116 research outputs found
The relativistic self-energy in nuclear dynamics
It is a well known fact that Dirac phenomenology of nuclear forces predicts
the existence of large scalar and vector mean fields in matter. To analyse the
relativistic self-energy in a model independent way, modern high precision
nucleon-nucleon () potentials are mapped on a relativistic operator basis
using projection techniques. This allows to compare the various potentials at
the level of covariant amplitudes were a remarkable agreement is found. It
allows further to calculate the relativistic self-energy in nuclear matter in
Hartree-Fock approximation. Independent of the choice of the nucleon-nucleon
interaction large scalar and vector mean fields of several hundred MeV
magnitude are generated at tree level. In the framework of chiral EFT these
fields are dominantly generated by contact terms which occur at next-to-leading
order in the chiral expansion. Consistent with Dirac phenomenology the
corresponding low energy constants which generate the large fields are closely
connected to the spin-orbit interaction in scattering. The connection to
QCD sum rules is discussed as well.Comment: 49 pages, 13 figure
Telomeric localization of the vertebrate-type hexamer repeat, (TTAGGG) ₙ, in the wedgeshell clam "Donax trunculus" and other marine invertebrate genomes
[Abstract]: The hexamer repeat sequence (TTAGGG) n, found at the ends of all vertebrate chromosomes, was previously identified as the main building element of one member of aHindIII satellite DNA family characterized in the genome of the bivalve mollusc "Donax trunculus". It was also found in 22 perfect tandem repeats in a cloned junction region juxtaposed to the proper satellite sequence, from which the DNA tract encompassing the clustered tandem copies was excised and subcloned. Here, the chromosomal distribution of (TTAGGG) n sequences in the "Donax" was studied by the sensitivity to Bal31 exonuclease digestion, fluorescence in situ hybridization (FISH) on metaphase chromosomes and rotating-field gel electrophoresis. To verify the occurrence of the hexamer repeat in the genomes of taxonomically related molluscs and other marine invertebrates, genomic DNA from the mussel "Mytilus galloprovincialis" and the echinoderm "Holothuria tubulosa" was also analyzed. The kinetics of Bal31 hydrolysis of high molecular mass DNA from the three marine invertebrates revealed a marked decrease over time of the hybridization with the cloned (TTAGGG)22 sequence, concomitantly with a progressive shortening of the positively reacting DNA fragments. This revealed a marked susceptibility to exonuclease consistent with terminal positioning on the respective chromosomal DNAs. In full agreement, FISH results with the (TTAGGG) 22 probe showed that the repeat appears located in telomeric regions in all chromosomes of both bivalve molluscs. The presence of (TTAGGG) n repeat tracts in marine invertebrate telomeres points to its wider distribution among eukaryotic organisms and suggests an ancestry older than originally presumed from its vertebrate distinctiveness.España. Dirección General de Enseñanza Superior; PB97-1136Cataluña. Direcció General de Recerca; SGR01-35
Antikaons and hyperons in nuclear matter with saturation
We evaluate the antikaon and hyperon spectral functions in a self-consistent
and covariant many-body approach. The computation is based on coupled-channel
dynamics derived from the chiral SU(3) Lagrangian. A novel subtraction scheme
is developed that avoids kinematical singularities and medium-induced power
divergencies all together. Scalar and vector mean fields are used to model
nuclear binding and saturation. The effect of the latter is striking for the
antikaon spectral function that becomes significantly more narrow at small
momenta. Attractive mass shifts of about 30 and 40 MeV are predicted for the
Lambda(1405) and Sigma(1385) resonances. Once scalar and vector mean fields for
the nucleon are switched on the Lambda(1520) resonances dissolves almost
completely in nuclear matter. All together only moderate attraction is
predicted for the nuclear antikaon systems at saturation density. However, at
larger densities we predict a sizable population of soft antikaon modes that
arise from the coupling of the antikaon to a highly collective Lambda(1115)
nucleon-hole state. This may lead to the formation of exotic nuclear systems
with strangeness and antikaon condensation in compact stars at moderate
densities.Comment: 49 pages, 13 figures, The revised manuscript contains additional
material at twice nuclear saturation density. An unexpected and novel
mechanism is unravelled that may have dramatic implications on the formation
of exotic nuclear systems with strangeness and antikaon condensation in
compact star
Nuclear energy density functional from chiral pion-nucleon dynamics: Isovector terms
We extend a recent calculation of the nuclear energy density functional in
the framework of chiral perturbation theory by computing the isovector surface
and spin-orbit terms: (\vec \nabla \rho_p- \vec \nabla \rho_n)^2 G_d(\rho)+
(\vec \nabla \rho_p- \vec \nabla \rho_n)\cdot(\vec J_p-\vec J_n)
G_{so(\rho)+(\vec J_p-\vec J_n)^2 G_J(\rho) pertaining to different proton and
neutron densities. Our calculation treats systematically the effects from
-exchange, iterated -exchange, and irreducible -exchange with
intermediate -isobar excitations, including Pauli-blocking corrections
up to three-loop order. Using an improved density-matrix expansion, we obtain
results for the strength functions , and
which are considerably larger than those of phenomenological Skyrme forces.
These (parameter-free) predictions for the strength of the isovector surface
and spin-orbit terms as provided by the long-range pion-exchange dynamics in
the nuclear medium should be examined in nuclear structure calculations at
large neutron excess.Comment: 12 pages, 5 figure
Stretching the Rules: Monocentric Chromosomes with Multiple Centromere Domains
The centromere is a functional chromosome domain that is essential for faithful chromosome segregation during cell division and that can be reliably identified by the presence of the centromere-specific histone H3 variant CenH3. In monocentric chromosomes, the centromere is characterized by a single CenH3-containing region within a morphologically distinct primary constriction. This region usually spans up to a few Mbp composed mainly of centromere-specific satellite DNA common to all chromosomes of a given species. In holocentric chromosomes, there is no primary constriction; the centromere is composed of many CenH3 loci distributed along the entire length of a chromosome. Using correlative fluorescence light microscopy and high-resolution electron microscopy, we show that pea (Pisum sativum) chromosomes exhibit remarkably long primary constrictions that contain 3-5 explicit CenH3-containing regions, a novelty in centromere organization. In addition, we estimate that the size of the chromosome segment delimited by two outermost domains varies between 69 Mbp and 107 Mbp, several factors larger than any known centromere length. These domains are almost entirely composed of repetitive DNA sequences belonging to 13 distinct families of satellite DNA and one family of centromeric retrotransposons, all of which are unevenly distributed among pea chromosomes. We present the centromeres of Pisum as novel ``meta-polycentric'' functional domains. Our results demonstrate that the organization and DNA composition of functional centromere domains can be far more complex than previously thought, do not require single repetitive elements, and do not require single centromere domains in order to segregate properly. Based on these findings, we propose Pisum as a useful model for investigation of centromere architecture and the still poorly understood role of repetitive DNA in centromere evolution, determination, and function
Nucleon-Nucleon Interaction: A Typical/Concise Review
Nearly a recent century of work is divided to Nucleon-Nucleon (NN)
interaction issue. We review some overall perspectives of NN interaction with a
brief discussion about deuteron, general structure and symmetries of NN
Lagrangian as well as equations of motion and solutions. Meanwhile, the main NN
interaction models, as frameworks to build NN potentials, are reviewed
concisely. We try to include and study almost all well-known potentials in a
similar way, discuss more on various commonly used plain forms for two-nucleon
interaction with an emphasis on the phenomenological and meson-exchange
potentials as well as the constituent-quark potentials and new ones based on
chiral effective field theory and working in coordinate-space mostly. The
potentials are constructed in a way that fit NN scattering data, phase shifts,
and are also compared in this way usually. An extra goal of this study is to
start comparing various potentials forms in a unified manner. So, we also
comment on the advantages and disadvantages of the models and potentials partly
with reference to some relevant works and probable future studies.Comment: 85 pages, 5 figures, than the previous v3 edition, minor changes, and
typos fixe
Novel simple sequence repeats (SSRs) detected by ND-FISH in heterochromatin of Drosophila melanogaster
<p>Abstract</p> <p>Background</p> <p>In recent years, substantial progress has been made in understanding the organization of sequences in heterochromatin regions containing single-copy genes and transposable elements. However, the sequence and organization of tandem repeat DNA sequences, which are by far the majority fraction of <it>D. melanogaster </it>heterochromatin, are little understood.</p> <p>Results</p> <p>This paper reports that the heterochromatin, as well as containing long tandem arrays of pentanucleotide satellites (AAGAG, AAGAC, AATAT, AATAC and AACAC), is also enriched in other simple sequence repeats (SSRs) such as A, AC, AG, AAG, ACT, GATA and GACA. Non-denaturing FISH (ND-FISH) showed these SSRs to localize to the chromocentre of polytene chromosomes, and was used to map them on mitotic chromosomes. Different distributions were detected ranging from single heterochromatic clusters to complex combinations on different chromosomes. ND-FISH performed on extended DNA fibres, along with Southern blotting, showed the complex organization of these heterochromatin sequences in long tracts, and revealed subclusters of SSRs (several kilobase in length) flanked by other DNA sequences. The chromosomal characterization of C, AAC, AGG, AAT, CCG, ACG, AGC, ATC and ACC provided further detailed information on the SSR content of <it>D. melanogaster </it>at the whole genome level.</p> <p>Conclusion</p> <p>These data clearly show the variation in the abundance of different SSR motifs and reveal their non-random distribution within and between chromosomes. The greater representation of certain SSRs in <it>D. melanogaster </it>heterochromatin suggests that its complexity may be greater than previously thought.</p
Momentum dependent mean-field dynamics of compressed nuclear matter and neutron stars
Nuclear matter and compact neutron stars are studied in the framework of the
non-linear derivative (NLD) model which accounts for the momentum dependence of
relativistic mean-fields. The generalized form of the energy-momentum tensor is
derived which allows to consider different forms of the regulator functions in
the NLD Lagrangian. The thermodynamic consistency of the NLD model is
demonstrated for arbitrary choice of the regulator functions. The NLD approach
describes the bulk properties of the nuclear matter and compares well with
microscopic calculations and Dirac phenomenology. We further study the high
density domain of the nuclear equation of state (EoS) relevant for the matter
in -equilibrium inside neutron stars. It is shown that the low density
constraints imposed on the nuclear EoS and by the momentum dependence of the
Schr\"odinger-equivalent optical potential lead to a maximum mass of the
neutron stars around which accommodates the observed
mass of the J1614-2230 millisecond radio pulsar.Comment: 25 pages, 11 figures, accepted for publication in Nuclear Physics
Cytogenetic analysis of Astylus antis (Perty, 1830) (Coleoptera, Melyridae): Karyotype, heterochromatin and location of ribosomal genes
Cytogenetic analysis of Astylus antis using mitotic and meiotic cells was performed to characterize the haploid and diploid numbers, sex determination system, chromosome morphology, constitutive heterochromatin distribution pattern and chromosomes carrying nucleolus organizer regions (NORs). Analysis of spermatogonial metaphase cells revealed the diploid number 2n = 18, with mostly metacentric chromosomes. Metaphase I cells exhibited 2n = 8II+Xyp and a parachute configuration of the sex chromosomes. Spermatogonial metaphase cells submitted to C-banding showed the presence of small dots of constitutive heterochromatin in the centromeric regions of nearly all the autosomes and on the short arm of the X chromosome (Xp), as well as an additional band on one of the arms of pair 1. Mitotic cells submitted to double staining with base-specific fluorochromes (DAPI-CMA3 ) revealed no regions rich in A+T or G+C sequences. Analysis of spermatogonial mitotic cells after sequential Giemsa/AgNO 3 staining did not reveal any specific mark on the chromosomes. Meiotic metaphase I cells stained with silver nitrate revealed a strong impregnation associated to the sex chromosomes, and in situ hybridization with an 18S rDNA probe showed ribosomal cistrons in an autosomal bivalent
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