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Studies on the mechanism of retinoid-induced pattern duplications in the early chick limb bud: temporal and spatial aspects.
All-trans-retinoic acid causes striking digit pattern changes when it is continuously released from a bead implanted in the anterior margin of an early chick wing bud. In addition to the normal set of digits (234), extra digits form in a mirror-symmetrical arrangement, creating digit patterns such as a 432234. These retinoic acid-induced pattern duplications closely mimic those found after grafts of polarizing region cells to the same positions with regard to dose-response, timing, and positional effects. To elucidate the mechanism by which retinoic acid induces these pattern duplications, we have studied the temporal and spatial distribution of all-trans-retinoic acid and its potent analogue TTNPB in these limb buds. We find that the induction process is biphasic: there is an 8-h lag phase followed by a 6-h duplication phase, during which additional digits are irreversibly specified in the sequence digit 2, digit 3, digit 4. On average, formation of each digit seems to require between 1 and 2 h. The tissue concentrations, metabolic pattern, and spatial distribution of all-trans-retinoic acid and TTNPB in the limb rapidly reach a steady state, in which the continuous release of the retinoid is balanced by loss from metabolism and blood circulation. Pulse-chase experiments reveal that the half-time of clearance from the bud is 20 min for all-trans-retinoic acid and 80 min for TTNPB. Manipulations that change the experimentally induced steep concentration gradient of TTNPB suggest that a graded distribution of retinoid concentrations across the limb is required during the duplication phase to induce changes in the digit pattern. The extensive similarities between results obtained with retinoids and with polarizing region grafts raise the possibility that retinoic acid serves as a natural "morphogen" in the limb
Bubble statistics and positioning in superhelically stressed DNA
We present a general framework to study the thermodynamic denaturation of
double-stranded DNA under superhelical stress. We report calculations of
position- and size-dependent opening probabilities for bubbles along the
sequence. Our results are obtained from transfer-matrix solutions of the
Zimm-Bragg model for unconstrained DNA and of a self-consistent linearization
of the Benham model for superhelical DNA. The numerical efficiency of our
method allows for the analysis of entire genomes and of random sequences of
corresponding length ( base pairs). We show that, at physiological
conditions, opening in superhelical DNA is strongly cooperative with average
bubble sizes of base pairs (bp), and orders of magnitude higher
than in unconstrained DNA. In heterogeneous sequences, the average degree of
base-pair opening is self-averaging, while bubble localization and statistics
are dominated by sequence disorder. Compared to random sequences with identical
GC-content, genomic DNA has a significantly increased probability to open large
bubbles under superhelical stress. These bubbles are frequently located
directly upstream of transcription start sites.Comment: to be appeared in Physical Review
A knowledge-based approach to VLSI-design in an open CAD-environment
A knowledge-based approach is suggested to assist a designer in the increasingly complex task of generating VLSI-chips from abstract, high-level specifications of the system. The complexity of designing VLSI-circuits has reached a level where computer-based assistance has become indispensable. Not all of the design tasks allow for algorithmic solutions. AI technique can be used, in order to support the designer with computer-aided tools for tasks not suited for algorithmic approaches. The approach described in this paper is based upon the underlying characteristics of VLSI design processes in general, comprising all stages of the design. A universal model is presented, accompanied with a recording method for the acquisition of design knowledge - strategic and task-specific - in terms of the design actions involved and their effects on the design itself. This method is illustrated by a simple design example: the implementation of the logical EXOR-component. Finally suggestions are made for obtaining a universally usable architecture of a knowledge-based system for VLSI-design
Conservation Strategies for West Indian Rock Iguanas (Genus Cyclura): Current Efforts and Future Directions
As a result of habitat loss and the negative impact of introduced mammalian predators and competitors, West Indian Rock Iguanas (genus Cyclura) are among the most endangered lizards in the world. Because they are important seed dispersers for native plants, their loss has serious ecological consequences for dry tropical forest and scrub habitats. Six of the nine species of Rock Iguanas are considered critically endangered by the IUCN (the World Conservation Union), with C. collei and C. pinguis numbering only a few hundred individuals in the wild and C. lewisi fewer than 25 individuals. Conservation recommendations include further research to better understand population dynamics and ecological requirements, establishment of new protected areas and stronger enforcement within existing ones, control programs for introduced species, captive breeding and headstarting where appropriate, and public education at the local, national, and international levels
Ribosome recycling induces optimal translation rate at low ribosomal availability
Funding statement The authors thank BBSRC (BB/F00513/X1, BB/I020926/1 and DTG) and SULSA for funding. Acknowledgement The authors thank R. Allen, L. Ciandrini, B. Gorgoni and P. Greulich for very helpful discussions and careful reading of the manuscript.Peer reviewedPublisher PD
Conservation Strategies for West Indian Rock Iguanas (Genus Cyclura): Current Efforts and Future Directions
As a result of habitat loss and the negative impact of introduced mammalian predators and competitors, West Indian Rock Iguanas (genus Cyclura) are among the most endangered lizards in the world. Because they are important seed dispersers for native plants, their loss has serious ecological consequences for dry tropical forest and scrub habitats. Six of the nine species of Rock Iguanas are considered critically endangered by the IUCN (the World Conservation Union), with C. collei and C. pinguis numbering only a few hundred individuals in the wild and C. lewisi fewer than 25 individuals. Conservation recommendations include further research to better understand population dynamics and ecological requirements, establishment of new protected areas and stronger enforcement within existing ones, control programs for introduced species, captive breeding and headstarting where appropriate, and public education at the local, national, and international levels
Effective Medium Theory of Filamentous Triangular Lattice
We present an effective medium theory that includes bending as well as
stretching forces, and we use it to calculate mechanical response of a diluted
filamentous triangular lattice. In this lattice, bonds are central-force
springs, and there are bending forces between neighboring bonds on the same
filament. We investigate the diluted lattice in which each bond is present with
a probability . We find a rigidity threshold which has the same value
for all positive bending rigidity and a crossover characterizing bending-,
stretching-, and bend-stretch coupled elastic regimes controlled by the
central-force rigidity percolation point at of the
lattice when fiber bending rigidity vanishes.Comment: 15 pages, 9 figure
Non-equilibrium mechanics and dynamics of motor activated gels
The mechanics of cells is strongly affected by molecular motors that generate
forces in the cellular cytoskeleton. We develop a model for cytoskeletal
networks driven out of equilibrium by molecular motors exerting transient
contractile stresses. Using this model we show how motor activity can
dramatically increase the network's bulk elastic moduli. We also show how motor
binding kinetics naturally leads to enhanced low-frequency stress fluctuations
that result in non-equilibrium diffusive motion within an elastic network, as
seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure
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