Before revision:  We demonstrate that chromosomes undergo prominent conformational changes when the two fibers form separate domains. Conversely, when small stretches of the thinner fiber are randomly distributed, they act as impurities and conformational changes can be observed only at small length and time scales. Our results bring a limit to the possibility of detecting variations in the behavior of chromosomes due to chromatin modifications, and suggest that the debate whether chromosomes expand upon transcription, which is fueled by conflicting experimental observations, can be reconciled by examining how transcribed loci are distributed. Finally, to validate our conclusions, we compare our results to experimental FISH data .
After revision:  We demonstrate that chromosomes undergo noticeable conformational changes only on length-scales smaller than 10^5 basepairs and time-scales shorter than a few seconds, and we suggest there might exist effective upper bounds to the detection of chromosome URLanization in eukaryotes. We prove the relevance of our framework by modeling recent experimental FISH data on murine chromosomes .