than 10
amino acids.
... ... *smallest cell-walled bacteria: The electron micrographs
in this report show various forms with diameters 0.2 to 0.5
microns. See related reports below concerning the reported size
of nanobacteria.
... ... *glomerular filtrate: A glomerulus is a tuft-like
structure composed of blood vessel capillaries or nerve fibers,
and in this context, a glomerulus is a blood vessel capillary
structure and part of the nephron, the fundamental filtration
unit of the kidney. The filtrate from kidney glomeruli consists
of small solute molecules filtered under pressure from blood.
... ... *bacteremia: This is a general term referring to the
presence of bacteria in blood.
-------------------
Related Background:
EVIDENCE THAT MARTIAN METEORITE AMINO ACIDS ARE CONTAMINANTS
As the subunits that compose protein polymers in living systems,
the detection of certain amino acids in a material is often
interpreted as indicating a possible biological origin. The
meteorite ALH84001, along with a number of other discovered
meteorites, has a composition that suggests it was apparently
ejected from the surface of Mars, and during the past year it has
been proposed that microanalysis of this meteorite indicates the
possible presence of bio-organics and biogenic fossils. This
proposal, however, has met with considerable controversy, and the
controversy is still in full force. ... ... Bada et al (4 authors
at 3 installations, US) now report that the amino acids present
in a sample of the ALH84001 meteorite appear to be terrestrial in
origin and similar to those found in the ice where the meteorite
was discovered, although the possibility remains that minute
amounts of endogenous amino acids are preserved in the meteorite.
The authors suggest that radiocarbon studies (cf. contiguous
report: Jull et al, Science 279:366 1998), coupled with their own
amino acid results, indicate that major and minor organic
constituents in the Martian meteorites are contaminants.
QY: Jeffrey L. Bada
(Science 16 Jan 98) (Science-Week 30 Jan 98)
-------------------
Related Background:
AN ARGUMENT FOR RELIC LIFE ON MARS
In 1984, a 1.9 kilogram meteorite the size of a potato (desig-
nated ALH84001) was found in Antarctica, and because of its
chemical composition the consensus is that this meteorite (and a
dozen similar meteorites) originated from the planet Mars. The
basis for the consensus is the detailed quantitative correspond-
ence of the trapped gases in the meteorites to Martian atmosph-
eric gases, and the specific distributions of oxygen isotopes. In
1996 a group of researchers, McKay et al (National Aeronautics
and Space Administration Johnson Space Center, US; Stanford
University, US) reported they had concluded that unusual charact-
eristics of the meteorite ALH84001 can be most reasonably
interpreted as vestiges of ancient Martian bacterial life. In
particular, the authors noted the presence of tubules 20 to 40
nanometers in diameter (called by some "nannobacteria"), and they
proposed these structures were fossilized bacteria or parts of
microorganisms. The report provoked considerable controversy when
it appeared, and the controversy continues, with many biologists
objecting to the interpretation of the rock data, and
particularly to the idea of "bacteria" 20 to 40 nanometers in
diameter. Now Gibson et al (National Aeronautics and Space
Administration Houston, US; University of Georgia, US), this
group including some of the authors of the 1996 report, in a
review of the evidence for relic life on Mars, consider the
ALH84001 meteorite not only the strongest evidence for Martian
relic life, but also for the possibility of present Martian
microbial life. The authors are hopeful that in 2005 a "sample
return" mission will be launched to robotically collect Martian
rocks and soil and return them to Earth.
QY: Everett K. Gibson
(Scientific American December 1997)
-------------------
Related Background:
EVIDENCE AGAINST NANOFOSSILS IN MARTIAN METEORITE
The term "nanofossils" (originally spelled "nannofossils" by the
group that introduced the term) refers to elongated microscopic
forms found in the Martian meteorite ALH84001. Several groups in
the space and geology communities have proposed these forms are
fossilized bacteria, but most biologists have rejected the idea
on the basis that the forms are too small to be bacteria and
should not be classified as such. Bradley et al (3 installations,
US) now report that new analysis of material from the ALH84001
meteorite indicates the majority of the elongated microscopic
forms can be resolved as either emergent substrate layers or
magnetite whiskers, rather than biogenic nanofossils. Their
report is followed by a response from McKay et al (3 install-
ations, US CA), some of the original proponents of the nanofossil
idea, and in their response McKay et al say the artifact
possibilities mentioned by Bradley et al are already known to
them, but are not related to their own observations. They add
that living bacteria as small as 70 nanometers in diameter have
been observed in mammalian blood, and that soil bacteria as small
as 80 nanometers have also been observed. The references for
these bacterial forms are one unpublished paper and two recently
published papers in Proc. Soc. Photo-Opt. Instrum. Eng.
3111:420,429 (1997). It is evident that the nanofossil contro-
versy has not yet been resolved.
QY: J.P. Bradley, Georgia Inst. Technol. 404-894-2000; David S.
McKay
(Nature 4 Dec 97) (Science-Week 26 Dec 97)
-------------------
Related Background:
A CONTROVERSY CONCERNING MINIMUM POSSIBLE DIMENSIONS OF BACTERIA
Apart from their heuristic significance, scientific controversies
can be either amusing or irritating. In recent months, a
controversy between some geologists and many biologists has
developed, and it is apparently irritating the biologists. The
issue concerns the minimum possible dimensions of bacteria. The
geologists are led by Robert L. Folk (University of Texas, Austin
TX US), and they have proposed that certain microscopic entities
found in the Martian meteorite ALH84001 are fossils of what they
term "nannobacteria" (their own unique spelling of the prefix
nano-), which they say are similar to those found in Earth
travertine and limestone rocks, and which have dimensions of 30
to 50 nanometers. This has caused a furor among biologists, whose
understanding of bacteria and life forms in general is that the
smallest dimensions possible for a life form with a bounding
plasma membrane is about 200 nanometers. In fact no membrane-
bound bacterium with dimensions less than 340 nanometers has ever
been identified, and one can make simple calculations that a 50
nanometer bacterium would not have enough internal volume to
sustain its chemistry. Folk published papers on the subject in
several geological journals in 1996, starting the debate, and in
the Letters section of the 20 June 1997 issue of *Science* the
debate continues, and this week it is being reported in the
popular media as a "debate about life on earth". What evidently
irritates biologists is the apparent misunderstanding by these
geologists of experimental methods in biology. Characterizations
of "living" vs. "non-living" by biologists are made on the basis
of experimental laboratory replicability of an organism, and not
on the basis of the visible structure of an entity. Which means
the geologists involved need to attempt to culture their
Earth-rock entities, and which means decisions that the Martian
meteorite's so-called "nannobacteria fossils" are actually such
will require demonstration of cultured entities with those
dimensions. Biologists are not unwilling to admit the existence
of new species of life forms, of which they have already
recognized several million entities, but they argue that one does
not classify pieces of rock as a life form on the basis of
structure alone.
(New York Times 29 Jul 97) (Science-Week 1 Aug 97)
7. ELECTROPHYSIOLOGICAL DEVELOPMENT OF AUDITORY HAIR CELLS
Evolution has resulted in the development of an array of
remarkable sensory systems in living organisms, but perhaps the
most exquisitely designed sensory systems are the mammalian
visual and auditory systems. Whereas the visual system is
constructed to respond with high sensitivity to input photons,
the structure of the auditory system has as its function the
detection and analysis of the mechanical vibrations produced by
sound waves. The sensory receptors in the mammalian auditory
system are the so-called "hair cells" of the *cochlea, cells with
extensions ("hairs"; *stereocilia) that respond to mechanical
vibrations of a surrounding fluid, the cells arranged in a
flexible sheet, and the physical properties of the sheet of cells
and their surroundings such that sounds of differing frequencies
produce maximum mechanical input at differing loci on the sheet,
the result a topological representation of the frequency spectrum
of input sound waves. The auditory hair cells are essentially
energy transducers, transducing mechanical energy into electrical
energy, and this electrical energy in turn exciting associated
nerve cells to produce a topological signal pattern conveyed to
the central nervous system for analysis. The electrical behavior
of sensory and nerve cells is dependent on the specifics of
various *ion channels in their *plasma membranes, and one
important question concerns the changes that take place in the
various cellular ion channels during *cell differentiation....
... Kros et al (3 authors at 3 installations, UK DE) now
present evidence for the development changes that occur in *inner
hair cells. The authors report that in mice, responses to sound
can first be recorded from the auditory nerve and observed
behaviorally from 10 to 12 days after birth, and that these
responses mature rapidly over the next 4 days. Before this time,
mouse inner hair cells have slow voltage responses and fire
spontaneous and evoked action potentials. During development of
auditory responsiveness, a large fast potassium conductance is
expressed, greatly speeding up the membrane time constant and
preventing action potentials. This change in potassium channel
expression turns the inner hair cell from a regenerative spiking
pacemaker into a high-frequency signal transducer.
QY: Corne J. Kros
(Nature 16 Jul 98 394:281) (Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *cochlea: The cochlea is essentially a canal in bone,
differing in morphology among the mammals. The human cochlea is a
cone-shaped cavity in the temporal bone, forming one of the
divisions of the labyrinth (internal ear), and consists of a
spiral canal that contains various structures, in particular the
spiral organ of Corti, which contains the auditory hair cells.
... ... *stereocilia: Nonmotile cilia. In this context, they
extend from hair cells, and when they are mechanically bent by
hydrodynamic waves, an electrical change in the hair cells is
produced.
... ... *ion channels: Ion channels are protein channels in cell
membranes that allow ions to pass from extracellular solution to
intracellular solution and vice versa. Most ion channels are
selective, allowing only certain ions to pass, and an individual
cell has ion channels with various ion selectivities. The
selectivity of an ion channel can be "gated", the channel
effectively opened or closed, and ion channels are said to
*voltage-gated or *ligand-gated, depending on how the change in
selectivity is provoked.
... ... *plasma membranes: This is the membrane that separates
living cells from their environment, consisting of lipid bilayer
and associated proteins, the ensemble approximately 75 to 100
angstroms in thickness. Similar membranes are also found within a
cell surrounding various organelles.
... ... *cell differentiation: This refers to the developmental
specialization of cells exhibited by changes in morphology,
biochemistry, function, etc.
... ... *inner hair cells: Sensory hair cells are present in
several sensory systems, including the auditory system, the
vestibular system, and in taste buds. There are two groups of
auditory hair cells in the cochlea, labelled "inner" and "outer"
hair cells according to their anatomical position.
... ... *voltage-gated: Refers to opening or closing of an ion
channel by changes in the electrical potential across the
membrane.
... ... *ligand-gated: Refers to opening and closing of an ion
channel by interactions between ligands and membrane receptors.
8. RELEASE OF TWO DIFFERENT TRANSMITTERS BY THE SAME NEURON
*Synaptic transmission in the central nervous system is mediated
by the release of *neurotransmitters into the *synaptic cleft and
the subsequent activation of *postsynaptic receptors. It is
widely accepted that individual neurons in the central nervous
system release only a single *fast transmitter. A single
neurotransmitter can coactivate multiple *ionotropic and
*metabotropic receptor types, and a fast neurotransmitter can be
coreleased with *neuropeptides. In the spinal cord and brainstem,
both glycine and gamma-amino-butyric acid (GABA) mediate
*inhibitory synaptic transmission. It is not known, however,
whether glycine and GABA are released from separate or
overlapping populations of *interneurons. ... ... Jonas et al (3
authors at University of Freiburg, DE) now report a study of the
possible corelease of fast neurotransmitters from synaptically
connected neurons in spinal cord slices. The authors report that
unitary inhibitory postsynaptic electric currents generated at
interneuron-motorneuron synapses consist of a *strychnine-
sensitive glycine-receptor-mediated component, and a
*bicuculline-sensitive GABA-receptor mediated component. The
authors suggest their results indicate that spinal interneurons
release both glycine and GABA to activate functionally distinct
receptors in their postsynaptic target cells. They also report
that a subset of *miniature synaptic currents also showed both
components, consistent with corelease from individual synaptic
vesicles. The authors further suggest that glycine- and GABA-
mediated cotransmission could, among other possibilities, support
the precise regulation of the time course of postsynaptic
conductance by the relative amount of glycine and GABA released
from the presynaptic interneuron, and that this could be of
critical importance for motor coordination and the generation of
locomotor patterns.
QY: Peter Jonas
(Science 17 Jul 98 281:419) (Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *Synaptic transmission: This is a general term referring
to the events mediating the membrane-to-membrane interaction
between a neuron and another neuron, or a neuron and a muscle or
gland cell, or a neuron and a sensory receptor cell. The input
cell is called "presynaptic" and the output cell is called
"postsynaptic", with the junction called the "synapse".
... ... *neurotransmitters: 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. At present acetylcholine, 5 amines, 4 amino acids, 2
purines, and more than 28 peptides are known to be
neurotransmitters.
... ... *synaptic cleft: The space approximately 20 nanometers
wide between the presynaptic membrane and the postsynaptic
membrane. It is into this space that transmitter substances are
released by the presynaptic cell.
... ... *postsynaptic receptors: Postsynaptic membrane entities
for which neurotransmitters are the ligands.
... ... *fast transmitter: Fast neurotransmitters are
neurotransmitters that produce short-lived (on the order of
several milliseconds or less) electrical changes in the
postsynaptic membrane. In contrast, the electrical changes
produced by slow neurotransmitters are of the order of hundreds
of milliseconds.
... ... *ionotropic: Receptors whose activation by
specific ligands alters various ion permeabilities that control
the electrical activity of the nerve cell.
... ... *metabotropic receptors: Receptors whose activation by
specific ligands alters the metabolism of the nerve cell (which,
among other things, may have long-term effects on ion
permeabilities).
... ... *neuropeptides: A relatively large class of transmitter
substances, some of which (e.g., endorphins) are apparently
involved in the translation of emotions into bodily events, and
are suspected of an involvement in various neuropsychiatric
diseases.
... ... *inhibitory synaptic transmission: The electrical
dynamics at the synapse are such that if a neurotransmitter
produces an increase in the polarization of the postsynaptic
membrane (hyperpolarization), the result is an inhibiting
component as far as any subsequent production of a postsynaptic
action potential is concerned. In the classic paradigm, a neuron
receives a variety of inputs from other neurons, some of which
are inhibitory and some of which are excitatory, and it is the
total effect of these multiple inputs that conditions the output
of propagated action potentials.
... ... *interneurons: Combinations or groups of neurons between
sensory and motorneurons that govern coordinated activity. Some
interneurons are inhibitory, involved in local feedback circuits
that may be simple or elaborate.
... ... *strychnine: A central nervous system stimulant that
blocks the inhibitory action of glycine at several postsynaptic
receptors in several central nervous system locations,
particularly in the spinal cord.
... ... *bicuculline: An alkaloid naturally occurring in several
plant species, it is a powerful convulsant that acts by
antagonizing the inhibitory neurotransmitter GABA.
... ... *miniature synaptic currents: These are electric currents
produced by changes in the postsynaptic membrane resulting from
the release of neurotransmitter from a single *synaptic vesicle.
... ... *synaptic vesicle: *Synaptic vesicles are the packets of
neurotransmitter substance formed in the presynaptic axon
terminals, and when transmitter substances are released, they are
released as packets, the vesicle membrane dissolving in the
synaptic cleft to release the transmitter molecules.
9. RHEUMATOID ARTHRITIS: A HUMAN PARVOVIRUS AS A CAUSATIVE AGENT
Rheumatoid arthritis is a disease of unknown etiology affecting
approximately 1 percent of the population worldwide. The disease
produces a long-term destruction of connective tissue that has
been considered to result from the body rejecting its own tissue
cells (autoimmune reaction). Inflammation of the synovial
membranes that line the joints is coupled with increased release
of synovial fluid, producing a thickening of the synovial
membrane, swelling of the joints, and in severe cases, soft
tissue tumors and bone and cartilage destruction. Supplementing
consideration of the disease as an autoimmune reaction, many
studies have suggested the role of an infective agent in the
etiology, and one of the suspects for such an agent is a virus,
because viral infections such as rubella (German measles), *human
parvovirus B19 (B19), cytomegalovirus (CMV), human *T-cell
leukemia virus I, and HIV (human immunodeficiency virus) often
cause an acute onset of *polyarthritis. Some cases with acute B19
infection are well known to present clinical symptoms resembling
rheumatoid arthritis, and the presence of B19 DNA has been
demonstrated in several autoimmune diseases, including rheumatoid
arthritis. ... ... Takahashi et al (10 authors at Tohoku
University, JP) now report the detection of human parvovirus B19
DNA in the synovial tissues in 30 of 39 patients with rheumatoid
arthritis, and infrequently in patients with osteoarthritis and
traumatic joints. The authors report the target cells of B19 were
*macrophages, *follicular dendritic cells, T cells, and *B-
cells,
but not synovial lining cells. The authors hypothesize that B19-
positive T-cells and macrophages infiltrate into the synovium and
recruit circulating *immunocytes, that synovial T-cells and
macrophages continuously activated by B19 secrete one or more
*cytokines that stimulate a variety of synovial cells, and this
leads to an excessive synthesis of inflammatory cytokines and
*proteolytic enzymes and finally causes the destructive
alteration of the joints.
QY: Kazuo Sugamura
(Proc. Natl. Acad. Sci. US 7 Jul 98 95:8227)
(Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *human parvovirus B19: This is a recently discovered
virus, the pathogen of the disease erythema infectiosum, an acute
viral disease characterized by mild constitutional symptoms and
a
spreading rash that begins on the cheeks. Localized outbreaks of
the disease are common among children and adolescents.
... ... *T-cell: (lymphocyte) Lymphocytes are a type of leukocyte
(white blood cell) responsible for the immune response. There
are
two classes of lymphocytes: 1) the B-cells, when presented with a
foreign chemical entity (antigen), change into antibody
producing
plasma cells; and, 2) the T-cells interact directly with foreign
invaders such as bacteria and viruses.
... ... *polyarthritis: Simultaneous inflammation of several
joints.
... ... *macrophages: Macrophages are amoeba-like leukocytes that
are able to surround and digest foreign entities such as
bacteria
and protozoa.
... ... *follicular dendritic cells: In general, a follicle is a
small secretory cavity or sac. Dendritic cells are mononuclear
cells with long cytoplasmic processes. They function as antigen-
trapping and antigen-presenting cells (antigen: a substance
foreign to the vertebrate host and capable of eliciting an immune
response). The shape of dendritic cells is "tree-like", hence
their name. They should not be confused with nerve cells, many of
which also have a "tree-like" shape, with processes called
"dendrites".
... ... *B-cells: (see *T-cell above)
... ... *immunocytes: A general term referring to any
immunologically competent leukocyte.
... ... *cytokines: A *cytokine is any substance that promotes
cell growth and cell division. Certain cytokines are endogenous,
and need to be controlled by cell regulatory mechanisms. When
these mechanisms fail, endogenous cytokines may be implicated in
serious human diseases such as rheumatoid arthritis (as presented
in this report), where apparently deregulated cytokines cause the
inflammatory response that produces the symptoms. As a promoter
of cell growth and division, a cytokine acts as a messenger to
cells, and the transmission of the message requires a binding of
the cytokine molecule to a cytokine-specific receptor on the cell
surface. This receptor is either a protein or a protein complex
or a part of a protein.
... ... *proteolytic enzymes: A general term for enzymes that
cause the breakdown of proteins via the hydrolysis of peptide
bonds.
10. PALEOLITHIC HUMAN POPULATION EXPANSION IN AFRICA
Human populations have undergone dramatic expansions in size, but
other than the growth associated with agriculture, the dates and
magnitudes of those expansions have never been resolved. Genetic
approaches to the study of human population expansions have
focused on variation at a single genetic locus, the "control
region" of *mitochondrial DNA. But in the study of demographic
history, single-locus investigations suffer from pronounced
statistical and biological limitations. The statistical problem
is that the conclusions rely on only one particular realization
of a gene genealogy, the "tree" determining the ancestral
relationships among a set of *alleles. The biological problem is
that there are a large number of functional genes in the
mitochondrion, and due to a complete linkage, a selective sweep
for any one of the genes may lead to a spurious signal of
expansion. ... ... Reich and Goldstein (University of Oxford, UK)
present two new statistical tests for population expansion, using
variation at a number of unlinked genetic markers to study the
demographic histories of natural populations. The authors report
that analysis of genetic variation in various aboriginal
populations throughout the world reveals highly significant
evidence for a major human population expansion in Africa, but no
evidence of expansion outside of Africa. The inferred African
expansion is estimated to have occurred between 49,000 and
640,000 years ago, certainly before the Neolithic expansions, and
probably before the splitting of African and non-African
populations. The authors suggest that in showing a significant
difference between African and non-African populations, their
analysis supports the unique role of Africa in human evolutionary
history. The authors also suggest that the missing signal in non-
African populations may be the result of a population bottleneck
associated with the emergence of these populations from Africa,
as postulated in the "Out of Africa" model of modern human
origins.
QY: David B. Goldstein
(Proc. Natl. Acad. Sci. US 7 Jul 98 95:8119)
(Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *mitochondrial DNA: Mitochondria are organelles of the
cell cytoplasm, and they are the principal energy source of the
cell, containing various enzymes involved in electron transport
and metabolic cycles. Mitochondrial DNA (sometimes denoted as
mtDNA), found in the mitochondria of all eukaryotes, is believed
to evolve in parallel with nuclear DNA, but since sperm lose
their mitochondria, it is inherited only in the maternal lineage
in animals. Until now, it has been mitochondrial DNA that has
been greatly exploited in studies of the evolution of humans.
... ... *alleles: An allele is one of two or more forms of a
given gene that control a particular characteristic, with the
alternative forms occupying corresponding loci on homologous
chromosomes.
-------------------
Related Background:
Y CHROMOSOME EVIDENCE INDICATES AFRICAN ORIGINS OF MAN
The Y chromosome is one of the two chromosomes that determine sex
in many animals, including humans, and it carries mostly male-
specific genes. Genetic polymorphisms are individual functional
variations of specific genes or genetic markers that occur in a
population with a significant frequency, e.g., more than 1%.
... ... At a recent symposium on human evolution (Cold Spring
Harbor Laboratory, NY US), a consensus was apparently reached
that current studies of human Y chromosome polymorphisms indicate
that the major human migrations that occurred had their source in
Africa, and that a small number of present African populations,
the Ethiopians, Sudanese, and south African Khoisans, possess
markers that have been conserved since that time. The data are
considered to confirm the recent mitochondrial DNA studies which
also indicate Africa as the source of human migrations. Some
paleoanthropologists are calling the Y chromosome results an
"unquestionable major breakthrough".
(Science 31 Oct 97) (Science-Week 21 Nov 97)
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