264 research outputs found
The Short-term Consequences of Nuclear War for Civilians
Much of the debate over nuclear weapons policy continues to revolve around discussions of the usefulness of nuclear attacks on military targets. Much less attention, however, is devoted to either the number or the importance of the civilian casualties that such attacks would cause. The short-term civilian casualties that would result from the use of nuclear weapons at three different levels of limited nuclear war are considered. These levels range from the employment of neutron bombs during an otherwise conventional battle in the Germanies to a nuclear attack against the strategic forces of the U.S. In addition, the consequences of all-out attacks by the superpowers on each other\u27s cities are briefly discussed. It is found that nuclear planners and strategists have almost always grossly underestimated the human costs of the use of nuclear weapons
The Hazard from Plutonium Dispersal by Nuclear-warhead Accidents
Nuclear weapons are carefully designed to have an extremely low probability of
exploding accidentally with an appreciable yield—even if they are involved in a
high-speed crash, struck by a bullet or consumed in a fire. The principal concern
when nuclear warheads are involved in such accidents is the possible dispersal of
plutonium into the environment. In particular, an explosion could disperse a
significant fraction of the plutonium in a warhead as particles of respirable size
DU Not a High Priority for Antinuclear Movement
Two years ago, members of anti-nuclear weapons
groups began to ask our views
about the alarm raised by the
International Action Center in its book,
Metal of Dishonor, about the use of depleted
uranium (DU) penetrators in anti-armor
munitions. We were asked whether the
hazard was so great that activists should
give priority to banning DU.
We read Metal of Dishonor and found
that, despite the contributions of physicists
and radiation-effects analysts, it contained
no quantitative risk estimate. We therefore
decided to provide the best one we could,
using information available in the literature
about the health effects of uranium and ionizing
radiation.
We concluded that, except for soldiers
in vehicles when they are struck, or individuals
who crawl around inside such vehicles
without adequate respiratory protection for
extended periods of time later on, the health
effects of DU are likely to be very small. The
radiation effects would be well below those
of natural background radiation and the
chemical effects would be well below the
thresholds for known toxic effects.
Contaminated armored vehicles and pieces
of depleted uranium, however, are potential
hazards and should be cleaned up or
buried—something which was not done in
most cases after Desert Storm and is only
being done now in Kosovo
Physical Constraints and Functional Characteristics of Transcription Factor-DNA Interaction
We study theoretical ``design principles'' for transcription factor-DNA
interaction in bacteria, focusing particularly on the statistical interaction
of the transcription factors (TF's) with the genomic background (i.e., the
genome without the target sites). We introduce and motivate the concept of
`programmability', i.e. the ability to set the threshold concentration for TF
binding over a wide range merely by mutating the binding sequence of a target
site. This functional demand, together with physical constraints arising from
the thermodynamics and kinetics of TF-DNA interaction, leads us to a narrow
range of ``optimal'' interaction parameters. We find that this parameter set
agrees well with experimental data for the interaction parameters of a few
exemplary prokaryotic TF's. This indicates that TF-DNA interaction is indeed
programmable. We suggest further experiments to test whether this is a general
feature for a large class of TF's.Comment: 9 pages, 4 figures; revised version as published in PNA
Opening of DNA double strands by helicases. Active versus passive opening
Helicase opening of double-stranded nucleic acids may be "active" (the
helicase directly destabilizes the dsNA to promote opening) or "passive" (the
helicase binds ssNA available due to a thermal fluctuation which opens part of
the dsNA). We describe helicase opening of dsNA, based on helicases which bind
single NA strands and move towards the double-stranded region, using a discrete
``hopping'' model. The interaction between the helicase and the junction where
the double strand opens is characterized by an interaction potential. The form
of the potential determines whether the opening is active or passive. We
calculate the rate of passive opening for the helicase PcrA, and show that the
rate increases when the opening is active. Finally, we examine how to choose
the interaction potential to optimize the rate of strand separation. One
important result is our finding that active opening can increase the unwinding
rate by 7 fold compared to passive opening.Comment: 13 pages, 3 figure
Salerno's model of DNA reanalysed: could solitons have biological significance?
We investigate the sequence-dependent behaviour of localised excitations in a
toy, nonlinear model of DNA base-pair opening originally proposed by Salerno.
Specifically we ask whether ``breather'' solitons could play a role in the
facilitated location of promoters by RNA polymerase. In an effective potential
formalism, we find excellent correlation between potential minima and {\em
Escherichia coli} promoter recognition sites in the T7 bacteriophage genome.
Evidence for a similar relationship between phage promoters and downstream
coding regions is found and alternative reasons for links between AT richness
and transcriptionally-significant sites are discussed. Consideration of the
soliton energy of translocation provides a novel dynamical picture of sliding:
steep potential gradients correspond to deterministic motion, while ``flat''
regions, corresponding to homogeneous AT or GC content, are governed by random,
thermal motion. Finally we demonstrate an interesting equivalence between
planar, breather solitons and the helical motion of a sliding protein
``particle'' about a bent DNA axis.Comment: Latex file 20 pages, 5 figures. Manuscript of paper to appear in J.
Biol. Phys., accepted 02/09/0
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Perchlorate Exposure Reduces Primordial Germ Cell Number in Female Threespine Stickleback
Perchlorate is a common aquatic contaminant that has long been known to affect thyroid function in vertebrates, including humans. More recently perchlorate has been shown to affect primordial sexual differentiation in the aquatic model fishes zebrafish and threespine stickleback, but the mechanism has been unclear. Stickleback exposed to perchlorate from fertilization have increased androgen levels in the embryo and disrupted reproductive morphologies as adults, suggesting that perchlorate could disrupt the earliest stages of primordial sexual differentiation when primordial germ cells (PGCs) begin to form the gonad. Female stickleback have three to four times the number of PGCs as males during the first weeks of development. We hypothesized that perchlorate exposure affects primordial sexual differentiation by reducing the number of germ cells in the gonad during an important window of stickleback sex determination at 14–18 days post fertilization (dpf). We tested this hypothesis by quantifying the number of PGCs at 16 dpf in control and 100 mg/L perchlorate-treated male and female stickleback. Perchlorate exposure from the time of fertilization resulted in significantly reduced PGC number only in genotypic females, suggesting that the masculinizing effects of perchlorate observed in adult stickleback may result from early changes to the number of PGCs at a time critical for sex determination. To our knowledge, this is the first evidence of a connection between an endocrine disruptor and reduction in PGC number prior to the first meiosis during sex determination. These findings suggest that a mode of action of perchlorate on adult reproductive phenotypes in vertebrates, including humans, such as altered fecundity and sex reversal or intersex gonads, may stem from early changes to germ cell development
The rise and fall of the ancient northern pike master sex-determining gene
The understanding of the evolution of variable sex determination mechanisms across taxa requires comparative studies among closely related species. Following the fate of a known master sex-determining gene, we traced the evolution of sex determination in an entire teleost order (Esociformes). We discovered that the northern pike (Esox lucius) master sex-determining gene originated from a 65 to 90 million-year-old gene duplication event and that it remained sex linked on undifferentiated sex chromosomes for at least 56 million years in multiple species. We identified several independent species- or population-specific sex determination transitions, including a recent loss of a Y chromosome. These findings highlight the diversity of evolutionary fates of master sex-determining genes and the importance of population demographic history in sex determination studies. We hypothesize that occasional sex reversals and genetic bottlenecks provide a non-adaptive explanation for sex determination transitions
Global Models of Runaway Accretion in White Dwarf Debris Disks
A growing sample of white dwarfs (WDs) with metal-enriched atmospheres are
accompanied by excess infrared emission, indicating that they are encircled by
a compact dusty disk of solid debris. Such `WD debris disks' are thought to
originate from the tidal disruption of asteroids or other minor bodies, but the
precise mechanism(s) responsible for transporting matter to the WD surface
remains unclear, especially in those systems with the highest inferred metal
accretion rates dM_Z/dt ~ 1e8-1e10 g/s. Here we present global time-dependent
calculations of the coupled evolution of the gaseous and solid components of WD
debris disks. Solids transported inwards (initially due to PR drag) sublimate
at tens of WD radii, producing a source of gas that accretes onto the WD
surface and viscously spreads outwards in radius, where it overlaps with the
solid disk. If the aerodynamic coupling between the solids and gaseous disks is
sufficiently strong (and/or the gas viscosity sufficiently weak), then gas
builds up near the sublimation radius faster than it can viscously spread away.
Since the rate of drag-induced solid accretion increases with gas density, this
results in a runaway accretion process, during which the WD accretion rate
reaches values orders of magnitude higher than can be achieved by PR drag
alone. We explore the evolution of WD debris disks across a wide range of
physical conditions and calculate the predicted distribution of observed
accretion rates dM_Z/dt, finding reasonable agreement with the current sample.
Although the conditions necessary for runaway accretion are at best marginally
satisfied given the minimal level of aerodynamic drag between circular gaseous
and solid disks, the presence of other stronger forms of solid-gas
coupling---such as would result if the gaseous disk is only mildly
eccentric---substantially increase the likelihood of runaway accretion.Comment: 23 pages, 20 figures, submitted to MNRA
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