10,580 research outputs found
A casein kinase 1 prevents expulsion of the oocyte meiotic spindle into a polar body by regulating cortical contractility.
During female meiosis, haploid eggs are generated from diploid oocytes. This reduction in chromosome number occurs through two highly asymmetric cell divisions, resulting in one large egg and two small polar bodies. Unlike mitosis, where an actomyosin contractile ring forms between the sets of segregating chromosomes, the meiotic contractile ring forms on the cortex adjacent to one spindle pole, then ingresses down the length of the spindle to position itself at the exact midpoint between the two sets of segregating chromosomes. Depletion of casein kinase 1 gamma (CSNK-1) in Caenorhabditis elegans led to the formation of large polar bodies that contain all maternal DNA, because the contractile ring ingressed past the spindle midpoint. Depletion of CSNK-1 also resulted in the formation of deep membrane invaginations during meiosis, suggesting an effect on cortical myosin. Both myosin and anillin assemble into dynamic rho-dependent cortical patches that rapidly disassemble in wild-type embryos. CSNK-1 was required for disassembly of both myosin patches and anillin patches. Disassembly of anillin patches was myosin independent, suggesting that CSNK-1 prevents expulsion of the entire meiotic spindle into a polar body by negatively regulating the rho pathway rather than through direct inhibition of myosin
Trace element abundance determinations by Synchrotron X Ray Fluorescence (SXRF) on returned comet nucleus mineral grains
Trace element analyses were performed on bulk cosmic dust particles by Proton Induced X Ray Emission (PIXE) and Synchrotron X Ray Fluorescence (SXRF). When present at or near chondritic abundances the trace elements K, Ti, Cr, Mn, Cu, Zn, Ga, Ge, Se, and Br are presently detectable by SXRF in particles of 20 micron diameter. Improvements to the SXRF analysis facility at the National Synchrotron Light Source presently underway should increase the range of detectable elements and permit the analysis of smaller samples. In addition the Advanced Photon Source will be commissioned at Argonne National Laboratory in 1995. This 7 to 8 GeV positron storage ring, specifically designed for high-energy undulator and wiggler insertion devices, will be an ideal source for an x ray microprobe with one micron spatial resolution and better than 100 ppb elemental sensitivity for most elements. Thus trace element analysis of individual micron-sized grains should be possible by the time of the comet nucleus sample return mission
Some system considerations in configuring a digital flight control - navigation system
A trade study was conducted with the objective of providing a technical guideline for selection of the most appropriate computer technology for the automatic flight control system of a civil subsonic jet transport. The trade study considers aspects of using either an analog, incremental type special purpose computer or a general purpose computer to perform critical autopilot computation functions. It also considers aspects of integration of noncritical autopilot and autothrottle modes into the computer performing the critical autoland functions, as compared to the federation of the noncritical modes into either a separate computer or with a R-Nav computer. The study is accomplished by establishing the relative advantages and/or risks associated with each of the computer configurations
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
A study of trace contaminant identification by microwave double resonance spectroscopy
Trace contaminant identification using microwave double resonance spectroscop
Hair radioactivity as a measure of exposure to radioisotopes
Since many radioisotopes accumulate in hair, this tropism was investigated by comparing the radioactivity of shaved with plucked hair collected from rats at various time intervals up to 24 hrs after intravenous injection of the ecologically important radioisotopes, iodine-131, manganese-54, strontium-85, and zinc-65. The plucked hair includes the hair follicles where biochemical transformations are taking place. The data indicate a slight surge of each radioisotpe into the hair immediately after injection, a variation of content of each radionuclide in the hair, and a greater accumulation of radioactivity in plucked than in shaved hair. These results have application not only to hair as a measure of exposure to radioisotopes, but also to tissue damage and repair at the hair follicle
Highly Ionized High-Velocity Clouds toward PKS 2155-304 and Markarian 509
To gain insight into four highly ionized high-velocity clouds (HVCs)
discovered by Sembach et al. (1999), we have analyzed data from the Hubble
Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) for the
PKS 2155-304 and Mrk 509 sight lines. We measure strong absorption in OVI and
column densities of multiple ionization stages of silicon (SiII/III/IV) and
carbon (CII/III/IV). We interpret this ionization pattern as a multiphase
medium that contains both collisionally ionized and photoionized gas. Toward
PKS 2155-304, for HVCs at -140 and -270 km/s, respectively, we measure
logN(OVI)=13.80+/-0.03 and log N(OVI)=13.56+/-0.06; from Lyman series
absorption, we find log N(HI)=16.37^(+0.22)_(-0.14) and 15.23^(+0.38)_(-0.22).
The presence of high-velocity OVI spread over a broad (100 km/s) profile,
together with large amounts of low-ionization species, is difficult to
reconcile with the low densities, n=5x10^(-6) cm^(-3), in the
collisional/photoionization models of Nicastro et al. (2002), although the HVCs
show a similar relation in N(SiIV)/N(CIV) versus N(CII)/N(CIV) as high-z
intergalactic clouds. Our results suggest that the high-velocity OVI in these
absorbers do not necessarily trace the WHIM, but instead may trace HVCs with
low total hydrogen column density. We propose that the broad high-velocity OVI
absorption arises from shock ionization, at bowshock interfaces produced from
infalling clumps of gas with velocity shear. The similar ratios of high ions
for HVC Complex C and these highly ionized HVCs suggest a common production
mechanism in the Galactic halo.Comment: 38 pages, including 10 figures. ApJ, 10 April, 2004. Replaced with
accepted versio
Half-life of the electron-capture decay of 97Ru: Precision measurement shows no temperature dependence
We have measured the half-life of the electron-capture (ec) decay of 97Ru in
a metallic environment, both at low temperature (19K), and also at room
temperature. We find the half-lives at both temperatures to be the same within
0.1%. This demonstrates that a recent claim that the ec decay half-life for 7Be
changes by $0.9% +/- 0.2% under similar circumstances certainly cannot be
generalized to other ec decays. Our results for the half-life of 97Ru,
2.8370(14)d at room temperature and 2.8382(14)d at 19K, are consistent with,
but much more precise than, previous room-temperature measurements. In
addition, we have also measured the half-lives of the beta-emitters 103Ru and
105Rh at both temperatures, and found them also to be unchanged.Comment: 6 pages, 6 figure
Molecular Characterisation of Bacteriophage K Towards Applications for the Biocontrol of Pathogenic Staphylococci
End of project reportThe aim of this work was to characterise staphylococcal bacteriophage (a bacterial virus) and to assess their potential as therapeutic agents against pathogenic strains of Staphylococcus aureus, particularly mastitis-causing strains. The project included the use of two newly isolated phage CS1 and DW2, and an existing polyvalent phage. The new phage were isolated from the farmyard and characterised by electron microscopy and restriction analysis. Both phage were shown to belong to the Siphoviridae family and were lytic for representatives of all three clonal groups of Irish mastitis-associated staphylococci. A cocktail of three phage (CS1, DW2 and K) at 108 (plaque forming units) PFU/ml was infused into cows teats in animal trials. The lack of an increase in somatic cell counts in milks indicated strongly that the phage did not irritate the animal. In addition, the most potent phage used in this study, phage K, was further studied by genome sequencing, which revealed a linear DNA genome of 127,395 base pairs, which encodes 118 putative ORFs (open reading frames)
A New Method for Obtaining Binary Pulsar Distances and its Implications for Tests of General Relativity
We demonstrate how measuring orbital period derivatives can lead to more
accurate distance estimates and transverse velocities for some nearby binary
pulsars. In many cases this method will estimate distances more accurately than
is possible by annual parallax, as the relative error decreases as t^-5/2.
Unfortunately, distance uncertainties limit the degree to which nearby
relativistic binary pulsars can be used for testing the general relativistic
prediction of orbital period decay to a few percent. Nevertheless, the measured
orbital period derivative of PSR B1534+12 agrees within the observational
uncertainties with that predicted by general relativity if the proper-motion
contribution is accounted for.Comment: 4 pages, latex, uuencoded compressed postscript + source, no figures,
uses aaspptwo.sty and dec.sty, accepted for publication in ApJL, omitted
reference now include
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