Large-scale stability of chromosomes

In addition to experimental observation, another important method of study is computer simulation, based on theoretical models of chromatin. A new study has extended work done previously, which used a simpler model of chromatin consisting up of a single fiber. In the new study, the filament could be made up of two types of fiber, one thicker and one thinner, in varying proportions. Experimental studies have indeed demonstrated the existence of two main types of chromatin, with thicknesses of 10 or 30 nm. In the study, the scientists made molecular dynamics simulations for model chromosomes in various conditions: they added increasingly large amounts of 10 nm fiber to the homopolymer chromatin, consisting of the stiffer 30 nm fiber only.

The model used in the new study, while a simplification of the real molecule, makes the simulation more realistic. The aim of the study was to evaluate whether small-scale modifications in the chromatin fiber lead to large-scale changes in the behavior of the chromosome. "Even after the introduction of the more flexible second fiber, the chromosome remains spatially stable" explains Ana-Maria Florescu, first author of the study and SISSA research scientist. "More in detail," adds Angelo Rosa, SISSA research fellow who coordinated the work, "we found that reorganization occurs only on spatial scales below 0.1 Mbp (million base pairs) and on time scales shorter than a few seconds."

One significant implication of this study concerns the techniques used for the experimental observation of the fibers: those most commonly used today have inadequate resolution to be able to observe this type of reorganization, so we aim to develop new methodologies (like the FISH technique based on oligonucleotides) able to visualize even genome distances smaller than 0.1 Mbp.