507 research outputs found
On pairwise distances and median score of three genomes under DCJ
In comparative genomics, the rearrangement distance between two genomes
(equal the minimal number of genome rearrangements required to transform them
into a single genome) is often used for measuring their evolutionary
remoteness. Generalization of this measure to three genomes is known as the
median score (while a resulting genome is called median genome). In contrast to
the rearrangement distance between two genomes which can be computed in linear
time, computing the median score for three genomes is NP-hard. This inspires a
quest for simpler and faster approximations for the median score, the most
natural of which appears to be the halved sum of pairwise distances which in
fact represents a lower bound for the median score.
In this work, we study relationship and interplay of pairwise distances
between three genomes and their median score under the model of
Double-Cut-and-Join (DCJ) rearrangements. Most remarkably we show that while a
rearrangement may change the sum of pairwise distances by at most 2 (and thus
change the lower bound by at most 1), even the most "powerful" rearrangements
in this respect that increase the lower bound by 1 (by moving one genome
farther away from each of the other two genomes), which we call strong, do not
necessarily affect the median score. This observation implies that the two
measures are not as well-correlated as one's intuition may suggest.
We further prove that the median score attains the lower bound exactly on the
triples of genomes that can be obtained from a single genome with strong
rearrangements. While the sum of pairwise distances with the factor 2/3
represents an upper bound for the median score, its tightness remains unclear.
Nonetheless, we show that the difference of the median score and its lower
bound is not bounded by a constant.Comment: Proceedings of the 10-th Annual RECOMB Satellite Workshop on
Comparative Genomics (RECOMB-CG), 2012. (to appear
Compound radiointerferometer with independent heterodynes for the investigation of emission sources radioimages
Compound radio interferometer with independent heterodynes to investigate radio emission source
Photodissociation of interstellar ArH+
Aims. Following the recent detection of 36ArH+ in the Crab nebula spectrum,
we have computed the photodissociation rate of ArH+ in order to constrain the
physical processes at work in this environment. Methods. Photodissociation
cross sections of ArH+ are computed in an ab initio approach including explicit
account of spin-orbit coupling. Results. We report the photodissociation cross
section of ArH+ as a function of wavelength. Photodissociation probabilities
are derived for different impinging radiation fields.The photodissociation
probability of for a very small unshielded cloud surrounded on all sides by the
unshielded InterStellar Radiation Field (ISRF) model described by Draine (1978)
is equal to 9.9e-12 s-1 and 1.9e-9 s-1 in the Crab nebula conditions. The
dependence on the visual extinction is obtained by using the Meudon Photon
Dominated Region (PDR) code and corresponding analytical fits are provided.
Conclusions. These data will help to produce a realistic chemical network to
interpret the observations. Photodissociation of ArH+ is found to be moderate
and the presence of this molecular ion is mainly dependent on the molecular
fractionComment: 11 pages, 6 Figures, Accepted in Astronomy Astrophysic
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