Science Progress (2001), 84 (4)
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Science Progress (2001), 84 (4), 255–266
The aperture problem for visual
motion and its solution in primate
cortex
CHRISTOPHER C. PACK
A primary function of the visual system is to analyze the trajectories of
moving objects. This seemingly simple process is complicated by theoretical
considerations, which show that measurements of the velocity of a moving
object are inevitably confounded with the spatial arrangement of its edges.
This type of confusion is likely to be detrimental to an organism’s survival,
and so must be resolved. This review describes some recent experiments
that demonstrate the existence and time-course of a solution in the visual
cortex of the macaque brain. Related work on perception, behavior, and
computational theory is discussed.
Science Progress (2001), 84 (4), 267–290
Direct imaging of single molecules:
from dynamics of a
single DNA chain to the study of
complex DNA-protein
interactions
BENOIT LADOUX1, JEAN-PIERRE QUIVY2,
PATRICK. S. DOYLE3, GENEVIEVE ALMOUZNI2 AND
JEAN-LOUIS VIOVY1
Recent years have seen significant advances in the characterization and
manipulation of individual molecules. The combination of single-molecule
fluorescence and micromanipulation enables one to study physical and
biological systems at new length scales, to unravel qualitative mechanisms,
and to measure kinetic parameters that cannot be addressed by traditional
biochemistry. DNA is one of the most studied biomolecules. Imaging single
DNA molecules eliminates important limitations of classical techniques
and provides a new method for testing polymer dynamics and DNA–protein
interactions. Here we review some applications of this new approach to
physical and biological problems, focusing on videomicroscopy observations
of individual DNA chains extended in a shear flow. We will first
describe data obtained on the stretching, relaxation and dynamics of a
single tethered polymer in a shear flow, to demonstrate that the deformation
of sheared tethered chains is partially governed by the thermally driven
fluctuations of the chain transverse to the flow direction. Next, we will
show how single-molecule videomicroscopy can be used to study in real
time DNA folding into chromatin, a complex association of DNA and proteins
responsible for the packaging of DNA in the nucleus of an eukaryotic
cell.
Science Progress (2001), 84 (4), 291–309
Three-dimensional reconstruction
of single particle electron
microscopy: the voltage sensitive
sodium channel structure
YUTAKA UENO1 AND CHIKARA SATO2
Single particle analysis in electron microscopy allows direct observation of
the reconstructed three-dimensional structures of protein molecules. This
method enables a more comprehensive study of membrane proteins which
have been problematic in structural studies using X-ray crystallography.
These membrane proteins include the voltage-sensitive ion channel proteins,
which play an important rule in neural activities, and have great
medical significance. The method described is supported by the development
of cryo-electron microscopy and the angular reconstitution method.
This review summarizes certain principles governing single particle analysis
employing angular reconstitution. This method was applied to our study
of the voltage-sensitive sodium channel, and the results are discussed. With
improvements in resolutions and statistical analyses, the single particle
technique is considered to be advantageous in studies of the structural
changes and molecular interactions of protein molecules.
Science Progress (2001), 84 (4), 311–334
Hydrogenation properties of
supported nanosized gold
particles
CHRISTIAN MOHR AND PETER CLAUS
Introduction
In the last few years, there has been growing interest in “nanosized”
structures in the range 1 to about 20 nanometers in many different
fields of research. This is also the size of metal particles usually used
in heterogenous catalysis. In general, such nanoparticles of metals
like palladium, ruthenium, nickel or platinum are used for hydrogenations1,
since on these Group VIII metals the dissociatively
adsorbed hydrogen is easily accessible. For a long time, only very
limited attention has been paid to realizing catalysis on the basis of
gold because of its electronic structure, namely the completely filled
d band ([Xe] 4f145d106s1), which is usually accompanied by very low
activities2. The situation has been changed since Haruta and coworkers
reported on CO oxidation at room temperature, that is feasible only
on very small gold nanoparticles on suitable supports3. This observation
was followed by enhanced search for other possible applications
in catalysis2. Unfortunately, focussing on oxidation reactions masked
the capabilities of gold in hydrogenation reactions, even though
there are some very promising first examples of possible applications
[2]. The speciality of these reactions, at least of the examples discussed
below, is the control of intramolecular selectivity rather than
maximum activities.
Science Progress (2001), 84 (4), 335–354
Oceanic slab melting and mantle
metasomatism
BRUNO SCAILLET1 AND GAËLLE PROUTEAU2
Modern plate tectonic brings down oceanic crust along subduction zones
where it either dehydrates or melts. Those hydrous fluids or melts migrate
into the overlying mantle wedge trigerring its melting which produces arc
magmas and thus additional continental crust. Nowadays, melting seems to
be restricted to cases of young (<50 Ma) subducted plates. Slab melts are
silicic and strongly sodic (trondhjemitic). They are produced at low temperatures
(<1000°C) and under water excess conditions. Their interaction
with mantle peridotite produces hydrous metasomatic phases such as
amphibole and phlogopite that can be more or less sodium rich. Upon
interaction the slab melt becomes less silicic (dacitic to andesitic), and Mg,
Ni and Cr richer. Virtually all exposed slab melts display geochemical evidence
of ingestion of mantle material. Modern slab melts are thus unlike
Archean Trondhjemite–Tonalite–Granodiorite rocks (TTG), which suggests
that both types of magmas were generated via different petrogenetic pathways
which may imply an Archean tectonic model of crust production
different from that of the present-day, subduction-related, one.