Topological constraints strongly affect chromatin reconstitution in silico

The fundamental building block of chromatin, and of chromosomes, is the nucleosome, a composite material made up from DNA wrapped around a his-tone octamer. In this study we provide the first com-puter simulations of chromatin self-assembly, start-ing from DNA and histone proteins, and use these to understand the constraints which are imposed by the topology of DNA molecules on the creation of a polynucleosome chain. We take inspiration from the in vitro chromatin reconstitution protocols which are used in many experimental studies. Our simulations indicate that during self-assembly, nucleosomes can fall into a number of topological traps (or local folding defects), and this may eventually lead to the forma-tion of disordered structures, characterised by nu-cleosome clustering. Remarkably though, by intro-ducing the action of topological enzymes such as type I and II topoisomerase, most of these defects can be avoided and the result is an ordered 10-nm chromatin fibre. These findings provide new insight into the biophysics of chromatin formation, both in the context of reconstitution in vitro and in terms of the topological constraints which must be overcome during de novo nucleosome formation in vivo, e.g. following DNA replication or repair

Implante de placa de silicone na substituição parcial da parede traqueal em caninos The sylicon slab implant in substitution of partial canine tracheal wall

Foram operados seis cães, SRD, no intuito de testar a viabilidade do uso de placa de silicone em casos de perda parcial da parede traqueal. Esses animais, submetidos a anestesia volátil, sofreram remoção de um terço da circunferência de três anéis traqueais a nível da região cervical média. No local dessa abertura foi implantada uma placa de silicone com dimensões compatíveis à mesma, fixada a cartilagem traqueal circunvizinha. Os cães foram observados num período de trinta dias, ao final do qual foram realizadas as necropsias com avaliação macro e microscópica do local do implante. Foi detectada a substituição do tecido removido, assim como a sua epitelização.<br>Six dogs were submitted to an operation to test the viability of sylicon slab in partial replecement of tracheal wall. These animais were anesthetized and had removed a third part with compatible dimension of the circunference of three tracheal rings at mid cervical region. At this place a sylicon slab was implanted and fixed to the surrounding tracheal cartilage. The dogs were observed during thirty days. At post morten, they were evalueted by gross and microscopic examination. It was detected the tissue replacement and the growing of the epithelium