15,076 research outputs found
Kinetic control of eukaryotic chromatin structure by recursive topological restraints
Chromatin structure undergoes many changes during the cell cycle and in response to regulatory events. A basic unit of chromatin organization is the nucleosome core particle. However, very little is known about how nucleosomes are arranged into higher-order structures in vivo, even though the efficiency and precision of cell division imply high levels of structural organization. We propose abandoning the current paradigm of chromatin organization based on thermodynamics of the lowest energy state and replace it with the idea of a topologically restrained, high-energy structure. We propose that DNA is subject to a recursive topological restraint, and is anchored by hemicatenates that are part of the chromosomal scaffold. Long-distance _cis_-regulation of transcription is a natural consequence of recursive topological restraint. This new theory of chromatin structure has a multitude of consequences for key aspects of cellular biology
Non-Isotropic Angular Distribution for Very Short-Time Gamma-Ray Bursts
While most gamma-ray bursts (GRBs) are now believed to be from cosmological
distances, the origin of very short-time GRBs is still not known. In the past,
we have shown that GRBs with time duration (T90) less than 100 ms may form a
separate class of GRBs based on the hardness and time distribution of these
events. We have also shown that the ln N ln S distribution is consistent with
the expectation of quasi-Euclidean distribution of sources. In this paper, we
report the study of the angular location of these GRBs showing a strong
deviation from isotropy within the Galactic coordinates of plus 180 degrees <
longitude < 90 degrees and -30 degrees < latitude < 30 degrees. We have studied
the rest of the GRBs and do not find a similar deviation. This further
indicates that the very-short GRBs likely form a separate class of GRBs, most
probably from sources of Galactic or near solar origin
On the origin of bimodal duration distribution of Gamma Ray Bursts
The modified version of a bullet model for gamma ray bursts is studied. The
central engine of the source produces multiple sub-jets that are contained
within a cone. The emission of photons in the source frame of a sub-jet either
takes part in an infinitesimally thin shell, or during its expansion for a
finite time. The analysis of the observed profiles of GRBs taken by BATSE leads
us to the conclusion that the latter possibility is much more favored. We also
study the statistical distribution of GRBs, in the context of their bimodality
of durations, taking into account the detector's capability of observing the
signal above a certain flux limit. The model with shells emitting for a finite
time is able to reproduce only one class of bursts, short or long, depending on
the adopted physical parameters. Therefore we suggest that the GRB bimodality
is intrinsically connected with two separate classes of sources.Comment: 12 pages, 12 figures; accepted by MNRAS. Small changes to match the
corre cted proof
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